MomusDesign
BENCHTOP CNC ROUTER PLANS v.2.1
Version 2.1 © copyright 2013 All rights reserved release date: March 30, 2013 www.momuscnc.com
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preface introduction machine specifications design goals CNC basics CAD/CAM workflow electronics enclosure design alterations structural design bill of materials parts list & schedules exploded views drawing sheets wood parts fabrication wood parts assembly metal parts fabrication metal parts assembly epoxy bed levelling cover installation electronics installation The purchaser of this document has express permission from the author to print a hardcopy for personal use only. This document may not be resold, distributed, or used for commercial gain. Commercial sale of components or assemblies derived from the information herein is forbidden without prior licensing agreement with Momus Design. IF THIS DOCUMENT HAS BEEN PURCHASED FROM ANY SELLER OTHER THAN MOMUS CNC, IT HAS BEEN AN UNAUTHORIZED AND ILLEGAL SALE. Please report any such activity to Momus CNC.
Xylotex installation Gecko G540 installation limit switches Mach3 setup machine alignment spoilboard first use suppliers ADDENDUM: Z axis thrust bearing
table of contents
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Preface to the 2nd edition Welcome to the second edition of the Momus Design CNC router plans. The machine described in these pages is an evolution and refinement of an earlier design, with numerous and significant changes throughout. The original machine was designed and constructed in 2008‐2009, with the plans debuting in the summer of 2010.
Bob Pavlik ‐2012
BENCHTOP CNC ROUTER PLANS
Many of the design improvements represented in this second edition would not have come about without the input and feedback of all those builders who constructed the first version of the machine. Especially significant was the communication between builders and myself that was made possible by the formation of a dedicated sub‐forum on CNCzone.com (http://www.cnczone.com/forums/momus_design_cnc_ plans/). The existence of this platform has become and integral and important aspect of both this design and the plans progressing forward. Please consider this as a welcome invitation to become a part of this community of builders and observers.
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After several years of using the machine, and following the progress of others who were constructing their own copies from the plans, it became apparent that there were many areas that could be improved. By the summer of 2011 it was felt that there were enough desired changes to warrant a full re‐design and refinement of the original concept.
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preface
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Introduction The machine plans contained in this book perhaps bear one significant difference from many others that are available. The difference is that these plans were not put together with the goal of selling plans. They are the by‐product of building a machine that came into existence because of the need for a functional tool.
Disclaimer: The author of this design is not a professional engineer. This manual outlines the construction of a hobbyist machine, designed and built by a hobbyist. Also note that CNC equipment can be dangerous machinery. In addition to the inherent dangers of operating any power tool, doing so remotely, via computer, adds another level of potential danger. Errors in programming machine movement can have catastrophic consequences.
So began the journey of designing and constructing my own machine, a process which ended up consuming several years. The resulting machine has far exceeded my initial expectations. The machine has performed superbly, proving capable of milling materials not typically handled by low cost DIY machines (such as aluminum,) and having accuracy that far surpassed original design goals.
BENCHTOP CNC ROUTER PLANS
The design of this machine began as many others surely do: by studying existing solutions with the intent of creating a copy of the best currently available. Unfortunately, none of the existing home‐built small‐ format CNC routers seemed suited to my particular needs and constraints. With a small workshop space available, limited equipment with which to build, the need to keep dust and noise contained, and a very meager budget, the existing options were quite limited.
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The design and fabrication of this machine was due to a specific need, primarily the building of models for architectural research, that focused on digital design processes and construction automation. The inherent nature of this research required the use of CNC machinery. Despite having access to large format CNC equipment, the flexibility of having a home‐workshop based tool seemed to be greatly advantageous.
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It is the responsibility of the reader to use the information presented in this manual in a manner deemed appropriate to personal judgment and safety, and use at your own risk.
introduction
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Machine Specifications:
ACCURACY Mechanical accuracy: Precision (repeatability):
+/‐ .005” +/‐ .001”
MACHINE CONSTRUCTION MATERIALS Machine base: Plywood Mechanical parts: Aluminum and steel. The machine is built entirely from standard stock material sizes of plywood, aluminum and steel. The stock material thickness is used for all critical dimensions. Materials were chosen for this design in order to give the highest possible machine rigidity for the cost. No MDF (medium density fiberboard) is used in the construction of the machine, as it is too flexible to result in a machine of adequate stiffness. DIMENSIONING Plans are dimensioned in Imperial measure. They are NOT currently available in metric. While the plans are not available in metric, numerous machines have been constructed in countries that use metric measure. This can be done either by adapting the plans for locally available metric sizes of materials, or by sourcing materials from the US (see list of suppliers at the end of the plans.) DRIVE METHOD X and Y‐ belt drive Z‐ precision acme leadscrew with anti‐backlash nut Belt drive can use either open‐ended belting or a closed belt, depending on availability. Z‐axis anti‐backlash nut is a commercially available product.
ADJUSTABILITY AND ALIGNMENT All bearings are provided with a micro‐adjustable setscrew for precise and easy adjustment. The design of the machine is based around a step by step alignment procedure, that allows for highly accurate setup with very simple tools. SPINDLE The machine is intended to use a trim router, such as the Bosch Colt, or Ridgid R2400/2401. MACHINE SPEED Jogging speeds of over 500 inches per minute. STEPPER MOTORS NEMA23 frame size. Minimum recommended size‐ 275 oz./in. RESOLUTION At 1/10 microstepping: X and Y axes‐ .001” Z axis‐ .0000625” Note that electronic resolution is not an indicator of the accuracy of the machine. However, it is a potential indicator of the speed of the machine. The finer the resolution, the slower the machine. MATERIALS THAT CAN BE CUT Wood Plastics Foam Aluminum Brass Circuit Boards Note that although the machine is capable of cutting aluminum, this machine is designed and intended primarily as a wood router. Spindle speeds of trim routers may limit the type of work that can be accomplished in materials other than wood.
Machine specifications
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Exact usable Z axis travel will depend on exact configuration of router mount, length of router bit, and thickness of spoilboard beneath the work piece.
LINEAR MOTION All axes are controlled by standard ABEC7 bearings riding on rectangular cold rolled steel rails.
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MACHINE SIZE cutting envelope‐ 16” x 16” x 5.5” overall machine size‐ 32.5”wide x 27.5”deep x 26” tall.
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Machine Design goals When this machine was designed, a list of goals and criteria were established at the very beginning of the design process. The overall goal was to create an inexpensive machine that could be fabricated in a home shop and perform like a machine costing many times more. Cost: The machine needed to be as inexpensive as possible, while meeting as many other goals. Target build budget was $400, excluding electronics. Tools: Construction had to be possible with minimal tools and equipment. It had to be constructable with basic tools that would not have a high level of accuracy in themselves. This would require creative build techniques to fabricate a machine that was more accurate than the tools used to build it.
Speed: Cutting speed needed to be considerably higher than most other inexpensive DIY machines, which often only moved at 20‐30 inches per minute. To facilitate running programs that contained tens of thousands of lines of code, a target speed of 150 ipm was set. Alignment: Many existing home‐built designs had no simple way of attaining accurate machine alignment. An easy alignment procedure was considered integral to a successful to a design. Bind free: As many existing designs had no way of being accurately aligned, they also often suffered from binding. Linear motion components that could not be brought into being exactly parallel or in‐plane would cause the machine to bind during travel along its axes. This binding could cause lost steps in the drive motors, potentially ruining a work piece.
BENCHTOP CNC ROUTER PLANS
Enclosed: Due to where the machine would be used, it was necessary to have a full enclosure to contain dust and sound.
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Accuracy: It needed to be able to cut to a tolerance of about +/‐.005". Higher accuracy wasn't deemed necessary as the machine was primarily intended to cut wood, which is not a material with high dimensional stability. However, a high enough accuracy was required to allow machining of mating parts that would be assembled, which requires more accuracy than simply carving or engraving a single piece.
Self‐contained: All wiring and electronics should be organized into a single unit, rather than having external components. Compact: To allow for easy use of the machine at different locations, it must require little or no disassembly/reassembly for travel.
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Attractive: The machine must be attractive and look more like a commercial product than something that was cobbled together.
design goals
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Desktop Manufacturing
However, having the ability to easily have these manufacturing capacities available raises questions of appropriate use of technology. If a part can adequately be made by more traditional hand crafted methods, it may be an inefficient use of time and resources to use a computer controlled machine. In addition to the initial time invested in machine construction, the fabrication of a part can require significant time spent at the computer. Even a simple part requires drawing or 3d modeling it, deciding on a machining strategy, generating toolpaths and G‐code from the drawing, and setting up the stock to be cut on the machine. The advantages comes in using the equipment for purposes that cannot be achieved easily by other means. CNC lends itself to jobs requiring high levels of accuracy, consistency between repetitive parts, and cutting complex geometries. These advantages are significant, and potentially transformative, for both hobby and business use.
More recently there has been a tremendous growth in Do‐It‐Yourself (DIY) home‐built CNC equipment. It is now a relatively straightforward process to generate the code (“G‐Code”) to control a CNC machine tool on a home PC, and output the signal through a parallel port or USB port to motion control motors. Depending on how complex and sophisticated the geometry of the parts being manufactured, this can even be accomplished with free software.
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In the early development of CNC, the numbers that were used to control a machine were hand‐coded and punched into a paper roll that was fed through a mechanical reader. The punched holes equated to discrete movement steps. While programming simple movements, such as straight lines, was easily accomplished, curved or free‐form geometry was much more difficult to achieve. With these complex shapes, the smaller the distance between the motion steps, the smoother the results, necessitating the calculation of thousands of movement points. With the advent of the computer came the ability to generate much more complex numerical code, resulting in very smooth machine motions.
The potential implications for this revolution in “desktop manufacturing” are huge. Transferring the manufacture of extremely complex parts from costly industrial settings, which was the only option in the recent past, to a low‐cost home shop opens up a world of possibilities. Many of the machines that have been constructed from the plans in this manual have found use in small home‐ based manufacturing businesses.
BENCHTOP CNC ROUTER PLANS
What was once a technology that existed strictly within industry, CNC (Computer Numeric Control) equipment has increasingly found widespread use in the home workshop. At its most basic, CNC is a method of using a numerical code to control a machine. Nearly any type of machine or configuration can be controlled this way. If it has a range of movement, whether linear or rotary, it can be controlled by a numerical code that instructs those movements. Therefore, CNC can be used on a wide spectrum of equipment, such as milling machines, lathes, plasma cutters, water jets, hot wire foam cutters, wire EDM, etc. Or, as is the case with these plans, a 3‐axis wood router.
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CNC basics
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SOFTWARE WORKFLOW
Design parts in software such as AutoCad, Rhino, Solidworks, TurboCad, etc...
TOOLPATHS (CAM) Generate movement of the cutting tool in software such as: MasterCam, RhinoCam, BobCad/Cam, etc...
MACHINE CONTROL Send cutting tool information to the machine with software such as: Mach3, emc2, TurboCNC, etc..
Creating the geometry for complex three dimensional surfaces requires a much more advanced software than is necessary for simple 2d linework. Software such as Rhino3d, Solidworks, or Inventor are powerful 3d modeling tools designed specifically to manipulate sophisticated topographical geometries. Cutting these surfaces is considered full 3d milling, as the Z axis of the spindle is moving in careful coordination with the X and Y axes to result in the desired shape. These surfaces are exported in file formats that preserve their topographical data.
1. Machine definition. The software needs to know the configuration of the machine itself. While many of the more basic programs are only capable of 3 axis milling, more advanced software can handle more machine axes, or alternate configurations of how those axes move. 2. Stock setup. The software needs to know how large your piece of stock is, so that it can calculate how much material might need to be removed from around the final part. This is less important in basic 2 ½ axis milling, where it often really does not matter how large the stock might be, as long as it is sufficiently large to allow clamping to the machine bed at a safe distance from the cutting path. Stock size is much more crucial when doing full 3 axis milling, as the increased amount Z axis movements creates more opportunities for collision between the cutting tool and the stock. So again, this is a situation of needing to match the abilities of your software to the types of parts that you want to make. Complicated parts may even need to have additional geometry drawn in the design software, to provide additional cutting surfaces to remove material for needed tool clearance. There are a couple of typical ways to enter stock size information within CAM software. One is to enter coordinates for the corner points of your stock size. Better CAM software can automatically detect the boundary size of your part and generate a stock size around that. In this case, it is often desirable to draw the uncut block of material in the design software, so that the CAM software creates the desired material size.
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DESIGN (CAD)
Designing parts to mill Parts that ultimately will be cut with the machine need to originate somewhere, and that typically happens within some sort of CAD (Computer Aided Design) software such as AutoCad, Rhino3d, Solidworks, AutoDesk Inventor, TurboCad, or even software such as Corel draw. This software may be either a 2d or 3d environment. The type of software required will be driven in large part by the type and geometric complexity of the work being designed. Very simple parts that are being cut from sheet stock can be designed in very rudimentary software that merely allows you to accurately draw two‐dimensional lines and export that information in an appropriate file format. Cutting parts of this nature is often referred to as 2 ½ axis milling, as most of the machine motion happens within only 2 axis. The spindle only moves up and down in the Z axis to enter the work at the beginning of the cut and to lift itself clear of the material at the end. Simple parts can be designed in one of the many freeware 2d drafting software packages available, as long as the design can be exported in a file format that is compatible with other software that will be used downstream in the workflow (the CAM software).
Toolpaths The next step in the CAD/CAM workflow is typically to generate toolpaths, or the movements of the machine that are necessary to cut your part. This can happen in CAM (Computer Aided Manufacturing) software such as Mastercam, RhinoCam, MeshCam, MadCam, or a range of other programs. The following are basic steps that are often part of the workflow within a CAM software package.
BENCHTOP CNC ROUTER PLANS
CAD/CAM Workflow
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CAD = Computer Aided Design CAM = Computer Aided Manufacturing
CAD/CAM workflow
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There are two methods of determining feed rates. The best method is to do a “chip load” calculation, which takes all of those factors above into account. The principle of this calculation is that it provides for an ideal quantity of material to be removed by the tool's cutting edge each time it moves through the material. This quantity is often published by the manufacturer of the cutting tool. This calculation will give a fairly accurate number for setting feedrate. The other method is an empirical process, where experience can provide an equally good feedrate number, or can aid in fine tuning the number that is arrived at by a chip load calculation. Unfortunately, in practice many other home‐built machines require neither method. They typically have a maximum travel speed that is far below running any danger of going too fast. So they can often simply be run at the highest feedrate that the machine will allow. If anything, many home‐built machines often have the problem of running so slowly that they can cause a poor surface finish, or even damage to the part or cutting tool, because they do not remove material quickly enough to keep the cutting edge cool When cutting wood this can cause burning of the material. 5. Toolpaths. The next step is typically to generate the actual paths that the tool will follow. With simple 2 ½ axis cutting this is a very simple process without many factors. Primarily all that matters is the size of the tool. More complex 3d cuts open up a wider array of cutting options. Complex parts may require determining a logical cutting strategy, to remove material in a series of stages that will result in the best surface finish or require the least time.
Plunging When milling part geometries where the tool can not simply enter the workpiece from the edge of the stock, such as when cutting a pocket, the tool needs to make some type of descending cut into the material. The simplest method is to move the tool straight down into the material, which may or may not be the most appropriate movement. Many cutting tools are not designed to be plunged straight down into material in such a way. A tool that can accommodate this move is referred to as “end‐cutting,” and can cut on its tip as well as its side. When the tool cannot be plunged, the tip will need to be gradually lowered into the stock as it is simultaneously moved in a sideways direction. This is called “ramping” into the material. The best CAM programs provide great control over how the tool can be ramped down into the material, including straight ramping and helical moves. It may also be desirable to avoid straight plunge cuts due to material properties. The grain of some woods may tear under such a tool movement. Direction of cut Any amount of experience with a hand‐held router will quickly reveal the difference between moving the tool from left to right and right to left along the edge of a piece of material. One direction will be much harder to control. With a hand‐held router you would typically move left to right to maintain the greatest control of the tool. This is called conventional milling. If moving in the other direction (“climb cutting”) the router bit may grab into the material and be pulled in an undesirable direction. However, when controlling the router with a machine the situation is not so simple. Depending on the circumstances, climb cutting may provide a much finer surface finish.
CAD/CAM workflow
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4. Cutting speeds and feeds. Depending on numerous factors (the type of material being cut, the type of cutting tool, the rpm speed of the spindle, how much of the tool is being engaged in the material, and the quality of finish desired), the feedrate must be set for how fast the tool is pushed through the material. On most home‐built machines the spindle speed is set at the router itself, so that does not need to be controlled through the software. The feedrate must be fast enough that the tool can efficiently eject the cut material away from the cutting area.
Roughing vs. Finish When removing large amounts of material quickly, a “roughing” cut is often used. This is a fairly aggressive cut designed to remove stock around the final geometry quickly and efficiently. It may have the tip of the tool engaged deep into the material, and when multiple parallel passes are required it may step over as far as the entire diameter of the tool between each pass. The roughing cut will typically then be followed by a finish cut which removes a much smaller final amount of material, both in depth of cut and distance of step‐over between passes. There may even be a change of cutting tools between the roughing and finish cuts.
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3. Tool size. The software needs to know the diameter and shape of the cutting tool. Many CAM packages have a library of tools from which you can pick the size and shape of your cutting bit. In others you may need to enter this information manually.
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While G‐code is a standard language, unfortunately each machine controller software often uses its own variation. The machine control software typically used by the home‐ builder runs on a version that is often very close to pure (canonical) g‐code. More specialized machinery, which has its own control electronics rather than using a PC for control, often has a correspondingly more specialized version of g‐code. Similar to the need for matching the design software to the type of parts that you want to create, it may take some necessary care to match CAM software to control software. Problems may be encountered with finding CAM software that can handle generating toolpaths for complex geometry, yet has proper post‐processing ability for control software such as Mach 3 or emc2, which are typical of what is used by the home user. Software that will accommodate very complex geometry may only have posts available for more industrial machine controls. This might mean learning enough g‐code to be able to manually edit and alter post‐processed code for use in PC based control software.
Software overlap Often, software use isn’t quite as direct as the workflow diagram might indicate. Many CAM software programs provide tools for doing basic CAD work. While this can be handy for making minor changes after importing geometry to the CAM program, it is rarely powerful enough to use it as the sole design tool. Anything more than minor changes are best done back in the original design software, and then re‐exported to the CAM program. Similarly, control software sometimes has basic tools for conversion of .dwg format line drawings to g code. Again, this isn’t the primary job of the software, so while it may work adequately for very simple jobs, it should not be relied on exclusively. Control software typically also contains a g‐code editor for manipulation of the code.
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7. Post‐processing. Once all of the above steps have been finalized, and the operator is content with the toolpaths, the final step is “post‐processing.” This is where the software converts all of the toolpath information into a format that can be read by other software that will control the movements of the machine itself. This is typically some variation of an industry standard language called “g‐code”, which is a simple text file. G‐code is nothing but a line by line set of numerical instructions for the machine to follow. It gives tool movement information in absolute coordinates, and may also provide information such as feedrate and spindle speed, and on more advanced equipment, moves such as automatic tool changes.
Machine Control software Mach 3, emc2, TurboCNC This software takes the code that was generated by the CAM software (g‐code) and outputs it as electronic signals that actually control the motors on the machine. This typically happens by sending “pulse” and “direction” signals through a parallel port cable to the machine electronics. Mach3 has come to dominate the DIY CNC market, as it is a very robust program and is affordably priced. Many people use the free demonstration version of the software, but it is recommend to purchase the full version if funds allow. The demo is crippled to running 500 lines of code. This might seem like a lot, and might be sufficient for milling very simple parts with mostly straight cuts, but it will quickly be found to be severely limiting for cutting anything more complex. Even small parts with very complex 3d shapes may easily require tens or even hundreds of thousands of lines of code. Another less known, but just as important, restraint of the demo version is that it limits the processing speed of how quickly it outputs signals to your machine’s motors. This can directly limit machine performance.
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6. Simulation. Many CAM programs have a simulation feature which will allow viewing a 3d computer simulation on the computer screen of the tool cutting the material. This can allow watching if the tool is moving in desired directions and sequences, and if there are any possible collisions between the machine and the stock. The best CAM programs will automatically detect these collisions and provide a warning.
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CAD/CAM workflow
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Learning G‐code Another alternative when cutting simple parts is to hand write g‐code. Doing this can eliminate the need for both design software and CAM software. Before the development of sophisticated CAM software, this was how numeric machine control code was generated. The number of applications of this technique are probably limited these days, but it can be a useful method to know. Even if hand writing numeric code is not a primary working method, understanding the G‐ code language can be useful as it can allow quick editing or modifying parts of the code such as feed rates, without having to go through the post‐ processing step again. Combined with other computer programming skills, learning g‐code could also allow writing scripts to generate toolpaths and g‐code from within 3d modeling software such as Rhino.
BENCHTOP CNC ROUTER PLANS
While software with advanced functionality can be very expensive, there is a substantial and growing quantity of lower budget programs available. Some of these are even free. While they are often lacking certain features, a significant amount of complex work can often be achieved with then by employing some creativity in how they are used. Workarounds can often be discovered that can compensate for functionality that they may lack. The supplier list at the end of this manual includes a list of software suppliers.
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Cost of software As might be surmised by this point, the cost of the software necessary to design, generate toolpaths, and then control the machine can be very, very expensive, especially if needing to create parts of any complexity. Before constructing a CNC machine, it is highly recommended to look into the costs of the software programs that might be required, as they can easily far exceed the budget for machine construction. Most software vendors have functioning demonstration versions available for download, and it is also recommended to try them before purchasing. Some are much more user friendly and intuitive than others, which have very steep learning curves.
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CAD/CAM workflow
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computer
While specific components of an electronic drive motion control system for a homebuilt CNC machine can vary widely, the abstract diagram to the left illustrates the basic principles of what is included. In general, these are:
motor drives & power supply
machine motors
1. A computer to send motion data to a hardware component called a “motor drive.” 2. A power supply to provide the required voltage & current to the motors. 3. Electronic motor drive(s) that forwards the motion data to the motors at the required voltage/current. 4. The motors. This is the general flow of information from the computer to the motors. In addition, there are typically hardware components to provide data feedback from the machine to the computer. All systems should be equipped with limit switches at the end of each axis travel, to provide safety to both the machine and operator. More sophisticated drive systems may have feedback sensors that give more accurate control of the motor positioning. Working backwards from the motion of the machine: Motors Most home‐built machines are controlled by stepper motors. These are simple type of DC motor that requires a pulse of electricity to move it one “step”. A typical stepper motor has 200 steps per revolution, so to cause continuing rotation in a stepper motor it requires a fast stream of discrete electrical pulses. The frequency of the pulses will determine the motor speed.
These motors are easy to electronically control via computer, and relatively inexpensive, but they do have some drawbacks. One is that there is the possibility of them “losing steps” under a load. This happens due to the stream of electrical step pulses continuing to flow to the motor even though it is temporarily being prevented from moving. Since the number of steps required to move the machine is very high, a very small number of missed steps may not have any noticeable impact on the finished part. On the other hand, enough missed steps may be catastrophic. In the best case, it may result in a less than perfect part, and at the worst it may result in machine collisions, since after the event that causes lost steps the machine location is not corresponding to where the software thinks it should be. Missed steps is a problem with stepper motors because they typically lack any sort of feedback mechanism. They simply do as they are told, and the control software has no way of recognizing any error that may occur, or way of correcting the motion. Some more advanced systems employ a sensor that informs the software of the position of the motor or machine, so that it can compensate for any deviation and return it to proper location. Another problem that steppers often suffer from is “mid‐ band resonance.” This occurs when the frequency of step pulses causes a dynamic resonance within the motor. This may cause it to move erratically or even lock up completely. This is obviously an even bigger problem than a few missed steps. Some stepper drives have circuitry that is designed to combat this phenomena. Many hobby level drives do not. Many industrial machines use servo motors rather than steppers. They do not run in discrete steps like a stepper motor, but rather are more similar to a conventional motor design. These can be either DC or AC designs, and typically run at a much higher rpm than steppers, necessitating a gear reduction of some sort. Due to both of these factors, servo motors consequently have a much smoother operation than steppers. The major advantage of servos is that they typically have a positioning feedback loop. They employ a device called an “encoder” that monitors the position of the motor. Any discrepancy between the theoretical position of where the motor should be, and where it is measured as being, can be compensated for, and brought back to the correct position. However, servos are still much more expensive systems to set up on a home‐built machine. They require more sophisticated electronics equipment to drive them and more knowledge to set them up and “tune” them.
machine electronics basics
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While an in‐depth discussion of machine control electronics hardware is beyond the scope of these plans, an overview is important for a basic understanding. For the most part, this discussion will stay somewhat abstract, although in the machine assembly instructions the specific installation of two types of drive boards, a Xyoltex and a Gecko G540, will be covered. Due to the complexity of choosing individual components that will function well together, it is highly recommended to purchase a pre‐packaged kit from a supplier that includes all of the electronics as a matched collection.
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Electronics
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Torque curve & power transmission
A belt drive was also felt to be advantageous over a lead screw system for this machine as it is more tolerant of misalignment. A lead screw must be aligned exactly parallel to the machine axis it is powering. By contrast, the belt can be out of alignment by a significant amount, with zero negative impact on machine accuracy or performance. Note that misalignment will have a significant negative impact on belt wear. Drives All motors that are used for motion control require some type of electronic drive board to control them. Drive boards take a variety of arrangements. They may incorporate control for several axes on a single board, or may be configured as an individual board for each axis. The advantage of an separate board per axis is that they can be replaced individually in case of damage.
Microstepping Another function of many drives is that they break up the number of steps per revolution that are required at the motor into a greater number. So for instance, a drive may have “1/8” or “8x” microstepping, which would effectively increase the number of steps per revolution that control the motor from its original 200 to 1600. This is advantageous in that it increases the resolution of the system and provides finer control over the movement of the machine. Note that these micosteps are typically not an exactly equal sub‐ divinding of the original 200 steps. Each microstep may vary from the others by a very small percentage. This discrepancy is typically so small that it is inconsequential and will not adversely effect the accuracy of the machine. However, when including microstepping in the calculation of machine resolution, it should be understood that this number has a very slight variability within it. Power supply This is as simple as it sounds. A transformer type power supply device is matched to the needs of the drive board(s) and motors. It is worth noting that many boards that operate by “pulse width modulation,” such as the Xylotex, actually perform most efficiently at the upper limits of voltage that they can handle. In other words, running them at lower voltage will not necessarily provide any additional protection for the drive board. Stepper motors also commonly require many times more voltage than their ratings may indicate. For instance, a stepper motor that is designated as a 2.5 volt motor may require a 24 volt power supply to efficiently power it.
electronics
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The belt drive system on this machine was designed with exactly these factors in mind. It is felt to be a good compromise between machine speed, power available to push a tool through the material it is cutting, and avoiding motor speeds that would be vulnerable to mid‐band resonance.
All drive boards do essentially the same thing. They receive input signals from the control software, which are low in voltage and current, and in turn output these signals to the motors with higher voltages and currents that they require for operation. As such, they mediate between the computer and the machine. Their in‐between position also allows them to handle signal inputs for additional functions such as emergency stop buttons and limit switches. Most drive boards are very vulnerable to any errors in mis‐wiring. Incorrect connections, or breaking a connection to the motors while under power, can cause an immediate destruction of the electronics on the board. Follow manufacturer directions very, very carefully.
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Like all electric motors, stepper motors have the characteristic of producing the greatest amount of torque at zero rpm. What this means is that the faster the motor turns, the less force it produces. Manufacturers provide graphs that show their motors torque output relative to rpm. Not all motors are created equal, as some have torque output that falls off much more quickly relative to rpm increase than others. Therefore,when designing a power transmission system it is crucial to know what rpm the motor will be turning for a given movement speed at the machine. “Gearing down” the system may not necessarily increase the machine’s “power” as the decrease in the motors available torque at a higher rpm may be greater than the mechanical advantage that is gained through gear reduction. However, even though the motor produces most torque when barely turning, the machine cannot be geared to maintain the motor at that speed. If the motor is turning that slowly then the distance between each “step” of its movement will translate to too large of a movement at the cutting tool. It will not have a fine enough cutting resolution. So the system becomes a compromise between several factors.
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Additional switches Drive boards or breakout boards will furnish some means of wiring in several important additional devices. An emergency stop button should be part of every system. It is typically a large red button with a mushroom‐shaped head, that provides an immediate way of shutting down the machine in case of an emergency. It should be placed in a location that is easily accessible while operating the machine. It can be wired to shut down all axes of machine movement, and can usually also be wired to shut down power to the router to kill the spindle movement. If at all possible, your e‐stop button should be wired in this manner.
The computer All of these inputs and outputs either originate or terminate in the control software in the computer. Most control software for home‐built machines is written to be used on PCs, although Mac versions are becoming available. The most popular, Mach3, is designed for Windows based machines, and others such as emc2 are Linux based. A computer with a parallel port for output to the machine electronics will typically be required. The computer need not be the latest model, and in fact an older model with Windows XP or even Windows 98 may be preferable with some software. A desktop model is generally better than a laptop, as most laptops do not have a high enough voltage output through their parallel ports (if they even have one) to perform well. They also often have power saving features to extend battery life, which interfere with the pulse timing of the control software.
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Breakout board Simple drive boards, such as those that have multiple axes self‐contained on a single board, may be designed for direct connection to the computer’s parallel port via a standard cable. When using multiple drives that each control an individual axis, an additional piece of hardware called a “breakout board” may be required. This is merely a device that connects to the computer via a cable (typically parallel port) and then provides multiple connections to allow wiring to the drives, emergency stop stop switches, limit switches, spindle control relays, etc. These boards also often provide an added layer of protection between the higher voltage drive boards and the vulnerable low voltage computer. They do this through optically isolated connections.
The other switches that should be wired into the system are limit switches. These are placed at the end of each axis’s range of movement, thus a 3 axis machine will typically have 6 switches. A limit switch on the Z axis, in the direction of movement toward the machine bed, may be omitted as it would require frequent repositioning due to changes in stock size, cutting tool length, etc. These switches will stop the motion of the machine if it unexpectedly reaches the end of a travel axis. This can prevent serious damage to the machine as well as guarding against personal harm from broken cutters. In addition to acting as safety devices, these switches can do double duty as homing switches. These are used to return the machine automatically to its home XYZ position. Most control software can be configured to use the switches in this manner.
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electronics
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Machine enclosure and sound transmission principles
The other type of sound control is between adjoining spaces. This is sound transmission. An example of this would be a situation such as adjoining rooms in a building, where it is desirable to have as little sound make its way from one room to another. This is a completely different situation from controlling sound within a space, and this is the type of sound control that the machine enclosure must provide, preventing the transmission of sound through it. The foam insulation used by many builders is not intended to control sound transmission, therefore it would be of significant use only if a listener were placed inside the machine enclosure. Otherwise, it will mostly serve to collect a lot of dust. There are three primary categories with which sound transmission can be controlled: 1. Distance 2. Isolation 3. Mass
The second category is very important when it comes to machinery. Vibrations can be transmitted through materials and cause new vibrations to be produced a distance away. These new vibrations produce sound. To combat this tendency, many pieces of equipment have isolation mounts that damp vibrations. These may be comprised of springs, hydraulic devices, or elastomeric materials such as rubber. Even if all sound could be contained within an enclosure, bolting the machine rigidly to a bench could have the effect of turning the bench top into a large sounding board. The third category is the one that is of primary importance to this design. Increasing mass is a very effective way of preventing sound transmission, therefore the ideal situation is to make an enclosure out of a thick, massive material. This has its obvious problems, such as weight, needing to open the enclosure, and needing to provide windows to see in. The window material will have a low mass, increasing the sound transmission of the enclosure as an overall assembly. Thicker window material can be used, at an increased cost. What is of most interest is a sub‐category of the principle of increased mass, and that is the removal of any gaps or cracks. A gap is a zone of zero mass and has a major consequence on sound transmission. It has such an impact that a 1/32" wide crack in the wall of a room can allow more sound through than the entire rest of the wall. To combat this, the enclosure must be thoroughly sealed. The tighter it is, the more effective it will be at preventing sound transmission. Some home‐builders who have installed a foam lining claim an improvement. Small gains may be seen for a couple of unintentional reasons. Depending on how it is attached, it may be preventing surfaces of the enclosure from vibrating and producing sound. In effect it is providing some damping. More importantly, it is helping with the mass issue. While it provides a small amount of direct additional mass, it is covering up crucial gaps. Unfortunately, better results could most likely have been achieved with a simple roll of tape or tube of silicone caulk.
sound control
MomusDesign
There are a couple of different types of “sound control.” First is the controlling of sound within a space. Sound waves bounce off of surfaces and it is often desirable to have a certain type of control over how this happens. The more times a sound bounces off of surfaces before it reaches a listener's ear, the longer the “reverberation time” is considered to be. Sometimes a longer reverberation time is desirable, such as in a concert hall, and at other times a short or zero reverberation time is needed, such as in a recording studio which must be free of echoes. Lining walls and ceilings with a material that absorbs sound waves will prevent them from being reflected back to a listener, and makes the space more acoustically “dead.”
The first category is obvious. The farther away a sound is placed, the quieter it is going to be. This category clearly isn’t of much help in designing a machine, as it can't simply be placed farther away.
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One of the important considerations while designing this machine was the necessity for it to be enclosed, for the containment of both dust and noise. There are many existing examples of home‐built machines that have also included a cover for these reasons. However, there generally seems to be a fundamental misunderstanding of the basic principles of sound control. This is evidenced by the technique that many of these builders have adopted: lining the inside of the enclosure with foam‐rubber acoustical "insulation” of the type with an “egg‐crate” texture. Unfortunately, this is an incorrect application of this material.
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The following section outlines design information, and a principal reason for including it here is to provide some knowledge of why the design exists as it does, and why some aspects can and some cannot be changed. Many of the other plans on the market make a selling‐point out of stating that they can be widely modified to suit individual needs. While this sounds attractive in principle, there are serious problems with this approach. The primary one is that changing the size of the machine can have serious negative consequences on its performance. Increasing a dimension can increase internal forces on machine components, as well as increasing the deflection of components. These stresses and strains can be many times in excess of the what would be encountered in the machine as it exists in these plans.
The assembly steps in these plans start with the fabrication of the wood components and finish with the metal ones. This sequence was chosen because it presents a smooth, seamless order of constructing the machine.
In short, you should not make changes to these plans unless you fully understand what the consequences might be. The sizes of all components have been optimized for the overall size and use of the machine. Proceed to make changes with extreme caution and at your own risk.
However, it is highly suggested to read through the entire set of plans and then decide if this sequence makes the most sense for you. If there is any doubt about the skills involved to construct the machine, then it is suggested to fabricate the metal components before the wood components. This sequence provides a couple of advantages: The first is that even though it may be more unfamiliar to many builders, metal can actually be easier to work. It has no perceptible grain and is therefore predictable in its behavior. Most of the metal parts are aluminum, which is a soft metal and has relatively low cutting forces involved, much like those of a hard wood. The second reason is that fabricating the metal parts will constitute the bulk of fabrication time and require the most patience to maintain build tolerances. The number of cuts, holes to drill and tap, and amount of hand filing and finishing will ultimately end up being a considerable amount of tedious time consuming work. This sequence will also quickly reveal the level of skills and stamina to complete the metal components. By the time all of the metal components are fabricated, the wood fabrication will most likely seem quick and easy.
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Assembly steps
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Can changes be made to the plans?
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design alterations and construction
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a. b. c. d.
Study existing designs that have been built by others. Construct something that looks similar. Be disappointed with some aspect of its performance. (Hopefully) build a second one.
Design can be a very complicated process, requiring not only significant knowledge but good decision making skills to balance the often conflicting criteria that are needing to be met. Progress in any field is typically incremental, with the breaking of revolutionary ground being the exception rather than the rule. Therefore, steps “a” and 'b” are both inevitable and desirable. The question becomes what knowledge and skills are necessary to successfully critique and improve upon existing designs? Design Process and principles The following is a brief attempt at explaining the thought process and rationale behind this particular machine design. This is provided for two reasons. First, to perhaps provide a general starting point and advice for those who decide to design their own machine. Secondly, to provide a foundation for those who may wish to make modifications to these plans. What follows is a list of points of understanding that were accumulated during the process of designing this machine. They range from observations of typical problem areas, to design philosophy, to structural formulas and engineering information. To those with engineering knowledge, please excuse any over‐simplifications of concepts. This information is aimed at those with little design expertise and is not intended to transform anyone into a capable engineer or designer. Rather, it is intended to illustrate how complex it can be to design a seemingly simple device and how quickly one can get in over their head, even when equipped with a little bit of over‐simplified information. Design hubris should be kept in‐check, in favor of the cautious and proven path.
Woodworking skills This may seem somewhat tangential to a design process, but it is not. Many builders of a CNC router, who are intending to use it to mill wood, possess very rudimentary woodworking skills. Many have never experienced trying to control a hand‐held router. It is important to have an intuitive understanding of the behavior of the material that will ultimately be milled with the CNC machine, and how it interacts with a cutting tool. A CNC router is a very advanced piece of woodworking equipment, and success in using it is going to be much greater if the operator has a solid understanding of woodworking basics and a feel for the material. Improving one's woodworking skills will quickly reveal important understandings of grain, how the tool wants to follow it, in which directions it wants to tear, and why it is important to control machine backlash some way other than through electronic compensation. The forces required to move a blade through wood, or to hold it back, may be unexpected, especially when it is spinning at 25,000 rpm in a router. Better woodworking skills will equate to a better feel for how to electronically control a cutting tool, and will also improve the build quality when constructing the actual machine. Use the router to build the machine If constructing a CNC router, then obviously at some point a router will need to be purchased for use as the spindle. By acquiring it early in the process, it can be used to fabricate the machine itself. Many of the wood parts for this machine can just as easily be cut with a router as they can a circular saw or a table saw. Several of the cutting operations, such as pockets, will actually require a router. With a guide‐fence and clamps, very accurate cuts can be achieved with the hand‐held router. Using the router while building the machine will quickly reveal the differences between moving the tool left or right along an edge (climb‐cutting vs. conventional cutting), the importance of feedrates and spindle speeds, how to plunge into a workpiece, the effect of different types of cutting bits, and the general forces involved.
design process
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The design of a home‐built machine often has a design process that follows these steps:
MomusDesign
Designing a machine:
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Stiffness comes from a combination of properties. The first is the flexibility of the material itself, due to a property called Modulus of Elasticity. The second factor is how that material is arranged in space, due to a property called Moment of Inertia (or more correctly, the Second Moment of Area.) Thus the first factor is based on material properties, and the second on geometrical properties. Because stiffness comes from a combination of these two factors, it means that to some extent having a surplus of one property can make up for a deficiency in the other.
Joints Connections between components are as important as material choice. There is a huge difference in the amount of force that can be transmitted through different types of joints. One big limitation in using wood for any structural application, whether a machine, a building, or a piece of furniture, is designing adequate joint details. Subtle differences in how pieces come together can have a significant effect on strength. Creating strong joints in a material such as MDF is no small undertaking, but it can be done. Basic structural design principles Some understanding of very basic structural principles will go a long way in designing a machine. A few simple experiments can help illustrate them. Attempting to twist a cardboard box with no top is relatively easy. Taping a top in place so that all six sides are solid planes makes it many times more resistant to being twisted. This has increased the torsional strength of the assembly. Attempting to flex a ruler that is lying flat and spanning between two supports is also relatively easy. Repeating this with the ruler standing on its edge shows no perceptible deflection. This has increased the bending strength of the member. Attempting to hold a piece of thin paper straight out, while holding at only one edge, and it will droop down. Putting a 90 degree crease, or a curve, in the paper that is perpendicular to the edge being held, and it will now rigidly cantilever out. This change in geometry has made it into a form‐ resistant structure.
MomusDesign
A lack of stiffness causes several problems. The obvious one is that if the machine deflects under a cutting load, then the tool isn’t in the spot that the computer thinks it is. In other words, your cut is not going to be accurate. The other big problem is “chatter.” A machine is a dynamic structure, meaning there is motion involved. That motion can cause vibrations and oscillations in the machine components if it can flex excessively. At best, experiencing chatter may mean reducing cutting speed or taking cuts that are not as heavy. At its worst it can destroy the cutting tool, the part being cut, or even cause damage to the machine.
What this means is that even a very flexible material can be arranged to produce a strong and rigid structure. This is why a machine built of MDF has to be so bulky, as it needs to position the material in a way that can overcome its inherent flexibility. Unfortunately this is still not as good as using an inherently stiff material in a geometrically efficient way. So yes, a somewhat rigid machine can be built out of wood. No, never as rigid as cast iron, nor as compact.
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Stiffness One of the biggest shortcoming of many home‐built machine designs is a lack of stiffness, or rigidity. Stiffness is the ability to resist deflection. Materials have “elastic” behavior. At its simplest, pushing on them causes them to deflect. Release the force and they spring back. Push too hard and it doesn’t spring back. When this happens, it has bent or broken, due to either exceeding the material's elastic limit and causing “plastic deformation,” or going beyond its yield point. ALL machines have some amount of deflection. A machine built of HDPE plastic has a very large quantity of it that may be easily seen by the naked eye. One built of MDF still probably has a considerable amount, and is often more than should be acceptable for construction of a machine. A machine that is built of massive cast iron and costs hundreds of thousands of dollars still has some deflection, it is just infinitesimally small relative to its cutting accuracy.
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Here is a basic list of aspects that should have somewhat closely matched accuracy levels: ‐Frame material. ‐Rigidity of frame design (NOT the same as frame material) ‐Ability to align and adjust the machine. ‐Type of bearings or guides. ‐Method of driving motion. ‐Backlash, or the amount of play in the machine. ‐Electronic resolution of steppers or servos. ‐Machine speed. ‐Spindle power.
A model built as just described can be pushed, twisted, and flexed to see where the design lacks stiffness. If it seems excessively flexible, making modifications to the physical model, or constructing a new version, can be a quick way of making significant advances in improving the design. This method will often reveal areas of weakness that were not anticipated, and this will be happening during a phase where they are easily corrected. Once an actual machine is constructed, making structural changes may be nearly impossible. Attempting to push a physical model to the point of breaking can also be instructive. If it takes significant force, and feels like it does not flex at all before catastrophically exploding, it is a good indicator of the overall stiffness of the assembly. If it softly crumples, or easily comes apart at the seams, more design work is probably required. It is possible to build a posterboard model that is nearly impossible to break in your hands.
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Before dedicating significant time to constructing a CNC machine, it can be very valuable to evaluate the design by constructing a small‐scale physical model. In addition to revealing potential problems with overall constructability or interference of parts, it can give a sense of its structural performance. By building it out of a material that has some inherent flexibility, it will be easy to witness how the geometry of the design effects the stiffness. Materials such as 1/16” chipboard (material that cereal boxes are made from, can be sourced at a good art supply store,) mat board, posterboard, or bristol board are excellent for this purpose. Avoid stiff materials like foam core. The more flexible the material, the easier to see an exaggeration of where the design will flex. A model does not need to be constructed so that parts slide or move, it can be fixed in mid travel. However, it should be constructed in a manner that mimics how the material will really come together at joints, as this can be a significant location of flex.
Matched level of components A common problem with DIY machine design is the necessity of specifying individual components that work well in conjunction with each other. A machine should have a consistent level of accuracy, or rather of inaccuracy, in its components throughout. A scenario like installing expensive high accuracy ball screws for motion control on an MDF machine will never realize the benefit of the high quality components. The amount of flex in the MDF material, or its dimensional instability due to moisture in the atmosphere, will far exceed the tolerances that the ball screws can attain. The levels of accuracy between the two components may be off by orders or magnitude. The quality level of all components needs to improve together in order to see improvements in overall accuracy of the system, otherwise money is merely being wasted.
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Building a model It is common to build a virtual 3d model of a potential design, with software such as SketchUp, Rhino3d, Solidworks, etc. However, one of the big problems that comes with the easy availability of 3d modeling software is that it does not give any sense of materiality and real‐world behavior. Every design works perfectly on the computer screen. The result being that endless variations of a design are drawn without a real understanding of its basic shortcomings, and few improvements are made from one iteration to the next.
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Degrees of constraint One common problem in many DIY designs is having the machine bind under a cutting load. Each axis of the machine needs to smoothly roll on a bearing system, and each axis needs to have bearings configured in a way that will constrain the motion so that it will only go exactly in the direction of that axis. This typically requires many bearings that are spread somewhat far apart. All works well with this arrangement until forces on the machine cause flexing, and some components are no longer in exact alignment with the direction of travel, causing binding. The arrangement of bearings on the Y axis of the design in this manual are somewhat unconventional. They are arranged so that the right side of the gantry is fully constrained against rotation in two directions. The gantry in effect is then cantilevered out from this set of bearings. The bearings on the left side of the gantry act as outriggers that stabilize its position. The bearing configuration on the left is not fully constrained in itself, but provide constraint to the overall system in the third direction of rotation.
Alignment. Aligning a typical home‐built machine can be problematic. Smooth bind‐free operation requires accurate alignment between bearing surfaces on each axis. It requires getting the machine parallel, square, and in‐plane. This can be a very difficult task to accomplish. Nearly all home‐built machines provide some means of providing adjustability, however, many use methods that make fine adjustment very difficult or impossible. Some do not provide enough directions of adjustability. The biggest problem is that nearly none of them provide any sort of reference plane to measure from. It does no good if the machine can theoretically be aligned, but there is no practical way of making measurements to find that alignment. This design attempts to solve these problems in several ways. Using the manufactured faces of stock metal pieces provides some amount of automatic alignment between parts. All of the bearings have a set screw to allow very fine adjustments of their pre‐load against the axis rails that they ride along. Finally, by pouring a thin self‐leveling layer of a very low viscosity epoxy on the bed of the machine, it can be used as a consistent reference plane from which to base all alignments.
design process
MomusDesign
‐The machine flexing: frame, motor mounts, etc. ‐Play: space between bearings and rails. ‐Linear motion inaccuracy: rails not straight, variation in dimension, not parallel or in‐plane. ‐Linear drive inaccuracy. ‐Electronics error.
Removing one direction of constraint from the left is intentional. It has been done this way so that the machine cannot bind if there is excessive flexing of the gantry. Flexing of the gantry will allow a small rotation in one direction to absorb this force. While under these conditions there may be some inaccuracy in the cut due to deflection of the gantry, the machine will keep moving and not bind. Accepting this very minor inaccuracy was felt to be a better situation than having the machine bind, which most likely would result in a completely unusable part. When a machine with stepper motors binds, what can often happen is that the computer keeps feeding the signal to the motors because it does not know that there is a problem. If the machine suddenly starts to move again, it is now receiving code that is out of sync with where in the cutting process it left off. The result is a large cutting error. Do not add additional bearings to the left side of the machine unless you understand the implications for doing so. It will require much more accurate alignment of the machine to prevent binding.
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Tolerance and accumulation of error Beyond creating a rigid machine that will not deflect under a load, there are other factors in design. One is the issue of tolerance. Many home‐built machine designs make claims like: “cuts accurate to .000025". A number like this is merely a theoretical electronic resolution of the stepper motors, and has no bearing on actual accuracy of the overall machine. It in no way translates to accuracy at the tip of the tool, which is where it counts. What does effect accuracy is the individual tolerances of the various parts of the machine working together. Sometimes inaccuracies will serendipitously cancel each other out, as an inaccuracy in one direction will be counteracted by an inaccuracy in the other direction, the net result being that the error is not large. But this is an uncommon situation, and the individual errors are more likely to accumulate and add to each other. So a theoretical accuracy tolerance for the machine is best arrived at by adding all of the possible causes of inaccuracy to arrive at a total possible inaccuracy number. Here are a few things that contribute:
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Structural design
Rubber Low density polyethylene HDPE Polypropylene Nylon MDF (wood composite) Oak wood (along grain) Pine wood (along grain) Magnesium metal (Mg) Aluminium alloy Brass and bronze Titanium (Ti) Copper (Cu) Wrought iron and steel
1,500‐15,000 30,000 200,000 217,000‐290,000 290,000‐580,000 530,000 1,600,000 1,300,000 6,500,000 10,000,000 17,000,000 15,000,000‐17,500,000 16,000,000‐19,000,000 30,000,000
Looking at this chart, the inherent problems of using materials like plastic or MDF for machine construction become immediately apparent. They are orders of magnitude more flexible than even the lowest modulus metals. The philosophy in the design of this machine was to use materials of high modulus wherever possible. One counter‐intuitive outcome of this is that it is often actually less expensive to use small quantities of higher modulus materials than large quantities of low modulus materials. Designing using higher modulus materials also allows the machine to be more compact, which can further help in reducing flex. It is important to understand that the ability for a material to resist deflection is not exactly the same as its “strength.” These are two separate measures of material properties. For our purposes, the ability to resist deflection is what is more important. If there is enough material to provide a stiff enough design, there is very little chance of it not being “strong” enough, so we can ignore that structural need.
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As mentioned earlier, there are two factors at play to achieve stiffness. One is a property called modulus of elasticity. This a property of the material and is essentially a measure of its flexibility. A higher number indicates a stiffer material. A stiffer material is obviously desirable when building a machine. The second factor at play is moment of inertia. This is a measure of stiffness of a shape or geometrical arrangement of the material in space. The same amount of a material can be used with differing amounts of efficiency. An example was given earlier of the bending efficiency of a ruler, comparing it when supported on its flat side vs. when on its thin edge. This phenomenon is happening due to the moment of inertia of the ruler being greater in one direction than the other. Another example would be to flex a thin steel rod. A steel rod that is about 1/4" in diameter can easily be flexed and bent by hand. However, that same quantity of material can be made into a large diameter hollow tube with a thin wall thickness. This tube would not be able to be flexed by hand. Both have the same amount of material, but the tube has a much higher moment of inertia.
To get an idea of the range of material stiffness, here is a modulus of elasticity chart. All values are in psi:
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Rigidity: For the design of the machine in these plans, rigidity became the primary design factor, as everything springs from the stiffness of the machine. Accuracy goes hand in hand with flexibility. It does not matter if your electronics can control motion to .0001" if your machine frame flexes 1/8" under a cutting load. This seemed to be the biggest downfall of the existing home‐built designs. Many machines were being built primarily of MDF (medium density fiberboard.) This material is by nature very flexible. The manner in which it was being arranged often did not help the situation.
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‐Section dimensions. Increasing its size in section will make it stiffer. A 1" x 1" bar will be stiffer than a ½" x ½" bar. ‐Section shape. Making the bar taller will make a much bigger difference in making it stiff than will making it wider. (Think back to the ruler example). Another good example of this is a floor joist. A 2x12 floor joist is going to be stiffer than a 2x6 floor joist. ‐Length. A longer piece is going to be more flexible than a short one. ‐End constraints. The bar in this example is just resting on a support at each end. This means it can rotate slightly as the bar deflects. If the bar was held rigidly at each end, like if it was welded solidly to another object, it would make it more resistant to flexing. ‐Load. A load that is spread out over the length of a member will cause less flexing than if it is all concentrated at the mid‐point.
The calculation of deflection is a three step process:
MomusDesign
To find solutions to these questions, two types of information are required. First is the modulus of elasticity, which is a property of the material. The second is the moment of inertia which is a property of the cross sectional shape of the part. This information works in combination with other factors such as the overall length of the part, how a load is applied, and how its ends are supported.
Example: A simple piece of metal bar stock that has each end resting on a support. A single load is pressing down in the center of it. What factors make it flexible or stiff?
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Moment of Inertia As briefly discussed on the previous page, the second factor in achieving stiffness is how the material is distributed in space. This is called moment of inertia, or more correctly, the second moment of area. As also mentioned, since most designs are driven by stiffness requirements rather than strength needs (the concept here is that if you built it so that it is stiff enough, it is going to automatically be strong enough) the focus here will be on stiffness. Unfortunately, comparing the moment of inertia of even very basic design options against each other requires some math. A few simple equations will be introduced here. Hopefully this will provide some (relatively) easy information to see the implications of alternate design decisions. These formulas will help compare scenarios such as increasing the dimensions of member, comparing solid shapes to hollow sections, and how much deflection increases if a span is increased.
1. Look up the material's modulus of elasticity in a chart. 2. Calculate the section modulus based on cross section shape. 3. Calculate the deflection. The deflection formulas take many of the factors such as end constraints and loading conditions into account, so it is just a matter of finding the formula that matches the situation.
structural design
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2
E=
Calculate moment of inertia
I=
cross sectional shape
30,000 200,000 217,000‐290,000 290,000‐580,000 530,000 1,600,000 1,300,000 6,500,000 10,000,000 17,000,000 15,000,000‐17,500,000 16,000,000‐19,000,000 30,000,000 30,000,000
max. deflection =
formula
W 4
a I = 12
a
=
W L3 48 E I
=
W L3 192 E I
=
W L3 3EI
L
a
simply supported at both ends, force applied at center of span.
3 bd I = 12
d
b 4 a I = 12
a
W
a
b 4
a
b
I = a12 b
4
L
fixed support at both ends, force applied at center of span.
a h
k
d
3 3 I = bd 12 hk
MomusDesign
LDPE HDPE Polypropylene Nylon MDF Oak wood Pine wood Magnesium Aluminium Brass & bronze Titanium Copper Wrought iron Steel
3
Calculate deflection
BENCHTOP CNC ROUTER PLANS
1
Find modulus of elasticity
b
W
I = 0.049 d
d
4
L
version 2.1 Units: All dimensions on this page are in inches.
D
d
4
4
I = 0.049 (D d )
fixed support at one end, force applied end of cantilever.
copyright 2013
Modulus of Elasticity units are in lb/in 2
structural design formulas
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Aluminum E=10,000,000
1.0
0.5
0.5
1.0
4
-simply supported at ends -force applied at center 1/2" square aluminum W= 10 lb at center L= 24" long
2
step
=
4
a I = 12
a I = 12
4 0.5 I = 12
4 1.0 I = 12
I = 0.00520833
I = 0.0833
W
W
L
L
W L3 48 E I
= (10) 24
138,240
3
step
W L3 48 E I
3
= (48) (10,000,000) (0.00520833)
MomusDesign
1
step
Now, lets change the material to 1" square aluminum. All else stays the same.
BENCHTOP CNC ROUTER PLANS
Example:
(10) 24
3
= (48) (10,000,000) (0.0833) 138,240
= 2,499,998
= 39,994,000
deflection = 0.055"
deflection = 0.003"
Note that the original example had over 17 times as much deflection as the second!
deflection example
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structural design conclusion
Among those other factors, it is unusual, indeed difficult, for any part to be designed so that there is only one type of stress involved. A good example of this would be a truss, which operates on the principle that its members are either in pure compression or pure tension. In practice this is very difficult to achieve and there is nearly always some amount of bending force introduced due to the realities of joint design. In the case of a CNC machine such as this one, the gantry experiences a combination of torsion and deflection forces. Not only do the cutting forces push on it causing it to deflect, but those forces are not aligned directly with its centerline. The cutting forces are cantilevered some distance down the Z axis. This eccentric loading introduces a torsional twist into the gantry.
A successful design also depends on creating joints that can transmit loads between parts in an effective and appropriate way. This is one other aspect that these simple calculations do not consider. Also keep in mind that all of this information pertains to static conditions. Remember that a machine tool is a dynamic condition and makes it much more complex to predict its structural behavior. Kinetic movements and rotating parts can introduce momentum, vibrations, and oscillations that can amplify static stresses to the point of breaking failure. Discussion of dynamic behavior is well beyond the scope of this set manual. As stated earlier, the intent of providing this information is twofold. First, it is here to illustrate that what may seem simple can be quite complex to calculate and predict. Please bear that in mind when tempted to make changes to the plans. The second reason for providing this knowledge is that it can give at least some sense of the implications of making changes. It is quite easy to do a few calculations and see that doubling the length of part makes it much more flexible. To directly see those numbers and be able to compare them may just aid in your judgment of how far dimensions can deviate from those in the plans before the cross sectional size of a member needs to change. In the ideal situation, this knowledge might help you improve upon the design as given
MomusDesign
Not only do cross section properties complicate the calculations, but there are complexities even within what may seem to be an adequate solution according to these calculations. For example, increasing the size of a hollow section while reducing its wall thickness will result in a stiffer member. However, there are limits to this. Wall thickness can not be reduced too far or the member will be vulnerable to buckling under a load. So while the basic deflections calculations show it to be adequate, there are other factors that may reject this as a solution.
Understanding and calculating combinations of forces can be very complex and is well beyond the average home‐builder's analytical abilities. It is important to keep in mind that this is nearly always the rule rather than the exception, so the structural calculations shown here should be viewed as a guideline for understanding some very basic principles. It can be used as a rough means of comparing the effects of changing sizes of members. This should in no way be seen as a definitive guide for structural design, if anything it should be taken as an illustration of just how difficult it can be to understand even simple loads on a part.
BENCHTOP CNC ROUTER PLANS
These calculations on the previous pages are adequate for very basic cross sectional shapes and simple loading situations. Essentially they are for finding deflection in simple types of beams. Unfortunately, the real world is rarely that cooperative. As soon as the cross section becomes more complex (like an I‐beam for example,) or multiple pieces are attached together (like the gantry on this design,) calculations become much more complicated. Determining the deflection in these composite assemblies is beyond the scope of this simple demonstration. Similarly, any introduction of loads other than the very idealized conditions shown in the formula diagrams also makes calculation tremendously more complicated and is beyond our scope.
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structural design conclusion
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GENERAL NOTES ‐ Do not measure off of printed drawings. Use dimensions as indiacted on drawings. Scale is not indicated on drawings due to variation in printer accuracy. Printing at 100% scale factor may not guarantee exactly 100% on paper!
Z
‐Do not make changes to the design without having a full understanding of their implications. This machine was carefully designed such that all components work together as a integrated whole. Changes may have unforeseen implications later in the build process, or may negatively affect the operation of the finished machine.
X
Y
rear
left
right
front
‐Counterbores. Many metal parts indicate counterbores. These are OPTIONAL. Counterbores are present to reduce the thickness of material to be tapped, to make hand tapping easier. ‐Filing to fit. If tolerances are not held accurately enough it may be necessary to file some parts slightly during assembly. ‐BE SAFE. Use good judgment while working and do not attempt anything that is beyond your ability or that may jeopardize your personal safety.
MomusDesign
‐Tolerances. An accuracy of +/‐ 1/32" is generally sufficient on metal parts. Any exceptions to this will be noted. Cut wood parts should aim for this same level of precision.
Naming conventions for axes and directions follow these standards throughout:
BENCHTOP CNC ROUTER PLANS
‐Dimensions on mechanical parts are given in Imperial decimal units. Dimensions are given to either 2 decimal place accuracy or may be given as full decimal equivalents to fractions. This does not indicate degree of tolerance required.
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general build notes
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METAL
6063‐T52 rectangular tube
STEEL
(2) @ 24.00"
A36 hot rolled angle
48.00” total
.25" x 2.50" CRS bar (1) @ 12.00” (1) @ 22.50”
6061‐T6 square bar
36.00” total
1.25" x 1.25" x .125" angle (2) @ 24.00"
(1) @ 24.00"
24.00” total
.75" x .75" bar
.25" x 1.50" CRS bar 1018 cold rolled rectangular bar
1.50" x 3.00" x .125" wall
48.00” total
(2) @ .75” (2) @ 1.25 (1) @ 2.50” (1) @ 6.75” (1) @ 7.25” (4) @ 7.375” (1) @ 8.50” (1) @ 10.75” (1) @ 20.75”
96.00” total
.125" x .75" bar (1) @ 5.625”
6.00” total
.1875" x 2.50" bar
.1875" x 3.00" bar (2) @ 24.00"
48.00” total
.375" x 1.25" bar Within each size of stock, individual part lengths are also listed. These quantities can be used if ordering material by individual part length. If ordering by individual part length, note that most suppliers will only guarantee cut accuracy to +/‐ 1/16", and cuts may not be perfectly square or have high finish quality.
6061‐T6 rectangular bar
(1) @ .75” (1) @ 1.00” (1) @ 1.25” (1) @ 1.875” (1) @ 2.50”
8.00” total
.75" x 1.00" bar
NOTE: Before ordering metal or other parts, consult the ADDENDUM on page 171 of this manual. It outlines upgrading the machine with thrust bearings on the Z axis (HIGHLY RECOMMENDED.) A separate bill of materials is listed for these parts in the addendum.
(4) @ 1.75” (2) @ 2.75” (1) @ 5.625” (1) @ 11.25”
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9.00” total
BENCHTOP CNC ROUTER PLANS
(1) @ 4.25” (misc) 4.5"
ALUMINUM
NOTE: Total quantity for each size of stock is listed. Total quantities typically include additional material length to include width of saw cuts. These quantities can be used when ordering material in bulk lengths. If ordering in bulk lengths, note that it is often less expensive to round quantities up to the nearest whole foot.
31.00” total
.75" x 1.25" bar (1) @ 2.00" (2) @ 2.25” (1) @ 6.75” (2) @ 4.25"
22.50” total
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.75" x 1.75" bar (2) @ 4.25
9.00” total
bill of materials 1
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(2) 4' x 8' sheets, 3/4" sanded finish plywood, such as “cabinet grade.” Note on plywood: While "cabinet grade" is specified, the important qualities to look for in plywood is that it has a sanded finish on both faces, preferably a high count of core plies, and core plies with no voids. Nominal 3/4" plywood is often marked and measures 23/32". This thickness, or close equivalent metric sizes, will not affect the construction of the machine base. To the greatest extend possible, the design allows for such small variation in material thickness.
PLASTIC
socket head cap screws (2) #10‐32 x .75” (7) 1/4‐20 x 1/2” (1) 5/16‐18 x 2.5” (4) 5/16‐18 x 3” soc. head cap screws (OR machine screws) (4) #4‐40 x 1” (8) #10‐32 x 1” (4) #10‐32 x 2.5"
hex head bolts (1) #10‐32 x .75” (or 5mm x 20mm) (14) 1/4‐20 x .5” bolts (modified, see page 86) (28) 5/16‐18 x 1.5” (8 modified, see page 86) (1) 5/16‐18 x 2.5” lag bolts (30) 1/4” x 1.25”
ELECTRONICS
nylon insert lock nut
Stepper motor choice may depend on the type of materials that you intend to cut with the machine. For very soft materials, such as foam, the machine may perform adequately with stepper motors with a lower rating than the suggested 275 oz./in. Stepper motors, motor drive, and power supply must all be carefully matched to each other for proper motor performance. Information on making these decisions is beyond the scope of this manual. For this reason, it is suggested that motors, drive(s), and power supply be purchased as a prepackaged kit from a reputable source.
HARDWARE grade 8 hardware (4) (8) (4) (4)
5/16‐18 x 3” bolts 5/16 washers 5/16 lock washers 5/16‐18 hex nuts
wood screws (1 lb. box) #10 x 1.25" (1 lb. box) # 10 x 3" nails (1 lb) 6d bright finish
(46) sealed roller bearings, ABEC‐7 skate 8mm x 22mm x 7mm (608 size) (2) 10 tooth timing pulleys, 3/8" wide, .200 XL pitch, bore dia. to match stepper motor shafts 400XL timing belt‐ 200 teeth x 3/8" width, McMaster part # 6484K445 500XL timing belt‐ 250 teeth x 3/8" width, McMaster part # 6484K451 OR 10 feet 3/8" wide x .200 XL pitch open ended timing belting.
soc. head cap screws (OR hex bolts) (1) 1/4‐20 x 1” (1) 5/16‐18 x 2”
(1) 24"' x 24"' sheet, polycarbonate. ~1/8" min. thickness, ~3/16" preferred.
Min. recommended stepper motor size: 275 oz./in.
MOTION
(4) (8) (48) (35)
#4‐40 #10‐32 1/4‐20 5/16‐18
flat washers (SAE) (4) #4 (20) #10 (100) 1/4” (100) 5/16 (6) 3/8" (modified, see page 86) lock washers (4) #4 (12) #10 (28) 1/4” fender washers (2) 5/16 set screws (grub screws) (50) #10‐32 x 3/8" (2) #10‐32 x 3/4" roll pins (4) 3/16" x 2" threaded rod (allthread) (96") 1/4‐20 (total length)
8" 3/8‐10 acme one‐start precision rod, McMaster part # 99030A327 (36" length) (1) delrin anti‐backlash nut w/ .925" square flange, DumpsterCNC part # AC38101‐LN (1) delrin shaft coupler, @stepper shaft dia., DumpsterCNC part # AC38101‐AC OR 8" 3/8‐8 acme two‐start precision rod, McMaster part # 99030A315 (36" length) (1) delrin anti‐backlash nut w/ .925" square flange, DumpsterCNC part # AC38082‐LN (1) delrin shaft coupler, @stepper shaft dia., DumpsterCNC part # AC38082‐AC OR 8" 3/8‐8 acme four start precision rod, McMaster part # 99030A303 (36" length) (1) delrin anti‐backlash nut w/ .925" square flange, DumpsterCNC part # AC38084‐LN (1) delrin shaft coupler, @ stepper shaft dia., DumpsterCNC part # AC38084‐AC Acme rod choice is a compromise between speed and resolution. Single‐start will have a finer resolution, but slow travel speed. Four‐start will move quickly with slightly less resolution. In general, the two or four‐start will still have enough resolution for most needs, and is thus recommended (four‐start preferred). Also, low torque stepper motors (below 300 oz/in) may require single start or double start, for additional mechanical advantage.
ELECTRICAL (12') stepper motor wire, cont. flex, shielded, McMaster part # 7673K44. Limit switch wire. (5) limit switches, SPDT submini lever switch, Radio Shack part # 275‐016 (or similar). Emergency stop switch.
MomusDesign
PLYWOOD
HARDWARE
BENCHTOP CNC ROUTER PLANS
NOTE: Consult ADDENDUM (page 171) for additional Z axis thrust bearing parts that are not listed in this bill of materials.
MISC. wood filler. carpenters wood glue. paint and primer. 24 fluid oz. (32 fluid oz. recommended) low viscosity epoxy resin & hardener, Ports and Pins>Input Signals The software must be configured to know when a limit switch is being activated, as well as knowing on which axis the event is occurring. The image to the right shows typical settings that correspond to the wiring diagram used on pages 151/152. The port# must correspond to which parallel port is being used to control the drive. The pin number corresponds to how the limit switches are wired through the drive (or breakout board.) On the Gecko G540 and Xylotex drives, certain terminal connections correspond to certain parallel port pin numbers.
Home Referencing: Config>Homing/Limits
BENCHTOP CNC ROUTER PLANS
MomusDesign
The machine can be instructed to sequentially move each axis in a determined direction, until it hits a “home” switch. The limit switches, configured above, can also be used for this homing operation. No separate switches are required. By checking “auto zero”, as seen in the image to the right, an axis will be included in the homing operation. The X and Y axes are typically homed in the negative direction, while the Z is homed positive. Due to the particular motor wiring, here it can be seen that the direction of two motors required reversing, in order to orient the correct positive/negative movement directions. Activating the REF ALL HOME command (circled in red in the lower right illustration), each axis will move at a reduced speed until it contacts a home switch. It will then back away from the home switch slightly. This position will be the zero coordinate for the axis. These three 0,0,0 locations set the “machine coordinates.” Soft limits: Config>Homing/Limits Rather than depend on limit switches for end of axis protection, travel limits can be set through the software. These are distances from the 0,0,0 machine coordinates. The advantage of using soft limits is that the machine will gently decelerate as it approaches this travel limit, unlike the abrupt stop caused by hitting a limit switch.
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Mach3 setup
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eq ua l
l ua eq
At this point the machine is basically finished and primarily needs alignment before use. The first alignment step will be to get the two X rails exactly parallel to the surface of the machine bed. This is done by measuring up from the bed at the front and rear of each rail, and adjusting them so that the measurements are equal. This can be accomplished to a high level of accuracy with some relatively simple tools.
BENCHTOP CNC ROUTER PLANS
MomusDesign
The image to the left illustrates an adjustable height gauge that was constructed from some scrap rectangular tubing and a length of threaded rod. The tubing size was 1 1/2" x 2". The surfaces of the tubing are very flat and allow the gauge to sit flush against the bed without rocking. The top of the threaded rod was filed to a shallow point. Note in the images to the right that the threaded rod was bent very slightly to the side so that it would reach exactly under the rails.
The height gauge is used in conjunction with a set of feeler gauges (middle left image.) These are simply thin flexible metal strips that are inserted into a gap to "feel" how wide it is. They can be purchased at any auto parts store for a few dollars. Adjust the threaded rod up under the front of the right X rail until it is almost touching. Ideally, leave about .010" of clearance. Use the feeler gauges to measure this gap. With trial and error, find the feeler gauge strip, or combination of strips together, that slides into the gap with just a faint amount of drag along its surface. Without changing its adjustment, reposition the height gauge under the rear of the right rail. Measure the gap between it and the rail with the feeler gauges. Loosen the bolts holding the rail to the rail angle and reposition it so that the gap is the same at the front and rear. This may take moving the height gauge from front to back several times and taking repeated measurements. With some patience, the rails can be very accurately positioned this way. Do the same for the left rail. The right and left rail should each be the same exact same height above the bed.
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alignment
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The two X rails should now both be parallel to the machine bed. The next adjustment is to set the Y rail parallel to the bed. This is done in a similar fashion, but we will now be adjusting bearings to effect its height. We will again be using the height gauge and feeler gauges, this time to measure the left and right sides of the Y rail (left and right images.)
BENCHTOP CNC ROUTER PLANS
MomusDesign
The four bearings that ride along the tops of the X rails will need to be adjusted (circled in the middle right image). All four will need to be exerting equal pressure downward on the rails so that the gantry will not rock. Again, this will require some trial and error and multiple repositioning of the measuring gauge during the process. Tighten the nuts on the bearing bolts so that they are just slightly snug and then adjust the set screw to reposition it before fully tightening the nut (lower right image).
While adjusting these four bearings to get the Y rail parallel to the bed, occasionally check that the front face of the Y rail is as close to perpendicular to the bed as possible (left photo). The closer to perpendicular, the easier it will be to adjust the Z axis later. Take your time with these adjustments and be patient. It may take multiple attempts to get the adjustment correct and may be frustrating and time consuming at first. The process will proceed faster as you develop a feel for adjusting the bearings.
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alignment
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Once you are satisfied with the adjustment of the four bearings that ride along the tops of the rails, next adjust the four that ride along the lower edge. Move the gantry so that the bearings line up with the four depressions in the inner skins, as shown in the lower right image. These are provided to allow clearance to get a wrench onto the heads of the bolts. However, as the bolt heads are thinner than normal, it will take a special wrench to fit onto them.
BENCHTOP CNC ROUTER PLANS
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There are a couple of solutions to this. One is to simply grind down the head of a standard wrench so that it fits. If you do this, grind the wrench slowly to avoid overheating the metal. Cool it often by dipping it into water. Another solution is to use a ready made wrench. Many power tools have special wrenches to fit their arbors that are thin. The lower left images shows a wrench for a router that was the required 1/2" size. Another option is to buy a bicycle “cone wrench” that is made for working on bicycle hubs (wrench with blue handle in image.) Buy one in a 13mm size. Note that this is a slightly loose fit on the 1/2" head (which is 12.7mm) but should work fine. These do not need to be excessively tightened, so no rounding should occur if the wrench is squarely on the bolt head.
Adjust these four bearings until they lightly touch the bottom edge of the rails. This also requires acquiring a feel for how tight against the rail they need to be. They need to be tight enough that there is no free play, but not so tight that they increase rolling resistance. A good test is that with the gantry stationary, you should still be able to grasp the bearing and turn it against the rail. It should slip against the rail when turned with some pressure. It should not slip very easily, but should not turn completely freely either. The tendency while adjusting may be to over‐tighten the bearings. It is easy to over‐tighten the bearings to the point where the stepper motors can not move the machine, or lose steps during movement. If in doubt, it is better to go with an adjustment that seems slightly too loose rather than too tight.
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alignment
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Most of the bearings on the machine are adjusted in sets, that simultaneously touch opposing sides of a rail. This may either be across the width (left illustration), or across the thickness (right illustration). Keeping this in mind can help during adjustments, as there are a couple of strategies that can be used to cause an adjustment to have the needed effect. Adjusting two bearings along one surface (the top two bearings in the upper left illustration) can cause either rotational shift or a parallel shift. Make these adjustments with the opposing bearings loose. Once bearings along one surface are satisfactory, adjust in the opposing set (middle left illustration.)
BENCHTOP CNC ROUTER PLANS
MomusDesign
If all four bearings are making the desired amount of contact, a rotational shift can be accomplished by leaving two opposing ones alone, while adjusting the other two equal amounts in the same direction. (lower left illustration.)
The next bearings to adjust will be the carriage bearings that ride along the Y rail. First adjust the four that contact the top and bottom edges (circled in the middle right image.) While adjusting these four bearings, also try to get the carriage itself perpendicular to the bed, as shown in the photo to the right. The closer to perpendicular the carriage can be set, the easier the alignment of the Z axis rail and Z screw will be later.
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alignment
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The Y axis bearings that ride along the front and rear faces of the rail are adjusted next. There are a total of eight bearings, that need to be adjusted as one group. Note that on the front ones, a single socket head cap screw secures two bearings at a time. Each bearing does have an individual set screw adjuster. You may find it easiest to adjust these double sets of bearings first (middle right image), and then adjust the rear ones in to touch the rail (upper right image.)
BENCHTOP CNC ROUTER PLANS
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The rear bearings also use bolts with thinner heads for clearance, so the thin wrench will be needed here too.
Like with the previous adjustment, check often during this adjustment to try and set the carriage perpendicular to the bed in the front/rear direction. Again, this will make later Z axis adjustments much easier, and will result in smoother Z axis operation.
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alignment
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The next adjustment will be to the Z axis rail. This needs to be adjusted so that it is perpendicular to the machine bed in both side to side, and front/rear directions. First adjust it side to side, using the four bearings circled to the right.
BENCHTOP CNC ROUTER PLANS
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Jog the Z axis down so that it is at the lowest point of its travel. Be careful that the rail does not move low enough that it it no longer captured between the top bearings. A small triangle or square can now be used as a gauge to set the rail perpendicular. When the Z rail is perpendicular to the bed, double check by flipping the triangle around and holding it up to the other side. If your triangle is not exactly a 90 degree angle, or the bed surface is uneven, then flipping the triangle to the other side will reveal the discrepancy, and that something is inaccurate.
Move the triangle or square to the front of the rail as shown in the image to the right. Adjust the eight bearings that ride along the front/rear surfaces of the rail to get it perpendicular to the bed. These also have paired bearings on long socket head cap screws, so use the technique that was used on the Y rail. All three of the machine axes should now be in proper adjustment and alignment.
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alignment
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The final adjustment is to the Z axis drive screw. There is no simple place to directly measure to find its proper positioning, so it will take some trial and error. Begin by jogging the Z axis to its lowest position. Tighten the #4‐40 x 1" machine screws and nuts holding the anti‐backlash nut so that they are snug enough to hold it in position, but it can still be repositioned with some slight force. Also snugly tighten the nuts securing the nut plate, and nut plate block. Each of these points can later provide adjustability. Standing in front of the machine, visually align the drive screw so that it is parallel to the Z axis rail. Move to the side of the machine and do the same in that direction.
BENCHTOP CNC ROUTER PLANS
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Jog the machine all the way to the top of its Z axis travel. It should not bind, and the sound from the stepper motors should stay consistent during the travel. Readjust as necessary. It will probably require jogging the Z axis up and down several times, readjusting each time, to finally get it correct. Once it is in a satisfactory position, tighten all of the nuts fully. On the anti‐backlash nut, be sure to either use thread‐locking compound (the removable type), lock nuts, or lock washers. These are highly vulnerable to vibrating loose, and their small size does not allow excessive tightening without stripping threads.
If all is adjusted correctly, the Z leadscrew should be parallel to the Z rail when viewed both from the front and from the side. They should remain parallel in both directions as the Z axis is jogged along its length. Binding may occur if the two are not parallel during the entire length of travel.
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alignment‐ Z screw
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Install the router mounts, as seen in the image to the left. Note that the mounts shown in this series of images are an older variation, and are not exactly the same shape as the current design. With the Z axis jogged to its lowest position, set a triangle or square against the faces that will contact the router body. Note if the square touches both evenly. If the mounts are not exactly perpendicular to the machine bed they can be shimmed into the correct position. Thin metal foil or paper can be used as shim stock.
BENCHTOP CNC ROUTER PLANS
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When satisfied that all of the clamping surfaces are perpendicular to the bed, install the router and front clamp halves. Tighten to secure the router.
Make sure that the router and cord clear all mechanical components as the Z axis moves through its full range of motion. The extended threaded rod that contacts the Z axis limit switch can also be used to manage the router cord. Be sure that it can not interfere with proper operation of the limit switch. Here a washer and nut were used to contain the cord.
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router and mounts
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Some method is required to secure workpieces to the bed area. The bed area should also be provided with a sacrificial “spoilboard” wear surface that can accommodate milling operations that cut through the entire thickness of the work. The spoilboard may be as simple as a piece of medium density fiberboard (MDF) that is fastened to the machine bed. Workpieces can in turn be screwed to this surface. A more versatile system is shown here, which consists of aluminum T‐track extrusions that are set within pieces of MDF. This provides a convenient and fast method of clamping that does not require sinking screws into the spoilboard surface. Many types of clamping accessories are commercially available for T‐track systems.
BENCHTOP CNC ROUTER PLANS
MomusDesign
The assembled pieces in the illustration to the left result in a working surface that measures 15” x 15”. This size will allow the cutting tool to just extend beyond each edge. This is necessary so that the router can be used to mill this surface flat after installation. The spoilboard can also be resurfaced after it wears during use. The assembly shown here used a piece of 3/4” plywood as the center layer. This was used rather than MDF as it is easier to screw into. Any screw that penetrates into MDF should be provided with a pilot hole to avoid damage, as the MDF does not easily displace material. The top MDF strips were fastened from the bottom, through countersunk clearance holes in the plywood layer. 1.500
.75
1.000
.50
1.000
.50
.75 1.500
.25 0.500
1.375 2.750
Typical extruded aluminum T‐slot track.
(2) pieces, MDF.
.75 1.5
.50 1
.25 0.5
2.75 5.500
.25 0.5
Thickness of spoilboard: The Z axis of the machine has a substantial amount of travel, and it should be kept in mind that the machine will be most rigid with the Z axis at its highest. Thus, it is beneficial to mount the workpiece as high as possible. If you anticipate cutting a consistent type of parts, such as mostly parts cut from 3/4” thick wood, then the spoilboard can be constructed with this in mind. It can be built to a thickness that raises the work to a desired height. The spoilboard could also be constructed in multiple layers, that could be removed to lower it when necessary.
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(4) pieces, MDF.
spoilboard
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The middle right image illustrates the finished spoilboard before mounting on the machine. Note that the top of the MDF surface is well above the top surface of the aluminum T‐track. This provides a safe thickness of wear surface that would need to be penetrated before the cutter would contact the metal track.
BENCHTOP CNC ROUTER PLANS
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The lower left image shows one type of clamp that is available for this T‐track system. Special T‐track bolts are also available, allowing more custom clamping configurations. It is recommended to use clamp components that are made of plastic, brass, or other soft metals where possible. This will reduce the danger that is possible if a clamp is accidentally hit by a cutting tool.
In the image to the right, the finished spoilboard is mounted on the machine bed. It was carefully located so that the cutting tool can cut move just beyond its perimeter for surfacing. It was also carefully positioned so that the T‐slots are as parallel to the X axis travel as possible. This makes it easier to clamp material to the spoilboard in a position that is known to be parallel to that axis.
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spoilboard
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Congratulations, the machine is now complete and ready to be used. After familiarizing yourself with a CAM application, and generating G‐code to cut a part, here is a suggested procedure for the first use of the machine. 1. Home the machine by using the Mach3 REF ALL HOME command. This will zero the Machine Coordinates. 2. Jog the machine so that the tip of the tool is in the location on the workpiece that corresponds to 0,0,0 as it was defined in the CAM software. When in that position, use the buttons circled in the image to the left to set the Work Coordinates. Note that this does not alter the Machine Coordinates, which are still used to control soft limit locations. 3. Jog the machine to make sure that soft limits are functioning correctly. The Soft Limits button (also circled in the left image) in Mach3 needs to be activated, which will be indicated by a green border around the button.
BENCHTOP CNC ROUTER PLANS
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4. Load a G‐code program. File>Load G‐code. The lines of code will appear in the window that is circled in the right image. 5. Run the program. For the first time running a program it is advisable to zero the Work Coordinate for the Z axis higher than the workpiece. This way you will be “cutting air” and have an opportunity to observe if the program seems to be running correctly. Keep a hand on the emergency‐stop button and click the Cycle Start button in Mach3 (also circled in the right image.) Watch to see that the axes appear to be traveling the correct distance. If there are any problems, the machine can be stopped by hitting the emergency‐stop button, or the red Stop button in Mach3 (circled at right.) 6. If all appears to be working correctly, set the correct Z axis Work Coordinate, jog the tool a safe distance above the workpiece, turn on the router spindle, and run the program. Congratulations again, you've just cut your first part. 7. It is advisable to measure the finished part, to compare its cut dimensions to its design dimensions. If there is any variation, the steps per inch values in the Mach3 motor tuning screens may need slight adjustment.
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This concludes the building of the machine, and hopefully begins many enjoyable hours of machining parts. Be creative!
copyright 2013
‐Bob Pavlik
[email protected]
first use
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SUPPLIERS Metals
Stepper Motor Drives
Epoxy
Online Metals 1138 West Ewing Seattle, WA 98119 800_704_2157 http://www.onlinemetals.com/
[email protected]
Xylotex, Inc. 2626 Lavery Court #307 Newbury Park, CA 91320 http://www.xylotex.com/
[email protected] http://groups.yahoo.com/group/Xylotex/
Jamestown Distributors 17 Peckham Drive Bristol, RI 02809 http://www.jamestowndistributors.com/ ‐MAS low viscosity epoxy
Speedy Metals locations in Wisconsin, Michigan, Texas http://www.speedymetals.com/
GeckoDrive Motor Controls 14662 Franklin Ave. Suite E Tustin, CA 92780 http://www.geckodrive.com/ http://groups.yahoo.com/
Dumpster CNC http://dumpstercnc.com/
[email protected] ‐anti‐backlash nuts ‐motor couplings McMaster‐Carr http://www.mcmaster.com/ ‐precision acme threaded rod ‐many misc. components Stock Drive Products http://www.sdp‐si.com/ ‐drive belt ‐timing pulleys Bearings VXB Bearings http://www.vxb.com/ Ebay vendor user id: irvineman store name: VXB Bearings Skateboard and Slotcar
CNCrouterparts http://www.cncrouterparts.com/ ‐GeckoG540/motor kit with molded cables Longs Motor Ebay vendor user id: longsmotor99 store name: Changzhou Longs Motor Co ‐motors and drives. Ships from China. Soigeneris http://www.soigeneris.com/ ‐Gecko G540 drives ‐G540 heatsink kit ‐EZ‐G540 DB‐9 connectors with potentiometer
Bolt Depot www.boltdepot.com ‐hardware BOLT IT UP Ebay vendor user id: 5137jones store name: BOLT IT UP ‐hardware Multiple Items Hubbard CNC Ebay vendor user id: carolbrent store name: HUBBARD CNC INC ‐wide selection of CNC parts
NOTE: Momus Design has no affiliation with any of the vendors on this or the following page. Inclusion here does not necessarily imply an endorsement.
parts suppliers
MomusDesign
Power Transmission
KelingCNC/Automation Technology Automation Technology Inc 2112 Stonington Ave Hoffman Estates, IL 60169 http://www.kelinginc.com/ ‐vwide selection of motors and drives
Hardware
BENCHTOP CNC ROUTER PLANS
Metals Depot 4200 Revilo Road Winchester, KY 40391 http://www.metalsdepot.com/
Ebay vendor: user id: polymerproducts store name: Polymer Products ‐low viscosity resin
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SOFTWARE CAD (Computer Aided Design) Software
CAM (Computer Aided Manufacturing) Software
Machine Control Software
‐AutoCad ‐AutoCad LT ‐Inventor (3d CAD) AutoDesk http://usa.autodesk.com/
‐SheetCAM 2.5d CAM http://www.sheetcam.com/
‐Mach3 CNC controller ArtSoft Newfangled Solutions LLC http://www.machsupport.com/
‐Google SketchUp Free 3d modeler Google http://sketchup.google.com/ ‐Rhino3d 3d NURBS modeler McNeel North America http://www.rhino3d.com/ ‐Blender free 3d modeler, less intuitive than Rhino http://www.blender.org/ ‐SolidWorks (3D CAD) Dassault Systemes http://www.solidworks.com/
‐MeshCAM 3d CAM, with indexed 4th axis capability http://www.grzsoftware.com/landing/ ‐Cut2d (2.5d) ‐Cut 3d (3d) ‐VCarve Pro (V‐carving, 2.5d) ‐Photo VCarve (converts images to toolpaths) http://www.vectric.com/
‐CNC Lite ‐CNC Plus CamSoft (951) 674‐8100 http://www.cnccontrols.com/ ‐DeskCNC controller seriall port based controller IMService http://www.imsrv.com/deskcnc/
Misc. Software ‐FreeMILL (free basic 3d CAM) ‐VisualMill (3d) ‐RhinoCAM (plug‐in for Rhino3d) http://www.mecsoft.com/
‐Deskengrave (free) Converts TrueType fonts to .dxf drawings http://www.deskam.com/deskengrave.html
‐DeskProto Entry Edition simple 2.5d, 3d http://www.deskproto.com/
‐ACE converter (free) Converts .dxf drawings to G‐code http://www.dakeng.com/ace.html
‐ArtCAM Express 2.5d, 3d http://www.artcamexpress.com/
‐G‐code to .dxf converter (free) Converts G‐code to .dxf drawings http://www.cnczone.com/forums/ opensource_software/8814‐g‐code_dxf.html ‐LazyCAM (comes integrated into Mach3) Basic .dxf drawing to G‐code. http://www.machsupport.com/
MomusDesign
‐TurboCAD CAM plug‐in available IMSI/Design http://www.turbocad.com/
‐PhlatScript free 2.5d CAM plug‐in for SketchUp http://sketchuppluginreviews.com/2010/04/30/ phlatscript‐google‐sketchup‐plugin‐review/
‐LinuxCNC (EMC2) http://www.linuxcnc.org/
BENCHTOP CNC ROUTER PLANS
‐DoubleCAD XT Free AutoCad LT type clone, 2d. http://www.doublecad.com/
‐CamBam Plus 2.5d, limited 3d CAM http://www.cambam.info/
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‐NCPlot (backplotter for G‐code verification) http://www.ncplot.com/
software suppliers
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Z axis thrust bearing addition:
# reqd. 1 1
BILL OF MATERIALS The materials for this upgrade are not included in the main Bill of Materials on page 27. These quantities should be added to that primary Bill of Materials when purchasing metal stock and hardware. Required is:
ALUMINUM .1875" x 2.50" x 2.25"
This addendum covers the installation of a pair of thrust bearing to the Z axis lead‐screw assembly. The original design of the Momus CNC router did not include a thrust bearing for several reasons: they would add to the overall cost of the machine, they add to the height of the Z axis while potentially reducing its travel, and require proper adjustment in order to be effective. A successful thrust bearing design would not only need to solve these issues, but in keeping with the rest of the design of the machine, would also need to able to be fabricated without special tools or equipment. However, there are several strong reasons for including such bearings. They relieve the stepper motor bearings of all axial force, which is beneficial as they have a low rating for loads of this type. Therefore, external thrust bearings will potentially increase the longevity of the Z axis stepper motor. The other reason is that a failure of the coupling between the motor and the lead‐screw could cause the router to plunge into the table. While this type of failure is unlikely, this thrust bearing upgrade will provide extra insurance against this happening.
ALUMINUM or hard plastic, such as ACETAL (delrin) .375" x 2.25" x 2.25"
BEARINGS (2) 3/8" x 13/16" x 9/64" unshielded needle bearings, VXB Item# Kit12703 (includes hardened washers) If ordering from a supplier other than VXB, be sure that the two hardened washers are included with bearings.
THREADED SHAFT COLLAR Delrin threaded collar to match lead‐screw pitch. Source: DumpsterCNC Alternatively, an unthreaded metal split shaft collar may be used. Hoever, this will be more difficult to adjust correctly.
Therefore, it is highly recommended that the following components be installed on Momus CNC routers. This upgrade is fully compatible with all previous versions of the machine. However, on machines constructed from older versions of the plans, it may reduce the Z axis travel by approximately 7/16", and will also raise the motor vertically by 3/8". This may cause cover interference on older machines that use quad‐stack stepper motors with a dual shaft. A thin shim between the cover and the machine base can be used to raise it for sufficient clearance. The assembly sequence shown here was photographed installing the parts onto an existing machine. The assembly sequence is identical on a new machine, only the gantry will not yet have been installed on the machine. Combine these instructions with those on pages 131‐132.
ADDENDUM z axis thrust bearings
MomusDesign
FABRICATED PARTS part # part name 46 thrust bearing plate 47 z axis motor spacer
BENCHTOP CNC ROUTER PLANS
LIST OF FABRICATED PARTS
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22 Z cable plate (existing)
BENCHTOP CNC ROUTER PLANS
MomusDesign
47 Z axis motor spacer
20 Z motor mount right (existing) 21 Z motor mount left (existing)
46 thrust bearing plate
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ADDENDUM exploded view
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172
material: 6061‐T6 alum. 1.125 2.25
.928
.25 DIA., 4 holes .438 DIA.
.928
1.856
stock size: .1875 x 2.50
1.856
# required:
1
.75 1.5
BENCHTOP CNC ROUTER PLANS
MomusDesign
1.125 2.25
.928 1.856
2.25
.75 1.5
4.5
.928 1.856
.25 0.5
Slight enlargement of this notch may be necessary to allow proper adjustment of lead‐screw. 2.25 4.5
.1875 0.375
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NOTE: Cut from 2.50" wide plate stock, as it is a more common stock size than 2.25"
ADDENDUM thrust bearing plate
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1.125 2.25
.928
material: 6061‐T6 alum. OR acetal plastic
.25 DIA., 4 holes
stock size: .375 x 2.50
.928
1.856
1.856
1.50 DIA. # required: 1.125
1
2.25
.928
BENCHTOP CNC ROUTER PLANS
2.25
MomusDesign
1.856
.75
4.5
1.5
.928 1.856
.75 1.5
2.25 4.5
.375 0.75
version 2.1 NOTE: Cut from 2.50" wide plate stock, as it is a more common stock size than 2.25"
ADDENDUM z axis motor spacer
copyright 2013
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The images to the left and right show the additional lead‐ screw components that are not shown in the main plans.
BENCHTOP CNC ROUTER PLANS
MomusDesign
As shown to the left, thread the Delrin collar onto the leadscrew, about 1.5". Slide one unlubricated needle bearing, and its hardened washers, against the top of the collar. Insert this assembly through the hole in the aluminum thrust bearing plate (part #46). Slide the other unlubricated bearing and hardened washers onto the top of the plate.
As shown to the right, thread the motor coupler onto the end of the lead‐screw. Thread it down the screw until the end of the screw is aligned with the bottom of the slot in the coupler. Gently tighten the coupler in position.
As shown in the left image, thread the collar back up the lead‐ screw until it firmly compresses the stack of components against the coupler. While keeping it firmly threaded against the parts, tighten it in position.
Keep the collar tightened in place, and remove the other components from the lead‐screw, as shown to the right.
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ADDENDUM thrust bearing assembly
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Thread the lead‐screw, with the collar installed per the instructions on the previous page, down through the anti‐ backlash nut, as seen in the image to the left. Thread it far enough that the top of the screw is approximately 1" below the bottom of the Z motor mounts. As seen to the right, lightly lubricate one of the needle bearings, and with it sandwiched between its hardened washers, slide it down the screw onto the collar. A light lithium grease can be used for lubricant. Be cautious to not use an excessive amount, that will get onto the surfaces of the collar. The collar should be kept clean.
As shown to the left, temporarily install the aluminum bearing plate with one or two of the 2.5" screws, and place the other lubricated bearing and washers on its top surface. Make sure its bore is aligned over the hole in the bearing plate.
BENCHTOP CNC ROUTER PLANS
MomusDesign
Raise the Z axis assembly up, so that the end of the lead‐screw is guided through the hole in the bearing plate, and through the bearing. While holding it in this position, thread the motor coupler onto the end of the screw. Thread it down the screw until it is firmly clamping the bearings and plate to the collar. This tightness is the pre‐load setting on the bearings, and should be tight enough to remove all slack. Maintain the coupler in this adjustment, and tighten its clamping screws.
Remove the temporary screws holding the bearing plate to the motor mounts.
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ADDENDUM thrust bearing assembly
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Set the new motor spacer (part #47) on top of the motor mounts.
BENCHTOP CNC ROUTER PLANS
MomusDesign
Set the cable plate (part #22) on top of the motor spacer. Place the stepper motor into position. Carefully raise the Z axis assembly up, so that the motor coupler is guided onto the end of the motor shaft. Be sure to carefully align the flat on the motor shaft with the set screw in the coupler (see page 131 for the addition of this set screw to the Delrin coupler.) It may take some wiggling to get the coupler to slide onto the shaft. Install the four screws that hold the motor to the mounts. Tighten them snugly in place.
Fully tighten the clamping screws on the coupler, so that the motor shaft is securely held. Tighten the set screw against the flat on the motor shaft. During the alignment procedure described on page 164, very slightly loosen the four screws holding the motor to its mounts so that the motor can be repositioned side to side for proper lead‐screw adjustment. Fully tighten the screws after this process. Do not loosen any of the clamping screws on the coupler or collar during this process, or the bearing pre‐load adjustment may be lost.
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ADDENDUM thrust bearing assembly
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The needle bearings that are used are an unshielded design, that must be protected against dust and cutting chips. The lower bearing can be protected by merely wrapping a length of PVC electrical tape around the circumference of the collar. Carefully set it so that it just lightly contacts the bottom surface of the bearing plate.
BENCHTOP CNC ROUTER PLANS
MomusDesign
The upper bearing can be protected by simply closing off the front of the cavity in which it sits between the motor mounts. This can also be done with tape, as shown to the right, or a blockoff plate can be fabricated.
After closing off the cavity, re‐install the Z axis limit switch. The limit switch placement will need to be adjusted from where it would be placed if no thrust bearings were used. This may mean only one screw will secure the bracket that is included with these plans.
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ADDENDUM thrust bearing assembly
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