Hi

I have the same problem when using a large cutter on my VMC.- any other mill is likely to exhibit the same problem to a greater or lesser degree. The basic cause is down to the size of cutter vs material width.

Assuming that the aluminium is 19mm wide, with a 40mm diameter cutter (3 teeth) you have an interrupted cut for much of the cutter's rotation due to the lack of teeth in contact with the workpiece. This is a particular problem if the cutter is on the centre line of the workpiece.. Exactly the same situation as the 'knocking' of an interrupted cut on the lathe.

Moving the cutter so that the tooth just skims to edge of the material should improve the situation, although it probably won't eliminate it altogether. An alternative would be to use a cutter with more teeth, such as a shell mill.

Good luck – please let us know how you get on.

I'm also experiencing this issue! I contemplated fitting a few 0.2mm shims either side of the two spindle parts to try and reduce this however sticking them in place is proving pretty difficult. Can you explain in more detail what you mean by moving the cutter to one side.

Thanks

Simon

Hi Simon

Ideally, you should have teeth in contact with the workpiece at all times. if you can eliminate the interrupted cut altogether, then it's far kinder to the machine and to your cutter – the surface finish will almost certainly be better as well.

I have a good quality inserted tooth cutter – 50mm dia with 3 teeth – that I hardly use due to this problem.

As Stuart says, the entry angle is also reduced dramatically which will reduce the knocking – reducing the feed rate should also help. Give it a try – an old machinists trick that really works.

Good luck.

My VMC knocked quite badly when I first got it. Traced the problem to the motor pulley. I slackened both pulley grub screws and then tightened the one opposite the key as tight as I could, then tightened the key side screw just snugly. Cured the knock.