Jon Elson wrote:
Cydrome Leader wrote:

I understand how much everything on a lathe, including the work can flex,
but this just doesn't make sense to me,

If I turn down say a 1/2 aluminum rod that's chucked and extends only a
half inch and dial in 0.01" of feed and measure the before and after
diameter with a micrometer the diamter decreases by 0.02" Perfect.

The part I don't get is if I take a smaller cut of say 0.001" it will
remove more than 0.002" of diameter. For example if I need to remove
0.003" of diameter, feeding the crosslide 0.001" will come pretty close.

It only seems to happen with very thin cuts. I'm only turning the
crossslide dial in one direction- there no backlash weirdness going on.
This is spring. The stack of carriage, cross slide, compound, toolpost,
toolholder and tool becomes quite flexible. It loads up as you take
initial heavy cuts, then when you take a very fine last cut, the spring
unloads and drives the cutter deeper than you wanted. All machinists
need to learn to compensate for this.

I was moving cutter over to the right past the work, so there wasn't a
buildup of tension or springiness that way. The post about backrack makes
sense though. There's nothing to keep the cutter from being pulled into
the work on a sherline. Mine has about 2-3 mil of backlash after some
adjustments, but there's no real positive lock of any sort, just tension
on a wedge shaped plastic gib. You can only get it so tight before it just
acts as a brake and nothing moves at all anymore.

Generally, the more rigid and massive the lathe, the smaller this effect
gets, but at some level it shows up on any lathe.

Jon

It's interesting process to try to tame thing thing.