Tapping plexiglass

[brad hays]

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.

[michael langman]

Brad, If the bolt screwing into the plexiglass is 3/8 inch or smaller I would use a coarse thread. 16 threads per inch or more would be sufficient. If the bolt is 1/2" or bigger I would use a fine thread size. 1/2" 20 tpi or 5/8" 16 tpi.

[Alex Zeller]

We have pieces at work that are threaded. We use heli-coils in them.

[Lee DeRaud]

We have pieces at work that are threaded. We use heli-coils in them.Parse that for me, please. Seems like, from the point of view of the plastic, a heli-coil is just a slightly larger screw of the same pitch.

[Malcolm Schweizer]

Parse that for me, please. Seems like, from the point of view of the plastic, a heli-coil is just a slightly larger screw of the same pitch.

The helicoil bears the friction when an application requires inserting and removing the screw repeatedly.

[Alex Zeller]

Parse that for me, please. Seems like, from the point of view of the plastic, a heli-coil is just a slightly larger screw of the same pitch.

A larger diameter also means more thread surface. I also believe that while the thread pitch is the same the threads themselves for a heli-coil are different that standard threads to get a better bite on the surface they are going into.

[Malcolm McLeod]

A larger diameter also means more thread surface. I also believe that while the thread pitch is the same the threads themselves for a heli-coil are different that standard threads to get a better bite on the surface they are going into.

You are correct about OD pitch, but the improved bite is largely one of unintended consequences. Helicoil inserts are designed so the coil ‘wire’ cross section, pitch, and internal major/minor diameters meets the intended thread standard (i.e. 1/2-13). This leaves the ‘outside thread’ of the coil at something other than a (SAE) standard - same pitch, same difference in major/minor diameters, but larger OD. And this is why the Helicoil kit comes with a custom tap - to match this OD/pitch.

[Malcolm McLeod]

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.

I’d try 1/4-20 (coarse) :: to insure sufficient wall thickness remains in the plexiglass. (Tapping 3” deep may be the biggest challenge .... be sure to back the tap out and clear cuttings. Often!)

[Lee DeRaud]

I’d try 1/4-20 (coarse) :: to insure sufficient wall thickness remains in the plexiglass.Upon further review of OP's post, I agree: he's threading a hole edgewise through the sheet, so 1/4" is pretty much the maximum. With 3" of thread, I'm pretty sure pull-out of the threads will not be an issue, so even smaller might be better, maybe 10-24 or 8-32, to maximize the overall structural strength around it.

(Tapping 3” deep may be the biggest challenge .... be sure to back the tap out and clear cuttings. Often!)
Drilling that hole is probably harder than tapping it...DAMHIKT.

[Malcolm McLeod]

Upon further review of OP's post, I agree: he's threading a hole edgewise through the sheet, so 1/4" is pretty much the maximum. With 3" of thread, I'm pretty sure pull-out of the threads will not be an issue, so even smaller might be better, maybe 10-24 or 8-32, to maximize the overall structural strength around it.

Drilling that hole is probably harder than tapping it...DAMHIKT.

I considered <1/4", but unlike polyethylene family, plexiglass/acrylic/polycarbonate are not known for their lubricity :: they'll gall easily and the flute relief on #10 and/or #8 taps will just make this worse IMHO. I'd want all the 'gap' I can get on a tap. Brittle material will also chip more on finer threads.

Whose signature line is "Sharp fixes all manner of (problems)"? ...New tap may be the ticket here!

As for drilling, I have experienced the same beating. I'd sincerely hope Mr. Hays can rotate a DP table to 90deg and drill straight; maybe a doweling jig as plan B? Slow RPM; limit heat build-up; cut a hole, don't 'melt' it in.