Press Reaction

Every press has a reaction time.  This is the time from when the control system sends its press fire signal to when the press goes from top to bottom of stroke and the tooling contacts the material.  Since speed · time = distance, the time delay results in a distance (length) delay (error).

Press Reaction applies to any type of press – punch, cutoff (shear), pneumatic, hydraulic, or mechanical – operating in an open loop flying die application.  The reaction time of a press determines how far from the original target the tooling will contact the material when the press is fired at a given line speed.  An improper (or lack of) reaction time compensation will result in a first part that is the wrong length, or an error in the distance between the punch pattern and the part length that seems to change with line speed.  When it comes to a punch, the punch pattern will shift away from or toward the leading edge of the part based on whether the roll former speeds up or slows down.

Quite often, technicians in the roll forming industry encounter press reaction when dealing with an open loop flying shear.  It’s typical for this application to produce a long part (or a few long pieces) when the operator starts the line, but then the part lengths seem to “settle out” once the machine is up to speed.  It’s possible for the parts to be short on startup, as well, but it depends on whether the length control system has reaction compensation built in and whether too little or too much compensation time is programmed.

Even though a punch press and a shear press are technically the same in terms of reaction time, the method to find reaction time can be confusing due to the nature of looking at the finished part with the human eye.  Most people struggle with the relationship between the two operations and what they’re seeing on a finished part.  For this reason, it is crucial to always handle issues with the shear, first, before attempting to correct problems with the punch.  It is very easy to find yourself chasing your tail if you do not approach these problems very logically and methodically.

That’s why this discussion is structured to discuss the shear, first, and then the punch, even though the final calculations are the same.  I’ll also cover a high-tech method that uses an oscilloscope for finding press reaction, as well as the low-tech method using measured parts and a formula for finding press reaction.  Either method should be perfectly fine, because if you’re running an application that relies on an open loop flying press, then your tolerances probably aren’t that tight, relatively speaking, to begin with.

Reaction by Press Function

  • Shear
  • Punch

Press Reaction Methodology

  • Oscilloscope
  • Calculated

Shear Reaction

When running an open loop flying shear, shear reaction is length error most often seen on start up.  That’s because the operator will typically perform a manual shear while the material is standing still in order to re-reference the cutoff to the material.  Most of the time this reference is unnecessary and simply generates scrap.  Unless the operator is threading up a new coil, or recovering from a material crash, it is completely unnecessary to reference the machine when re-starting after a halt.  Regardless, firing the shear when the material is standing still creates a situation where you have a cut that has no reaction time.  That is, since there is no motion (speed), then the time delay inherent in the press is irrelevant.  A speed of 0 · any reaction time = 0″ of distance error.

But then the operator puts the line into Run, and when the shear press fires to cut the first part, it misses the target by the reaction time.  If the programmed reaction compensation in the length control system was set to 0 seconds, then the part produced will have the maximum length error possible for the current speed of the line.  Because the first cut on the leading edge didn’t include any reaction time, and the second did, the part will be long by the reaction time multiplied by the current line speed.

Assuming the machine is up to full speed by the time the control system cuts the first part, subsequent parts will be “good”, because each cut will have the reaction time of the shear press built in.  The leading edge of the part will be off by the reaction time, but so will the trailing edge of the part, so when the operator measures the part, both ends have the same amount of error in the same direction and the part will measure to the correct length.  In this scenario, if the machine were to cut a part as it’s coming to a stop, the part produced would actually be short.

Using an Oscilloscope to Measure Reaction

If you’re familiar with the operation of an Oscilloscope (O’scope or scope), then this is probably the best method for measuring reaction time.  It’s extremely accurate, and if there are any variances in the timing of the press, you will immediately see them on the scope.  This is a tremendous benefit when trying to isolate length variance problems.

In order to use an O’scope to measure press reaction, you must have a scope with two input channels, a magnetic base (like the type used with dial indicators), and a high-speed proximity sensor.  The sensor should have a switching time of 0.0001 s or less, with a repeatability of 0.00005 s or better.  You will also need a bracket to affix the proximity sensor to the magnetic base so that it can be positioned to detect the shear at the bottom of the press stroke.

Lock the magnetic base to the die base plate.  Mount the proximity sensor to the base and position it so it only turns on when the cutoff is at the bottom of the press stroke.  Wire the proximity sensor for power, and connect one channel of the scope to the output of the proximity sensor.  Connect the other channel of the scope to the shear fire output of the control system.  At this point, you are ready to directly measure the time difference between when the system fires the press and when the blade reaches bottom of stroke.  You can perform this test with the machine standing still by manually firing the shear through the control system.  This method has the added benefit of not generating any scrap.

Perform this test several times to verify that the timing of the press is consistent.  Variances in the time delay will directly equate to length error on finished parts.  If you were to fire the shear manually 2o times and you saw a total variance on the screen of 0.03 s between all the firings, you could multiply this time by the maximum line speed an calculate your length variance.  For instance, assuming a timing variance of 0.03 s and a maximum line speed of 100 fpm (20 ips), then 20 ips · 0.03 s = 0.6″.  It’s likely the operator would measure at least ± ¼” of variance on finished parts from this machine.

Assuming the shear press is consistent, or consistent enough to produce in tolerance parts, simply measure the time difference on the screen to get the shear reaction time.  Most oscilloscopes have slider bars on the screen that can be positioned to display the exact time difference between the bars.  It would be this time difference that you would program into your length control system as the “Shear Reaction” parameter, assuming the system allows you to compensate for reaction time.  If not, get used to throwing away parts on start up.

There are many inexpensive oscilloscopes on the market today.  I personally carry a small, USB scope in my laptop case.  The scope is a very small box with connections for two test probes, as well as the USB connection for the laptop.  The scope comes with software that turns the laptop into the “screen” and controls for the scope, as well as allowing direct data-logging capability.  I believe every manufacturing facility should have at least one O’scope on premises for troubleshooting purposes.

Calculating Shear Reaction

In order to calculate shear reaction, you must first ensure you don’t have a length variance already in the system.  It is impossible to calculate an accurate reaction time around a variance.  The best method to verify consistency is to program several parts, and then run the line.  Watch the line speed indicator to verify that the machine has reached the speed you want to run, and verify the speed is consistent.  If it’s not consistent, you cannot calculate a consistent reaction time.  Inconsistent line speeds are almost always the result of poorly mounted encoders, or it indicates that the encoder bracket is loose, or has developed mechanical backlash.  Sometimes, inconsistent line speeds are the result of a loose or faulty tachometer on the back of the main mill motor.

Once the machine is running at a consistent speed, have the operator catch at least 5 parts as they are produced.  Measure each part carefully.  Verify the lengths are consistent.  At this point, consistency is more important than accuracy.  If your lengths are inconsistent, it’s probably due to problems with encoder tracking.  It’s possible the inconsistency is due to an inconsistent press, but the only way to be sure of this is to use an Oscilloscope in the method described above (Using an Oscilloscope to Measure Press Reaction).  Outside of mechanical problems with the press (sticky valve, blown seals, worn clutch/brake system), it’s possible the firing circuit, itself is the problem.  It is crucial to eliminate mechanical relays and PLCs from the shear fire circuit whenever possible.  Only solid-state devices should sit between the length control system and the press solenoids.

Assuming you have consistency in line speed and part lengths, you can set up a test to find shear reaction:

  1. Start by programming a part length that is long enough for the machine to be at speed prior to the press firing to cut off the first part.  The machine does not have to be run at full speed for this test.  That’s because speed · time = distance, and if your length control system compensates for reaction time, then it is always performing this calculation for you based on the current speed of the line.  What matters for the test is that the machine reaches whatever speed you set before the first part is cut in automatic.  If this isn’t happening, you must either program a longer part for the test or slow down the roll former.
  2. Once you’re confident the machine will be at speed for the first automatic cut, set any values for reaction time to 0 s.  You don’t want to try to calculate around an existing error.
  3. Program the machine to run at least two parts.
  4. Perform a manual shear to ensure your first part will have the shear reaction error on it.
  5. Run two parts.  Make sure to record the line speed the machine reaches before it cuts the parts.
  6. Catch the parts in the order they are produced.
  7. Measure each part and record its length.
  8. Finally, perform the following calculation:

((dfirst – dsecond) ÷ line speed) · 5 = Shear Reaction

Where:

dfirst = Length of first part

dsecond = Length of second part

line speed = speed of machine in fpm reached just prior to first automatic cut

5 = Line Speed Conversion Constant

Once you have calculated this value, you can enter it into the length control system’s Shear Reaction parameter, and perform the test again.  This time, both the first and second parts should come out at the same length.

If your shear die is boosted, you will need to either disconnect the boost from the die, or set the parameters to zero to keep the control system from firing it.  If your material is too flimsy to support the weight of the die, you will need to either run the test at a slower speed, load a heavier gauge for the test, or you will need to crash the material in the die.  Crashing material is unpleasant and frustrating, but if you can verify that you get consistent lengths and that once up to speed your lengths are accurate, then you can technically perform this test with a single part – the first one that comes out, because that part will come out fine with your reaction time in the form of extra length and the jam up will occur on the entry side of the die.

Keep in mind that the press reaction time is based on consistency – the idea that nothing about the press changes from day-to-day.  If you are running a mechanical press in single-stroke, then each firing of the press is wearing down the brake and the clutch.  Over time, the reaction time can change due to that wear.  It might need to be recalculated, especially whenever you replace the clutch/brake.  The same is true for a pneumatic or hydraulic press.  If a hose is changed, any variance between the length of the old hose and the length of the new hose will result in a timing change in the press reaction.  The reaction time will need to be recalculated each time a component is replaced.

Example 1

The operator of a roll former with an open loop flying shear complains that the first part is always long on start up.  You have the operator run the machine and catch at least five parts to check length.  You throw out the first 3 parts, but every other part is coming out within ± 1/64 on a tape measure.

You check the Shear Reaction parameter in your length control system, and it has a value set, but when the operator references the shear and starts the line, the first part is indeed longer than subsequent parts.  You change the Shear Reaction to 0 s, and ask the operator to slow the line down to around 50 fpm for a test.

You hand-program the control system to produce a 60″ part in order to avoid making too much scrap, but when you have the operator run the line again, you can hear the shear fire before the line reaches a stable 54 fpm.  So, you change the current length to 120″ to ensure the machine is fully at speed before the shear cuts.

You ask the operator to re-reference the machine and run two parts.  He does, and this time you can see that the machine gets to a stable 54 fpm just before you hear the shear fire.  The machine makes two parts and the operator stops the line.

You record “54 fpm” in your notebook and measure both parts.  The first part measures 121-5/8″ (121.625″).  The second part measures 120-1/4″ (120.25″).  You record both lengths and perform the calculation:

((dfirst – dsecond) ÷ line speed) · 5 = ((121.625 – 120.25) ÷ 54) · 5 =

(1.375 ÷ 54) · 5 =

0.0255  · 5 = 0.1273 s

 

Punch (Press) Reaction

Once you’re confident that your shear lengths are accurate and consistent, then you can focus on the punch press.  If you have an Oscilloscope, simply refer to the section above (Using an Oscilloscope to Measure Reaction).  Those instructions work for any type of open loop flying press application with the exception of a kicker (see below).  If you don’t have an O’scope, or you aren’t comfortable using one, then you must calculate press reaction much like you would shear reaction.

Calculating Press Reaction

Press reaction works just like shear reaction, but keeping the punch synchronized with the length and making sure you have all the right holes to measure can be tricky.  The first step in using this method is to verify the physical distance from the shear at home position to the punch at home position.  The best way to do this is to load material into the machine through both presses and step through the following procedure:

  1. Once you’ve jogged material through the entire machine, perform a standing cut.
  2. Perform a standing punch.
  3. Jog the punch hole all the way through the machine and past the shear.
  4. Perform another standing cut to manually cut the piece off.
  5. Measure the distance from the hole to the leading edge.

The point of reference on the hole should be the same point the operator typically references when he measures a part.  If the operator measures from the leading edge of the part to the leading edge of the hole, use that distance.  If the operator measures to the center of the hole, measure that distance.  If different operators measure to different locations, make sure there’s a part print available at the machine and standardize practices across shifts.

However you choose to measure your reference points, program the measured distance into the length control system.  Do not use a tape measure to try to physically measure the distance using the tooling, itself.  This is very difficult, a little unsafe, and if your punch is upstream from the forming rolls, then you aren’t taking into account stretch.  By jogging material through the machine, you are stretching the distance that you will later measure, and everything will work out.

Next, you’ll need to program a part that is long enough to accommodate all the holes that need to be punched, as well as the physical distance from the shear to the punch.  The easiest way to do this is to program a part length equal to the distance you just measured, plus 130″.  If it’s not possible to physically run that long of a part, you will need to program a part length that is at least 130″ long, but is also a multiple of the physical distance from the shear to the punch.  If you find yourself in this situation, you must temporarily lie to the length control system and define both the shear and the punch at 0″.

The final trick is to program a punch pattern that calls for a leading edge punch at 60″, and another at 120″.  This will give you two automatic press firings from which you can measure to find your reaction time.

Make sure to set the press reaction parameter(s) to 0 s, and be ready to record the speed of the line when the test is run.  Make sure clean material has been jogged through all the presses again, and that you’ve performed a standing punch and a standing shear.  This will re-reference the system for the part that you want to run, and the standing punch becomes the reference point from which you will measure the two automatic punches the control system will make.  Run the part.  Make sure to record the speed of the line.

When the part comes out of the shear, you should have three holes on the part – the first hole is your manually punched hole while the machine was standing still, and the other two are the automatic punches made by the control system.

Measure the distance from the first manual hole to the first automatic hole.  Record this distance.

Measure the distance from the first automatic hole to the second automatic hole.  Record this distance.

Perform the following calculation:

((dfirst – dsecond) ÷ line speed) · 5 = Press Reaction

Where:

dfirst = Distance from first manual hole to first automatic hole

dsecond = Distance from automatic hole to second automatic hole

line speed = speed of machine in fpm reached just prior to first automatic punch

5 = Line Speed Conversion Constant

Once you have calculated this value, you can enter it into the length control system’s Press (Punch) Reaction parameter, and perform the test again.  This time, both the first and second automatic holes holes should come out at the same distance.

If your punch die is boosted, you will need to either disconnect the boost from the die, or set the parameters to zero to keep the control system from firing it.  If your material is too flimsy to support the weight of the die, you will need to either run the test at a slower speed, load a heavier gauge for the test, or you will have an extremely difficult time calculating an accurate press reaction.

Keep in mind that the press reaction time is based on consistency – the idea that nothing about the press changes from day-to-day.  If you are running a mechanical press in single-stroke, then each firing of the press is wearing down the brake and the clutch.  Over time, the reaction time can change due to that wear.  It might need to be recalculated, especially whenever you replace the clutch/brake.  The same is true for a pneumatic or hydraulic press.  If a hose is changed, any variance between the length of the old hose and the length of the new hose will result in a timing change in the press reaction.  The reaction time will need to be recalculated each time a component is replaced.