On 2011-12-19, wrote:
On Sat, 17 Dec 2011 18:40:16 -0500, "Jim Wilkins"
wrote:

[ ... ]

Mine turns at about 50 RPM in back gear and still demands close attention
and quick reactions to stop at the end of the thread. I shut off the motor
when it's about one thread away and then lift the belt tension lever to stop
the spindle quickly.

The spindle is fairly easy to turn by hand if the bit is sharp and the chip
1-3 thousandths thick. It's certainly less effort than cutting the full
depth thread all at once with a die. Could you make up a safe balanced hand
crank for the spindle and use the power only to run the bit back to the
start?

OK, I had a go on a piece of scrap today. Everything went fine until I
cut the first pass at 40 tpi. Re-reading the chart I corrected the
gears and tried cutting ostensibly 20 tpi. I did, BTW what you
suggested and run the lathe under power both ways just finishing the
last three threads manually.

And backing the cutting tool out before running in reverse?
There is usually some backlash in the leadscrew/half-nuts interface, so
it will follow a slightly different track in reverse, and will tend to
dull the cutter and do weird things to the thread.

The big disappointment was that the thread ended up 21 tpi. Once I was
able to get the die on it I was able to correct that with a resulting
pattern of periodic double cuts and a variable major diameter (reminds
me of beat frequency oscillators). I can get the nut on but it is not
pretty!

Strange. Is this with a quick-change gearbox, or a set of loose
gears which you have to change on the far side of the headstock? If the
latter, I would suggest that you go through the gears and count the
teeth on each, and compare them to what the manual claims they should
be. This is *every* gear on the way from the spindle to the leadscrew,
not just the ones you expect to change.

Another possibility is that instead of using the half-nuts to
engage the drive, (if your lathe is so equipped) you have the feed
clutch engaged instead. This does not make the carriage move a precise
distance per spindle rotation -- and might have been about the ratio you
would get between your intended thread with the first setup and what you
got.

The other thing that I did not expect was that starting with a rod of
0.493" the final major diameter (before the die application) was
0.507". Even after filing it was still 0.502".

The tool is not cutting -- it is mashing metal out of the
groove, causing it it build up on the crest of the threads. This may be
happening in the reverse feed if you did not back the tool out of the
groove before reversing.

So the big question: Is there a reason for the 21 tpi *other* than
change gears?

I forget what model lathe you have, so I don't know what
features to expect. Perhaps it is made for metric, and you can't get a
precise 20 TPI without playing games with conversion gears -- or it may
be set up for inch threads, but there are conversion gears in the gear
train.

If you want to be sure what you are getting, set up for 20 TPI,
and set up a dial indicator on the bed to measure the travel of the
carriage. Engage the half nuts, zero the indicator, and rotate the
chuck precisely one full turn. This should move 1/20 of an inch, or
0.050". If it doesn't, check the gears -- all of them. Count the
teeth. If it is a lathe which has not been used for cutting threads
before, it may have come with a gear with the wrong number of teeth, and
the number stamped on the gear may be what you really wanted, not what
you got. :-)

Mark a tooth with layout die before counting so you don't count
the same tooth twice.

Michael Koblic,
Campbell River, BC

BTW what's with the deafening silence on the question as to why these
threads are UNF rather than UNC? Could it be that it does not matter?

I don't remember even seeing the question. :-)

It does not matter what you call it when cutting it on the
lathe.

It only matters when you are trying to *buy* the tap and/or die.
National Fine or National Coarse are just names for what family the
threads are in. What matters are the diameter and the TPI (or in metric
threads, the pitch (mm per turn). In your case, 1/2-20, which is NF
(The 'U' in front comes from "Unified", when thread standards between
the US and the UK were somewhat merged.

Good Luck,
DoN.

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