Advice on machining plastic


Hey guys,

Figured I might get a good advice from someone on this forum. I'm try to get a good 3d surface finish on some test parts I made. Material is 3D printed PETG which should be similar to machining ABS. I'm using a 2 flute 1/4 ball mill amorphous diamond coated from Harvey. Stepover of the pattern is 0.5mm but i feel i could go a bit higher and not have a scalloping effect. I don't have air cooling at the moment (compressor is defective) but i might try this. It feels like it's re-cutting chips. See pictures ... any advices?


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when your 3d milling its a toss up between how fast you want to generate your profile and surface finish quality. if you want a better finish a general rule of thumb would be the slower the feed rate and finer the step over the better the finish you will get. I would slow the feed down first then possible try a finer step over if it's still not giving you a good finish. getting some mist or coolant would help too. cutting dry is not ideal by any means.


When ever I would cut plastic (Delrin and Teflon) on the cnc mill I used 2 or 3 flute super sharp high speed steel endmills. You can also use a shop vac to suck away the chips as thy are cut.


looks like you have an adaptive clearing path to get the bulk of the stock removed that's fine. another thing to would be to leave a small amount of stock for a finish pass with your ball nose. keep what you have for a roughing tool pass and leave maybe .005 or less for the final finish pass at a much slower finish feed rate. you should be able to set stock to be left easy enough with fusion 360 in the radial and axial section of stock to leave.

a shop vac is not a bad idea either as suggested above to keep things clear. chips being recut will never help the situation.