[tbaker2500]

Help! I need some body. Help! Not just anybody... (Maybe a machinist?)

I was cutting block acrylic today. Airblast, no melting problems, good chip clearing. 1/2, 1/8" ball, 1/16" sq, all cut fine. This is a pour mold for urethane, with a complicated 3d surface. All the other passes went fine, except for the 1/8" sq em. 2fl, carbide, I think it was 20ipm, 40% WOC, .05" DOC 4500rpm. It was a long pass, granted, about 1.5 hours. I find that my atrax EM shattered about halfway through the pass. I found the pieces, and the cutting corners have chipped off badly. No melted plastic in the slightest. I put in a SGS series 52 (I think, maybe 54?) carbide EM. Both of these had about a 3/4" stickout as required by the mold. I finish the pass. Afterwords, I inspect this bit to find both of it's cutting corners also chipped away.

This flummoxes me a little. This is just acrylic, with a fairly light cut. How could it possibly be causing the EM to chip?

I checked the runout, and it was 1.5 thou. Not good. It is a really old holder and collet, in bad shape. I've ordered some new collets and a new holder (Prices are good, down to $35 for a ER20 holder!) just to move forward. So that definitely contributed to the issue.

The only other pass of this length was the 1/8" ball, carbide, similar feeds and speeds. I was about a 2 hour run, with no problems. It is in a much newer holder and collet. Didn't check the runout on it.

Anybody have any other ideas as to what is causing this? Thanks for ideas.

[mike sr1]

I would be suspect of contaminated stock, I would inspect the cut closely for some sort of contamination.

I chipped a .5 carbide on aluminum tooling plate, there was a piece of foreign material in it.

It doesnt seem logical cutting plastic??

[tbaker2500]

Thanks for the suggestion. Being crystal clear acrylic block 1" thick, I wasn't able to see anything wrong inside it.