On Tue, 01 Feb 2005 20:53:20 -0800, Grant Erwin
wrote:

Derek wrote:
I have a small HF lathe that my kid & I putz on, no high precission
work etc.

I'm keen to learn any tricks from the old masters regarding turning a
piece down to a specific diameter. For example, say you need to turn
a 10 mm rod down to 8.5 mm, are there any fancy compound slide angles
that equate closely to the dial, or do you guys just "shave and
measure"?
Bear in mind this is rudementry bushings & such, but I would like
to learn a better way.

Any tips much appreciated.
Thanks.

From my personal archives:

'make your final cut and the cut just before the final cut the same
way, i.e. use power feed on both or neither and make sure your depth
of cut is about the same for both. if you want your final cut to be 5
thou then stop rough cutting when you are at least 10 thou away (and
make a pass or two without changing the depth after your last rough
cut), and then make a pre-final cut of half the remaining depth, then
re-measure what is left and make the real final cut.

The problem you are having is that the actual cut depth is not a
linear function of tool bit position. There are all kinds of non-
linear digging in, and machine deflection effects that can happen,
that are not worth trying to predict, hence the above technique can
help."

From my personal experience: when you are starting your final cut, turn
just a little bit, maybe 1/16", then MEASURE. If you're right on, keep
going, otherwise FIX IT.

GWE

I haven't used my lathe much lately (a 9" SB), so I went out this
morning and tried to see how close I could get. I turned a piece of
brass using the 3 jaw to near 1/4" for an inch or so, then mounted a
1/4" collet in place of the 3 jaw for turning the other end to as
close a 1/4" as I could. I pretty much followed Grant's method.
Getting to the final cuts I pretty much ignored the dial graduations
and just nudged the cross feed a little. The result was a diameter of
..2498 inches, with a runout of about .0001 inches, and a taper over
and inch or so of 0.0002 inches, using a Starret 436X micrometer.
Surprisingly, the runout on the other end using the 3 jaw was only
about .0002 to .0004.

Tom