On 2011-12-20, wrote:
On Mon, 19 Dec 2011 06:17:00 -0500, "Wild_Bill"
wrote:

[ ... ]

Thanks. Pretty much all that I did except for the indicator thing.

I could only find a scanty reference to the depth of each pass - they
recommended 0.003" on the compound. That makes it about 0.0025" depth
of cut per pass. For the 1/2"-20 the depth of thread is about 0.060"
so that is about 25 passes. Soudns about right? Does this vary with
how beefy one's lathe is?

Depends on how beefy the lathe is (including the toolpost and
the length of threading tool extension, how long the section of workpiece
is compared to the diameter (the skinnier, the shallower the pass, but
if you can support the other end with a live center, you can get away
with a bit more), the material being threaded (I like 12L14, but if you
are planning to weld to it, forget that material. :-)

If you have 3" or less of the 1/2" stock sticking out, you can
likely do it as follows:

1) Light scratch just to verify that the threading gears are
set right -- compare the scratches to a thread gauge).

2) I would probably do four passes of 0.010" actual depth

3) And then finish a couple of 0.005" passes. (Make sure you
know whether your lathe cross-slide and compound are calibrated
in terms of diameter off the workpiece, or radius. (This is why
the threading gauges have a "double depth" column as well as the
"single depth" one.

The threading depths there are with free machining metals, 12L14
steel, 360L brass, 6061T6 brass. If you have some of the gummy "steel"
from Home Depot -- experiment until you find values which work for you.

And be sure to lubricate the threads each pass. Some of the
Rigid high sulfur pipe threading oil for steel, perhaps some Molly-D,
nothing for the brass, and just keep spritzing with WD-40 or kerosene
for aluminum. I've also got some really high sulfur cutting oil which
brushes on with an acid brush and stays put. It is called Sul-Flo, and
is only sold in rather expensive quantities, so our metalworking club
got together a group purcase, with the one who wanted it the most (from
using it before) doing the sub-dividing into gallon plastic bottles. It
looks just like flowers of sulfur (the fine yellow powder) in a thick
oil. It is really nice when cutting tough steels.

I tend to use pre-formed insert tooling specifically for
threading most of the time, though I have ground my own Acme threading
tools with the proper relief angles for the particular diameter and
pitch I was threading.

Good Luck,
DoN.

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