Tool Choice for Half-Inch Acrylic

Advice on selecting bits that will do clean work machining thick acrylic on the CNC router. April 17, 2009

Question
I'm looking to cut 1/2" thick acrylic between 35,000 and 40,000 rpm. I was told a 1/4" dia. 3 flute, roughing bit will give me the best edge finish and the highest cut speed. All I can find is 3/8". Any help would be appreciated.

Forum Responses
(CNC Forum)
From contributor R:
Why are you cutting at such high rpm's? You are likely to experience melting issues at those speeds. As far as tools go, Onsrud makes a very nice line of cutters specific to plastics.

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From contributor S:
I have never cut 1/2 thick acrylic, but we cut 1/4 quite often and the best bit I have found it an "O" flute. I agree you should talk to Onsrud, since they make their living selling bits.

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From contributor M:
Contributor R is right on. I would take a serious look at a 3/8 spiral O from Onsrud and lower your RPMs substantially, like by a factor of 4 or so. I can't imagine not melting the plastic at 35 to 40KRPM, or for that matter that your spindle even rotates that fast. I don't cut acrylic that terribly often, but did just do a job with 3/8 material. 18000RPM and 800 IPM worked well for me with a spiral O, though I am quite sure that could be improved upon for speed if you have a substantial amount of material to cut and are willing to experiment a little.

With a 1/4 " bit you will very likely need to double pass the material to get decent chip ejection. With the 3/8 you should be able to do it in one pass without clogging up the works. I don't know of a bit that will leave a better finish than an O flute in acrylic. For what it is worth, Onsrud makes a sweet 1/4 spiral O.

Also, when going for speed, consider climb cutting the acrylic. It is actually brittle to tooling when the chip is too large, and I had a good deal if difficulty with micro-shattering near the edges of a sheet when conventional cutting. Not so much on interior cuts. Let us know how you make out, maybe we can learn something from you!

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From contributor G:
A ruffing tool will not give you the best edge finish - a single O flute is the best tool. We also have slow helix tools both in 2 or 3 flute, based on what your part size is. What machine do you have that will run 35-40K rpm's?

On that thick of material the 1/4" with 3 flutes and at that rpm is a recipe for melting and throwing hot chips back onto your cut surface. Generally you will not be able to cut acrylic as fast as wood. A fast cut will produce pocks and chips. An Onsrud "O" flute is a good choice but you should be looking at larger diameters. You will also be able to get by cheaply using an Onsrud 48-076 carbide tipped 1/2" dia. 18,000 rpm at 250-300IPM and then possibly follow that up with a cleanup pass taking off .050" of material. The best I've gone with (for the money) is a 52-638 3/8" 2 flute upspiral. Again 18000rpm at 250-300ipm. If you're using a higher grade cast material, your edge will be smooth as glass. These feeds may seem slow but try it and see. Lots of air on your cutting tool will do wonders.