How to make Left handed threads on a lathe

[BobinWa]

Ok guys help me out here, I need to make a left handed metric thread bolt 1.00-10. My Grizzly lathe has the settings to make the threads but I do not understand the proceedure to make left handed? Your assistance is most appreciated.


BobinWA

 

[bridgeportbj]

Ok guys help me out here, I need to make a left handed metric thread bolt 1.00-10. My Grizzly lathe has the settings to make the threads but I do not understand the proceedure to make left handed? Your assistance is most appreciated.


BobinWA

Bob
Cut in reverse left to right.
BJ

 

[Tony Wells]

The process is basically the same. There are a couple of ways to do it. Most popular is probably using a thread relief and starting at the relief and spindle normal rotation, reverse the direction of the leadscrew. Of course, this requires a bit of nerve and confidence, because if the relief is fairly narrow like it usually is, you don't have much of a chance to change your mind if you miss the mark on the threading dial. But if you start out slowly, you'll get the hang of it.

On faster pitches than yours, (I suspect you meant m10-1.0) on some inserted tools and also hand ground bits, the helix angle requires a change in clearances. Obviously, the thread is running at the same angle as a RH thread, but the lead "leans" to the other side and the tool must not be allowed to drag on the flank of the thread. I don't think you'll have a problem with standard tooling with a 1.0 pitch.

 

[toolman49]

G'Day Bob,
I would also set the compound to 29 1/2 degrees facing the tailstock (opposite of RH threading).
Regards,
Martin

 

[Ray C]

You're in luck. Left hand metric is easier than right hand.

Don't forget that with metric threads, you never disengage the halfnut until the threads are completely cut.

For left hand metric, you will cut from left to right.

0) Set the speed to something you're comfortable with (slow).
1) (the lathe is not on). Pick your starting point (usually a recessed shoulder) and dial in a small amount of engagement.
2) Turn on the lathe and make the cut but, at the end of the cut, do not disengage the halfnut; instead, kill the motor.
3) Dial-out the bit to clear the piece. Keep track of your depth.
4) Run lathe in reverse to backup the bit to the starting point.
5) Dial-in more depth and repeat from step #2.

Only when you're done can you disengage the halfnut. If you have a thread chaser, it makes no difference what number you engage the halfnut. Anywhere will do fine.

If you had a metric leadscrew, you could avoid all that hassle and use the halfnut lever like normal. I'm half tempted to get a metric leadscrew as I cut metric every month or so.

Ray

 

[valleyboy101]

Hi Ray,

I cut a 10mm x 1.0 RH metric thread tonight. I engaged the half nut at the same point on the dial on each pass and had no problem. Was it just BS luck? I don't have a metric lead screw and never thought that I should leave the nut engaged.
I'd appreciate your advise.
Thanks,
michael

 

[Ray C]

I haven't put much mind to it but suspect it depends on the pitch of the leadscrew. Cutting metric threads is something I've only done in the last year or so. When I first used a lathe in the 70's, metric wasn't a consideration. Years later when I got an Atlas, the apron gears were so dicey, I avoided threading at all cost. On my PM, this is how it needs to be done -and perhaps I wongly advised thinking this was the general case for machines with SAE leadscrews. I'll think it over and try to find-out one way or another.

Ray

Hi Ray,

I cut a 10mm x 1.0 RH metric thread tonight. I engaged the half nut at the same point on the dial on each pass and had no problem. Was it just BS luck? I don't have a metric lead screw and never thought that I should leave the nut engaged.
I'd appreciate your advise.
Thanks,
michael

 

[Ray C]

OK, as promised, I thought about this and drew some conclusions and to confirm my feeble thinking, did some net-searching.

Yep, in the general case, you need to keep the half-nuts engaged the whole time. There are some exceptions based on tricks you can play depending on the leadscrew pitch and the length of where you decide to disengage/re-engage the leadscrew. For example, with certain teeth gears and an 8TPI screw, every 5" happens to be an exact length where you get away using the standard technique. Generally speaking, since the "restrictions" of doing it that way suck just as much as keeping the halfnuts engaged, I'll stick with the way I know.

-Sure would be nice to have a metric leadscrew but, swapping the halfnuts would not be easy on my machine.


Ray

 

[8ntsane]

This chart may help you as well. Might help you from the fear of a crash into the chuck by threading away when possible.

 

[DMS]

I think cutting from the back side like the setup 8ntsane show's in his diagram (top, 3rd from the left) is easiest. It is most like cutting a right hand thread, except your spindle direction is reversed (and the feed direction of course). It has the advantage of being able to engage the half nut before engaging the piece. It may be tricky depending on your cross slide travel.