Pete Snell wrote:

Peter Fairbrother wrote:

I'm doing facing cuts and the tool is grabbing because the rake is too
positive (I think. There's a graunchy noise and the feed goes slack for a
bit. Worse near the center, I'm not sure why. Chinese minilathe, 0.5 mm cut,
brazed carbide tool on cast iron).

It's not really practical to change or regrind the tool. I'm dead on center
height, and wondered whether changing the height might make it better by
changing the effective rake somehow?

If so, which way? And why? I tried to work it out but got confused, and came
up with two different answers.

Is this a BIG facing job? (more than a couple of inches in diameter?)

90 mm x 55 mm rectangle shape, and that may be a reason for it getting worse
at the middle - I can't get the speed up very high, as it's an odd shape
with the CoG off-axis in a 4-jaw independant and it vibrates too much.

I can get an okay finish with a very fine finishing cut or two, but I have
to do a few of these and they don't have to be measured, just flat.

If so, you may have to increase the spindle speed to keep the cutting
speed in the desirable range as you move from or toward center.

Sounds good when it's hissing, or actually more like air whooshing through a
carb in a sweet engine and it's one more nice point about having a
variable speed - you don't have to stop the lathe to change gear or even
stop the feed, you just turn the speed knob up until the swarf starts
burning.

Is that last bit right by the way? I've been doing it that way all day and
the carbide tool seems undamaged, I figured if the swarf wasn't burning the
tool wouldn't get too hot. No lube used (or needed), I'm cutting good
quality fine-grained grey cast iron, and the lathe doesn't have a coolant
system.

I'm afraid I'm abusing the poor wee lathe though!


--
Peter Fairbrother