CNC Tooling for Large Solid Wood Parts

Step-cutting with a relatively light bit can break the tool. Instead, rough out with a large, slow helix bit and come back for a light finish pass. September 24, 2006

I have a job cutting 1-3/4" thick poplar into chair legs. I am having trouble with tooling breaking. Using a Whiteside braised carbide straight flute. Making three passes at 350IPM, 24000rpm and climb cutting on my CNC. I have broken one tool and had to slow feed way down. Getting a good finish, but tools are screaming way too much. I really need to stick with a 1/2" bit, but any suggestions are appreciated.

Forum Responses
(CNC Forum)
From contributor C:
Without having more info, I think it is safe to say your part is not held securely enough, which would easily cause bit breakage. Tool screaming is most likely your part vibrating against the cutter. Your part has to be absolutely rigid when held.

From the original questioner:
No, the parts are extremely rigid. I have to pry them off the fixture even after I turn the vacuum off. I think it is just too long of a tool to run as fast as I am trying to do. I am going to try conventional cutting on Monday and see if that helps.

From contributor A:
We process a lot of lumber products. Had a job with 2 1/4" thick red oak radial handrail. We initially were step cutting, as you are, and we kept having problems with the tooling tips overheating and dulling. Our tooling supplier suggested we use a 3/4" diameter downshear rougher bit and cut it in one pass, which worked very well. We then used a slow helix bit to remove a final 1/16" of material. We haven't step cut any lumber since.

From contributor J:
The screaming is probably from way too high an RPM. At only 350ipm, you're not getting enough chip load. Slow to 15000 rpm.

From contributor G:
I suggest using a slow helix ruffer solid carbide. It will give you a great finish. The CRU-500-3SH has a 1 7/8 ( 1/2" OD ) cut length and can complete your machining in two passes @ 500IPM @ 14000Rpm's. You will not have any screaming or bit breakage.

From the original questioner:
Just an update for all those who offered advice. I ended up with an Onsrud 60-317 running 24000RPM @ 350IPM full 1.7 depth and coming back to remove the .05 skin at 500IPM. Great finish, no vibration, no screaming. Thanks all for all the advice and help.