Probably a dumb question for most of you but here it is. I need to cut some
internal metric threads with a 1.5 pitch.
Now if I remember right, I read on this forum that the threading dial for
starting the next pass on single point cutting wouldn't work on metric
threads. If this is true then how do you know when to engage the feed for
the next pass?
Thanks for the help in advance,
Depends on the lathe. For a lathe with standard threading capabilities,
that has had metric added after the original design, the SOP is to never
release the split nut. Instead, you stop the spindle at the end of the
thread, reverse it to get back to the start point, then restart the
spindle in forward for the next pass.
Unless you have a metric lathe (with a metric leadscrew) you cannot use
the thread chasing dial
On an imperial machine (with an imperial leadscrew) when threadding a
metric pitch,diametrial pitch,module, or inch pitch, once you engage
the half-nut, you must leave it engaged. You will have to reverse the
lathe and pull your tool out at the end of the thread.
inch pitch is different from threads per inch don't get them confused.
your thread chasing dial is only good for TPI settings.
If you are threading a nut right through you shoud have an easy time,
if it's a blind hole, you are gonna have a hard time with it.
hope this helps
Correct on a non-metric lathe. If you have a metric lead-screw that's a
I went the el-cheapo route when I got my lathe and didn't spring for the
version with the half-nut. In my case (and yours) the procedure is to
_never_ disengage the half-nut. Instead you have to push the button to
stop the lathe at the end of the cut, roll it back by hand, re-do the
I'm thinking of making myself a selection of left-cutting threading
tools so I can make the cut on the backside of the work with the tool
moving to the right -- this will allow me to start the tool by hand
where it makes a difference, and let it run off the end where it doesn't.
My other thought is to (a) make a crank handle for my spindle or the
idler pulley and turn the spindle by hand while cutting threads or (b)
install a DC motor, put an encoder on the spindle, and use a controller
to stop the motor at the right spot each time. Choice (a) will take
less time to make, longer to use each time, and carries the risk of
forgetting to disengage the crank for that "special excitement" that we
all want to avoid. Choice (b) promises to be lots of work.
Tim, one note that you might want to remember on your lathe, it will
make you life easier.
Even though you do not have a thread chasing dial, you can still
dissengage on threds that are a multiple of the leadscrew.
For example if your lathe has 4 TPI, you can thread
4,8,12,16,20,24,28,32 etc and disengage because the lathe will follow
the same path every time you fully engage the half nut.
Your lather probably has 6 or 8 TPI leadscrew so you can figure out
multiples and save you self some time and grief.
go back and read my post Dave, i'm not talking about threading metric
on an imperial lathe, i'm giving Tim specific tips on his lathe for
thradding without a thread chasing dial cutting TPI's of multiple's of
the leadscrew. I explained how to thread metric on an imperial lathe
(among other pitchs) on my first post.
Choice B isn't that bad.
Mounting a DC motor is trivial, and you can build a DC powersupply for
pretty cheap if you go the Variac and Diode route. Sure this dosen't give
you constant speed, you will slow down a bit when you take a cut but it's
just too easy and cheap. At low speed a simple microswitch with a bit of
overtravel will stop you in the same place every time and if you want to get
fancy you can make it trip a relay that will automaticaly reverse the motor.
Another cool thing about a DC motor on your lathe, is that you can just
increase the speed as you crank in on the crossfeed while doing cutoff or
facing work :-)
This is the URL to an article I wrote for our Live Steam web site about
doing DC motors..
had several of those old tape spindle motors like the one I put on my
lathe, and just finished mounting it on my 1J Bridgeport... Works like a
charm.. I'n 1:1 pulley mode I go from dead stop to 3000RPM..
A friend just finished sticking a 1/4HP DC motor on his drill press, he's
happy as a clam and he's headed for a DC motor on his Bridgeport.
If you don't have a threaded on chuck (I do :( ) Threading and boring on
the 'backside' is the way to go.
Now wait, you can use the thread dial with odd pitch threads[3 1/2 for
instance], as long as you always return to the same number, it makes no
sense to me that metric would be any different.
Second, if you could not do that, could you not put witness marks on the
leadscrew, chuck, and set an indicator or stop on the back of the
carriage, and it would HAVe to be in the same spot, no?
THink about it this way. If you open the half nuts and move the carriage
until the dial rotates one full turn and then close the half nuts
you have moved the carriage some increment of the lead screw pitch.
Probably 1 inch in this case. IF you are threading a .8 MM pitch
will the toolbit still line up with the thread. No way.
Good response Chuck
The leadscrew is revolving at a predetermined rate (for the pitch to be
cut) . If the pitch of the leadscrew doesn't divide into the pitch
selected, it cannot be split using the half nut. and furthermore you
can actually use the half nut for splitting multiple start threads of
certain pitches example, 6TPI - 2 start , the machine is set to cut 3
TPI assume that the lathe has a 4TPI leadscrew, you would cut one
thread on any main division (1,2,3 or 4) and it will follow the same
path, to split the thread for two starts the second lead will have to
be cut on 1 1/2, 2 1/2, 3 1/2, or 4 1/2. there are still quarter
divisions, engaging on any of there will wipe out your thread in this
case, but in some cases the thread can be split up to 16 time
(depending on the TPI of course, but alot of threads can be split 4
times for a 4 start thread)
What I did when I wanted to cut a "inch" thread on a metric lathe. I
engaged the half nut
with the dial set to zero. At the end of the first pass I disengaged the
half nut, backed out
the cutter and stopped the lathe. I then reversed the lathe and when the
dial again read zero
I engaged the half nut and continued to run the lathe in revers until it
was ready for the next pass.
It was tricky but worked.
doesn't follow, many of my available pitches are not divisable by the
leadscrew pitch, my old monarch has a 4 tpi leadscrew. it has available
2 3/4 , 13, 14 tpi threads; not divisible. If ther is a reason you cna'
tuse the half nuts, this cannot be it.
I don't think some of you guys really know how the thread chasing dial
works , so here are the rules of the thread chasing dial.....
_thread (lead) to cut_ on an Inch leadscrew (eg 4TPI)
multiple of the lead screw (on a 4tpi machine cutting 4,8,12,16,20 etc)
you can engage the dial anywhere it fully engages
Even number of threads can be engaged on any division (1, 1-1/2, 2,
2-1/2, 3, 3-1/2, 4, 4-1/2) remember not the quarter divisions that are
Odd number of threads can be engaged on any main division (1, 2, 3, or
fractional half pitches like 1-1/2, 6-1/2, 10-1/2 etc, can only be
engaged on opposite divisions (1 & 3) or (2 & 4) or (1-1/2 & 3-1/2)
Fractional threads that are not half pitches like 2-3/4, 11-1/4 etc can
only be engaged on one division, once you pick a division, you must
stick with it.
Metric, Diametrial pitch, Module pitch, and Inch Pitch, once engaged
cannot be disengaged, you will have to reverse the lathe and backout
the tool *do not disengage*
some exceptions to these rules are thread chasing dials with divisions
that are not 24 divisions (some TOS lathes have 16 divisions)
It's not rocket science, and this does work.