Punch Press Tools – Lessons Learned

In 35-125, we do lots of flashlight trainings.  Once or twice a year I have to find another shop willing to do me a favor by laser cutting clear flashlight lenses.  I thought that making them with a press in our shop could be a fun project.

 

CAD Model Link.

Spring Housing CAD model link

 

Design Goals:

  • Make the lenses
  • Make all lens features in one hit, instead of reindexing blanks.
  • Strip both the lens and the material from the punch tools.
  • Minimize fingers between punch and die.

 

My initial design used a couple stacks of belleville washers around screws or shoulder bolts. After fighting with those washers, I don’t think I’d use them for much beyond indicating bolted joint or bearing preload. Stacked up inside a hole or around a screw they have a tendency to get stuck on screw threads or poor surface finish, and then I got a bunch of kinked belleville (former) springs.

 

The main takeaway from this project was about punch and die clearance – it seems like a tighter clearance would give better shearing, but it makes it really hard to get everything together and makes the parts much harder to strip off of their tools. After consulting Dayton Lamina I realized that the clearance I initial designed for was WAY too tight. I made everything to be a close slip fit, and I could have a great deal more clearance at 5% of the material thickness. On the second iteration, all the punch and die interfaces were given %5 of the material thickness (0.060”) on each side. That makes about .006” on diameter, which is easy to hit with an end mill or a reamer.

 

Materials:

The parts for this tool were made mostly from Tight-Tolerance Hardened Multipurpose 4140 Alloy Steel Sheets and Bars. They are really nice, and at Rockwell C30 are soft enough to machine with high speed steel tools. (High Speed Steel tools have a hardness around Rockwell C60.)  We have nice sets of high speed steel reamers and drills in 35-125, and I didn’t want to order specific carbide tooling for all the holemaking on this project. Round punch blanks were made from drill rods. They can be purchased in most fractional sizes and are as hard as high speed steel tools.  They can also be machined into special drills if needed.

 

Design:

I wasn’t sure how long the drill rods as punches would last, so I made the punch holder out of three pieces with holes on the split lines between each part. That way I could remove and replace punch blanks without pressing them in and out of a housing.

All of the other parts ride on two quarter inch pins pressed into the die plates, 4 inches apart on the centerline of the tool. This allows all of the layers of the tool to be precisely located without a lot of fussy adjustments.

 

Stripping:

Punching through the material is only the first half of the process. Next you have to get the part off of the punch. Typical punch press tools do this in an orientation like this:

 

 

This design is pretty straightforward for a single punch, but gets harder to implement for a tool like mine because of the combined features. I tried a couple versions that were a round or hex button in the middle of the lens. These didn’t strip the lens well and tended to bend it like a diaphragm, making it bind on the hole punches even more.

 

I tried a number of urethane rubber sheets for the bottom stripper, and they worked well.  They initially got made as a byproduct of trying urethane sheets in the top cavity as a captive spring.  The 5 ton arbor press tended to squish those rubber sheets into useless stringy rubber bits.

 

The top stripper was where I found out all about throw, preload and durability of die springs. This is where I kinked and ruined my belleville washers. I ended up finding a spring with a 500lb/in spring constant, which turned out to be stiff enough to strip the lens out of the top cavity.

 

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