Issue with reaming

[BeerFizz]

Hi,

I have a cnc'd mini mill with a belt drive spindle.

I'm trying to make precise hole (or as close as possible) to .375" in half inch polycarbonate.

I have a .375 reamer, .374 and a .373

Currently I am trying with the .373 reamer. I have pocketed a thru hole with a diameter of .370". The reamer GCode has a plunge feedrate of 4. When I run the GCode the reamer chatters and squeals its way through and the resultant hole is way way over sized.

I have the reamer as far up into the collet as it will go and I put a dab of oil around the hole before starting to ream. The spindle is running pretty much as fast as it will go.

What am I doing wrong

Thanks for all help
Phil

[kimpgray]

Are you going too fast ? rememberyou have maybe 6 blades in a reamer so feed per rev is gonna need to be adjusted for many blades ,,,, unlike 2 for a drill ? Maybe you need to leave more material in for reaming ,,, this may stop the chatter ? good luck from UK

[sti2011]

Sounds like your rpm is definetly too high. Polycarbonate will melt and smear if your sfm is too high. I would try about 800-1000 rpm with flood coolant and a feed of around 10-14 IPM. You might also want to leave a bit more in the hole before reaming. Poly does like to be cut and not rubbed. If you don't have flood coolant use any fluid on hand in good amounts to keep the heat down. A heavier cut will also hopefully put heat in the chip rather than the part.

[BeerFizz]

Thanks for all the answers.

I finally put a tach on the spindle and calibrated it so to speak.

After several more tests, about 500 RPM with a feed of 4 IPM cut a beautifully reamed hole. A perfect fit for my precision ground shaft.

You live and learn!
Thanks again for all the help
Phil