Try getting a real tool steel blank from one of the online suppliers,
MSC, J&L, etc., and grind your tool from that. You can even get round
blanks for certain types of HSS. I like Tantung blanks for internal
threading, they seem to last a long time. Used to cost 2-3X as much
but lasted 10X as long. I've done 2-start threads, but nothing that
small. There's some fancy driver plates you can use that will let you
index your work 180 degrees while leaving the gear train engaged.
Instead of building one of those, I carefully marked the tumbler and
headstock gears, cut the first thread, then disengaged the tumbler and
flipped the work a half-turn and re-engaged. It worked. Make sure you
have a male thread to gauge against.
A 10-pitch thread in a 5/16" hole has a pretty steep helix angle,
about 5.8 degrees. If your cutter isn't ground with this helix angle,
it will either chip or barf the thread.
Back the cutter well out of the thread before returning to start for
Thanks Don. So does this mean that on the lead side of the tool the
relief angle should be undercut by about 6 degrees more and with the
trailing side about 6 degrees less. Hell I wish I could draw on this
Wow, that sounds very difficult for single-point threading! The small
diameter plus steep thread pitch means the tool has to be steeply
releived on TWO sides at once. This may be a case where you have
to do it with a tap-like tool. How about cutting the male version
of the thread, then grind away some flutes to make it look like a
There was an article in either Home Shop Machinist or Machinist's Workshop /
Projects in Metal a year or two ago about cutting small double lead internal
threads. If I remember correctly, the author used a straight flute tap with
the proper TPI and gound off all but two adjacent points near the tip of one
flute, ground some extra clearance on the leading flanks and cut both leads
(starts) at the same time. The first tip cuts one lead and the second tip
starts cutting half of a pitch away from the first making the second lead,
no need to index the part 180 degrees or make compound / cross slide
adjustments for the second lead. From your description it sound like your
cutting a 5/16-20 double lead (2 start) thread. A 1/4-20 tap might make a
good tool, grind as described above and thread with the lathe set for 10TPI.
Since the lathe is set to 10 TPI, you're doing a 5/16"-20 2 start
Depending on your lathe, you may be able to do each lead from a
different mark on the threading dial. I do 4 start 36 TPI threads all
the time, setting the lathe to 9 TPI and cutting each lead the same
depth by using each of the 4 marks on the threading dial. Just happens
to work well on my setup.
The tool needs extra clearance. Imaging looking straight on at an
internal threading tool, pointing towards you. On most tools, you see
the line come straight down from the cutting point. For a multi lead,
the line you see should go down and to the right. That is, you need
some extra clearance below the trailing side of the V. The amount can
depend on the thread.
\ | / <- front view of regular internal thread tool
\ \ / <- front view for multi-start
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