Half nut skipping still

Vintage lathes 😢

I have a problem with my Zyto lathe in that the half nut jumps, maybe 1 in 5 times I engage it. I thought it was due to the little rods in the cam being bent, but I’ve fixed that (see wonky threads thread) and it is still doing it, it’s a problem it has only developed recently.

I have tried all sorts of things, adjusting the gibs, tightening the spring that holds the lever, really everything.

sometimes it engages very smoothly, other times there is quite a “click” and it skips. I know it is skipping because I’ve tried turning threads and it misses the thread track.

I know the upper half nut has a partially knackered thread, but it has been like that since I got the lathe and it has worked previously.

If anyone has any thoughts they’d be much appreciated.

There comes a time when the half-nuts need attention/renewing. But maybe there is something stopping or holding full engagement - an over-centre device, perhaps?

Its either too worn or it needs a good cleanout after being used for a while

small particles can become compacted in the thread

This is assuming it's 100% engaged when you throw the lever, the Drummond M has a smooth upper nut and a proper lower nut, so only the lower half engages but the proper support of the upper half is very important because of the leadscrew flexing when under load

If they dont pinch-hold the worm securely the half nut thread pops out

Edited By Ady1 on 25/11/2021 23:18:36

In your picture, the pegs seem to be a rather loose fit in the cam slots. Could this clearance allow the half-nuts to jump out of engagment?

What is an over-centre device?

I know the half nut needs sorting out, but I think I’ve done something else to it which is causing this specific problem. I’d love to get new half nuts but being 80 years old, replacements are a bit thin on the ground. The bottom and top half nuts are symmetrical in every way, and I’ve toyed with the idea of casting some new ones from the bottom one, but that’s a whole new project in itself, and something I know even less about than machining.I watched an interesting YouTube video of a chap in the US replacing lead screw threads on a Logan lathe, which was interesting but probably beyond my capabilities, particularly if my threading mechanism is out of action,

I am pretty sure that the lower half nut is engaging slightly before the upper one. It may just be a question of lubrication and getting the gibs tightened correctly. Or it may be something to do with the lead screw - I’ve probably disturbed that recently when messing with the change gears.

😟

That’s interesting, I thought the same, but the threads they screw into are 3/16, they came like that, and it used to work, so I think that’s probably not the cause.

I cleaned it all out thoroughly when I took it to bits.

but I think you are right about the misaligned engagement,

Do you have any photos showing the half nut mechanism dismantled? I presume each half nut runs in a dovetail slide; if so, do those two grub screws allow any adjustment? Maybe slacken one half nut off to make engagement "loose" so the two half nuts don't try to compete together when you engage the lever. If the half nuts pivot rather than slide, maybe you could make an adjustable cam; that's the solution I tried when fixing a boxford.

..a couple of observations, don't know whether of any value or not..

You suspect the bottom one engages first - many lathes of a similar pattern use shims between the apron and the saddle to ensure that the halfnuts are centrally-disposed to the leadscrew and can close on it together..

..if you loosen-off the apron (beware of shims already in place) and then close the half-nuts onto the screw, does a gap appear between the saddle and top of apron?

..before you do this, it might be worth checking the fit of the damaged half-nut with the leadscrew, just to make sure that there are no burrs causing it to 'ride' on the threads.. ..if it's been closed with enough force to bend the cam pins in some previous life, anything is possible, sometimes a little restoration of the thread form is necessary

That’s a really interesting idea. I will go and try it now. Ive had the apron off many times, and there were no shims, but I’ve always tightened it fully, if slightly loose, it may behave differently.

I’d be very interested in ideas to restore the thread that don’t involve needing a working half nut,

That’s a really interesting idea. I will go and try it now. Ive had the apron off many times, and there were no shims, but I’ve always tightened it fully, if slightly loose, it may behave differently.

I’d be very interested in ideas to restore the thread that don’t involve needing a working half nut,

Another test is to observe the leadscrew as the half nuts are fully closed: does it move up or down? If so, adjustment as suggested by DiogenesII is indicated.

Looking at the picture of the lever, it appears to be in the disengaged position.

Where are the pins in the engaged position?

They should not be bearing against the end of the slot.

Look for any burrs or bent pins, if you have not already replaced them.

If you did, (Judged by your thread on "Wonky Threads" ) are the threads concentric with the body of the studs?

Is there anything limiting the travel of the lever, or the studs driving the half nuts to prevent full engagement of both halves?

HTH

Howard

It was moving up. So I have shimmed the apron with feeler gauges, and it seems (touch wood, pending further testing) to have cured the engagement problem. I’ve engaged it now maybe 30 times and hit the number on the thread dial indicator every time, no jumping or skipping.

it it still stiff on disengagement, which is probably the same problem in reverse - one nut disengaging but the other not. I will look into that further.

Thanks for the tip.

Even though I seem to have made progress, there is still a problem with this lever in that the engagement position is too low (about 9 o’clock). Also, it has a system whereby a spring is I think designed to cause it to lock in the on or off positions. I will post some pics later. By the way, yes, I have fixed the pins, they are straight now 😎

Stupid question, but for these styles of lathes, is “down” usually engaged or disengaged for the half nut?

thanks

Steve

Maybe the bent pins are evidence of a previous owner having this problem and leaning on the lever to prevent skipping.

Ideally, both half nuts will move symmetrically to engage simultaneously.

Hopefully adjustment, somewhere, will solve the problem.

Howard

By the looks of the picture, rotating the lever anti clockwise (down ) would lower the top nut into engagement, whilst disengaging the lower one, which seems illogical.

One thought.

The pins seem to have lots of clearance in the slots.

Is it possible that the pins should be 1/4" diameter, with a 3/16 thread on the end to increase the linear moment of the pins for given angular movement of the lever?

Howard

I would expect a spring to open the nuts when they were expected to disengage, you don't want then to stay engaged when disengagement is selected. The slots in the cam disk (for want of a better description) look far too big for the diameter of the pins, looks like poor design as it is so a spring pushing the pins against one surface (in this case the outer one) makes sense. As the slots do not look like they are the same width along their lengths this would increase the movement between open and closed and so adjustment of the disengaged position will change the angle of the lever upwards when engaged. The change in the curve of the outside surface of the slots also looks like an over-centre design where once the half nuts are engaged the movement of the pins is reduced or even reverses slightly.

I would get some springs, adjust for full engagement at the point where the curve changes or just beyond it and see if there is full disengagement with the lever down.

Martin C

A simple 1:1 lever to operate one of the halves would cure my complaint of a lack of logic.

It could even be that the operating lever was replacement made by a previous owner, but not very well. Note the chatter marks on the chamfer.

Certainly, having the pins a close fit in the slots will give increased travel to the half nuts, which may help. rather than trying to make a lever with narrower slots, larger pins, turned down for the thread may be the easier path to a cure...

Removal of the Apron and a close examination of the half nuts and the operating mechanism would seem to be an the next item on the list of operations in the investigation.

Careful reassembly, together with some checking of components ought to arrive, eventually, at a solution.

Keep us posted on your findings!

Howard