Threading trouble

Hi All,

So - this might emphasize why I need a new lathe....but lets see....

I rarely have a need for thread cutting as dies typically work for the sizes I need, however I recently made myself a 80/63 compound gear to convert the lathe from imperial to metric threads so thought I would have a go. The pitch seemed to work very well when I tried a 1.5mm setup.

The process I did is as follows - offset compound slide to 30deg (I was doing a standard ISO thread), turned down stock held in a collet to required size and created relief groove, took a scratch cut with half nuts engaged at no.1 on thread dial, disengaged half nuts and wound back cross a little, moved carriage back, would in the cross slide to the same spot and would in the compound a little and then re-engaged the half nuts at the same thread dial number..... result = new thread cut!!!

The only way I can get the thread to cut the same point each time is by never dis-engaging the half-nuts. This makes the thread dial pointless! Any ideas why i have this problem? I can only assume its a work half-nut/leadscrew combination? Or am I missing something?

Thanks

Matt,

The likelyhood of having the half nuts on an imperial leadscrew lathe re-engage at exactly the same position on your thread when using metric conversion gears is close to zero.

Keep using the "always engaged half-nuts/reversing lathe " method.

Edited By mahgnia on 02/10/2020 03:04:59

That would explain it then!

Only trouble is, i don't have a reverse so have to manually wind back the spindle which is a pain in the arse!

There is a way of doing it but its too complicated to describe from memory. It's in Martin Cleeves book Screwcutting in the Lathe. Otherwise on a Myford, disengage the back gear and wind lathe backwards by hand. Use the leadscrew handwheel once it's in motion.

Hi Matt,

You don't need to keep the half nut engaged when cutting a metric thread on an imperial lathe if you have a thread dial fitted. The procedure to follow is:

After setting the tool to cut the correct depth, start the lathe in the forward direction and when the thread dial reads 1, engage the thread nut.

When the tool has advanced to the end of the thread, disengage the half nut and turn the motor off.

Withdraw the tool to clear the work.

Start the lathe in reverse and when the thread dial reads 1 engage the half nut.

Run the lathe in reverse until the tool clears the work.

Turn the lathe off but leave the half nut engaged.

Set the tool to the new depth and start the lathe in the forward direction.

Repeat till the thread is cut to size.

This method is useful because disengaging the half nut stops the tool a lot quicker than waiting for the lathe to run down giving a more accurate stopping point.

This won't fix your problem though if you can't reverse your motor. It may be cheaper to get someone who knows what they are doing to rewire your existing motor through a reversing switch than getting a new lathe.

Regards,

Alan

turned down stock held in a collet to required size

This suggests the workpiece is not particularly large in diameter. Dies are available for all standard metric threads at reasonable cost.

There is no alternative other than driving the lathe in reverse with the half nuts engaged, as far as I know.

There are umpteen threads on this very subject if a forum search is undertaken.

There are loads of youtube examples of threading in this manner.

Nothing different than for those with metric lathes making imperial threads.

Sorry, but no alternative to cut your thread so you will have to get used to it. Making a handle to drive the spindle would help your plight.

There is a way, for certain threads whose Synchronous Distance is equal to less than two revolutions of the threading dial and only using proper 127 tooth gearing, not any of the other approximations such as 63T etc. But it takes Cleeve several pages to describe it in his book and I'm still not sure I get it. So not too practical for us scrubbers. He operated on a higher screw-cutting plane than the rest of us.

But another more practical alternative is to use a hand crank on the lathe spindle so you can cut by hand and reverse by hand. (EDIT : As I just noticed NDIY already suggested.)  Probably no slower than using back gear.

I have found you can disengage back gear and wind the lathe chuck backwards by hand without disengaging the half nuts but it's a PITA. Cranking the leadscrew handwheel does help once things are rolling. I reckon a hand crank in the end o fthe headstock spindle would be much better. But I now have a reversing switch so havent pursued. it.

Edited By Hopper on 02/10/2020 09:02:44

Hi Alan, Guys,

The modern equivalent to the old chalk mark !

For what its worth, I just plunge straight in, but I do have reverse and try to cut away from the chuck with the tool at the back for external threads.

A winding handle is a useful thing to have. I used an old washing machine pulley. You can see the handle at the bottom of the wheel.

The brass collar is to stop you pushing the mandrel in too far and the wheel hitting the gear cover.

The expander is a simple tapered plug.

John, like the idea for recycling an old washing machine pulley.

Brian

Seems safer than a crank handle if you forget and leave it in place while starting the lathe up too.

Metric threads are specified in pitch rather than turns per, and it does make threading awkward compared with imperial. The simple answer is to leave the half-nuts engaged.

But my metric WM280 has a thread-dial that works on many pitches, and my metric mini-lathe came with 3 different cogs that could be fitted to the turns dial to increase the range of pitches indicated. Rarely used because my mini-lathe was a bit too fast for comfy power threading, so I normally hand-cranked it as described above, and simply wound the cutter in and out rather than watch the dial.

My WM280 is slow enough for powered threading, and it has a metric thread-dial, but I rarely use it. I prefer to keep the lead-screw engaged and cut threads in reverse. The advantage of reverse threading is there's no risk of smashing into the chuck. I save time by powering the cutter back to the beginning, but it's easy to stop short of the shoulder and finish positioning by twirling the chuck by hand. Helps to own a lathe equally happy in forward and reverse - controls all work in either direction, same torque from the motor, and the chuck can't unscrew!

Guess what? Metric thread dials don't work on the Imperial threads metric machines happily produce. So I cut imperial threads in the same way as metric, half-nuts engaged all the time.

Dave

Edited By SillyOldDuffer on 02/10/2020 11:56:35

All good if you have a reversing switch but the OP doesn't -- that's the problem.

Matt, are you talking about threading on your South Bend lathe? If so, you can reverse the leadscrew using the tumbler reverse, unless it is missing. True, you do have to loosen a lock screw behind the gear cover on the headstock, and move the tumbler gear lever, but not such a big deal once in a while. I leave halfnuts engaged when doing non-die single point threading on my SB lathe just FYI.

I keep reading that many lathes are too fast for threading. I don't do a lot of threading, but when I do it tends to be relatively large - M12 and under I do with taps/dies.

This

is the clapper bolt for a bell and is threaded M20 at each end. As the short end has about 25mm of thread, winding it by hand was NOT an option. I started slowly at about 75rpm and kept increasing until I still felt comfortable which was over 150rpm. Part was supported by the live centre as it was just too big to fit through the headstock. The smaller threads I have cut were done at higher speeds which not only got the job done quicker, but produced a better finish.

Edited By Nicholas Wheeler 1 on 02/10/2020 12:25:47

Nicholas, you must have some quick reactions!

Tony

Thankyou everyone....as always, i learnt something and this is the power of this forum/community!

Nice idea with the washer pulley like that one......

I can reverse the leadscrew by shifting the tumbler as suggested by Jeff, i guess i have to be careful about not moving any gear positions whilst doing so however.

I also like the idea to disengage the halfnuts in the screw relief to prevent chuck crash and then wind back the spindle without moving the carriage until the thread gauge aligns again. i will give that a try!

Thanks for all the suggestions, i will practice some more over the weekend!

This has cropped up before, and I seem to remember there is something odd about the tumbler gear operation on the Southbend.

But I'll ask the question anyway - if you move the tumbler gear to reverse the leadscrew does that not lose the registration between the leadscrew and the spindle? 'Cos if so you just screwed (pun intended) up your thread!

Hi Simon, yes, you do have to be very careful that no gears in the leadscrew geartrain shift when you rotate the tumbler. At one point in the tumbler motion the gears disengage and if re-mesh is not perfect the gears can shift 1/8 or so of a tooth width. Just takes some practice so they don't shift relative to each other. Some people have built fancy clamping jigs to hold the gears while the tumbler is moved, but I'm not sure these can be depended on to not slip unless clamped hard, which is risky to the gear teeth.

On some large thread cutting I have gotten away with just leaving everything in mesh and reversing the motor while in back gear. Very small risk of chuck unthreading while in slow back gear and with tool pulled back with cross slide a known amount (so you can set depth of next cut).

A Mandrel Handle is a valuable aid when screwcutting. It allows the mandrel to be rotated at a low speed, and should remove any fears when cutting up to a shoulder.

Also, as in this case, the direction of rotation can be reversed without problems.

It may not be as fast as under power, but avoids damaged tooling and workpieces.

The extra time spent buys a lot of piece of mind!

This assumes that we are not working on piecework or daywork rates!

Howard

Jeff -

Your last paragraph: "... tool pulled back a known amount..."

With respect, don't you mean, "having noted the feed in" on the cut just finished? So always relying on the feed setting not the wind-out point.

Otherwise you can end up with problems from the backlash.

Once the tool is clear of the work, if the lathe is in back gear on low speed and started from rest, the chuck's inertia is highly unlikely to be enough for it to loosen.

When using this technique I have the machine running so slowly I can run the tool back in near to the work before it passes the end of the bar. This means I don't become confused by how many complete turns I need give it, especially as my lathes have a lot of back-lash in the cross-slides. (How do I know? Guess...) My lathes do have thread-dials though, and inverters allowing very low spindle speeds without under-running the motors.