Case Studies

Open Loop Flying Die Destruction

A machine builder, a die maker, and an over-the-road trailer manufacturer walk into a project together... This 3 year disaster began with a phone call to help program a control system for a roll forming line with a servo flying cutoff and an open loop flying punch.

The machine builder couldn't get the punches to show up consistently and they wanted my help to resolve the problem.  We weren't getting anywhere over the phone, but that's because the builder wouldn't give me clear details on what was happening.  Soon, they were insisting that I perform an on-site visit for free.  They had purchased my company's controls, but they did the setup themselves.  We were willing to do the trip based on a "Gentleman's Agreement" (our fault, we pay - your fault, you pay), but they refused.

For 3 years I would receive an occasional phone call from the machine builder or the die maker, as they attempted to get their final payment on this production line, but the customer held them accountable and refused to pay until it worked as quoted.  Finally, out of desperation the machine builder finally agreed to pay for my time, but insisted I had to personally show up to troubleshoot.

When I arrived, the customer was furious.  When I walked into the Plant Manager's office, he was in mid-rant at a representative of the machine builder, a rep from the die maker, and his company's own Project Manager.  When we walked out to the machine, I asked about a parts cage right next to the machine.  He said that cage was full of parts specifically for the line I was on-site to look at.  Over the next two days, I watched the punch system destroy itself a dozen times.

Blocks were cracking.  The heads of bolts were shearing.  The destruction was unbelievable.  This wasn't a single flying punch press, but 3 hydraulic cylinders mounted in a single carriage.  The entire punch system - presses, dies, platens - weighed over 600 lbs!

The mind-blowing thing about this setup was the die maker did not provide for any method to accelerate the die to material speed.  The act of the punches entering the steel and the material dragging the die up to speed was how it "accelerated".  When I stood back and watched, I could see the entire setup shift and flex as the top platen's inertia was overcome by the bottom of the system accelerating based solely on the punches entering the steel.  No wonder they were breaking things!

It took me 2.5 days, but after isolating the 3 presses and getting shifting results on the part, I realized I had been looking at the problem all along.  After making a change and running a test, I was measuring the distance between punches when I realized there was a scrape mark prior to the leading edge of the first punch.

The concept here is the same as a magician who lies down on a bed of nails.  The nails are pointy and sharp, but by spreading his body weight across a larger surface area, the nails don't pierce his flesh.  In this case, these punches were large, rectangular tools.  When we fired a single press, the holes were right on target, but the more presses we fired, the farther back the punch locations would shift.  But they weren't really shifting - the multiple punch tools hitting the moving material at the same time spread the force over a greater surface area.  The presses had to build more pressure in order to bite into the steel enough that the heavy mass would begin to accelerate and then the punches would pierce the steel.

Once I pointed out what was happening, the die maker admitted they needed to add a closed loop accelerator to that portion of the machine.  A couple months later that's what they did to resolve the problem.


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