Need help: square hole in plastic

[Marco Bernardini]

I have to make some square 20×20 mm holes into mystery plastic (the shop where I got the offcut is specialized in delrin, nylon, teflon, HDPE, so it is one of these: I'd go for HDPE).
My idea is to sharpen the end of a 20×20 iron tube, to make a sort of punch, and press it into the plastic using the bench vise as a press.
Someone has a better solution not involving a mill?
Thanks in advance!

 

[Terrywerm]

Your plan is certainly worth a try, but it may work better in a press. Might be too much for a vise.

Otherwise, drill a round hole first, then follow with your punch to square it out. Slightly less cutting force required that way.

 

[xalky]

Your plan is certainly worth a try, but it may work better in a press. Might be too much for a vise.

Otherwise, drill a round hole first, then follow with your punch to square it out. Slightly less cutting force required that way.

I agree with drilling a hole first. Unless you have a die on the other side without a hole drilled you might just blow out the back of the delrin. It might blow out anyways,although HDPE is pretty soft. A square broach would be a better approach IMO. Depending on how many you need to do, a square rasp might work well, if you can find one thats the right size.

 

[Marco Bernardini]

Your plan is certainly worth a try, but it may work better in a press. Might be too much for a vise.

Otherwise, drill a round hole first, then follow with your punch to square it out. Slightly less cutting force required that way.

Thanks!
This could be a nice reason to persuade wifey I absolutely have to build a 20 tons press! :whistle:
But since I've no spare time to make that, the solution of the round hole sounds pretty good. I think I'll add a wood frame around the plastic to prevent deformations, too.

 

[jgedde]

I have to make some square 20×20 mm holes into mystery plastic (the shop where I got the offcut is specialized in delrin, nylon, teflon, HDPE, so it is one of these: I'd go for HDPE).
My idea is to sharpen the end of a 20×20 iron tube, to make a sort of punch, and press it into the plastic using the bench vise as a press.
Someone has a better solution not involving a mill?
Thanks in advance!

They key question I have is how thick is the plastic?

John

 

[Marco Bernardini]

I agree with drilling a hole first. Unless you have a die on the other side without a hole drilled you might just blow out the back of the delrin. It might blow out anyways,although HDPE is pretty soft. A square broach would be a better approach IMO. Depending on how many you need to do, a square rasp might work well, if you can find one thats the right size.

I can look for some pieces of iron (full, not pipe) and bolt them together leaving a square space as die: this will be easier than to make a square hole.
If I find a square piece of 20×20 full iron I can also try to convert it into a broach with the angle grinder.
I don't have to make many holes, maybe two will be enough (must check after the first two!), so a dedicated rasp seems a bit excessive.

- - - Updated - - -

They key question I have is how thick is the plastic?

John

Oh, I forget to add this peculiar detail, thanks for remembering it!
It's 20 mm thick.

 

[Rapscallion]

Marco, do you have a lathe?
If you have a lathe you can "shave" it to shape after setting it up in the chuck. Drill and/or bore to 20mm and shape it with a blank 10mm HSS tool in the toolpost by ramming it while moving the tool over on the cross slide until in the corner. Turn the chuck 90 degrees and repeat the process for each corner. This can be a long process but, it can also be very neat and precise.

 

[Marco Bernardini]

Marco, do you have a lathe?
If you have a lathe you can "shave" it to shape after setting it up in the chuck. Drill and/or bore to 20mm and shape it with a blank 10mm HSS tool in the toolpost by ramming it while moving the tool over on the cross slide until in the corner. Turn the chuck 90 degrees and repeat the process for each corner. This can be a long process but, it can also be very neat and precise.

No lathe… and not even a planer!
Anyway to have a few machines helps to keep the brain awake to find alternative solutions )

 

[Bill C.]

I have to make some square 20×20 mm holes into mystery plastic (the shop where I got the offcut is specialized in delrin, nylon, teflon, HDPE, so it is one of these: I'd go for HDPE).
My idea is to sharpen the end of a 20×20 iron tube, to make a sort of punch, and press it into the plastic using the bench vise as a press.
Someone has a better solution not involving a mill?
Thanks in advance!

May have to use a heat gun to warm up your punch. I agree with other posters best to use a arbor press if you got one. That way you can clap the punch in the ram part of the press and use a plate with a hole a little larger than diagonal diameter of the punch to support the plastic. Good luck.

 

[Marco Bernardini]

May have to use a heat gun to warm up your punch.

Good idea, thanks!
I'll try on a scrap, before…