I tried using an airline with the feed/speed I quoted and it did improve the finish a little, but the finish was good enough without and I couldn't have gone any faster without the tool deflecting too much, hence I didn't bother with it. My machine isn't especially rigid, otherwise it may have been more worthwhile.

Quote Originally Posted by CLaNZeR View Post
You will need to start with shallow depth cuts and slow feedrates and see what you can push both to, to suit your machine and avoid chatter.
The risk here is if you start will too low a feedrate (without also using a very low spindle speed, but there's a limit to how slow you can go with the VFD driven spindles) you can risk the material melting on to the cutter which can break the cutter just as fast as going too fast. This is probably what's happening to Craig as with a 4mm cutter at 200hz (12000rpm) the feedrate should be around 950mm/min for single flute or 1900mm/min for two flute.
When trying a new material or tool I always start with 'normal' or recommended feedrates and a shallow cut, then gradually increase the depth of cut.

Either way it will help a lot to see a picture of the machine this is being cut on to give an idea of how rigid it is.

Quote Originally Posted by craigrobbo View Post
also tried 250-280hz on 1.5kw spindle(not sure what that relates to in RPM but i guess about 12k - 15k)
You can press the >> key on the VFD to scroll through the readings until you find RPM if you're not sure. An easy way to remember it is that you know the spindle must do 24000rpm at 400Hz, since that's the maximum, so just work it out proportionately from that. Hence 250Hz is 250/400*24000=14400rpm and 280Hz is 16800rpm.