Tapping plexiglass

[bradleyheathhay]

Say you had a 1/2" thick sheet of plexiglass and needed to tap a hole that was drilled beginning on the edge and continuing in 3 in. What type of thread density would you choose, coarse or fine? The strain on the anchor will be fairly minimal.

[China]

I would probably get really technical grab whatever I had that would accommodate what I was going to screw into it

[BobL]

By plexiglass I assume you mean acrylic?

Either way 3" seems a lot unless it was carrying a serious load.
I would think 1" should be more than enough for a light load and for that I would tend to use a coarse thread.

It would be helpful to know more about the load/angles etc.

I would do some test holes and threads in some scrap.
Maybe have a read of something like this
Drilling Acrylic: The Ultimate Guide - Wee Tect

That article mentions using sharper drill bits than usual, but that is mainly to guard against mangling the breakthrough - as you sound like you are drilling blind holes you can use standard drill bits (I have only ever used regular drill bits. A drill press will give you better control than a hand held drill.

The article doesn't mention a lube but I have used kerosene, for cutting, drilling and tapping.

[elanjacobs]

Either way 3" seems a lot unless it was carrying a serious load.
I would think 1" should be more than enough for a light load and for that I would tend to use a coarse thread.

Yeah...3" is a lot of thread...
I'd say 1/4" 20tpi is probably a safe bet

[Chief Tiff]

Unless you’re using a plexiglass screw to hold your load the usual arguments regarding higher tensile strength (fine thread) versus resistance to thread stripping (coarse thread) aren’t going to apply. On the assumption that you’re using a steel screw or bolt you’ll want to be cutting a coarse thread because the plexiglass threads are going to fail well before the fastener starts to look even moderately concerned.

As for threaded depth; the entire load is generally only taken up by the first seven threads in like-for-like threaded materials. Despite the yield strength and ductility of the plexiglass being way different to a steel screw engaged in it I doubt that tapping to ten or twenty threads deep will actually resist pull out any more.

But anyhoo; please tell us some more. What are you trying to fasten here? We may be able to offer an alternative...

[russ57]

I think i did tap some acrylic a while ago. Seem to remember that the hole size is important, don't drill undersize. I guess it doesn't flow like metal does.

I think I'd be aiming for 4 or 5 mm, 1/4 gives only 1/8 wall thickness.

[BobL]

I think i did tap some acrylic a while ago. Seem to remember that the hole size is important, don't drill undersize. I guess it doesn't flow like metal does.

I think I'd be aiming for 4 or 5 mm, 1/4 gives only 1/8 wall thickness.

1/8" wall thickness is 3.2mm which is OK.
Coarse thread M4 is 0.7mm and M5 is 0.8mm pitch, M6 is 1mm which would be a little easier to deal with.

All 8 screws into the 20mm thick acrylic used on this OH TS guard are M6 (20mm long).
TSguard2.jpg