Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
WAVEFORM SIGNATURE
ANALYSIS
In pressworking, a waveform signature is a pictorial or graphical representation of
relative movements or amplitude displayed as a rectangular coordinate line-chart. The X
or horizontal axis represents units of time, distance, displacement or degrees of
crankshaft rotation. The Y or vertical axis usually displays force or amplitude.
Stress Strain Curves
A familiar example is the stress Vs strain curves or signature produced by a tensile
testing machine. Figure 1 is a simplified curve of a typical stress strain signature.
Tensile testing machines produce a signature of this type. Test samples or coupons are
increasingly loaded until the sample pulls apart.
Tensile test curve signatures provide important information about the formability and
strength of the test coupon. The point on the vertical axis where pronounced elongation,
or movement to the left occurs, is the yield strength of the material. The peak unit stress
or applied load is the ultimate tensile strength.
The amount of elongation that occurs before fracture is an important measure of material
formability. The material yield and ultimate tensile strengths are useful to predict the
force required forming the material. The stress strain curves or signatures for various
materials are available as an atlas listing over 600 examples in a reference publication
published by ASM International. 1
Charting Tensile Testing Machine Waveforms
Tensile testing machines have a load cell directly in line with the test coupon. The load
cell output signal is proportional to the stress or force applied to the sample at any
moment.
A sensor that provides a signal that is proportional to the distance the coupon is stretched
measures the strain or displacement of the test coupon. Zero force (stress) and distance
(strain) correspond to the lower left-hand corner of the chart.
Waveform Signatures of Cutting Loads
Tensile testing machines are of robust construction and operate smoothly. There is very
little shock or machine vibration superimposed on the stress strain waveform chart.
1
H. Boyer, "Atlas of Stress-Strain Curves", ASM, International, Materials Park, Ohio, © 1986. This
reference lists over 600 stress-strain curves for ferrous and non-ferrous materials.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Figure 1. A chart of the stress strain signature of a test coupon stretched to failure.
Smith & Associates
Except for superimposed noise signals, similar stress strain relationships to those
observed with a tensile testing machine occur when punching or shearing metal. In each
case, the process results in a fracture or separation of the material.
Punching Thick Carbon-Steel, a 1989 Webster Industries Case Study
Figure 2 illustrates the waveform signature of the stress-strain relationship when cutting
off and piercing two holes in a chain side bar. The material is AISI-SAE 1039 fine
grained steel 0.500-inch (12.7 mm) thick by 3.0-inches (76.2 mm) wide. A chart recorder
paper speed of 200 mm (7.874 inches) per second gives good detail.
Like the tensile test chart, (Figure 1), the vertical axis indicates stress or force. However,
the horizontal axis represents time rather than displacement.
The press speed is 60 strokes per minute (SPM). Even at 200 mm per second, the
waveform trace distance from initial contact of the punch on the work until it breaks
through is very short. The portion of the waveform from initial punch contact to
breakthrough occurs in 5 mm (0.20 inches) or 0.025 seconds.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Figure 2. The actual waveform signature of a combined piercing and cut off operation
having excessive snap through or reverse load. Webster Industries/Smith & Associates
Figure 3. Waveform signature of the operation illustrated in Figure 2 after modifying the
die by adding timing and balanced shear. Webster Industries/Smith & Associates
The punching waveform exhibits a sharp negative spike below the zero trace at break
through. This is due to a sudden release of the energy stored in the press and die as
strains and/or deflection of the various parts.
Analysis of the Amount of Stored Energy
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
The magnitude of the actual energy released increases as the square of the actual tonnage
developed at of final breakthrough. The actual energy is:
FxD
E = ———
2
(EQUATION 1)
In this Equation: F = Pressure at moment of breakthrough in short tons. (Lbf X 2,000)
D = Amount of total deflection in inches.
E X 166.7 = Energy in foot-pounds
or:
F = Pressure at moment of breakthrough in Metric-tons (kgf X 1000).
D = Amount of total deflection in millimeters.
E = 9.807 = Energy in Joules or Watt-Seconds
Note: (1 Foot Pound = 1.356 Joules or Watt-Seconds)
(1 Joule or Watt Second = 0.7376 Foot Pounds)
(1 Inch-Ton = 166.7 Foot-Pounds)
Importance of Timing of Break Through
In timing punch entry or die shear, give consideration to provide for a gradual release of
the force developed. With the exception of high-speed applications, little shock load
occurs by the impact of the punch on the stock. In fact, when the punch first contacts the
stock, the kinetic energy of the slide is an additional energy source.
To complete the work, the flywheel supplies energy. As this occurs, the press members
deflect. An analysis of the quantity of energy involved will show why a gradual
reduction in cutting pressure prior to snap through is very important. A general rule for
snap through or reverse load that a press can withstand without sustaining damage is 10%
of rated press force or tonnage. Reverse loads significantly higher than 10% of total
capacity may damage the machine. An especially critical part is the connection, which
may fail and allow the slide to fall.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Simplified Square-Law Relationship Example
For example, using English units: if 400 tons resulted in 0.080-inch total deflection to cut
through a thick steel blank, the energy released at snap through, from the formula, is
2667 foot pounds. 2 3
Careful timing of the cutting sequence results in a reduction of the tonnage f snap
through to 200 tons—the reduction in shock and noise is dramatic. This is because half
the tonnage produces only half as much deflection or .040-inch. The resultant snap
through energy is only 667 foot-pounds or one-fourth the former value.
Actual Die Timing Improvement
Timing shear angles and punch entry sequences to provide a gradual release of force
prior to snap through is a straightforward way to reduce the shock and noise associated
with this problem. The simplified analysis of the square-law relationship applies in our
case study.
The operation uses a 300-ton (2,669-kN.) straightside press. The allowable reverse load
is 30 tons (267 kN.). Point (A) of Figure 2 illustrates a peak load of 191 tons (1,699 kn.),
which is well within press capacity.
The reverse load (B) is 87 tons (774 kn.), which is nearly three times the allowable
amount. Reworking the die on the repair bench is the solution. Here, shortening one
punch 0.312-inch (7.92 mm) and grinding the balanced-angular shear on the punches was
done. Balanced shear was also ground on the parting punch.
Figure 3 illustrates the improvement achieved by modifying the tool. The peak tonnage
decreases to 82.8 tons (737 kn.) that is less than half the initial value. The reverse load
decreases to 22 tons (196 kn.) or about one fourth the former value.
Timing Results Agree With Square Law Formula
This and the documented results of many other tests show snap through reductions
conforming closely to the square law formula. Relocation of all strain sensors provides
clean signals for chart recordings free of noise, as explained in references. 2 and 3.
2
D. Smith, “Die Design Handbook”, Section 4, Shear Action in Metal Cutting, The Society of
Manufacturing Engineers, Dearborn, Michigan, 1990. Anthony Rante, PE. Manager of Mechanical
Engineering, Danly Machine, Chicago, Illinois, is thanked for reviewing the Editor's formulas and
examples.
3
D. Smith, “Using Waveform Signature Analysis to Reduce Snap-Through Energy”, SME Technical
Report MF90-11, Society of Manufacturing Engineers, Dearborn, Michigan, © 1990. The report has
extensive background into the problem and solution including employee-training issues.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Figure 4. The waveform signature of a combined punching, cut off and joggle bending
operation: the part, an engineering-class chain component, parts from the bar stock in this
combined operation. Webster Industries
Waveform Signature of Combined Cutting and Bending
Figure 4 illustrates the waveform signature of a combined punching cutting off and
joggle bending operation. Here, AISI-SAE 1039 steel 0.500-inch (12.7 mm) thick by
2.000-inch (50.8 mm) wide has two holes punched, and a 0.562-inch (14.27 mm) joggle
formed. The part, an engineering-class chain component, parts from the bar stock in this
combined operation.
Plastic Chart Overlays
Webster Industries located in Tiffin, Ohio uses a portable chart recorder to check every
set up. This insures that optimal die timing is used. It is especially important not to
exceed 10% of press capacity as snap through load.
Webster Pressroom Employee Training
A customized training program quickly developed carried out assures active employee
involvement. Due to the many problems with tooling and new technology, a great deal of
additional shop floor interactive training assured employee confidence in the procedures.
Therefore, the diesetters who build up the tooling from standard components, and the
diemakers who build the tooling have an excellent insight into the physical principles that
underlie the successful pressworking of heavy metal. The tonnage monitor readings and
chart recorder data are now trusted to provide the process control information needed to
produce parts of consistent high quality while avoiding equipment damage.
To simplify chart interpretation, colored plastic overlays on overhead projector film stock
serve for ease of evaluation. These overlays (Figure 5) permit rapid evaluation of peak
positive and negative loads.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Figure 5. Plastic overlay printed on overhead projector film stock for evaluating peak
positive and negative loads. Smith & Associates
Putting Theory into Action
Classroom training begun and a chart recorder selected. The tonnage monitor
manufacturer has a standard modification provide analog outputs. Tonnage monitors
with built in waveform displays provided by many equipment producers. The equipment
available to us lacked that feature in 1989.
Modifying Tonnage Monitors for Chart Recorder Output
The tonnage monitor manufacturer provided instructions for modification of the tonnage
monitors to obtain the analog signals from each channel, as well as the sum channel. The
analog direct current (DC) voltage level from zero to 100 percent tonnage is zero to -4
volts DC. An Amphenol ® type connector was recommended for the stamping shop
environment.
To avoid the need to re zero the chart recorder each time that it moved to a different
press, a 4.7-microfarad DC blocking capacitor placed in the signal line from each
channel. A larger value of capacitance may be necessary for very slow operations.
Calculate this by using the time constant formula based upon the number of seconds a
press cycle takes and the input impedance of the chart recorder.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Chart Recorder Selection
Required bandwidth or frequency response and ease of use are the two most important
considerations when selecting a chart recorder for signature analysis. The nominal
frequency response of a tonnage monitor is at least 1 KHZ.
Electro-mechanical pen and ink recorders typically have a frequency response that
extends to approximately 125 HZ. This is sufficient for observing slow phenomena such
as conventional drawing embossing and coining at low press speeds. For analysis of fast
rise time events such as those generated by snap through release as well as cam and draw
ring impact, a chart recorder having frequency responses of at least 1 kHz is desirable.
Lower frequency response will not display the magnitude of shock and impact problems.
A two-channel recorder is a minimal requirement for a comparison of the coincidence of
events on two different channels. An important application in press troubleshooting is
making sure that all corners of a slide reach bottom dead center at exactly the same time.
Timing problems caused by a twisted crankshaft or a partly sheared key is identified by
an A to B comparison of exactly when peak tonnage was developed on two or more
channels.
A four-channel recorder will permit observation of four channels
simultaneously, which often will speed up signature analysis work.
The requirements decided upon were:
1. Thermal paper to avoid the mess and clogging problems associated with pen and
ink recorders.
2. At least two channels.
3. Automatic identification of voltage range, paper speed, date and time.
4. At least 1 KHZ real time frequency response.
5. A wide range of speeds and voltage input levels to permit other uses in the plant
such as power line monitoring.
6. Portability.
7. Battery powered operation.
An existing oscilloscope with camera was considered, but it was decided that the cost of
film to record the amount of data to be gathered would be prohibitive. Further, a chart
recorder permits real time recording of a number of sequential events.
Digital signal acquisition and storage done in conjunction with a personal computer was
also considered. This was ruled out because it would require the operators to be trained
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
in basic computer skills in addition to signature analysis.
equipment requiring portable operation would be greater.
Further, the amount of
Upon review of available equipment, a four channel 25 KHZ real time chart recorder
having all of the required features was selected. Upon installing recorder interfaces on
each of the monitors, and placing an Astro-Med Mark 4 chart recorder on order, training
classes started. An instrument cart having pneumatic tires was also purchased. A
protective cover of 0.250" (6,5 mm) clear acrylic plastic was fabricated in the tool room.
To avoid the need to plug the unit into AC power which could cause ground loop noise
pick up, a 12 volt automotive storage battery was placed on the bottom shelf and
connected to the recorder by a cable with a quick disconnect plug. At this point, with
classroom, training about halfway complete, hands on training was started and theory put
into practice.
The battery-powered recorder is mounted on a cart having all needed supplies. It plugs
into the press electronic force monitor to obtain readings. To speed up the analysis, laser
printed plastic overlays are used to interpret the readings. The waveforms illustrated in
Figures 2, 3 and 4, each have the appropriate overlay for the press tonnage in place.
Interpreting the Reading
The diesetter simply overlays the chart recording with the appropriate plastic overlay to
determine both the peak positive and negative or reverse load. While the machines are
quite robust, to avoid maintenance problems, the reverse or snap through load is
maintained within 10% of press capacity.
Long Term Results
Webster Industries started this program in 1989. All training and full implementation
was accomplished in 1990. Five years later every job is checked with the chart recorder
and records of all results maintained. This information in invaluable when responding to
rush orders for the hundreds of types of engineering class chain produced at Webster.
The following positive results were achieved:
1. The annual cost of perishable tooling breakage dropped from over $18,000.00 to
zero.
2. By optimizing die timing, all jobs could be run at full press speed resulting in at
least a 25% productivity improvement.
3. Freedom from double breakage in pierced holes because of faster press speeds.
4. Improved set up repeatability resulting from verification of the exact duplication
of a previous set up that produced high quality chain components.
5. Optimizations of punch to die clearance on all pierce and shave operations by
means of waveform signature analysis.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
6. Enhanced record keeping and analysis of the factors needed to duplicate quality
part runs.
7. The press shop is a quieter place to work now that snap through energy is under
control.
8. Elimination of press damage other than normal wear saving of over $100,000.00
per year.
9. Improved scheduling flexibility due to freedom from press breakdowns.
10. The pressroom is no longer a production bottleneck.
11. Throughput of stampings has increased every year without the requirement of
additional labor or machinery.
Analysis of Factors Leading to Success at Webster
President Fredric Spurck, Vice President George Tolford, Engineering Manager John Lay
and Quality Control Manager Joe Wise all gave their unequivocal support to the
recommended changes in maintenance and manufacturing methods. Management
prioritized training as an immediate requirement. Management simply refused to accept
any form of mediocrity in the way that they did business. Factors insuring success
include all of the following
1. The absolute support of top management.
2. Analysis of the results of the confidential pressroom employee interviews and
adoption of the recommended action plan.
3. Management attended the training sessions.
4. There was a skilled literate work force to train.
5. Attendance of training sessions was mandatory.
6. Assignment of an extremely project oriented individual, Quality Control
Manager, Joe Wise, to act as in-house facilitator for training and project
implementation.
7. Every employee must follow written procedures detailing how to accomplish
every production and maintenance task correctly and safely.
8. Support of the labor union for management's training objectives.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
9. The requirement that the use of the chart recorder to insure that previous good set
ups are duplicated and that there is no excessive snap through done for every set
up. This procedure continues on all jobs to this day.
Diagnosing Hydraulic Overload Problems
Process variation problems often occur in presses equipped with hydraulic overload
systems. Waveform signature analysis is an excellent tool to help pinpoint the cause.
Waveform signatures taken directly from load cells placed on strong supports in the die
space is the procedure used in the case study. However, charting successive hits from the
analog voltage output of a dedicated press force monitor can also show variation from
stroke to stroke.
Figure 6. A waveform signature illustrating a partial dumping of a hydraulic overload
system on a toggle press during three successive cycles: the result is serious product
variation. Smith & Associates
Case Study of Triple Action Toggle Press Hydraulic Overload Problem
Here, the complaint was a serious variation of the depth of the embossed features in
automotive inner doors. The machine is a Danly 1000-ton (8896 kn.) 120-inch (3048
mm) wide triple-action toggle press.
Both the outer slide or blankholder, and inner slide have toggle mechanisms to provide
dwell at the bottom of stroke. Both slides have hydraulic overload protection. In case of
an overload, hydraulic cylinders in the slide of this under-driven machine are designed to
dump, providing approximately 0.750-inch (19.05 mm) of additional shut height. Figure
6 illustrates a waveform signature taken from one of four 250-ton (2224 kn.) load cells
used to test the inner slide.
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
As the toggle mechanism cycles, the slide dwells on bottom by means of idle points in
the driving linkage. A double hump variation in pressure of equal amplitude during the
stroke is normal for this type of slide actuation.
The waveform in Figure 6 is abnormal. Reading the illustration from left to right: during
the first waveform (A), the cylinder is leaking down under load, resulting in a loss of
force during the dwell portion of the cycle. Near the end of the second cycle (B), the
relief valve dumps suddenly. During the third cycle (C), the cylinder is recharging. The
pressure is seen increasing during the dwell on bottom.
Analysis of the Problem
Clearly, the force variation is caused by the hydraulic overload problem. This in turn is
responsible for the inconsistent depth of part embossment. A number of factors have
contributed to this problem. The press is several decades old. Any maintenance is
generally in response to a breakdown.
The lube-oil, which is also used to charge the hydraulic overload cylinders, is not
changed unless the press is torn down to repair a broken part. The recommended antifoaming additives are not used. Contamination with water-based drawing lubricants is a
factor. The cylinder packing used is the least costly obtainable, not what the
manufacturer recommends. There are many other related causal factors responsible for
the pressure variation problem and the overload system tripping erratically.
The most serious maintenance error found was a jumper that defeated the limit switches
that signal the press to stop if an overload dumps. This can result in unsafe press
overloading as well as machine and die damage. In this case, a direct result is the
production of thousands of dollars in scrap parts, some of which may have been
assembled into automobiles.
In this case study, portable instrumentation is used to highlight a problem. The overall
root cause, however, are poorly organized maintenance management practices, and a lack
of systematic employee training in machine maintenance skills.
Another Hydraulic Overload Problem
The chart recording shown in figure 7 was made with a pin and ink two-channel recorder.
The machine is a Danly 1000-ton (8896 kn.) 120-inch (3048-mm) wide triple-action
toggle press. These machines give decades of good service if they are not abused and
routine maintenance is regularly performed.
This machine had provided several decades of service with little more than breakdown
maintenance performed. Indeed, there was ample evidence that it had been abused by
overloading and lubrication failures. Management tolerated the practice of jumpering
important safety systems such as slide position limit switches, lube pressure switches,
and hydraulic overload dump switches.
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Waveform Signature Analysis C013.DOC © 1989-2001
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Figure 8 shows an example of a severe hydraulic overload leakage or bleed down
problem. Note the pronounced tilt shown as the inner slide is on dwell. The probable
cause is a cylinder packing failure or dump valve leakage.
Figure 7. Example of a press having a number of mechanical and hydraulic overloads
problems. Smith & Associates
This machine also has a severe misalignment problem. The punch plate was shimmed
0.105-inch (2.667 mm) on the left side in order to uniformly set embossments in an
automotive package tray panel.
Figure 8. An example of a severe hydraulic overload leakage or bleed down problem as
evidenced by the tilt shown as the inner slide is on dwell. Smith & Associates
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Figure 9. Shudder die to a severe mechanical bind in the press gibbing due to press
misalignment. Smith & Associates
Avoid Breakdown Maintenance
In the writer’s opinion, based on many consulting jobs involving stamping process
problems, there apparently is an unfortunate misunderstanding on the part of
management in some large stamping plants. Apparently, some managers believe that a
stamping press is doing an acceptable job so long as it will cycle up and down.
Whenever a machine is not maintained and used in accordance with the builder’s
specifications, all of the principles of excellent manufacturing are being completely
ignored. This management style is totally unacceptable in the present world economy.
Figure 10. Double-action press waveform of an inverted stretch-form die. Both the
inner and outer slide signatures are illustrated with the press tonnage curve limits
superimposed. Toledo Transducers, Inc.
Monitoring Double Action Press Tonnage Curves
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Waveform Signature Analysis C013.DOC © 1989-2001
Smith & Associates, 530 Hollywood Drive, Monroe, MI 48162-2943. Rev June 28, 2001
Large conventional double-action drawing presses used to stamp automotive body panels
is designed to develop full force a short distance from bottom of stroke. The blank holder
is typically designed to exert full tonnage only 0.250 to 0.500-inch (6.35 to 12.7 mm)
above bottom dead center (BDC).
Specially-Adapted Force Monitoring Equipment
Figure 10 illustrates the waveform of an inverted stretch-form die running in
conventional double-action press. Both the inner and outer slide signatures are shown,
with the press tonnage curve limits superimposed. 4
The press tonnage curve limits of both the inner and outer sides are compared to the force
of each slide at many points during the press cycle. This is accomplished as follows:
1. The tonnage curve data supplied by the press manufacturer, in terms of maximum
force versus distance from bottom dead center (BDC), is programmed into nonvolatile digital memory in the press tonnage or force monitor.
2. A rotary resolver, connected to the press crankshaft, sends angular position data
to a specially adapted press tonnage or force monitor.
3. Force data taken from strain gages or sensors on the press pull rods, pitmans or
eccentric straps, is converted to an accurate force value for both the inner and
outer slides.
4. In the event that the force throughout the critical die closure portion of the cycle
should violate the maximum value specified by the press manufacturer, an alarm
signal is sent to stop the press.
Correcting Press Force Curve Violations
In the event that a force or tonnage curve overload condition is detected, waveform
analysis is used. A display illustrated in Figure 10 is acquired and shown on the screen
of a portable personal computer. This data can be stored on a magnetic disk, for detailed
analysis and/or hard copy generated at the press on a portable printer having graphics
capability.
Plant Wide Monitoring Capability
The Toledo Transducers system is termed critical tonnage curve monitoring. In some
plants where this system is installed, the waveform signal is collected from presses plant
wide by daisy chaining all monitors together with data transfer cable by means of the
industry standard RS422 data protocol.
4
B. Mettert, "Load Sensor Placement and Tonnage Data for Underdrive Presses", SME Technical Paper
MS90-384, Society of Manufacturing Engineers, Dearborn, Michigan, 1990. This paper explains in detail
the benefits; techniques and advantages of mounting metal carrier strain gages on under driven press pull
rods compared to other methods. Ford Motor Company extensively uses this procedure.
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Waveform Signature Analysis C013.DOC © 1989-2001
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The RS422 cable is terminated in a PC, which can perform plant wide monitoring. If the
PC is configured to act as a file server, the waveform signal can be observed at any PC
having the copyrighted Toledo Transducers software if connected to the plant digital
communications network.
Tonnage Curve Correction Measures
If a tonnage curve violation is found, one or more of the following corrective measures
should be taken:
1. The engagement of nitrogen cylinders may be stepped to lessen the initial force.
2. If the press is equipped with an Eaton Dynamatic ™ constant energy drive, the
press may be slowed down upon initial draw ring contact to lessen problems with
bounce or rebound.
3. Die modification to lower the draw ring should be carried out if it has more travel
than necessary.
4. A press with a better force versus distance from bottom may be used if available.
5. The press may be modified, in some cases, by installing stronger parts to increase
the tonnage curve capability.
6. Use a hydraulic press, which has no tonnage curve limitation.
Press Controls with Waveform Signature Readout
Figure 11 illustrates the operator terminal of a press control system having an integrated
force monitor. In addition to press and auxiliary equipment control functions, the force
monitor portion of the system has useful capabilities that include:
1. A built in screen with waveform signature capability.
2. Selectable high limits at a number of crankshaft angular positions in the press
cycle.
3. Presettable reverse load alarm monitoring.
A system of this type is especially useful for progressive die and other combined
operations where cutting, snap through and forming forces are present. The alarm
window set points can be stored by job number to speed up diesetting operations.
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Waveform Signature Analysis C013.DOC © 1989-2001
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Figure 11. The operator terminal of a press control system having an integrated force
monitor: a built in flat panel screen provides waveform signature display in color and
multiple set point alarm capability. Link Systems, Inc.
Do You Need a Pressroom Wide Monitoring System?
Press force monitors having remote monitoring capability are available from several
manufacturers. Force data and, in some cases, waveform signatures can be exported to a
remote monitoring location. While the technology is quite feasible, the desirability and
cost effectiveness of gathering such data in a central command center is not universally
accepted as a useful management tool. 5 6 7
Some managers believe that force-ponitoring equipment is a management tool to prevent
press overloading damage. Experience has shown that efforts to use the monitor for this
purpose alone usually fail. The typical result is that the equipment is disabled, and not
used.
5
D. Smith, “How to Improve Hit-to-Hit Time with a Tonnage Monitor”, SME Technical Paper TE88780, Society of Manufacturing Engineers, Dearborn, Michigan, © 1988.
6
D. Smith, “Quick Die Change”, Chapter 28, instituting a Tonnage Meter Program, The Society of
Manufacturing Engineers, Dearborn, Michigan, © 1991.
7
D. Smith, “Visual Indicators in the Workplace”, The Fabricator, The Fabricator's and Manufacturer's
Association, International, Rockford, Illinois, January-February 1993.
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Waveform Signature Analysis C013.DOC © 1989-2001
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Properly used, the tonnage or force monitor is a tool for the pressroom work force to set
up, troubleshoot, and control the stamping process. Case studies show that training,
employee trust and teamwork are the essential factors in achieving the goal of improving
the efficiency of stamping operations.
Monitoring Waveform Changes
Signature Technologies of Dallas, Texas has developed a patented method of measuring
small deviations that occur within a repeating series of waveform signatures. As we have
discussed, the simplest forms of tonnage monitors provide only a peak force reading and
an electrical output that can be used to stop the press in the event of a preset force is
exceeded and/or press over capacity condition occurs.
Establishing a Mask of High and Low Limits
The Signature Technologies establishes process control boundaries, which function as
high and low, alarm set points around the entire waveform signature. Figure 12
illustrates the press force signature running properly within a mask of control limits.
Figure 13 illustrate discrete signature points and the corresponding high and low limit
points. The data-sampling rate required depends on the machine speed and amount of
detail in the operation being performed. As shown, acquisition resolution of individual
data points within 0.022 degrees is possible depending on the signature window size and
press speed.
Monitoring the Waveform Signature
Figure 12. The basis for Signature Technologies signature analysis system plots within
mask or control limits having both a high and low signature limits. Signature
Technologies, Inc.
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Waveform Signature Analysis C013.DOC © 1989-2001
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Figure 13. Discrete signature points and their corresponding high and low limit points.
The data-sampling rate required depends on the machine speed and complexity of the
signature. Signature Technologies, Inc.
Figure 14. An alarm condition resulting from a signature fault. Note the record of the
conditions under which the fault occurred displayed on the left side of the screen.
Signature Technologies, Inc.
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Data Display
In addition to trip or machine shut down function, additional high and low limit points
can be provided to warn of changes in process operating parameters that indicate a drift
of any of the signature points.
The system features a color computer monitor display. The signature together with the
warning and limit points are shown in contrasting colors together with machine counts
and many other types of useful data.
Figure 14 indicates an alarm condition resulting from a signature fault on the left
(downstroke) side of the signature. Note the record of the conditions under which the
fault occurred displayed on the left side of the screen.
Die and Press Protection System Features and Limitations
Some force monitoring systems provide alarm outputs for additional conditions, which
include in part:
1. Excessive amounts of reverse or snap through load.
2. Violation of available press tonnage as a function of crankshaft degrees of
rotation or distance from bottom of stroke.
3. Failure to achieve a preset minimum force.
4. Exceeding a preset load within a user definable portion of the press-working
stroke.
Several types of die and press protection systems have been in use for many years. In
addition to force monitors, the use of press hydraulic overload systems, stretch-links and
shear-collars are employed.
Problems involving misfeeds and mislocated stock in progressive die and transfer press
applications can cause tool damage in the process of tripping the overload set point on a
force monitor. In many operations, just one bad hit can damage the die. The result is an
increase in product variation, and need for prompt bench repair. Double hits involving
very heavy stock can result in catastrophic press and die damage.
Excessive Snap-Through Loads that do not go Negative
Rapid energy release typical of snap-through at the completion of heavy cutting does not
necessarily cause a negative load on the press. The press structure stores energy as
deflection—avoid the sudden release even though a forming load is in process through
optimal die timing of the cutting load. Figure 15 illustrates a rapid release sufficient to
damage press parts.
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Figure 15. Example of a sudden snap-through energy release occurring at the start of a
large forming load. Although the sudden downward excursion does not go negative, the
energy release damages the press. Toledo Transducers
Limitations of Hydraulic Overload Protection
Hydraulic overload systems are designed mainly to protect the press. The trip points are
often adjusted to loads of 50 percent or more of press capacity.
The value of hydraulic overload systems is undisputed. Indeed, such systems should be
specified wherever possible for gap frame and solid frame straightside presses that are
liable to become stuck on bottom dead center. However, high forces may cause
expensive damage to delicate tooling although press damage is avoided.
Shear Collars and Stretch-Links
Both straightside and gap frame presses may incorporate a replaceable steel shear ring.
The ring has a stepped diameter. One is placed under each connection. One side is
machined out to provide an area that will shear under a press overload condition, causing
the ring to collapse.
Under driven presses may incorporate a stretch-link in line with each pull rod. This
device is shaped much as a dumb bell used for calisthenics. It is retained by split collars,
which are designed to facilitate replacement.
Both shear-collars and stretch-links provide a degree of protection for the press.
However, because they are subjected to large cyclical loads, the force required to deform
the device changes with time.
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Waveform Signature Analysis C013.DOC © 1989-2001
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There is a serious problem with this protection on presses with multiple connections. If
only one device yields, the slide will be tipped out of alignment severely. This may not
be noticed until severe scoring of the gibbing, and/or die damage has occurred.
Conclusion
In the author's experience, training all pressroom personnel in the fundamentals of
pressworking operations is a part of the process of improving efficiency. This training
must impart knowledge of how presses, dies and auxiliary equipment work. Employee
training leading to increased responsibility is the goal. In many cases, it is far better
economy to train employees to control the process at the press, than to transmit and
analyze data at a remote location. The latter approach often is, by definition waste
because it does not add value to the product.
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