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

..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

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

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

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

My thoughts are that if the engagement mechanism is not backing off when under load, then the leadscrew is flexing downwards because of the shape of the worn nut threads. The Smart & Brown model A also uses a part section nut, but any flex in the leadscrew is stopped by a bronze slipper opposite the nut. I wonder if you could fit something on your Zyto?