1/8" "O" Flute Bits Breaking - 1/2" Black Acrylic

[Ross Moshinsky]

Material: 1/2" 2025 Black Acrylic
Hold Down: Lots of carpet tape
Machine: CNC Routerparts 48x96 NEMA34 motors
Spindle: 3.5hp Porter Cable

I'm cutting out some shapes and letters out of 1/2" black acrylic. I bought two brand new 1/8" "O" flute bits from Centurion tools. I followed Onsrud's recommended 18,000rpm (actually 19k on this router), 75-100ipm, and .07-.1 depth. Ramp is 1.5". Both broke within probably 6 inches of cutting while testing. I throw in a 1/8" ball end mill (I sadly don't have a regular 1/8" end mill) and it can do the job without much of an issue. Edge isn't perfect, but it's definitely acceptable, especially if I do a little sanding. To get the job done, I'm going to finish with the 1/8" ball end mill or get a 1/8" end mill but I'd like to figure out why I can't use the "right bit" for the job.

My guess is that the collet has too much run out and it's causing the bits to break. I've broken 3 of these "O" flutes so far (2 on acrylic, 1 on Corian). Considering they are about $25 a bit, I'd like not to break another one.

Thoughts?

[Scott Shepherd]

Did you double the feed on the ballnose? If not, then you put a ballnose in and ran it at 1/2 the feedrate of the o-flute, assuming the o-flute is a single flute and the ballnose is a 2 flute. If that's the case, then consider that. Having said that, I don't run o-flutes that small, but I do run 2 flutes that small at 15,000 RPM and 2 IPS, normally taking about 1/8" DOC and I don't think I've ever broken one, ever. I've gone through probably 40 of those (tend to swap them out often to keep the sharpest ones for certain jobs) in the last 2 years.

[Ross Moshinsky]

I kept all the settings the same. Just put in the 2 flute ball nose. I'm thinking that the 1/8" o-flutes are just not worth the effort. I can't imagine running 12-14 passes just so the bit won't break. I think with a really tight machine, they can work, but my machine isn't really tight. I'm thinking they are just too fragile to run effectively on my setup.

[Paul Phillips]

Ross, it must be your machine then, I use them all the time with great success, my settings- 80-100ipm, 24000rpm, .125 depth per pass. Not sure what you're cutting but as a general rule, for Acrylics it's best to use the largest diameter bit that you can but sometimes the detail necessitates smaller bits, another general rule, never cut a pass deeper than the diameter of your bit, ie, 1/8"bit=1/8"pass.
Hope this helps.

[Ross Moshinsky]

I'm going to go ahead and order myself a precision collet for the holidays and see what kind of difference it makes. The machines you're running are significantly more industrial than mine. Based on the cut quality I'm getting, I'm thinking the run out in the collet are just sloppy enough to create additional stress on the bit and as a result, it's breaking.

I would have loved to use a 1/4" bit on this job but it was all lettering which wouldn't allow it. Plus I really jammed everything into a 4x8 sheet so the few parts I could have used a 1/4" bit, I honestly didn't have the extra space for the bit. I had no interest in buying another sheet of the material. It was relatively expensive and I'd rather have the money than 3/4 of a sheet sitting around.