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Plexiglass + holesaw = broken plexiglass?

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Victor Bravo

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Roto-Zip with a template or radius arm/compass, take it to about 1/16" below final diameter.Then "finish" cut the hole with a 2in drum sander, gently "started" at an angle in the hole, and the edge of the drum moved around slowly in a circle, then finally slowly work the drum in until it can be i nserted andit sands the funal inside diameter hole.
 

BJC

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Roto-Zip with a template or radius arm/compass, take it to about 1/16" below final diameter.Then "finish" cut the hole with a 2in drum sander, gently "started" at an angle in the hole, and the edge of the drum moved around slowly in a circle, then finally slowly work the drum in until it can be i nserted andit sands the funal inside diameter hole.
I’ve found that a 2” drum sander makes an over-sized 1 1/2” hole.

Edit: Sorry, VB, the smart Alec in me just jumped out.


BJC
 
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pictsidhe

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When I want to drill acrylic and I only have the wrong tools, I find that running them backwards usually works, though it's not pretty. For drilling small holes, a regular drill bit running backwards will melt it's way through. I've never had a problem drilling UK car registration plates that way.

When trying to run forwards, it's when the tool bites in that cracks happen. I find it really, really hard to prevent that happening on any tool with positive rake. I have done it, but not with a good enough success rate to want to try it on anything expensive.

If you use a tool with a pilot drill, clamp a piece of wood to the back for the pilot to run in, and swap the pilot drill for a piece of rod after drilling the pilot hole. This helps with cutting holes in any material.

Practice!
 

Kyle Boatright

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As others have suggested:

A large pilot hole then a drum sander to enlarge to desired size.

This is a safe approach for a critical hole.

Also, get the work area and the plexi really warm. 70' is OK. 90' or 100' is better...
 

PTAirco

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Step 1: Throw out the Plexiglas.

Step 2: Buy some Lexan or Makrolon (polycarbonate).

I have seriously had it with acrylics. No matter how careful you are, it will crack eventually. Yes, its scratch resistance is a few percent better, but overall it is a lousy material compared to polycarbonate. There is a reason eyeglass lenses are almost all polycarbonate now.

If you do enjoy torturing yourself and you need to stick to plexiglass - I have given up on drilling holes. I melt them with a soldering iron. Rarely do they ever need to be a very precise diameter. I melt them and then carefully deburr with a unibit. Deburring is mandatory. I have used a flycutter to do larger holes, but even then warming the material up is helpful. And it only works when you can get it under the drill press.
 

BoKu

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Step 1: Throw out the Plexiglas.

Step 2: Buy some Lexan or Makrolon (polycarbonate)...
If you do this, be prepared to increase the thickness substantially to achieve the same stiffness. Polycarbonate has a much lower modulus than acrylic, so it will deflect a lot more and require more in the way of support structure. Also, make sure that the polycarbonate has UV and scratch resistant coatings. Bare polycarbonate is very prone to discoloring with prolonged exposure to sunlight, and it is more susceptible to scratching than acrylic.

Anecdote: Back when I was campaigning an HP-11 sailplane, I removed the original forward canopy transparency and replaced it with one made of polycarbonate. But I found that at it would flex inward at high speed, and when I flew through hail it would shimmer like a soap bubble with each hailstone strike. After about four years it was so brown from sun exposure that it was useless when flying into the sun. So I dug out the original 1965 acrylic transparency from under my sports car, where I'd been using it as a drip tray. I cleaned off the oil, polished it up with Mirror Glaze, reinstalled it, and it was on the glider and looking sharp when I sold the rig about seven years later.

--Bob K.
 
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Jonny C

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Jun 17, 2014
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Thanks for all your helpful tips.

I've just tested a new (cheap) unibit in plexiglass and it worked perfectly. My drill press on its slowest setting of 600 RPM and a battery drill at about 300 RPM both drilled through it very easily in seconds. I was never worried about cracking - with the lightest force it cut easily and didn't feel at all like it was grabbing or scraping the plastic.

I also had a small piece of 1/4" nylon on which I tested the unibit. The unibit couldn't really bite into the nylon without a lot of force so I pre-drilled half way through with a normal 3/32" bit. This didn't work because the unibit stopped at the bottom of the hole so I pre-drilled drilled all the way through. The unibit then worked brilliantly in the nylon. A perfect hole in seconds. (It was necessary to 'dip' the drill bit in and out a few times - even at 600 RPM the nylon started to melt if I tried to go through in one pass.)

I'd previously used holesaws in nylon which gave an inaccurate diameter hole and took ages. I wish I'd known you could get 2" unibits! Nylon bearings will be a lot easier in future.
 

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