cutting holes with an endmill on a rotary table

[picclock]

Pretty standard sort of operation. Work centred on rotary table and drilled to 12mm. Hole is then to be enlarged to required size. But, however I try to do this I get rubbing and squeaking from the endmill. Normally I open up the centre hole to 20+mm by using an endmill as a drill - likely bad practice but always works no problem. So if I start with a 20mm hole and use a 14mm endmill to enlarge the hole by moving the x axis on the mill and turning the rotary table all hell breaks loose with rubbing and squeaking. Depth of cut seems to make no difference, however when the hole exceeds a certain diameter it behaves normally. Its as though there should be a ratio of mill to hole size that would prevent this happening.

I can't help thinking that apart from the irritating noise this is likely blunting the cutter.

Any enlightenment on this most welcome.

Best Regards

picclock

[Chuck in E. TN]

Is it possible the material is work hardening when the hole is drilled? What material?
Chuck

[SemiSkilled]

A 14mm cutter in a 20mm hole sounds big as if you draw two circles with the 14 mm touching the inside of the 20 it look like a lot of engagement, in other words the cutter is taking a bite for a long time a 6 or a 10mm looks better.

Lee

[picclock]

Hi Chuck - The material is mild steel, but it does the same on Aluminium. Pretty certain no work hardening is taking place. Typical cut depth of 0.5mm so it would be gone after 1st cut.
Hi Semiskilled - One would think that if the cutter was run at full depth (d/4) on a straight piece of steel the cut length would be a lot longer (11mm), with 0.5mm engagement 6.6mm. Also depth of cut does not affect rubbing/squeeling.

Many Thanks

picclock

[tekfab]

Have you tried different spindle speeds ? although the book might say for that dia the cutter should turn at x rpm  when your avtually doing the job you sometimes have to go by feel and sound, as you say that as the hole gets bigger things even out it may well be that in your machine with that set-up you need to experiment, i would !

Mike