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Anyone have tips for 3D finishing pass?

PostPosted: Tue Aug 27, 2019 7:11 pm
by loopacrylics
Hello I am machining out a large piece of acrylic. Its about 24" X 36" x 2". I did a roughing pass with no issues. Now I am having an issue with my finish. My 1/8" ball nose that I use for finishing is way too short to finish this whole job. The 1/4" ball nose bit that I use is much longer but its screaming at me when I try and run a finishing program. Could I use a 1/2" ball nose to try and do a finishing pass with that then come back with my 1/4"? Its just when I try to make 2 separate 3d finishing tool paths the 1/4" and the 1/2" bit are going down the same depth so would it even do anything? Also any feeds/speeds advice is greatly appreciated. Like I said my 1/8" works fine for this @ 23000RPM 40IPM and my 1/4" is @ 23000RPM 60IPM which has a very loud whine when passing through.

Re: Anyone have tips for 3D finishing pass?

PostPosted: Tue Aug 27, 2019 7:50 pm
by Adrian
Generally when a bit is screaming it means the RPM is too high and/or the feed rate is too slow. With 3D work, especially on a finishing pass, it's hard to get any high feed rate because of the amount of 3 axis movement.

Re: Anyone have tips for 3D finishing pass?

PostPosted: Tue Aug 27, 2019 10:21 pm
by Xxray
Along with feed rates/rpm issues, a screaming bit is indicating it could be dull and/or is biting off more than it can chew. A good plexi cut will produce nice consistent chips rather than dust or melted goo. Is this intricate 3D work or just a general shape ? With plexi, you usually want low rpm's and high feed rate as possible to minimize gumming/melting issues but it can take some tinkering to find a sweet spot - In all instances, a good sharp bit is required for optimal results.

Re: Anyone have tips for 3D finishing pass?

PostPosted: Wed Aug 28, 2019 4:37 pm
by adze_cnc
Also, ball-end bits don't really have much cutting edge at the tip. If you have lots of near horizontal cutting you could get chattering, bad surface finish, melting, etc. In wood this is not too much of a problem as it will just burnish the material.