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Abstract— Load monitoring in metal stamping presses has been

an important method of measuring press performance for

decades. This monitoring capability is now particularly of use in

the increasingly automated manufacturing environment. Modern

control systems can use load sensing data to improve press

performance and diagnose reliability issues. This paper identifies

the main factors that contribute to press loading, presents load

sensing system options, and explores the implementation of load

data into automated process control systems.

The main focus of the examples in this paper is a custom metal

stamping press in use at a third-tier automotive supplier. Some of

the systems and methods discussed in this paper were put into use

and tested on this press.

Index Terms—Stamping press, strain gauge, load sensing,

progression die, Wheatstone bridge.

I. INTRODUCTION

etal stamping presses are widely used in current

manufacturing processes. These machines make

everything from deep-drawn automobile body panels to the

change in our pockets. The stamping press is not a new

technology at all. Presses have been rhythmically shaking

factory floors for decades. As sensing and control technology

have advanced, so has the capability of stamping presses. The

automated production lines of today can benefit from

improved load sensing on stamping presses.

The loads stamping presses are subjected to are dynamic, and

depend upon multiple factors in the press setup, die design,

and material used. There are also multiple methods used to

measure press loads. Most sensing methods use some type of

transistor to convert displacement measurements into electrical

signals. Once the load data is determined it can be applied in

several different ways ranging from simple fail switches to

complete statistical analyses. This paper presents the causes of

press loading, load sensing methods, and application of load

data into press control systems. The study will begin with an

overview of the topic then focus on a specific example using a

press from a local manufacturer.

M. De Kam is a student at Calvin College in Grand Rapids, MI with a

Bachelor of Science of Engineering with a concentration in mechanical

engineering.

e-mail: mjd2@calvin.edu

II. CONTRIBUTING FACTORS IN PRESS LOADING

A. Working Metal Properties

The properties of the metal being punched, cut, and formed by

the stamping press have a great impact on the load the press is

subjected to. The shear strength of the working metal will have

a large effect on the press loading. The shear strength is

determined mostly by the toughness and hardness of the metal.

Soft metals like aluminum take little force to punch and form.

Hard metals such as steel will require a higher press loading.

For each type of metal there are also differing levels of

hardness. Higher hardness alloys will require higher press

loadings.

Stock thickness will also determine the press loading. As the

stock metal becomes thicker, the press will experience more

force.

Finally, the length of the cuts being made in the stock will

contribute to the final press loading force.

The relationship between the preceding properties of the stock

material and the press loading force may be calculated using

the following equation:

F S t L This equation relates the load force (F) to the shear strength of

the material (S), the thickness of the material (t), and the

length of the cut (L).

B. Cutting Clearance

The clearance between the diameter of the punch and the

diameter of the blanking die will affect the overall press

loading. The clearance between punches and die holes depends

on the thickness and hardness of the stock material. The

following equation relates these variables to clearance:

c = a · t

The clearance (c) depends on material thickness (t) and an

allowance constant (a). The allowance constant is based on

material properties and increases with hardness as shown in

the chart below.

Metal Group a

1100S and 5052S Aluminum 0.045

2025ST and 6061ST

Aluminum; prass; soft cold-

rolled steel, soft stainless

steel

0.060

Cold-rolled steel, half-hard;

stainless steel, half-hard and

full hard0.075

Fig 1. An allowance constant chart for the clearance equation.

Metal Stamping Press Load Sensing: Integration

of Load Measurement in Press Control Systems Matthew De Kam

M

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C. Tool sharpness

It is straightforward to see the relationship between tool

sharpness and the press force necessary. Dull punches and dies

will contribute to increased press load. It is helpful to be able

to sense the load on the press in order to evaluate if it is time

to sharpen punches and dies.

D. Cutting Shear

Good die design will make use of a shear angle on punches

and cutting surfaces. The angle creates a scissors-like action

between the punch and the die. The cutting operation is then

spread out over a longer time, and the instantaneous cutting

length is much smaller. A good die designer will extend this

principle to the entire die by staggering the height of each

punch. This method ensures that all the punches are not

engaging the stock at the same time. As an added bonus the

phenomenon of negative press loading can be used as an

advantage with this die design feature. Negative press loading

occurs because of the tremendous energy that is stored up

when pushing the punch through the material. When the

material yields and the punch breaks through a force equal and

opposite to the initial press load must be used to stop the upper

portion of the press and pull it back up. If one punch is just

starting to penetrate as the previous punch has fully yielded the

material then the reverse loading will be absorbed in pulling

the next punch through the material. Taking advantage of this

concept can greatly reduce the press loading. The following

diagram illustrates punch staggering.

Fig 2 A diagram of typical punch staggering.

D. Ram Velocity

The last contributing factor to press loading is the velocity of

the press ram. Increasing the velocity of the ram will increase

the force according to the following equation:

f mtv

d

d

Most people want to maximize the output of their presses, so

increasing the cycle time is not an option. However, sensing

the load as the press velocity varies is beneficial to find the

maximum allowable speed that the press can reliably run at for

a certain product.

In terms of control systems this is the one factor of press

loading that could be adjusted by using an automated control

system. The motor speed could be controlled by a system

which continuously checks the press load curve.

III. LOAD SENSING METHODS

A. Strain Gauge Systems

Strain gauge load sensing systems are the standard method of

load sensing in solid mechanical members. These

mechanical/electrical signal translators increase in resistance

as the member they are rigidly attached to stretches due to an

applied load.

Fig 3. A uni-axial strain gauge. As the gauge is stretched or compressed the

metal in the grid becomes more narrow, or thicker. This change in thickness

alters the resistance to current flowing through the gauge.

Different arrangements of strain gauges can be used depending

on the geometry of the press. Clusters of gauges will generally

account for more of the strain in press members and therefore

result in a more accurate reading of load. Before deciding the

location of the strain gauges it is generally helpful to have a

stress-strain analysis of the press structure. This type of

analysis (normally Finite Element Analysis) will indicate any

stress concentrations, and where on the press an accurate

reading of strain might be taken. For a straight sided press the

four columns or tie rods at the four corners of the press are

generally the best place to sense the load.

Strain gauges need some additional circuitry to power the

system and condition the signal. The change in resistance of a

strain gauge is typically very small compared to the nominal

resistance. In many situations the resistance only changes by

three percent. This obstacle is overcome by using a

Wheatstone bridge and an amplifying circuit.

Fig 4. A Wheatstone bridge circuit in a quarter-bridge setup.

The Wheatstone bridge outputs a voltage that is proportional

to the change in resistance of the strain gauge. This signal is

then amplified and fed to the control system or a digital

readout.

B. Load Cells

Small load cells are also used to determine press loadings.

Load cells can be placed between the die and the press ram to

determine the press loading. These load cells output a voltage

that is proportional to the loading of the press. This voltage is

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conditioned with amplification and filtration circuitry. These

types of miniature load cells are generally used during die

setup, and are not generally intended for permanent control

applications.

C. Piezoelectric Transducers

Piezoelectric transducers are a more recent technology that is

useful for determining press loading. The transducers are

mounted to the press frame in the same positions as strain

gauges. These transducers are easier to mount, and require

much less additional circuitry to condition the signal. The

output signal is strong, clean, and can in some cases be

outputted directly to a digital readout.

D. Calibration

Each of the load sensing methods mentioned requires a precise

method of calibration in order to obtain meaningful results.

This calibration must be very accurate to allow the load

sensing system to display true data. Calibration is generally

performed by applying several known loads to the press. These

known loads should be in the middle of the expected load

range of the press. By applying known loads and comparing

them to the load sensing system readouts a calibration curve

for the system can be obtained. This calibration curve is then

used to interpret all further results of the load sensing system.

Current calibration techniques are complex, and the hardware

required can often cost as much as the load sensing system.

IV. LOAD SENSITIVE CONTROL SYSTEMS

A. Digital Readout

One option for using the load data is simply to have a digital

readout near the press controls. When the operator of the press

sees any strange load profiles or any high load values the press

can be shutdown and adjusted.

B. Load Limit Switch

The load data could be integrated into a press control system

by configuring the press controls to stop the press if a certain

maximum load is exceeded. The press PLC would receive an

analog input from the load sensor. If the analog input exceeds

a certain preset value, the press should shut down.

C. SPC Analysis Tracking

Another option for integrating the press load data into the

control system is to perform a complete SPC analysis on the

data. This option would require the system to record all the

past load profiles. These profiles could be parameterized and

compared to each other. In this way the software could learn

what a normal load curve is for a given product and warn the

user of any load curves that are outside of the control limits.

This option is useful because it provides a record of process

quality which is increasingly desired in the manufacturing

world.

D. Variable Velocity Press

A final method of integrating the load data into the press

control system is to create a feedback loop that would alter the

speed of the press motor to compensate for excess load. This

system would know the desired load based on the SPC data,

and could alter the press velocity to keep the process within

the control limits. This option has complications because there

are many components that actually contribute to the press

loading besides ram velocity.

V. INDUSTRY APPLICATION

A. Background

Currently in the manufacturing industry, a third-tier

automotive supplier uses metal stamping presses extensively in

production. These presses generally run progression dies as

explored previously in this paper. This manufacturer is

interested in determining the actual press loading so that it can

better evaluate the condition of its many stamping processes.

The improved diagnostics are focused on providing more

accurate assessment of die wear and press performance.

B. Press Design

The presses used by this manufacturer are primarily of the

same design. The press design was done specifically to

provide this manufacturer with a quality press for a low,

predictable cost. Two models with two different tonnage

ratings are primarily used in this facility; a twenty-ton press

and a sixty-ton press. These presses are of a unique design, as

can be seen in the figures below.

Fig 5. Progression die metal stamping press.

The geometry of these presses made stress and strain

calculations difficult. An approximate cross-sectional area of

the links which hold the top and bottom half of the press

together was obtained and used to perform preliminary

calculations. A Finite Element Analysis of this press design

would be ideal for finding the stress concentrations, and

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determining the most reliable place to locate the load sensors.

However, in this case such an analysis was no longer available.

C. Load Sensing Application

One press, rated at sixty tons, was chosen for the application of

this load sensing system. The maximum rated load was used to

calculate estimates of the stress and strain in the press.

Cross Sectional Area

Tie Bars Rear

At 6 .5 Ar 10 34( ) .5 2

Axs 4At Ar

Tonnage Rating

Fpress 120000 (This v alue corresponds to 60 tons)

Stress

Fpress

Axs

E

7.143105

Fig 6. Strain calculations based on maximum rated press load.

These estimates were then used to size a strain gauge load

sensing system. This load sensing system used the following

components:

E = excitation

voltage

S = signal out

Wheatstone Bridge

Connection

E+

S+

E-

S-

CX

RO

+

_

G

G

78L05

CB

V+

V-

R

CF

V

G

SU1

Power In

Ground

Signal Out

Strain

Gauge

Strain Gauge Circuit Schematic

Fig 7. This sensor circuit incorporates a typical Wheatstone quarter-bridge

with an amplification circuit.

D. Control System Application

Given the sixty-ton stamping press and the strain gauge load

sensing system, each of the control system options were

simulated using modeling software. To obtain meaningful

results in the simulation, a transfer function for the mechanics

of the press was needed. Given the geometry of the press drive

system a transfer function was calculated that traced the

motion from motor velocity to die velocity.

Fig 8. Stamping press drive linkage. The geometry of this dynamic system

was used to find the transfer function for the press.

m 30 rm3.607

2 rf

26.742

2 rc 4.5

m 3000f rm

m

rf

c t( ) f t

xc t( ) rc cos c t( ) xb t( ) xc t( )

yb t( ) .45xb t( )

0 0.5 1 1.54

2

0

2

4

yb t( )

tvb t( )tyb t( )

d

d

0 0.5 1 1.510

0

10

vb t( )

t

ab t( )tvb t( )

d

d

f t( ) m ab t( )

0 0.5 1 1.51 10

5

0

1 105

f t( )

t

Fig 9. These calculations show the typical position, velocity, and force

profile for the given drive system. These calculations were used to find a

transfer function for the press.

The simulations performed in this study used an ideal force

profile as the desired signal. In this signal the force of

stamping is negligible.

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Fig 10. Desired press force profile created using signal builder modeling

software.

1) Digital Display

This simple type of control system relies heavily on the

machine operator to monitor the load, and take action if it

exceeds the presses limitations. This system was simulated

using Simulink software.

Fig 11. Block diagram for digital display control system.

In all of these simulations the disturbance signal was generated

using the signal builder in the software package. This signal

was modeled to represent the force encountered under typical

stamping conditions.

Fig 12. Disturbance signal for all simulations. This signal simulates both the

positive and negative force encountered by the press ram during the piercing

operation.

2) Load Limit Switch

A basic model of this type of control system was constructed

in Simulink. This model simply received the most recent press

load curve and tested to see if the load was within specified

constant lower and upper bounds. These bounds were set at

the specified press load rating.

Fig 13. A load-limiting control system. This system simply cuts power to the

press when the load becomes larger than the specified limit magnitude.

This system successfully simulates how a press control system

could simply stop the process if the magnitude of press loading

became too large. The following output shows how this system

Responds to a press load over the given limit:

Fig 14. System output for a load exceeding the specified load limit of sixty

tons.

This system demonstrates how the control system would

respond to a press loading greater than the rated press limit.

This type of system could easily be integrated into the

automated control system of this manufacturer.

3) SPC Analysis Tracking

If the load curve results from each press hit were recorded in a

central database, this control system could easily take

advantage of Statistical Process Control to better evaluate this

stamping process. Having multiple load curves stored in

memory would allow a control system to calculate upper and

lower control limits for the stamping process.

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Fig 15. This feedback control system keeps the press load within acceptable

limits.

These limits would be based on actual manufacturing floor

stamping data instead of idealized design calculations. Once

stable control limits for the process are established the control

system could stop the process, or alert an operator if the

control limits are exceeded. This type of statistical process

tracking is valuable for keeping accurate records of quality

data, which many customers are now demanding along with

the product.

4) Variable Velocity Press

Current press technology incorporates the flexibility and

accuracy of servo controls into stamping applications. A servo

motor can provide variable velocity at different points in the

stamping process. A key attribute of this technology is that

servo motors can vary the velocity of the press action while

still offering nearly the same amount of energy. This means

that the punching and forming points in the stamping process

can be slowed down to decrease press load while still not

having much of an impact on overall press cycle time.

In this situation a feedback control system is ideal to keep the

press from exceeding load limits. The following diagram

shows one system of this kind:

Fig 16. This feedback control system keeps the press load within acceptable

limits.

This control system takes a desired force profile and amplifies

it to correspond with the rated load. A disturbance signal is

then added to the force signal to represent the force

encountered when forming the metal. This signal is then

integrated to obtain a velocity profile. The signal is run

through the press transfer function and then goes through a

derivative to get it back into force form. The feedback loop

has the function of smoothing the disturbance force and

conforming it to the desired press load curve. The following

simulation outputs compare the desired load profile with the

actual load profile seen by the press:

Fig 17. System input and output load profiles.

It can be seen from these graphs that using a servo motor to

control the velocity of the press can keep the press within its

load limitations. This control system also reduces the negative

load experienced by the press just after piercing the metal.

This feature will extend the life of the press greatly.

Although this system does smooth the disturbance signal it is

still relatively noisy. Also it is still possible to run the press

beyond its load limitations with this control system if the

disturbance force becomes too high. An additional component

can be added to the control system to ensure that the load will

never exceed the set limits. If a saturation block is included

before the integration block the force signal can be strictly

limited not to exceed the sixty ton limit.

Fig 18. This feedback control system saturates the press load to keep it

within acceptable limits.

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When a large disturbance force is applied to this system it is

limited to not exceed sixty tons. This disturbance force would

exceed the load limits when applied to the previous

unsaturated control system.

Fig 19. The output of the saturating load control system. Notice the smooth

peak stays below sixty tons.

This saturation effect depends on the ability of the servo motor

to supply enough energy to the system to complete the work

even at the reduced velocity.

E. Press Design Constraints

While the servo motor powered press offers some distinct

control advantages, it also constrains the design of the press.

Typical mechanical presses which run on constant speed

motors utilize a flywheel, clutch and brake to generate, store,

and apply energy to the process. The constant speed motor

turns a large flywheel. Energy is stored in the large rotating

mass of the flywheel. When the press is called into action the

brake grabs the flywheel and send the press ram into motion.

Fig 20. Conventional press design, and a servo press design.

The ram stroke then takes a great deal of energy out of the

rotating flywheel. In this situation if the press cycles too often

the flywheel will not have enough time to restock its energy by

getting back up to speed. This can cause the press to stall on

jobs which a well under rated press loading. A press with a

servo motor is able to supply a nearly constant rate of energy

with variable velocity. In this type of design the mechanical

advantage is usually in a mechanical linkage. There is no need

to have energy stored up in a large moving mass in this type of

design.

The press studied in this case is a combination of these two

techniques. The motor only runs when the press ram is in

motion. However energy is stored up in the moving mass of

the large steel blocks which contain the cam profiles. In a

conventional press the motor is constantly running and turning

the flywheel. Further study would need to be done in order to

determine if this press would be a candidate for a servo motor

drive. A changeover to servo technology could be as easy as

changing the drive motor to a servo motor if the mechanics of

the press allow for it in this case. The required mechanical

advantage may not be present in this press design as it is in the

designs outlined below.

Fig 21. Alternative servo press designs.

An additional benefit of a servo controlled press is the ability

to accurately sense the press ram position and dwell at certain

points in the stamping process. This ability to dwell during a

stamping process allows secondary operations to be performed

on the part while the press is in the middle of its cycle. An

example of this type of secondary action could be punching a

hole in the side of a part using lateral cylinders or cams.

VI. CONCLUSIONS

In conclusion load sensing technology can increase the

capability and quality of a metal stamping process in many

ways. The most promising form of load sensing utilizes a

feedback control system in conjunction with a servo motor to

control the press ram velocity. However, many other solutions

exist in the range of no load sensing to load sensing, control,

and documentation. Each company should evaluate the costs

involved with such a project to ensure that it is worthwhile to

develop such a sensitive technology on the well established

process of metal stamping.

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Matt DeKam was born in Sioux Falls South

Dakota in the United States of America on October

18, 1981. He graduated from Southwest Minnesota

Christian High School, and studied at Augustana

College. He is currently pursuing an Engineering

degree at Calvin College in Grand Rapids,

Michigan.

His employment experience includes designing

and building industrial control panels for Affinity

Solutions in Sioux Falls, SD, and manufacturing

engineering work at Innotec in Zeeland Michigan.