There's a much simpler method; one which makes it impossible (!) to
make the error highlighted by Jim Stewart whilst at the same time it
keeps track of the depth of thread by direct reading of the cross-slide
dial - not possible if you angle the compound slide. I use it; by
coincidence I used it this afternoon to cut a half inch BSP internal
thread to a shoulder. The method was described in a letter to Model
Engineer magazine (M.E.) by Duncan Webster of Cheshire, UK as follows:
"In Mr. Ellis' article "Letter to a Grandson" about screwcutting (M.E.
4128, 8th September 2000) he describes two options for incremental tool
feed between cuts: straight in and angled top-slide. As he rightly
points out, the straight-in method requires the tool to cut on both
flanks causing the chips to collide and jam and, incidentally making it
impossible to have any top rake on the tool. The so-called half-angle
method is fine except that you can't use it on the Myford ML7 lathe as
the top-slide won't go round that far; also you no longer have a dial
that tells you how deep you have cut.
The following method is not new, it was shown to me by an old chap who
had been apprenticed in a Merseyside shipyard in the 1920s and it has
been described in M.E. before but seems not to have caught on.
Set the top-slide (compound slide) parallel to the job, its usual
position for plain turning. Wind in the cross-slide until the tool
just touches the job. If the cross-slide index dial can be zeroed,
then do so, otherwise wind the top-slide forward until its dial reads
half the reading on the cross-slide. Ideally, zero both if they are
adjustable.
Move the saddle to the right, clear of the workpiece; wind in the
cross-slide say ten thou(thousanths of an inch) and wind in the
top-slide half of this distance. Engage the leadscrew half-nuts and
take the first cut. Move the saddle back to the start point, wind the
cross-slide in a further cut (say ten thou) and the top-slide forward
by half of this amount and take another cut. You will note that the
top-slide keeps a record of how you are going so there's no need to
remember the cross-slide reading and the total depth of cut is
indicated on the cross-slide dial.
Almost all of the cut is on the left of the tool; the right flank has
only a minute shaving cut. You can therefore have top rake on the tool
which makes life a lot easier when cutting steel. Using this method on
my ancient Myford, I quite happily take 20 thou cuts for the first two
or three, reducing thereafter, but only getting down to fiddly little
ones as I approach final size.
Yes, you do need to have a decent dial on the top-slide; a useful
exercise in turning and dividing if you haven't."
Duncan A. Webster
Cheshire.
Regards,
Ian