HIGH SPEED STAMPING PROCESS IMPROVEMENTTHRU FORCE AND DISPLACEMENT MONITORING

Rich Grogan

Helm Instrument Co., Inc.361 W. Dussel DriveMaumee, OH 43537

Abstract

Practical methods can be used and associated numerous benefits derived from the implementation offorce sensors and displacement sensors on high speed stamping dies. Such dies are typically of themulti-station “progressive die type”, and are operated in the 200-1000 SPM range. The parts made inhigh speed stamping dies are normally small and very complex, with a large number of intricate formingoperations involved. Examples of the wide variety of parts that can be made include electronic connectorpins and sockets, integrated circuit lead frames, and terminals for electrical cable assemblies. Force anddisplacement monitoring can be used to improve the overall high speed stamping process, as aids toachieve faster and better machine “set up”, and also as an online production monitor. The productionmonitoring benefits include tooling protection and enhanced part quality. Guidelines have beenestablished for locating the force sensors and displacement sensors on high speed dies. Various types ofinstrumentation display and control features are available, with strong emphasis on the defined relation-ship 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 30years. In its infancy, the technology incorporated simple bolt-on force sensors mounted on the machineframe and basic "peak force" monitors, used primarily for press overload protection on metal stampingpresses. As the technology advanced over the years, the types of machines being successfullymonitored have expanded greatly. They now include not only stamping presses, but also forgingmachines, die cast machines, injection molders, assembly machines, compaction presses, and slideforming machines. With the advent of very expensive and sophisticated tooling, and also the greatemphasis placed on part quality, the focus of force monitoring has shifted from press overload protectionto tooling protection and achieving enhanced part quality. This is particularly true for high speedstamping operations with multi-station progressive dies. In such operations, numerous complex andprecise forming steps are involved, cycle rates typically exceed 200 parts per minute, and extremelyhigh quality is expected. Force monitoring technology is commercially available for high speedstamping operations, so that those manufacturers can achieve the process improvements of better “setup”, tooling protection, and improved product quality.

There are many down-to-earth practical reasons and associated benefits for monitoring the forcesdeveloped 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 goeshand in hand with the "reduce waste" item listed above. What we can measure, we can control. Thequality of a finished, formed part has been shown to be closely linked and dependent upon the forcerequired to form the part. In essence, by using force measurement as an indicator of part quality foreach formed part, and by controlling that force within a certain narrow window, better and moreconsistent quality parts can be produced and off-tolerance parts minimized.

Yet another good reason for monitoring force is to achieve predictive tool maintenance and predictivemachine maintenance. Many variables can affect the force required to make a part on a high speedprogressive die. These include such items as machine condition, tooling condition, material characteris-tics, and lubrication. By establishing initial "good part" force values and force signatures for a givenmachine running a given part, and monitoring subsequent force values and signatures, changes overtime can be used to predict when a tool may need to be changed or a machine re-worked. The idea isto monitor the press forming operations on a continuous basis and to look for small changes in the forcevalues, indicating non-critical tooling or machine conditions that can be scheduled for correction beforemajor problems develop.

Repeatability of the forming process is a very important element for proper machine / tool operationand the production of consistent quality parts. Force monitoring of the high speed stamping processprovides a very useful tool that relates forming force to the consistency of the operation. Force valuesthat show great repeatability and little variation indicate a highly repeatable and stable productionprocess. By monitoring the force developed to form each part on a continuous production basis, andestablishing 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 thefinished part and to a proper machine and tooling “set-up” condition. When the machine and tooling areinitially adjusted using conventional techniques to allow for "good part" production, the force values andsignatures for that condition can be stored. When that same part is to be run again, those stored "goodpart" force values and signatures can be recalled and used as a “set-up” reference for the present forcevalues and signatures. The initial "good part" condition thus becomes a “set-up” tool for the machineoperator, allowing him to make machine/tooling adjustments to duplicate the previous good run, and getback to the desired operating condition. This yields a more accurate “set-up”, with the additional benefitof time savings. In the absence of force monitoring, tooling “set-up” on a typical high speed progressivedie may take 8 hours or more, sometimes with a “by guess or by golly” approach. Relating force valuesto a good “set-up” condition can reduce this time considerably, saving expensive labor and allowing formore 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 speedstamping machine is directly related to the force required tomake it. Measurement of the forming force provides anobjective, scientific criteria to define a "good part" and "inspec" condition. From a control standpoint, if the measuredforce for each part stays within a narrow band called the"quality window", the production of good parts that meetspecification can be maintained. The allowable size of thisquality force window must be determined, often by trial anderror, 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 haschanged (increased or decreased), the monitoring system can not necessarily identify the particularculprit 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 tobetter control the process. If the change in force is small, resulting in a force value still within the "qualitywindow" and parts still within spec, such a small change can be used to indicate a tooling wear ormaterial variation condition. A larger change in force, resulting in a force value outside the "qualitywindow" and parts out of spec, can be used in the control process to divert off-tolerance parts. Beyondthat, in the event of several consecutive off-toleranceparts, an adjustable "fault counter" can be used tostop the machine so that corrective action can betaken by the operator. At this point, the condition isconsidered to be serious enough that the machineshould be stopped before a lot of off-tolerance partsare made, and before an unchecked condition deterio-rates further with possible resulting tool and/or ma-chine damage. FORMING FORCE "QUALITY WINDOW"

OVER (HIGH)

OK

UNDER (LOW)

First . . . you must make a good force measurement. There are many useful things that can be donedownstream 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 forcemeasurement. The crux of proper force measurement on a high speed stamping machine is where tomeasure those forces. For successful monitoring of the process, the force must be measured in alocation where it relates directly to the forming operation and the part being made. Force sensorlocation is crucial in order to properly link the force measured to the quality of the product. Dependingupon 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 theselend themselves very nicely to making force measurements on high speed stamping machines, andhave demonstrated very good performance in many field installations. The two types of sensors thatare most commonly used in the industry are strain gage and piezoelectric force sensors. Each type ofsensor has its own unique characteristics, with corresponding benefits and also limitations for machineforce measurements.

The first type to be discussed is the strain gage forcesensor. Strain gage technology has been in existence forsome fifty years, and is now extremely well perfected. Astrain gage force sensor typically incorporates a precisionmachined metal structure, to which is bonded small foilresistive strain gage sensing elements. The metal struc-ture, typically machined from heat treated stainless steel,is designed to deflect under load in the “elastic” region ineither tension or compression. The location at which thestrain gages are bonded usually has a reduced crosssection, in order to maximize deflection at the gages andincrease the output of the sensor. In practice, most commercially available strain gage sensorsincorporate "dual element" strain gages, with multiple such gages bonded to the same structure and allwired together to form a "Wheatstone Bridge" circuit. Thus, each gage bonded to the structure includestwo foil resistive sensing element grids. One grid is aligned with the primary deflection axis (tension orcompression), and the other grid (called the Poison gage) is aligned 90 degrees to the primary deflectionaxis. Each grid is made of a copper alloy material such as constantan, and is arranged in a serpentinefashion. The copper alloy material is specially formulated to allow for a very precise and linear changein resistance when physical deflection (elongation or contraction) occurs. The manufacturing processfor 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 physicallyattached to the machine and/or the tooling in a location that will physically deflect as the machineoperates to make a part. The deflections are typically very small, on the order of perhaps 10-100millionths of an inch per inch (micro-strain), but measurable physical deflections nonetheless. As themachine deflects when a part is formed, the sensor structure with the attached strain gage sensingelements 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 appliedto 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.

"DUAL ELEMENT"FOIL STRAIN GAGE

TENSION /COMPRESSIONDEFLECTION

AXIS

Most strain gage force sensors have the internal strain gageelements wired in a full "Wheatstone Bridge" circuit configura-tion. This requires a minimum of four individual foil resistivesensing elements which undergo deflection under a loadedcondition. For proper operation of the bridge, opposing gageelements undergo the same type of deflection (both in eithertension or compression), and adjacent gage elements undergodissimilar deflection (one in tension and the other in compres-sion). The full "Wheatstone Bridge" arrangement provides thepositive features of high output, temperature compensation, andbending compensation. There are four external wiring connec-tions to the bridge. Two of these are from the regulated DCpower 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 loadedcondition, the bridge is unbalanced, and a millivolt signal proportional to the applied force appearsacross the signal lines. At the monitoring instrument, this signal is conditioned and amplified, andultimately 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 theprogressive die assembly. The most basic version of the strain gage sensor is the simple “bolt-on” typefor machine frame installation. In general, due to its low cost and ease of installation, it should be thefirst considered choice for force monitoring on any type of press or manufacturing machine.

The basic bolt-on strain gage force sensor istypically mounted directly to the machine frame.This is the most common method of force sen-sor installation on conventional metal stampingpresses, such as the “Gap-Frame” press shownin the illustration. On such presses, two bolt-onsensors with a two channel instrument are typi-cally used to monitor “left side” and “right side”loads. For straightside presses with fourcolumns, four bolt-on sensors with a four chan-nel instrument are used to monitor “individualcorner” loads. This approach has been success-fully used on many types of presses for over 30years. Depending upon the particular press andthe parts being run, this may be a workable arrangement. It relies upon consistent and reasonably linearpress 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-onframe mounted device is referred to as "parallel" type force sensor, and experiences only a small

2 CHANNELFORCE MONITOR

FRAME-MOUNTEDFORCE SENSOR

"GAP-FRAME" STAMPING PRESS

LOADGARD

fraction of the total force developed in the machine frame. For this reason, a field calibration procedureutilizing calibration load cells and portable instrumentation are typically used on such installations tocalibrate the sensors to a known load condition. In general, the bolt-on force sensor approach performsvery well on conventional stamping presses, where the ram movement during the forming operation isonly in a single direction, and predictable deflections are generated in the frame due to the formingforce.

Machine frame sensors may be considered and havebeen used with some success over the years on high speedstamping machines. However, due to the machine/toolingconfigurations and the high speed dynamics typically in-volved in such operations, machine frame sensors areuseful practically for press overload protection only. Ma-chine frame sensors for high speed operations are ex-tremely limited in their capabilities for tooling protection andpart quality control. This is due partly to the fact that manyhigh speed progressive dies require small tonnage com-pared to the capacity of the press in which they are run. It isnot uncommon for five ton progressive dies to be run inpresses ranging from 30-50 tons in capacity. This results inlow press frame deflections, and corresponding low machine frame sensor output signals that may beunusable from a monitoring standpoint. Another factor results from the condition that most high speedprogressive dies have many, many complex forming stations. A machine frame sensor relies on pressframe deflection to generate a force signal, which is a “composite” signal relating to several tool stationsin a multi-station tool. Because such sensors are so far removed from the precision forming that occursin any particular tool station, it is unlikely that a frame-mount sensor will yield a significant signal changedue to a fault (chipped punch, scrap in die, etc.) in an individual station. Thus, such faults would goundetected. An additional reason why machine frame sensors are quite limited for high speed stampingrelates to the sensor signal components from the acceleration/deceleration dynamic effects in the pressframe itself at high speed. As shown in the typical “Frame-Mount” sensor signal from a multi-station toolat 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 criticalin-die forming signals that are actually desired to be monitored.

In view of the many limitations associated with frame-mountsensors, “In-Die” force sensing offers the maximum toolingprotection and part quality control benefits for high speedstamping. “In-Die” force sensing consists of locating a sensorwithin or beneath an individual tool station, in order to generatethe strongest possible signal and one that relates most directlyto the particular forming operation. By placing the sensor asclose as possible to the actual “point of operation”, subtle faultsoccurring in the individual station produce a significant signalchange that is detectable at the monitor. The “In-Die” sensorsignal shown is from the same 560 SPM press and tool as the“Frame-Mount” sensor signal above. The signal was generatedfrom 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 readilyapparent that the “In-Die” signal is much “cleaner”. It offers an “individual” signal relating directly to thestaking operation, and exhibits a stable “zero” line between cycles with no “ringing”. As such, it is atremendously useful forming force signal that can be used for the detection of a wide variety of formingfaults. 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 manyyears on a wide variety of multi-station tools. The force sensing technology developed for those generalstamping applications can also be applied to many different types of high speed operations, includingcoining, embossing, bending, staking, and hole piercing. The “In-Die” sensing approach consists oflocating force sensors within or beneath critical tool stations, to separate multi-station forming loads, andto 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 ofstrain gage load cells placed into machined pockets beneath individual secondary operation dies orpunches. The other approach, involving the “Die Plug” sensor, has been in existence for about tenyears, 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 approachconsists of mounting such a cell within or beneath anindividual tool station. This “load cell” approach allowsfor a very direct and high force signal output even onlight load applications, and also allows for the separa-tion of force signals on multi-station tools. Such cellsallow for a “calibrated” force readout directly in poundsor tons. Strain gage load cell design is a well perfectedscience. Strain gage load cells incorporate strain gagesensing elements bonded typically to cylindrical or ring shaped steel structures. The completed straingage force sensors or “load cells” can be made in many different physical shapes, sizes, and capacityratings. 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 thecapacity rating of the cell. Load cells designed for higher force values are typically physically larger thansmaller capacity cells. All of the strain gage sensor principles as outlined above apply to strain gageload cells. These devices generally incorporate a reduced cross-sectional area groove concentricallymachined at the middle of the cell structure. This provides a physical protected location for the straingages, and also determines the actual force rating of the cell.

Strain gage load cells typically operate under a “compression” load-ing condition, with the compressive deflection of the reduced area gagesection resulting in a proportional output signal from the multiple bondedstrain gages wired in a full "Wheatstone Bridge". These cells arereferred to as "series" type force sensors. As such, all of the force to bemeasured is transferred through the cell. This type of cell is usuallyfactory calibrated under test loading conditions before being shipped forfield installation. There are several quality manufacturers of strain gageload cells in the industry. Many of these offer standard "catalog" load cells that are pre-engineered, andmay be produced on a stock basis by the manufacturer. A wide variety of sizes and capacities of suchload cells is available. For those applications where a strain gage load cell is required, it is stronglyrecommended to initially verify if a standard device will physically fit into the machine and have thedesired capacity rating. This allows for the quickest and most economical installation. In the event thata 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” typedevice 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 verysimple, in that it is simply glued with industrial epoxy into a small hole machined beneath a tool station.

As the tool (die or punch) is involved in performing the operation, verysmall 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 theapplied tool force.

Since the “Die Plug” sensor is available in strain gage andpiezoelectric versions, each has its own characteristics. Thestrain gage version functions like a miniature strain gage loadcell, with all of the associated properties. The piezoelectricversion incorporates a very highly sensitive piezoelectric sens-ing element. This element is typically made of a crystalline orceramic material. It is the same basic type of sensing elementthat is used in extremely sensitive microphones. A piezoelectricsensing element self-generates a proportional voltage outputsignal when it undergoes physical deflection. Its extremely highoutput, great sensitivity, and wide operating range make it a good choice for certain machine forcemeasurements. 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 operationmonitoring, it is recommended that the piezoelectric “Die Plug” sensor be reserved for very light loadand/or very high speed operations (exceeding 1200 SPM). Regardless of whether a strain gage orpiezoelectric “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 firstinvolves installing the sensor in the machined tooling hole to the proper depth. The front sensingelement should be centered as closely as possible beneath the tool to be monitored, and within thecompressive "footprint" of the tool. The second step involves injecting a special high strength epoxypotting into the injection tube. This allows the epoxy to backflow around the sensing element and tocompletely fill the hole, permanently encapsulating the sensing element into place. The final stepincludes cutting off the excess tubing that projects beyond the sensor hole, and connecting the cable tothe monitoring instrument. Modular wiring interconnect systems, including miniature tool-mounted

"DIE PLUG" FORCE SENSOR

connectors, plug-in cables, and junction boxes, are available to easily route the sensor wiring to themonitoring instrument. Due to its small size and high sensitivity, the "Die Plug" type of sensor can beused very effectively for high speed progressive die force monitoring, especially where installationspace 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 criticaltool stations.

The quantity of “Die Plug” force sensors and their locations in progressive dies depends largely onthe particular operations taking place. Not all stations necessarily need a sensor. However, it isrecommended that one sensor always be located very near the first die station. This location providesa means for detecting variations in material such as thickness, hardness, temper, etc. Other sensorlocations can include forming stations where early characteristics of punch and die wear can beidentified, as well as faults such as damaged tools, scrap in die, etc. Monitoring such stations later in theprogression 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 asmall bracket, with multiple forming operations. In general, it is most desirable to locate the sensorsbeneath 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. Thiscan occur when the stationary die tool area does not provide a clean “force path” to the sensor. In thosecases, the moving side punch tool can be utilized for the sensor location.

PROGRESSION

(4) IN-DIE FORCE SENSORS

MONITORMATERIAL THICKNESS

AND HARDNESS

MONITOR BENDING

MONITOR PIERCING

MONITOR BENDING

A two channel approach is popu-lar for small, high-speed progres-sive dies which have a number ofstations in a very small area. Thefirst sensor is typically located ator near the first station to monitorfor gross faults such as misfeedsand material variations. The sec-ond sensor is typically located ina station towards the end of theprogression that relates to the fin-ished part quality. Chipped or bro-ken punches, scrap, misfeeds, orother process variations takingplace along the progression willbe detected as the strip or productexits the die.

Having located the right type of force sensors in the proper locations to generate meaningful outputsignals, a monitoring device of some sort is needed to complete the system. The force monitoringinstrument performs certain basic functions that can be depicted in a block diagram. These functionsinclude:

- 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 thedisplay meter almost always displays the force value for each machine cycle in a digital format. For astrain gage force sensor, this usually represents a calibrated or “actual force” value. For a piezoelectrictype force sensor, this represents an uncalibrated “reference” force value. Another common feature isthat the instruments typically monitor and display the peak force value developed in the machine or thetool. 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 allof the various types of monitors, from the simplest to the most advanced, are generally available in bothsingle and multi-channel configurations. The number of channels is dictated by the maximum numberof tool stations with corresponding sensors to be monitored. This becomes a matter of economics, andselecting 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 discreetlyadjustable high and low alarms. In the event of either a high or low alarmcondition, indicating a change and a possible fault in the process, an alarm relaywould fire to stop the process. This allows for corrective action to be taken.

A refinement in the way of force monitors is a micropro-cessor circuitry "self- programming" unit. Based upon aninitial "good part" operating condition, the operator initi-ates 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 moni-tored with respect to the "good part" condition.Microprocessor-based units offer the user some very pow-erful force monitoring capabilities, such as "good part"self-programming and automatic alarm setting, with aminimum of operator involvement.

A recent advancement in force monitoring instrumentation is the“PLC-based” force monitor. As opposed to the other instrumentoptions, which are dedicated specialized units, the “PLC-based” unitincorporates force monitoring into a standard PLC rack. This is doneby plugging force signal conditioning “modules” into the PLC rack,and configuring the PLC program for the desired force display,monitoring, and alarm functions. Such an approach can yield signifi-cant savings, since the force monitoring hardware is simply therequired number of “modules” installed into an existing machinecontrol PLC rack, or a new PLC rack if required. Also, if an existingPLC 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 cansave considerable monies and space, compared to the more traditional type of dedicated force monitor.This is especially true on systems with many channels.

As stated previously, the quality of the finished parts is directly related to the forming force requiredto make them. The primary and perhaps most important function of the "force sensor/monitoringinstrument" combination is to monitor forming forces for all parts on a continuous production basis, andto verify if those forces go beyond the force "quality window". In that event, corrective action in the wayof off-tolerance part diversion or machine stoppage is initiated. Another important function is to use themonitor as a “set-up” tool to achieve better and quicker machine/tool “set-up”. This is done by recordingforce values during initial “set-up” of a given part to a known “good part” condition. With such “goodpart” force values thus established during “set-up”, that same tool can be adjusted during subsequentset-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 machinecannot be monitored for a 5% load change, when the process itself normally involves a 20% loadvariation 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 berecognized that every forming operation, even the most precisely controlled, will have some variation inthe cycle-to-cycle forming forces. The idea is to control those "normal variation" process variables asclosely as possible, and then to set the monitor alarm limits beyond the normal variation levels.

Processor

Display

LOW

HIGH1 2

SetupRun

Strain GageInput (2)

2 ChannelStrain Gage Module

PLC-BASEDFORCE MONITOR

One of the newer and most exciting features avail-able in advanced load monitors is a "tracking alarm"function that monitors each forming cycle throughoutthe entire stroke. Peak force monitors capture anddisplay the peak force value for each machine cycle,and monitor with respect to that peak force. As usefulas that function is, the newer "tracking alarm" typemonitors extend the force monitoring function muchfurther. Based upon a learned "good part" force signa-ture, the "tracking alarm" monitor establishes a highand low alarm band that tracks this signature through-out the entire forming cycle. The signature for eachsubsequent part is then compared to the learned "goodpart" signature, and a change in load at any point along the signature that goes beyond the trackingalarm band is recognized as an alarm condition. This allows for the detection of very subtle faults in theprocess that may not be manifested as a change in the peak force. Thus, greater sensitivity to loadchanges 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 stampingpresses for approximately 20 years. Although the sensors and monitoring electronics have beencontinually upgraded, with corresponding performance improvements, the basic system componentsand operation have remained relatively unchanged. Displacement monitoring systems can be used togood benefit on a large variety of metal stamping operations. However, the performance features andbenefits are most fully realized when applied to high speed progressive dies. Displacement monitoringtechnology should not be regarded as competing directly with force monitoring technology. Since eachhas its own unique benefits, limitations, and cost factors, they should be considered as complimentarytechnologies that can be used together for enhanced stamping process improvement and part qualitycontrol.

A displacement sensor die monitoring system is very simple, mak-ing it easily installed and very cost effective for monitoring highspeed progressive dies. A typical system consists of one or morenoncontact precision “Eddy current” displacement sensors, with acorresponding 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 withthe power supply and one channel, and additional channel units thatcan stack on top. Each channel has a digital meter to showdisplacement deviation values, and also the alarm setpoints.

In practice, the sensors are mounted in a protected location on the stationary die or die shoe. Foreach 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 nominalsensor-to-target displacement gap (typically .040”-.060”) at bottom dead center. During normal machineoperation with parts being made, the system monitors the sensor-to-target separation gap between theupper and lower dies, and looks for subtle deviations in stripper position or die height at each sensorlocation. Using sensors with high frequency response, and microprocessor-based electronics, thismonitoring 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 faultoccurs in the process, causing the sensor-to-target displacement gap change to exceed the alarmsetpoint, an alarm relay would activate to stop the machine. In this way, the displacement monitor canbe 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 firstoption involves mounting the targets on the spring-loaded stripper. This is the most typical and widelyused location, and is the best for pulled slug and scrap-in-die detection. The second option involvesmounting the targets on the upper die assembly itself. This provides the best performance for thedetection of tooling/forming faults such as broken dies and coining depth abnormalities.

The illustrationshows a typical instal-lation of the sensortarget on a spring-loaded stripper. Thetool station representsa hole pierce or blank-ing operation, with aslug ejected duringeach stroke. Under a“normal” condition asshown at bottom deadcenter, the strippermaintains uniformparallel contact on thematerial strip. Thesensor-to-target dis-placement gap is veryrepeatable and con-sistent for this“normal” condition.

This illustration also showsthe stripper mounting loca-tion for the sensor target,but with an abnormal“pulled slug” condition. Ifthe pulled slug falls any-where on top of the materialstrip, which is a quite likelylocation, the additional in-terference from the slugpresence causes the strip-per to become cocked at anangle. For the displace-ment sensor closest to theslug, the sensor-to-targetdisplacement gap increasesappreciably. Provided thatthe alarm limits at the moni-tor have been set properlyand that the displacementchange exceeds the“normal” condition limit, an

alarm would then activate to stop the press. After the machine operator has cleared the slug from thedie, the alarm can be reset and normal press operation resumed. This stripper mounting arrangementfor the sensor targets has proven extremely successful in the field in detecting pulled slugs and scrap inthe die for many types of dies, different materials, and material thicknesses.

The other option for sensortarget mounting is to locatethem on the upper die as-sembly itself. This mount-ing location is the best onefor detecting faults involvedin bottom form operations,such as coining, embossing,and bending. The tool sta-tion in the illustration repre-sents a coin station with thesensor mounted on the up-per die. For this type ofapplication, a broken die orpunch would create achange in the coin depthand in the sensor-to-targetdisplacement gap, resultingin an alarm shutdown. Itshould be noted that thissensor option on the upperdie is the only one availablefor dies with non spring-loaded “box” strippers.

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

In terms of the displacement sensor monitor performance, there are typically two different modes ofoperation. These modes relate to the “normal” condition benchmark displacement values to which thecurrent 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 othermode 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 outlinedbelow.

Conclusion

As stated previously, both force monitoring and displacement monitoring technologies are commer-cially available for high speed stamping operations. Their functions compliment each other, and theycan both be used to achieve substantial process improvements. A key element in using thesetechnologies 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 monitoringfor changes outside normal limits, many crucial types of forming faults can be detected. These includesuch 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