circular pocket cutting chatter

[awjareme]

I am just learning some features of our machine Milltronics RH33 that we recently got. I am trying to bore out some holes in a 3/4" plate in which i pre-burnt 2 holes to say 1.5"dia. Finish bore is 2.251" The holes were cut using plasma and of course all the slag has been removed. I am trying to use and endmill and circular interpolate the bore. The problem i am having is when using the HSS endmill everything went way to slow and the endmill didnt last 3 bores. I then went to a carbide insert endmill, and there is way to much chatter in the bore. I have tried speeds from 4-15IPM, 600-1500rpm and cut widths from .010-.100 with no luck. It is extremely loud and does a real poor job. What am i doing wrong? Is there a better cutter for this? We plan to drill and use a singal point boring bar in the future, but i'd like to be able to figure this out as some times we do large bores 16-24" in shallow parts (.5-1.5")
Oh ya i was cutting at a z-depth of .300" at a time, also tried .100" and no diff.

[christinandavid]

Try a HSS or solid carbide ripper cutter (or even a 'tight' helix coated carbide endmill) to semi-finish the bore in a single pass. They have a better cutting action, exerting less pressure and will therefore not hammer the component as much.

Indexable tools seem to be primarily designed for full engagement (with stable conditions) and will not enjoy unsupported cutting on a flame-cut surface.

Conventional-milling (rather than climb-milling) may be beneficial to tool life in either circumstance - the cutting edge is then not immediately engaging with the flame-cut surface and may wear more evenly/predictably.

DP

[fizzissist]

Depending on the material, plasma cutting can be a real problem. You've got extreme heat, and real rapid cooling, and that leaves little bundles of really hard not-joy. Death to cutters.

If you're using carbide, you'll have to treat it like a seriously interrupted cut in hard material. For HSS, I'd try a rougher, you know, the ones that are bumpy.

With HSS you'll probably want to conventional cut...

As for the chatter, the old rule is decrease the rpm, then increase the feed...but in this case, you probably need to increase rigidity with more clamping and support for the part closer to the holes.

All too often, we found that plasma roughing caused more problems than it solved. If you've got the CNC, then just do a circular ramp in at 3 degrees downfeed, and cut the thing from start to finish in a fraction of the time spent screwing with the plasma. After all, you gotta clamp it and cut it anyway.

Waterjet is way more expensive, but for big holes or patterns, there's no heat affected zone (HAZ) to have to deal with, better precision, and no funky edges that can interfere with clamping or locating.

[NW_Phil]

try running a coated/uncoated cobalt rougher around it first. cutting plasma burned edges can do a number on carbide endmills. rough it then finish with carbide. alot of times i wished for the plate to either be waterjet or let me drill it out.