What is 5 Axis CNC Machining

 ASSIGNMENT  ON  5-AXIS MACHINING 

 SUBMITTED BY-SUHAIL ABROL  E.NO-92/08  ROLL NO.-11

What is 5 Axis CNC Machining? Before understanding 5 axis CNC machining, you¶ll have to understand what a normal 2 axis and 3 axis CNC machining is. A 2 axis CNC machining system has the ability to modify objects on a plane. This plane has only two coordinate systems, the x and the y axis. Objects can be moved on a plane. If the plane is upright then an object can be modified top to bottom and left and right or forward and backward. On a 3 axis CNC machine, an object can be modified in space. It can be modified, left and right, top to  bottom, and forward and back. You may be wondering, isn¶t a 3 axis CNC machine enough to handle any given object. Since it can modify an object in space then it must be able to transform an object at any given point. All this is correct but there is still a need for 5 axis CNC machining. Why? On a 2 axis CNC machine, you can modify objects in a plane. On a 3 axis CNC machine, you can modify objects in space. On a 4, 5, 6, and so on and so forth axis CNC machine, you¶ll be able to modify an object on several axes. Modifying an object on the x, y, and z planes may not be enough to create a  perfect sphere. Maybe a 45 degree angle axis will help in smoothening out the sphere. Going further an opposite 45 degree angle axis may also help do the job and so on and so forth. The more axes on a CNC machine translates into a more complex piece of equipment. For 5 axis CNC machining equipment, you can take advantage of the extra two axes aside from the x, y, and z planes. If  you are making a piece of art, you can rotate the object in two more ways than the usual x, y, and z movements. The extra axes may be a circular axis, a diagonal axis, or whatever. As long as the extra axes help in the  production of the final product, you can always have a 5 axis CNC machining tool handy. The extra axes on a 5 axis machine are commonly called the Q and the B axis. The Q axis is normally associated with the rotation of the product whilst the B axis is associated with the tilting of the product. This is the nomenclature that is used in many 5 axis machines but it can always vary from machine to machine. If you are having a hard time picturing what 5 axis CNC machining is all about then think of an airplane. Imagine the number of ways it can move in the air. First, think about the plane moving forward in the forward-backward plane. This is the first movement and your first axis. Second, think about the plane going up and down in the altitude plane. This is the second movement and your second axis. Third, think about the plane turning left and turning right. This is the third movement and your third axis. Here comes the tricky part. Now you have to imagine the plane rolling like a barrel. This is the fourth movement and your fourth axis. When a plane rolls from being upright to being upside down, its axis is entirely different from the other  three.

The fifth movement is even trickier. When a plane lands, imagine the nose tilting upward but not increasing in altitude. This is the fifth movement and it is your fifth axis. The 5 axis CNC machining equipment can modify an object in the same manner that the plane moves around space.

Terminology The angles A, B, C designate rotations about the X, Y, and Z axes, respectively.  Note that all standards define the positive direction (i.e., increasing angle) as CCW rotation of the cutting tool. This direction is easily found by holding your right hand with your thumb points in the positive direction of the linear axis and then your curled fingers show the positive rotary direction. For dual rotaries the name of the second axis is based on its orientation when the first axis is at zero.  Note that the direction of any axis is defined as the direction of the tool (regardless whether the tool or the table moves), therefore the direction of rotary tables is reversed - so use your left hand instead.

Head Rotation - Axes & Directions Four different five-axis machining center designs:

Design #1: Rotary Table + Pivoting Spindle Head Like many other horizontal machining centers, this one places a 360-degree, B-axis rotary table beneath the workpiece. This table doesn't just index, it can also feed through a cut. One example is a turbine housing. On a part like this, the same hole appears at various locations around the OD. When this is the case, a machine with this design can position itself from one hole to the next with a move in only one axis. Any other type of five-axis machine would move from one radial hole to another on a cylindrical  part using moves in at least two axes, maybe more. But on a rotary table/pivoting head machine, the tool only has to be tilted to the correct angle for the hole one time, and the spindle head only has to be  positioned in X, Y, and Z one time. Drilling a sequence of holes then becomes a matter of feeding in, retracting, and indexing only in B to reach the next hole.

The result is a more repeatable process. More axes of positioning would only compound the opportunities for positioning error to affect the move. Another strength of this machine design relates to workpiece size. The fewer rotary axes move the workpiece (as opposed to the tool), the better the machine can accommodate large parts. This machine does rotate the work-piece in B, so the part's swing is limited in this axis. However, because this is the only workpiece pivot, the machine handles tall workpieces effectively. Five-axis machines placing both  pivots at the table generally are limited to workpieces that are small relative to the linear travels. But the design of this five-axis machining center leaves the workpiece more fixed, allowing the machine to take on very tall cylindrical parts.

Design #2: Double Rotary Table Horizontal machining centers with B-axis rotary tables are often available with a secondary rotary axis in the form of a 360-degree, A-axis unit that can be mounted on the main table like a tombstone. General Tool's version of this configuration comes from a CNC horizontal boring mill from Giddings & Lewis (Fon du Lac, Wisconsin). On this machine the main table is so large that the A-axis unit can be positioned across a wide range of locations, increasing flexibility. Effective programming, however, requires the  programmer to know precisely where the face of the A-axis table locates with respect to the pivot in B. In  practice, this often means the program is written to assume a specific location for the A axis, leaving the operator(s) setting up the machine with the time-consuming step of positioning the A-axis module  precisely to match this requirement. An ideal part for this machine is one that presents a ring of holes to the spindle, particularly if that part is a cylindrical one that also requires machining around its OD. This machine has no enclosure, so it cuts some of the shop's largest parts when the A-axis unit is not in  place. Equipped for five-axis machining, it faces more restrictive limits on workpiece size. When the Aaxis unit is in place, the size of the workpiece is limited not only with respect to its swing about the A axis, but also according to how large a part it's practical to suspend from the surface of the horizontal table. However, the large amount of XYZ travel remaining around this smaller five-axis part helps to make this model the shop's best five-axis machine for the use of long tools or extensions, particularly at odd angles. Other machines don't offer enough travel to back the part away from the spindle to leave room for a long tool. This is particularly true of machines that pivot at the spindle head, because the spindle must then be  backed away along an interpolated path in the linear axes to match the orientation angle of the tool. But on the machine with two pivots at the table, making room for a long tool requires only a move in Z. This  particular machine has 36 inches of travel in the Z axis, complemented by 40 inches of travel in the W axis. In addition, a non-pivoting spindle head leads to less number crunching during and after rotary axis moves. Tool location doesn't have to include trigonometry-induced variations, so any tool offset can be  just a one-time adjustment in X, Y, or Z. This makes each tool path command easier to compute. The work is easier for the CNC and the CAM software. But the trade-off is that this machine may make the  programmer's work harder. Programming a double rotary table machining center is challenging enough that an inexperienced programmer might not be able to make the most efficient use of this machine. The challenge relates to visualization. Bud Schaefer is an experienced programmer with General Tool, and he says even he sometimes has trouble programming machines of this type.

It can be hard to picture, he says. You move in B, but then the pivot point for A moves as well.

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Allowing for the workpiece to move through compound angles in the course of the machining cycle introduces variations in the positions of various features that can be time-consuming just for the  programmer to think his way through. Give me a five-axis machine with at least one pivot in the spindle head, Mr. Schaefer jokes. I may have to insist on qualified tools, but at least I can picture what's happening to the workpiece. "

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But he knows some jobs simply demand a fixed spindle head. A five-axis job requiring heavy cuts is an example of this. No five-axis machine can take a deeper cut than what the rotary axes are able to support, and the bearings for a rotary table are typically much larger than the bearings for a pivot at the spindle head. Many machining center builders have succeeded in making their spindle head pivots far more rigid than the smaller bearing size would suggest. Nevertheless, Mr. Kramer feels safer assigning the heavy cutting five-axis jobs to a machine like this one, where the spindle has no freedom to tilt whatsoever.

Design #3: Double Pivot Spindle Head In fact, the need to optimize rotary axis rigidity compelled General Tool to choose a positioning-only design for its five-axis machine with two pivots at the spindle head. This machine, a Versa machine 6040 machining center from Versamill (Mt. Carmel, Illinois, Versamill is no longer in business) could have  been outfitted with a spindle head capable of feeding with the rotary axes, instead of just positioning. However, this would require servo axes to hold the spindle orientation during linear cuts. The positioningonly head offers more rigidity because it can hold each orientation with a hydraulic clamp. And because General Tool sees five-axis machining primarily as a way to save on setup time and tooling costs, the sacrifice for choosing a positioning-only machine is not a large one. This machine mates a 360-degree, C-axis pivot with a ±135 degree pivot in B (though a C-axis index could also locate this pivot in the A axis). If placing both pivots at the spindle in this way places any limitation on cutting force, that trade-off is repaid in flexibility. Any five-axis machine with a rotary table tends to favor round parts. However, the design of the double spindle pivot machine makes it ideal for   parts that are decidedly not round. For example, this is General Tool's most effective machine for singlesetup machining of long aerostructure parts, particularly ones with odd-angle holes along the length. The machine also does have a role in round part machining. With the agility of its spindle head, this machine can do what none of General Tool's other machining centers can do so well²machine features on the ID of a cylindrical part.

Design #4: Rotary Table + Table Trunnion In A Compact Machine This design is similar to the double rotary table approach in that it places two pivots under the workpiece, none in the spindle head. The Model DMU 70V vertical machining center from Deckel Maho (DMG America, Schaumburg, Illinois) combines a 360-degree, C-axis rotary table with a 180-degree trunnion. This trunnion axis, referred to as B, departs from the standard labeling convention for rotating axes. The center of rotation for this B axis sits at a 45-degree angle with respect to Y (see Figure 4). "

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The rotary axes are built into a vertical machining center platform to achieve a five-axis machine with a footprint no larger than a mid-sized VMC. The machine does still offer programmers the same challenge as a double rotary table machine where visualizing the work is concerned. However, in this case, the fixed

spindle results in a small and accessible five-axis machine which nevertheless can take relatively deep cuts. General Tool bought this machine primarily because it was small. All of its other five-axis machines are much larger, and therefore prohibitively expensive to run for smaller, more inexpensive parts. By contrast, the rotary table/trunnion machine can't accept large workpieces, but makes five-axis machining of smaller parts much more economical. Plus the vertical design makes the machine easy for operators to load and unload, allowing the shop to machine a run of work-pieces in a way that none of General Tool's other five-axis machines makes practical. The shop also considers this its most precise five-axis machine. In practice, pivoting spindle head designs lose accuracy through uncertainly in the tool offsets. Similarly, a double rotary table machine has uncertainty in where the A axis has been positioned with respect to the pivot in B. That's why General Tool tends to assign parts requiring continuous five-axis machining to this machine, whenever size  permits. Often, size does not permit. Five-axis jobs are usually big jobs. However, that may be because five-axis machines have historically been big, built-to-order machines. Today, more builders are offering five-axis machines that depart from that tradition. The variety of low-cost, standard five-axis machining centers now available may allow shops to apply five-axis machining to classes of parts²small, low-cost components²falling outside of five-axis machining's traditional niche. If so, then more shops will be turning to five-axis machining to let them work more productively, and more shops will find themselves evaluating competing five-axis designs in much the same way that General Tool has. What Do The Letters Mean?

Just as most machining center builders define their linear axes the same way²that is, the Z axis is the one  parallel to the spindle center line²they also tend to define rotary axes the same way. y y y

On most machines, the A, B, and C axes correspond to X, Y, and Z in this way: The A axis pivots around a center line parallel to the X axis. The B axis pivots around a center line parallel to the Y axis.

There are a few ways to build 4 and 5-axis machines: y y

y

Use

a single or dual rotary table Add tilt axis B Add rotary table A Add 5th rotary A-axis for the spindle Add a CA head

Rotary Table & Trunnion

The advantage of rotary tables is that they can easily be installed when needed.  Note that rotary table B is usually used on horizontal machines, mainly for cutting heavy parts from different sides.

Rotary table A is usually used with vertical machines to cut cams and roller dies. Dual rotary tables are usually AB for horizontal machine and AC for vertical machine, but other  configurations are used as well.

BA dual table for VMC

AC dual table for VMC

Rotary Head

For vertical machines the head can be tilted by B-axis, which turns about the Y-axis. The tilted head reduces the rigidity less than any other head. Automatic tool changer can be used same as with 3-axis machines. Adding 5th axis A enables tilting the tool about the X-axis. This BA head is less intuitive to move manually than a CA head.

BA head for VMC

CA Head for VMC & gantry

CA Head CA and CB heads are same heads with a different setup. When C=0, CA is set so that the 5th axis turns about X-axis and CB is set so that the 5th axis turns about Y-axis.

CA Head Geometry

A CA head is mostly used for vertical and gantry machines while CB head is used for horizontal machines. The head with rotary axes provides tilt and swivel for the cutting tool. The difference between CA and CB heads is how the zero position is set.

CA Head Coordinates

Pros & Cons The advantages of 5-axis machine are: y y y y

Less

setup - in one setup many surfaces and holes can be machined. Face mill can be used instead of a ball nose with better finish. Removing more stock with fewer cuts. Cutting internal sharp corners.

There are some disadvantages though: y

Reduced rigidity.

y

Reduced working envelope.

y

Reduced feedrate.

y

Hard to implement automatic tool changer.

y

For

y y y

compact heads - no taper, collets are used. Singularity (see below). Total accuracy depends heavily on the rotary axes accuracy and setup. Accuracy depends on accurate tool length data.

Pivot length The perfect orientation mechanism will change the tool orientation without changing its location. In such a case changing orientation doesn't require any linear move. This is impossible to achieve because of mechanical constrains. For A or B tilt there is a certain distance between the center of rotation (pivot) and the tool tip. This is called pivot length. In some machines it is reduced to a small distance by a clever design. However, tool length changes this length and therefore tool length setting must be accurate.  Note that for 3-axis milling incorrect tool length setting is compensated by the home setting and at worst it will change the depth of the cut. With rotary head incorrect tool length changes the shape as well. Just imagine a sphere cut with a CA head - its radius accuracy depends upon an accurate setting of the tool length. For CA head X and Y each may also have a pivot length, either by design or because of misalignment (offset). For BA head, B may have X offset and A has some distance from B-pivot. The total Z-pivot length is therefore affected by both tool length and A-axis orientation. Longer Pivot Length causes: y

Reduced rigidity

y

Reduced

y y y

accuracy Reduced cutting feed Reduced working envelop Changing orientation requires bigger changes in position

CNC Milling

Computer Numerical Control (CNC) Milling is the most c ommon form of CNC. CNC mills c an perform the functions of drilling and often turning. CNC Mills are classified according to the number of axes that they possess. Axes are labeled as x and y for horizontal movement, and z for vertical movement, as shown in this view of a manual mill table. A standard manual light-duty mill (such as a Bridgeport) is typically assumed to have four axes: 1. 2. 3. 4.

Table

x. Table y. Table z. Milling Head z.

The

number of axes of a milling machine is a common subject of casual "shop talk" and is often interpreted in varying ways. We present here what we have seen typically presented by manufacturers. A fiv e-ax i s CNC milling machine has an extra axis in the form of a horizontal pivot for the milling head, as shown below. This allows extra flexibility for machining with the end mill at an angle with respect to the table. A si  x-ax i s CNC milling machine would have another horizontal pivot for the milling head, this time perpendicular to the fifth axis. CNC milling machines are traditionally programmed using a set of commands known as G-codes. G-codes represent specific CNC functions in alphanumeric format.