LOL, well I'll try. I only cut left handed threads once and that
was over a year ago. I think it was just luck that my screw came out
perfect. On both right and left threads you still cut on the front
side which means the cutting happens when the spindle is turning
towards you. The only difference is that in a left handed thread you
start cuts at the head stock end and set the carriage to move towards
the tail stock as the spindle revolves in its normal forward manner.
The tool bit just rides in the cut groves when you crank the spindle
backwards to the beginning of the cut before you advance again. Since
I used a handcrank I never had to back out the cutter bit (60 degree
point held 90 degrees to the stock being cut) and I've never had to
use the threading dial. If you're using a HSS bit you'll want to have
the compound set at 29.5 degrees to the left. If you're using a
carbide cutter then you MUST have the compound set at zero degrees or
you'll break off the tip of your tool when you crank in reverse to
come back to the starting point even if you are only advancing .001"
for each cycle of the carriage. (If you're using the threading dial
and retracting the bit during the return stroke you can safely off set
the compound to 29.5 degrees even with a carbide cutter.) Hand
cranking a long screw (like a lead screw) is long HARD work that will
give your arm a workout but there is a much less chance of making a
mistake. There are charts that tell you how far you'll need to advance
your bit. If you're coming in at 29.5 degrees you'll need to advance
the bit twice as far as opposed to coming in at zero degrees on the
compound. The easiest way to gage how close you are to being finished
is to use a test nut when your screw starts looking almost finished.
In the case of a left handed lead screw thread, simply buy a spare
cross slide nut before you start and use it as your gage. In the case
of a right hand thread, just obtain the proper size regular nut before
you start. After you finish a cut and it seems you're getting close to
the final size, back out your tail stock center and see if your nut
will start on the thread. A very tight rough fit is good. At that
point you only need to smooth up your thread with a threading file or
proper sized die to get a perfectly made screw. If you have neither a
file nor die, just go through several cycles without advancing the bit
and then clean up your threads with a wire brush followed by a little
light fine sanding to polish the OD of the threads. Newly cut threads
can also be lapped for a really perfect (very smooth) fit. Just coat
your thread with a very fine abrasive (automotive chrome polish works
well) and run your nut up and down the thread. (This lapping operation
is preformed after you remove the screw from the lathe.) Clean and oil
your new screw and you're all finished.
My first try at making a left handed cross slide lead screw turned
out perfect. After that I decided to make a perfect (right hand
thread) compound lead screw. It that case I ended up with FIVE (5) bad
(undersized) screws before I succeeded in making a perfect screw. I
guess the stars and planets weren't aligned right for first time
perfection on that job. Sometimes thread cutting can be challenging
and very frustrating.
--- In 7x12minilathe@..., Amaradio William A Jr
<wamarad@s...> wrote:
Bruce, Can you explain How you cut threads, left or
right on a lathe. I understsnd that the lead screw
controls the pitch of the thread. but how do you cut a
thread and spiral to the next thread? Do you back off
the 29.5degree cutting tool by visual observation?I
understand this is why there is a a thread dial on a
lathe. Many machinists don't use them, How do they cut
threads? Are there any simple .easy to understand
,step by step books or articles on threading . I own
a Cummins 7x10 and a Jet 9x20 and can't cut threads on
either of them. Thankyou Bill Amaradio
--- Bernard Goguelet <phantomcow3@c...> wrote:
Bruce Leo Hartmann wrote:
Yep, I made a new 10X1mm left hand lead screw
for the cross slide on
my old style Homier. I used 9/16" oil hardening
(although I didn't
harden the screw) drill rod. I made it 3/4" longer
than standard and
.007" larger in diameter. It cut very smoothly and
turned out perfect.
I did first order a 21 tooth gear from Uncle Rabid
and I made (and
used) a hand crank for the threading operation. No
problem at all.
Since you're doing 20 tpi you won't need any
special gears. A standard
thread file or 7/16" LH die will make the final
clean up much
smoother. Make sure your tailstock center is in
perfect alignment with
your headstock. You do not want ANY taper.)
--- In 7x12minilathe@..., Bernard
Goguelet
<phantomcow3@c...> wrote:
Has anybody cut left hand threads using their
mini lathe?
I am making a new leadscrew for this lathe,
7/16-20. At school i
threaded a left hand screw for the first time,
but this is an
industrial
machine. I could wait until monday to cut my
leadscrew, but im feeling
impatient.What has been peoples reactions to
left hand threads cut on
this machine?
-Bernard
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well, I decided i will use the lathe at school. I
dont trust my
headstock and tailstock alignment just yet. And the
one at school has a
taper of .002 over 24". Considering my new leadscrew
is not even half of
that, i can live with it :).
-Bernard
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