Interesting plastic drilling

[GrahamRounce]

Interesting? A disc of perspex, 8mm thick. I drilled a 2mm hole all the way through, then a shallow 3.5mm one for the screw head. Put a drop of superglue into the bigger hole and pushed the screw through so it sat flush.
Then noticed all the junk that seems to have penetrated and spread into the perspex itself! Note especially the hair-fine slivers apparently floating several mm into the bulk! Any ideas?

 

[Lons]

Hi Graham

Drilling acrylic (it will be cast acrylic at that thickness) is never a clean hole as it slightly fractures as it cuts, normally not a problem but putting superglue into it affects the acrylic and penetrates the cracks. Superglue also turns white in that situation and I'd suggest the brown marks are from residue off the drill bit and or screw if you inserted that while the glue was wet..

You shouldn't really use superglue on acrylic unless it's one of the specialised varieties as it reacts chemically and weakens it.

 

[niall Y]

Hmm .... I used to work with Perspex, but in the days before superglue. In this instance I would have drilled , threaded and countersunk. the screw .
The problem is obviously the superglue which is a form of Acrylic itself, though it doesn't seem to have reacted very well with the Perspex disc. You might have done better with "Tensol" with some Perspex swarf dissolved into it
I am far from being an expert on the chemistry of Perspex, but I seem to recall that flat sheet - as opposed to components like thin tubes, have slightly different formulas. They certainly smelt different when you cut into them, with the tube having that rather unpleasant smell that is more akin to superglue.

 

[Vulcan]

You have to work with what you’ve got but I always avoid Acrylic. PETG is much nicer to work with.

 

[GrahamRounce]

I should have said there could have been (apparently was) crud on the drill and also the wood supporting it while drilling.

It's the way tiny pieces have migrated into the perspex - or you think those are fractures, not pieces? But "floating", disconnected ones?

PPS, I'm not complaining, it's not a problem. I'm just curious.

 

[Keith 66]

They look like fractures caused by the drill pulling itself into the hole. If you want acrylic to drill cleanly take a normal twist drill & grind a negative rake flat on the cutting edge, you can also use a diamond file. A drill ground this way scrapes rather than cuts & will drill fast clean holes in acrylic without breaking it.
The two types of Acrylic are Cast & Extruded, not sure what the chemical difference is.

 

[Lons]

Definitely fractures in my experience and the crud has migrated into those pulled in by the superglue which soaks in an melts into the fractures. If you drip superglue around the top surface of a hole it will often craze the surface.
Like Niall I worked in the industry for several companies as branch manager of plastics distribution companies, long before superglue but we cut quite a lot and fabricated some We distributed ICI perspex and I've visited the factories on numerous occasions but to long ago for me to remember the chemical differences however the properties of cast sheet and extruded is different and that of course includes extruded tube, as Keith says, your nose is a good guide as it is with many plastics when heated.

I'd also have drilled tapped and countersunk as suggested and while what Keith said about grinding drill bits is absolutely correct, I never bother as is occasional use and just ensure the bit is sharp, clean and speed is slow.

 

[Cabinetman]

Also possible to heat it up in an oven and push the screw through then let it cool, it will push up the surrounding surface though.
It’s a memory plastic so all sorts of clever uses can be made of it.
Ian

 

[Mike Fletcher]

In my experience, use a sharp drill bit with little pressure when drilling especially when exiting the hole. Make sure the drill bit is clean. Then, I use a small torch to heat up the area around and in the hole, reducing (eliminating) the sharp edges that can cause cracks. You will get some crazing but with a little practice to get the right amount of heat this can be reduced.

 

[Lons]

In my experience, use a sharp drill bit with little pressure when drilling especially when exiting the hole. Make sure the drill bit is clean. Then, I use a small torch to heat up the area around and in the hole, reducing (eliminating) the sharp edges that can cause cracks. You will get some crazing but with a little practice to get the right amount of heat this can be reduced.

Have you had a go at flame polishing edges Mike? Remarkably easy with a little practice and fascinating to do it gives great results. You can only do it with cast acrylic as extruded behaves differently.

 

[Mike Fletcher]

Yes, I flame polish (haven't heard it called that, thanks) edges after filing blade marks away. As you probably know, it gives great looking results.

 

[Keith 66]

I used to keep a set of drills specially ground for acrylic on my technicians desk at school otherwise the kids would have busted every bit of acrylic they ever drilled! Feeding the drill slowly is something they just didnt get!

 

[Lons]

Yes, I flame polish (haven't heard it called that, thanks) edges after filing blade marks away. As you probably know, it gives great looking results.

Yes have done it many times. It weakens the surface slightly but as you said leaves a nice finish.

 

[throbscottle]

I looked at a PC case mod project some years ago. Lots of drilling of plastics, acrylic included. The modder said instead getting very expensive special drill bits for the job, drilling into concrete with normal bits before drilling the plastics with them gave lovely clean holes! Also easier than regrinding them.

 

[Phil Pascoe]

I often found where appearance wasn't necessarily the most import consideration that it was better to melt the holes through than to drill them.

 

[throbscottle]

+1 for melting! My soldering iron comes in for serious misuse when Mrs runs out of plant pots!

 

[cowtown_eric]

I take ordinary drill bits and grind off the sharp edge with a dremel such that it is maybe a slightly negative rake-all the same as a plexiglass saw blade.

This scrapes rather than cuts

Once modified, they get a tag of tape saying "plexiglass" and tossed in a small drawer of specialized drill bits.

Plexi responds quite favorably to a card-scraper, and I as well like the flame finish, but be sure you have a water spritzer and we rag to extinguish small flames. This stuff once it starts to burn gets really going real fast.