High Speed Stamping Process Improvement

HIGH SPEED STAMPING PROCESS IMPROVEMENT THRU FORCE AND DISPLACEMENT MONITORING

Rich Grogan Helm Instrument Co., Inc. 361 W. Dussel Drive Maumee, OH 43537

Abstract Practical methods can be used and associated numerous benefits derived from the implementation of force sensors and displacement sensors on high speed stamping dies. Such dies are typically of the multi-station “progressive die type”, and are operated in the 200-1000 SPM range. The parts made in high speed stamping dies are normally small and very complex, with a large number of intricate forming operations involved. Examples of the wide variety of parts that can be made include electronic connector pins and sockets, integrated circuit lead frames, and terminals for electrical cable assemblies. Force and displacement monitoring can be used to improve the overall high speed stamping process, as aids to achieve faster and better machine “set up”, and also as an online production monitor. The production monitoring benefits include tooling protection and enhanced part quality. Guidelines have been established for locating the force sensors and displacement sensors on high speed dies. Various types of instrumentation display and control features are available, with strong emphasis on the defined relationship between the forming force or tool displacement and resultant part quality.

Force Monitoring Force monitoring technology for manufacturing machines has been in existence for approximately 30 years. In its infancy, the technology incorporated simple bolt-on force sensors mounted on the machine frame and basic "peak force" monitors, used primarily for press overload protection on metal stamping presses. As the technology advanced over the years, the types of machines being successfully monitored have expanded greatly. They now include not only stamping presses, but also forging machines, die cast machines, injection molders, assembly machines, compaction presses, and slide forming machines. With the advent of very expensive and sophisticated tooling, and also the great emphasis placed on part quality, the focus of force monitoring has shifted from press overload protection to tooling protection and achieving enhanced part quality. This is particularly true for high speed stamping operations with multi-station progressive dies. In such operations, numerous complex and precise forming steps are involved, cycle rates typically exceed 200 parts per minute, and extremely high quality is expected. Force monitoring technology is commercially available for high speed stamping operations, so that those manufacturers can achieve the process improvements of better “set up”, tooling protection, and improved product quality. There are many down-to-earth practical reasons and associated benefits for monitoring the forces developed on high speed stamping presses. These include the following crucial items: -Improved Process Reliability / Part Quality -Reduce Machine Set-Up Time -Reduce Waste -Improved Production Control -Allow For Unattended Operation -Enable Analysis of Machine Condition An additional reason for high speed machine monitoring is to reduce off-tolerance parts. This goes hand in hand with the "reduce waste" item listed above. What we can measure, we can control. The quality of a finished, formed part has been shown to be closely linked and dependent upon the force required to form the part. In essence, by using force measurement as an indicator of part quality for each formed part, and by controlling that force within a certain narrow window, better and more consistent quality parts can be produced and off-tolerance parts minimized. Yet another good reason for monitoring force is to achieve predictive tool maintenance and predictive machine maintenance. Many variables can affect the force required to make a part on a high speed progressive die. These include such items as machine condition, tooling condition, material characteristics, and lubrication. By establishing initial "good part" force values and force signatures for a given machine running a given part, and monitoring subsequent force values and signatures, changes over time can be used to predict when a tool may need to be changed or a machine re-worked. The idea is to monitor the press forming operations on a continuous basis and to look for small changes in the force values, indicating non-critical tooling or machine conditions that can be scheduled for correction before major problems develop. Repeatability of the forming process is a very important element for proper machine / tool operation and the production of consistent quality parts. Force monitoring of the high speed stamping process provides a very useful tool that relates forming force to the consistency of the operation. Force values that show great repeatability and little variation indicate a highly repeatable and stable production process. By monitoring the force developed to form each part on a continuous production basis, and establishing the initial “set-up” condition for the machine to allow for "good part" production, proper "cycle-to-cycle" repeatability and associated good part quality can be maintained.

From a machine “set-up” standpoint, the concept of "duplicating a previous run" is very important. This also hinges on the principle of the forming force value being closely related to the quality of the finished part and to a proper machine and tooling “set-up” condition. When the machine and tooling are initially adjusted using conventional techniques to allow for "good part" production, the force values and signatures for that condition can be stored. When that same part is to be run again, those stored "good part" force values and signatures can be recalled and used as a “set-up” reference for the present force values and signatures. The initial "good part" condition thus becomes a “set-up” tool for the machine operator, allowing him to make machine/tooling adjustments to duplicate the previous good run, and get back to the desired operating condition. This yields a more accurate “set-up”, with the additional benefit of time savings. In the absence of force monitoring, tooling “set-up” on a typical high speed progressive die may take 8 hours or more, sometimes with a “by guess or by golly” approach. Relating force values to a good “set-up” condition can reduce this time considerably, saving expensive labor and allowing for more machine “up time”. Some force monitors allow for the storage and recall of such force-based “set-up” criteria by job number or part number.

The quality of a finished formed part on a high speed stamping machine is directly related to the force required to make it. Measurement of the forming force provides an objective, scientific criteria to define a "good part" and "in spec" condition. From a control standpoint, if the measured force for each part stays within a narrow band called the "quality window", the production of good parts that meet specification can be maintained. The allowable size of this quality force window must be determined, often by trial and error, for the particular part and its associated specifications.

Since many variables in the process can affect the forming force, in the event that the force has changed (increased or decreased), the monitoring system can not necessarily identify the particular culprit responsible for the change (machine condition, tool condition, material property variations, etc.). However, the system can definitely detect that the force has changed due to some process change, resulting in a quality change of the product. The detection of such a change in force can be used to better control the process. If the change in force is small, resulting in a force value still within the "quality window" and parts still within spec, such a small change can be used to indicate a tooling wear or material variation condition. A larger change in force, resulting in a force value outside the "quality window" and parts out of spec, can be used in the control process to divert off-tolerance parts. Beyond that, in the event of several consecutive off-tolerance OVER (HIGH) parts, an adjustable "fault counter" can be used to OK stop the machine so that corrective action can be UNDER (LOW) taken by the operator. At this point, the condition is considered to be serious enough that the machine should be stopped before a lot of off-tolerance parts are made, and before an unchecked condition deteriorates further with possible resulting tool and/or machine damage. FORMING FORCE "QUALITY WINDOW"

First . . . you must make a good force measurement. There are many useful things that can be done downstream from the force measurement in terms of manipulating the data and controlling the process. However, the success of these downstream functions all hinges on initially making a valid force measurement. The crux of proper force measurement on a high speed stamping machine is where to measure those forces. For successful monitoring of the process, the force must be measured in a location where it relates directly to the forming operation and the part being made. Force sensor location is crucial in order to properly link the force measured to the quality of the product. Depending upon the machine configuration and the tooling, there are various desired and proven sensor locations.

There are several types of force sensing elements that are commercially available. A couple of these lend themselves very nicely to making force measurements on high speed stamping machines, and have demonstrated very good performance in many field installations. The two types of sensors that are most commonly used in the industry are strain gage and piezoelectric force sensors. Each type of sensor has its own unique characteristics, with corresponding benefits and also limitations for machine force measurements.

The first type to be discussed is the strain gage force sensor. Strain gage technology has been in existence for some fifty years, and is now extremely well perfected. A TENSION / COMPRESSION strain gage force sensor typically incorporates a precision DEFLECTION machined metal structure, to which is bonded small foil AXIS resistive strain gage sensing elements. The metal structure, typically machined from heat treated stainless steel, is designed to deflect under load in the “elastic” region in either tension or compression. The location at which the "DUAL ELEMENT" strain gages are bonded usually has a reduced cross FOIL STRAIN GAGE section, in order to maximize deflection at the gages and increase the output of the sensor. In practice, most commercially available strain gage sensors incorporate "dual element" strain gages, with multiple such gages bonded to the same structure and all wired together to form a "Wheatstone Bridge" circuit. Thus, each gage bonded to the structure includes two foil resistive sensing element grids. One grid is aligned with the primary deflection axis (tension or compression), and the other grid (called the Poison gage) is aligned 90 degrees to the primary deflection axis. Each grid is made of a copper alloy material such as constantan, and is arranged in a serpentine fashion. The copper alloy material is specially formulated to allow for a very precise and linear change in resistance when physical deflection (elongation or contraction) occurs. The manufacturing process for strain gage sensors includes the bonding of strain gages to the structure with a special adhesive, wiring, cable attachment, and the application of special protective “potting” over the gages.

The principle of operation of the strain gage sensor is very simple. The sensor structure is physically attached to the machine and/or the tooling in a location that will physically deflect as the machine operates to make a part. The deflections are typically very small, on the order of perhaps 10-100 millionths of an inch per inch (micro-strain), but measurable physical deflections nonetheless. As the machine deflects when a part is formed, the sensor structure with the attached strain gage sensing elements also deflects, typically in tension or compression. The sensing elements ultimately deflect, producing a resistance change for each grid proportional to the deflection and to the actual force applied to the structure. This minute but measurable change in resistance for the various grids wired into the "Wheatstone Bridge" result in a proportional millivolt output signal from the overall sensor.

Most strain gage force sensors have the internal strain gage elements wired in a full "Wheatstone Bridge" circuit configuration. This requires a minimum of four individual foil resistive sensing elements which undergo deflection under a loaded condition. For proper operation of the bridge, opposing gage elements undergo the same type of deflection (both in either tension or compression), and adjacent gage elements undergo dissimilar deflection (one in tension and the other in compression). The full "Wheatstone Bridge" arrangement provides the positive features of high output, temperature compensation, and bending compensation. There are four external wiring connections to the bridge. Two of these are from the regulated DC power supply in the monitoring instrument, and provide the necessary "excitation" voltage to the bridge. The other two are the output connections or "signal" lines. Under a “no load” and un-deflected condition, the bridge is electrically balanced, and there is no output signal on the signal lines. Under a loaded condition, the bridge is unbalanced, and a millivolt signal proportional to the applied force appears across the signal lines. At the monitoring instrument, this signal is conditioned and amplified, and ultimately used to drive the display meter. To summarize, strain gage force sensors have the following characteristics and features: -Consistent repeatable output under dynamic and static loads -Full bridge configuration allows for temperature and bending compensation -Recommended for moderate to heavy loads -Suitable for slow speed to high speed operations (up to 1200 SPM) -”Peak Force” monitoring or “Signature Analysis” monitoring capability -True load signature for analytical work -Require external excitation voltage

Strain gage force sensors can be mounted to the high speed stamping machine frame or within the progressive die assembly. The most basic version of the strain gage sensor is the simple “bolt-on” type for machine frame installation. In general, due to its low cost and ease of installation, it should be the first considered choice for force monitoring on any type of press or manufacturing machine.

The basic bolt-on strain gage force sensor is "GAP-FRAME" STAMPING PRESS typically mounted directly to the machine frame. This is the most common method of force sensor installation on conventional metal stamping 2 CHANNEL presses, such as the “Gap-Frame” press shown FORCE MONITOR in the illustration. On such presses, two bolt-on LOADGARD sensors with a two channel instrument are typically used to monitor “left side” and “right side” loads. For straightside presses with four columns, four bolt-on sensors with a four channel instrument are used to monitor “individual corner” loads. This approach has been successFRAME-MOUNTED FORCE SENSOR fully used on many types of presses for over 30 years. Depending upon the particular press and the parts being run, this may be a workable arrangement. It relies upon consistent and reasonably linear press frame deflection under a forming load condition to provide the force measurement output signal. A minimum amount of press deflection is required for this arrangement to work properly. The bolt-on frame mounted device is referred to as "parallel" type force sensor, and experiences only a small

fraction of the total force developed in the machine frame. For this reason, a field calibration procedure utilizing calibration load cells and portable instrumentation are typically used on such installations to calibrate the sensors to a known load condition. In general, the bolt-on force sensor approach performs very well on conventional stamping presses, where the ram movement during the forming operation is only in a single direction, and predictable deflections are generated in the frame due to the forming force. Machine frame sensors may be considered and have been used with some success over the years on high speed stamping machines. However, due to the machine/tooling configurations and the high speed dynamics typically involved in such operations, machine frame sensors are useful practically for press overload protection only. Machine frame sensors for high speed operations are extremely limited in their capabilities for tooling protection and part quality control. This is due partly to the fact that many high speed progressive dies require small tonnage compared to the capacity of the press in which they are run. It is not uncommon for five ton progressive dies to be run in presses ranging from 30-50 tons in capacity. This results in low press frame deflections, and corresponding low machine frame sensor output signals that may be unusable from a monitoring standpoint. Another factor results from the condition that most high speed progressive dies have many, many complex forming stations. A machine frame sensor relies on press frame deflection to generate a force signal, which is a “composite” signal relating to several tool stations in a multi-station tool. Because such sensors are so far removed from the precision forming that occurs in any particular tool station, it is unlikely that a frame-mount sensor will yield a significant signal change due to a fault (chipped punch, scrap in die, etc.) in an individual station. Thus, such faults would go undetected. An additional reason why machine frame sensors are quite limited for high speed stamping relates to the sensor signal components from the acceleration/deceleration dynamic effects in the press frame itself at high speed. As shown in the typical “Frame-Mount” sensor signal from a multi-station tool at 560 SPM, a background acceleration/deceleration frame signal and between cycle press “ringing” signal are superimposed upon the forming force signal to be monitored. Thus, spurious signal factors, along with the possible detrimental signal effects of hitting on stop blocks, can often “mask” the critical in-die forming signals that are actually desired to be monitored. In view of the many limitations associated with frame-mount sensors, “In-Die” force sensing offers the maximum tooling protection and part quality control benefits for high speed stamping. “In-Die” force sensing consists of locating a sensor within or beneath an individual tool station, in order to generate the strongest possible signal and one that relates most directly to the particular forming operation. By placing the sensor as close as possible to the actual “point of operation”, subtle faults occurring in the individual station produce a significant signal change that is detectable at the monitor. The “In-Die” sensor signal shown is from the same 560 SPM press and tool as the “Frame-Mount” sensor signal above. The signal was generated from a strain gage force sensor located beneath an individual staking station. In comparing the “In-Die” sensor signal to the “Frame-Mount” sensor signal, it is readily apparent that the “In-Die” signal is much “cleaner”. It offers an “individual” signal relating directly to the staking operation, and exhibits a stable “zero” line between cycles with no “ringing”. As such, it is a tremendously useful forming force signal that can be used for the detection of a wide variety of forming faults. These include material variations, misfeeds, damaged tooling, worn tooling, and scrap in die.

In the general metalworking industry, “In-Die” force monitoring has been successfully done for many years on a wide variety of multi-station tools. The force sensing technology developed for those general stamping applications can also be applied to many different types of high speed operations, including coining, embossing, bending, staking, and hole piercing. The “In-Die” sensing approach consists of locating force sensors within or beneath critical tool stations, to separate multi-station forming loads, and to achieve the highest degree of tooling protection and enhanced part quality. The two most common types of “In-Die” force sensors include conventional strain gage load cells, and an “implant” type of “glued-in” sensor called a “Die Plug”. The load cell option involves the use of strain gage load cells placed into machined pockets beneath individual secondary operation dies or punches. The other approach, involving the “Die Plug” sensor, has been in existence for about ten years, and is rapidly becoming the sensor of choice. This is due to its low cost and ease of installation.

The conventional strain gage load cell approach consists of mounting such a cell within or beneath an individual tool station. This “load cell” approach allows for a very direct and high force signal output even on light load applications, and also allows for the separation of force signals on multi-station tools. Such cells allow for a “calibrated” force readout directly in pounds or tons. Strain gage load cell design is a well perfected science. Strain gage load cells incorporate strain gage sensing elements bonded typically to cylindrical or ring shaped steel structures. The completed strain gage force sensors or “load cells” can be made in many different physical shapes, sizes, and capacity ratings. The load cell structure is precision machined from a material such as 17-4 PH stainless steel. The physical size of the structure controls the deflection that it experiences under load, and therefore the capacity rating of the cell. Load cells designed for higher force values are typically physically larger than smaller capacity cells. All of the strain gage sensor principles as outlined above apply to strain gage load cells. These devices generally incorporate a reduced cross-sectional area groove concentrically machined at the middle of the cell structure. This provides a physical protected location for the strain gages, and also determines the actual force rating of the cell.

Strain gage load cells typically operate under a “compression” loading condition, with the compressive deflection of the reduced area gage section resulting in a proportional output signal from the multiple bonded strain gages wired in a full "Wheatstone Bridge". These cells are referred to as "series" type force sensors. As such, all of the force to be measured is transferred through the cell. This type of cell is usually factory calibrated under test loading conditions before being shipped for field installation. There are several quality manufacturers of strain gage load cells in the industry. Many of these offer standard "catalog" load cells that are pre-engineered, and may be produced on a stock basis by the manufacturer. A wide variety of sizes and capacities of such load cells is available. For those applications where a strain gage load cell is required, it is strongly recommended to initially verify if a standard device will physically fit into the machine and have the desired capacity rating. This allows for the quickest and most economical installation. In the event that a standard design is not available to fit the application, custom load cells can often be provided.

As opposed to a conventional strain gage load cell, the “Die Plug” force sensor is an “implant” type device that is roughly half the cost of a load cell. It incorporates a highly sensitive strain gage or piezoelectric sensing element that is located at the front of a tube. The sensor cable runs along the tube, and "self-centering" legs are attached to the outside of the element. The application of the sensor is very simple, in that it is simply glued with industrial epoxy into a small hole machined beneath a tool station. "DIE PLUG" FORCE SENSOR As the tool (die or punch) is involved in performing the operation, very small deflections that occur in the tool holder or support steel where the "Die Plug" is located also cause minute physical deflections of the sensor. This yields a dynamic output voltage signal that is proportional to the applied tool force.

Since the “Die Plug” sensor is available in strain gage and piezoelectric versions, each has its own characteristics. The strain gage version functions like a miniature strain gage load cell, with all of the associated properties. The piezoelectric version incorporates a very highly sensitive piezoelectric sensing element. This element is typically made of a crystalline or ceramic material. It is the same basic type of sensing element that is used in extremely sensitive microphones. A piezoelectric sensing element self-generates a proportional voltage output signal when it undergoes physical deflection. Its extremely high output, great sensitivity, and wide operating range make it a good choice for certain machine force measurements. However, the piezoelectric sensor is rate-sensitive, responds only to dynamic forces, and is not well suited to calibrated “actual force” measurements. In general, for high speed operation monitoring, it is recommended that the piezoelectric “Die Plug” sensor be reserved for very light load and/or very high speed operations (exceeding 1200 SPM). Regardless of whether a strain gage or piezoelectric “Die Plug” is used, the measurements are almost always of the uncalibrated “reference” force value type.

There are three very basic steps involved to properly install a "Die Plug" force sensor. The first involves installing the sensor in the machined tooling hole to the proper depth. The front sensing element should be centered as closely as possible beneath the tool to be monitored, and within the compressive "footprint" of the tool. The second step involves injecting a special high strength epoxy potting into the injection tube. This allows the epoxy to backflow around the sensing element and to completely fill the hole, permanently encapsulating the sensing element into place. The final step includes cutting off the excess tubing that projects beyond the sensor hole, and connecting the cable to the monitoring instrument. Modular wiring interconnect systems, including miniature tool-mounted

connectors, plug-in cables, and junction boxes, are available to easily route the sensor wiring to the monitoring instrument. Due to its small size and high sensitivity, the "Die Plug" type of sensor can be used very effectively for high speed progressive die force monitoring, especially where installation space is limited and the forces involved may be small. For many high speed prog die applications, the “Die Plug” sensors are typically located in the stationary tooling “hard plate” beneath individual critical tool stations.

The quantity of “Die Plug” force sensors and their locations in progressive dies depends largely on the particular operations taking place. Not all stations necessarily need a sensor. However, it is recommended that one sensor always be located very near the first die station. This location provides a means for detecting variations in material such as thickness, hardness, temper, etc. Other sensor locations can include forming stations where early characteristics of punch and die wear can be identified, as well as faults such as damaged tools, scrap in die, etc. Monitoring such stations later in the progression allows for real time “in process” inspection of the finished part quality. Typical “Die Plug” sensor locations for a high speed progressive die are shown in the illustration. The part being made is a small bracket, with multiple forming operations. In general, it is most desirable to locate the sensors beneath the stationary tools, avoiding flexing cables at high speed. Depending upon the tool geometry, it is sometimes necessary to locate one or more “Die Plug” sensors beneath moving punch tools. This can occur when the stationary die tool area does not provide a clean “force path” to the sensor. In those cases, the moving side punch tool can be utilized for the sensor location.

PROGRESSION

MONITOR PIERCING

MONITOR BENDING

MONITOR MATERIAL THICKNESS AND HARDNESS

MONITOR BENDING

(4) IN-DIE FORCE SENSORS

A two channel approach is popular for small, high-speed progressive dies which have a number of stations in a very small area. The first sensor is typically located at or near the first station to monitor for gross faults such as misfeeds and material variations. The second sensor is typically located in a station towards the end of the progression that relates to the finished part quality. Chipped or broken punches, scrap, misfeeds, or other process variations taking place along the progression will be detected as the strip or product exits the die.

Having located the right type of force sensors in the proper locations to generate meaningful output signals, a monitoring device of some sort is needed to complete the system. The force monitoring instrument performs certain basic functions that can be depicted in a block diagram. These functions include: - Force measurement - Signal conditioning - Load display - Alarm capability and adjustment - Alarm firing (control) - Alerting the operator (control) - Stopping the process (control) Several levels of force monitoring instruments are available. One common feature to all is that the display meter almost always displays the force value for each machine cycle in a digital format. For a strain gage force sensor, this usually represents a calibrated or “actual force” value. For a piezoelectric type force sensor, this represents an uncalibrated “reference” force value. Another common feature is that the instruments typically monitor and display the peak force value developed in the machine or the tool. An exception to this is the most advanced type of monitor, which incorporates "tracking alarms", and monitors the whole “force signature” throughout the entire forming cycle. It should be noted that all of the various types of monitors, from the simplest to the most advanced, are generally available in both single and multi-channel configurations. The number of channels is dictated by the maximum number of tool stations with corresponding sensors to be monitored. This becomes a matter of economics, and selecting the most critical stations to monitor in order to get the “most bang for the buck”.

The simplest type of force monitor is a basic peak force monitor with discreetly adjustable high and low alarms. In the event of either a high or low alarm condition, indicating a change and a possible fault in the process, an alarm relay would fire to stop the process. This allows for corrective action to be taken.

A refinement in the way of force monitors is a microprocessor circuitry "self- programming" unit. Based upon an initial "good part" operating condition, the operator initiates a sampling routine at the unit whereby it "learns" a "good part" force value based upon the sample taken. Subsequent machine cycles are then automatically monitored with respect to the "good part" condition. Microprocessor-based units offer the user some very powerful force monitoring capabilities, such as "good part" self-programming and automatic alarm setting, with a minimum of operator involvement.

A recent advancement in force monitoring instrumentation is the “PLC-based” force monitor. As opposed to the other instrument options, which are dedicated specialized units, the “PLC-based” unit incorporates force monitoring into a standard PLC rack. This is done Processor by plugging force signal conditioning “modules” into the PLC rack, and configuring the PLC program for the desired force display, Display monitoring, and alarm functions. Such an approach can yield signifiStrain Gage cant savings, since the force monitoring hardware is simply the Input (2) required number of “modules” installed into an existing machine PLC-BASED FORCE MONITOR control PLC rack, or a new PLC rack if required. Also, if an existing PLC machine control is involved, the display for that control can often be used to display the force information as well. This modular “building block” type of approach can save considerable monies and space, compared to the more traditional type of dedicated force monitor. This is especially true on systems with many channels. 2 Channel Strain Gage Module

HI GH

1 2

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As stated previously, the quality of the finished parts is directly related to the forming force required to make them. The primary and perhaps most important function of the "force sensor/monitoring instrument" combination is to monitor forming forces for all parts on a continuous production basis, and to verify if those forces go beyond the force "quality window". In that event, corrective action in the way of off-tolerance part diversion or machine stoppage is initiated. Another important function is to use the monitor as a “set-up” tool to achieve better and quicker machine/tool “set-up”. This is done by recording force values during initial “set-up” of a given part to a known “good part” condition. With such “good part” force values thus established during “set-up”, that same tool can be adjusted during subsequent set-ups by duplicating those force values. This allows for a more scientific and predictable “set-up” procedure, resulting in time savings and a dimensionally good part.

In terms of effective force monitoring, the level at which the alarm limits are set is crucial. A machine cannot be monitored for a 5% load change, when the process itself normally involves a 20% load variation from machine sloppiness or other factors. To do so would create continual nuisance alarms, unwanted machine shutdowns, and ultimately defeat the purpose of force monitoring. It should be recognized that every forming operation, even the most precisely controlled, will have some variation in the cycle-to-cycle forming forces. The idea is to control those "normal variation" process variables as closely as possible, and then to set the monitor alarm limits beyond the normal variation levels.

One of the newer and most exciting features available in advanced load monitors is a "tracking alarm" function that monitors each forming cycle throughout the entire stroke. Peak force monitors capture and display the peak force value for each machine cycle, and monitor with respect to that peak force. As useful as that function is, the newer "tracking alarm" type monitors extend the force monitoring function much further. Based upon a learned "good part" force signature, the "tracking alarm" monitor establishes a high and low alarm band that tracks this signature throughout the entire forming cycle. The signature for each subsequent part is then compared to the learned "good part" signature, and a change in load at any point along the signature that goes beyond the tracking alarm band is recognized as an alarm condition. This allows for the detection of very subtle faults in the process that may not be manifested as a change in the peak force. Thus, greater sensitivity to load changes throughout the entire forming cycle and better process control can be achieved.

Displacement Monitoring Displacement sensor die monitoring systems have been commercially available for metal stamping presses for approximately 20 years. Although the sensors and monitoring electronics have been continually upgraded, with corresponding performance improvements, the basic system components and operation have remained relatively unchanged. Displacement monitoring systems can be used to good benefit on a large variety of metal stamping operations. However, the performance features and benefits are most fully realized when applied to high speed progressive dies. Displacement monitoring technology should not be regarded as competing directly with force monitoring technology. Since each has its own unique benefits, limitations, and cost factors, they should be considered as complimentary technologies that can be used together for enhanced stamping process improvement and part quality control.

A displacement sensor die monitoring system is very simple, making it easily installed and very cost effective for monitoring high speed progressive dies. A typical system consists of one or more noncontact precision “Eddy current” displacement sensors, with a corresponding number of instrumentation channels for the sensors. The monitoring instrument is typically configured in a modular “stackable” arrangement, with a main “base” unit at the bottom with the power supply and one channel, and additional channel units that can stack on top. Each channel has a digital meter to show displacement deviation values, and also the alarm setpoints.

In practice, the sensors are mounted in a protected location on the stationary die or die shoe. For each sensor, a mating steel “target” is mounted to the spring-loaded stripper plate or the upper die. During initial system set-up with the material strip in the die, the steel targets are adjusted for a nominal sensor-to-target displacement gap (typically .040”-.060”) at bottom dead center. During normal machine operation with parts being made, the system monitors the sensor-to-target separation gap between the upper and lower dies, and looks for subtle deviations in stripper position or die height at each sensor location. Using sensors with high frequency response, and microprocessor-based electronics, this monitoring process can be done with resolution to 1/1000mm (.00004”) and at speeds up to 2500 SPM. Each channel of the monitor has its own adjustable plus/minus alarm setpoint. In the event that a fault occurs in the process, causing the sensor-to-target displacement gap change to exceed the alarm setpoint, an alarm relay would activate to stop the machine. In this way, the displacement monitor can be used to detect a variety of different forming faults, including: ♦ Misfeeds ♦ Pulled slugs ♦ Scrap in die ♦ Material changes ♦ Stroke deviation ♦ Misalignment between punch and die ♦ Coining depth abnormalities ♦ Broken punch or die assemblies ♦ Double hits

There are two basic options for the location where the sensor targets can be mounted. The first option involves mounting the targets on the spring-loaded stripper. This is the most typical and widely used location, and is the best for pulled slug and scrap-in-die detection. The second option involves mounting the targets on the upper die assembly itself. This provides the best performance for the detection of tooling/forming faults such as broken dies and coining depth abnormalities.

The illustration shows a typical installation of the sensor target on a springloaded stripper. The tool station represents a hole pierce or blanking operation, with a slug ejected during each stroke. Under a “normal” condition as shown at bottom dead center, the stripper maintains uniform parallel contact on the material strip. The sensor-to-target displacement gap is very repeatable and consistent for this “normal” condition.

This illustration also shows the stripper mounting location for the sensor target, but with an abnormal “pulled slug” condition. If the pulled slug falls anywhere on top of the material strip, which is a quite likely location, the additional interference from the slug presence causes the stripper to become cocked at an angle. For the displacement sensor closest to the slug, the sensor-to-target displacement gap increases appreciably. Provided that the alarm limits at the monitor have been set properly and that the displacement change exceeds the “normal” condition limit, an alarm would then activate to stop the press. After the machine operator has cleared the slug from the die, the alarm can be reset and normal press operation resumed. This stripper mounting arrangement for the sensor targets has proven extremely successful in the field in detecting pulled slugs and scrap in the die for many types of dies, different materials, and material thicknesses.

The other option for sensor target mounting is to locate them on the upper die assembly itself. This mounting location is the best one for detecting faults involved in bottom form operations, such as coining, embossing, and bending. The tool station in the illustration represents a coin station with the sensor mounted on the upper die. For this type of application, a broken die or punch would create a change in the coin depth and in the sensor-to-target displacement gap, resulting in an alarm shutdown. It should be noted that this sensor option on the upper die is the only one available for dies with non springloaded “box” strippers.

In terms of the recommended number of sensors for particular dies, those guidelines have been well established by the displacement monitor equipment manufactures. In general, the very small dies can utilize only one sensor with good results. The large and more complex dies typically incorporate two sensors, which would be mounted along diagonal corners of the die set. Using the fewest sensors, this approach provides the best overall coverage for abnormal forming conditions that can occur from side to side or front to back. The very largest dies typically incorporate four sensors, with one mounted at each corner of the die set.

In terms of the displacement sensor monitor performance, there are typically two different modes of operation. These modes relate to the “normal” condition benchmark displacement values to which the current displacement values are compared. One mode is called a “Mean Value Comparison Mode”, which incorporates a stroke-to-stroke “rolling average” for the “normal” condition benchmark. The other mode is called an “Absolute Value Comparison Mode”, which incorporates a learned sample for the “normal” condition benchmark. The operating features and benefits of each particular mode are outlined below.

Conclusion As stated previously, both force monitoring and displacement monitoring technologies are commercially available for high speed stamping operations. Their functions compliment each other, and they can both be used to achieve substantial process improvements. A key element in using these technologies is to relate in-die forming force and upper to lower die displacement to finished part quality. By comparing “normal” condition force and displacement values to current ongoing ones, and monitoring for changes outside normal limits, many crucial types of forming faults can be detected. These include such things as damaged tools, tooling wear, pulled slugs, scrap in die, and material changes.

Helm Instrument Company, Inc. 361 W. Dussel Drive, Maumee, OH 43537 Phone 419-893-4356 FAX 419-893-1371