"Jon Elson" wrote in message
...
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.

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

Jon

After taking and measuring the 0.01" cut, run the bit back over the
work and see how much more it removes.

On my old lathe the first fine cut after roughing down removes more
than the dial shows, but subsequent finishing cuts match the infeed.

jsw