Every once in a while, we would get extra fill-in jobs at my company. Last night, we were punching a hole pattern on a plexi-glass panel using a .312 diameter punch.

I measured the hole size with a caliper and a dowel pin and it was closer to .300 in size.

What would cause that to happen? Is there a special rule of thumb that I ought to use as far as using bigger hole size punches to get a desired sized hole diameter for plexi?


Moderator note: I change the title of this thread

I wonder if the plexiglass punched cleanly, or if the material rolled?

Hole punches are designed for metal: the metal shearing process is designed to cut about halfway, and then the rest of the thickness shears off. If there is any dullness the metal is inclined to roll, and this misshapened part measures differently. This could be happening with the plexiglass too, possibly through heating or residue coating the punch and die, or if these two parts are mismatched or worn.

Just tossing out my thoughts.

Jennifer, that makes sense. That's about the best explanation! :thanx:

The other thing you may want to look is die clearance.

How thick was the material?

I wonder also whether the Plexiglass is compressing as the punch goes through and then expanding again as the the punch retracts - thus making a smaller hole. I used to be involved in punching copper and aluminium (many years ago) and we had a similar problem.

Every once in a while, we would get extra fill-in jobs at my company. Last night, we were punching a hole pattern on a plexi-glass panel using a .312 diameter punch.

I measured the hole size with a caliper and a dowel pin and it was closer to .300 in size.

What would cause that to happen? Is there a special rule of thumb that I ought to use as far as using bigger hole size punches to get a desired sized hole diameter for plexi?

Plexi-glass (trade name) is a thermoplastic. During my tenure on Indian Railways, I used Acrylic sheet, sheet made from different thermosetting resins and Fiber glass for pilfer proofing. The problem of shrunk hole size (using a punch) is common to all thermoplastics. The reason is that the punch is slightly tapered at the edge, therefore when the punch comes in contact with the sheet, the impact is on a smaller than the intended dia of the hole. So the 'punch' shears slightly smaller hole, and the body of the 'punch' penetrates as a 'force-fit', thus swelling the hole. Upon retrieval of the 'punch' the hole shrinks back, resulting in smaller hole size.

Remedy lies in -
1. Use of oversize punch.
2. Use drill in stead of punch.
3. Use laser for accuracy as well neatness.

Hope this helps.

Umang :D

Plexi-glass (trade name) is a thermoplastic. During my tenure on Indian Railways, I used Acrylic sheet, sheet made from different thermosetting resins and Fiber glass for pilfer proofing. The problem of shrunk hole size (using a punch) is common to all thermoplastics. The reason is that the punch is slightly tapered at the edge, therefore when the punch comes in contact with the sheet, the impact is on a smaller than the intended dia of the hole. So the 'punch' shears slightly smaller hole, and the body of the 'punch' penetrates as a 'force-fit', thus swelling the hole. Upon retrieval of the 'punch' the hole shrinks back, resulting in smaller hole size.

Remedy lies in -
1. Use of oversize punch.
2. Use drill in stead of punch.
3. Use laser for accuracy as well neatness.

Hope this helps.

Umang :D

Good post Umang.

I would add that depending on the thickness of the thermoplastic you may need to use a cooled drill. Also drilling may discolor the internal walls of the hole which may present an appearance failure mode. We had a similar problem and found a laser to be the best solution.

Dave

Good post Umang.

I would add that depending on the thickness of the thermoplastic you may need to use a cooled drill. Also drilling may discolor the internal walls of the hole which may present an appearance failure mode. We had a similar problem and found a laser to be the best solution.

Dave

Thanks Dave,

You are right that the thickness does matter, and the drill, with or without coolant, discolors the hole's wall. Though a bit expensive, Laser is the ideal solution. :agree1:

I wonder also whether the Plexiglass is compressing as the punch goes through and then expanding again as the the punch retracts - thus making a smaller hole. I used to be involved in punching copper and aluminium (many years ago) and we had a similar problem.

Presently I am using punches on my sheet metal products, but the shrinkage is very nominal (comparative to thermoplastics). And now I am getting the punches from the market, that are designed to absorb the variation due to shrinkage.

Umang :D