Tim Wescott wrote:
On Thu, 04 Apr 2013 16:07:22 -0400, Jim Wilkins wrote:

"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.

Note too that with some material/tool combinations there's a minimum
depth of cut that you can achieve. This mostly happens with materials
that have a lot of flex for their strength, and tools that are not as
sharp as they need to be for that material.

are there any sort of rules of thumb for cutting vs rubbing depths?

I mostly have this issue when machining plastic, or when I have not been
diligent about sharpening my HSS tools.

I have noticed that the generic looking 1/4" HSS tools with all sorts of
bevels behave quite differently than one indexable HSS insert (little
diamond shaped things)holder I have where the top of the insert is
completly flat. The only bevel is on the edge of the cutter itself.

It seems to leave a nicer finish on aluminum under pretty much any
condition. Is this possibly due to the less backrake?