Worm wanted for lathe

I have a long-overdue job that needs to be done to restore this lathe. The feedshaft worm is badly worn and spares appear unavailable. I've been on the Denford Data website where there appears to be a number of knowledgeable and helpful guys and I'm hopeful that I can get the original drawings for the worm. With or without the drawings I would like to get a new worm sourced or made and I'm wondering if anyone can make a suitable recommendation. I'd expect to pay a fair rate but we have to be realistic as this is not a top-end lathe.

Incidentally, by visual inspection it is my presumption the worn worm is the cause of partial engagement (and unwanted disengagement). The worm wheel looks to be in much better condition but it remains in the gearbox and taking a photo is tricky at present. However, I would have expected the worm wheel to wear at a faster rate than the worm but maybe I've got that wrong.

I'm based in Dorset.

Thanks for reading,

Brian

I would suggest identifying the worm's and wheel's vital statistics and seeing what replacement stock items may be available and suitable for modifying to fit. You might need go nearest-metric and it could entail making or modifying other parts, but a bit more easily and potentially a lot more cheaply (say around £80 - £100 for a stock worm and wheel?) than having one-off specials made commercially.

Don't replace the one without the other.

This is similar to a problem I have with a Denbigh horizontal mill whose long-feed worm wheel is there (part of the lead-screw nut) but its worm and drive are long-lost. Another contributor here suggested I adopt that solution, turning the original wheel down to create a seating for a suitably bored-out new wheel, bought together with its appropriate worm.

One advantage is that you know the worm and wheel are matched, and anyway, as I think he pointed out to me, there is no use in trying to match a new worm to a wheel that will also be worn, and may be of different pressure-angle to boot.

The steel wearing much more severely than the bronze is not unusual, and some combinations of bronze and steel are worse than others for decades of living together in perfect harmony. The lathe's makers would have used the best but assumed a finite life. I don't know why but I suspect the softer material eventually becomes a de facto lap.

One point to watch - it applies to my mill but I can't quite be sure from your photos - is the hand of the worm. Most stock worms and wheels are right-hand thread. So if your lathe's parts are LH, the transmission stockists might have the right diameters but need to make or order the LH versions specially. If so I'd naturally expect a delay while a few more orders for south-paw worms and wheels come in.

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Worms with paws, North or South...? I'll keep my eyes open when I'm next digging the garden.

A long shot, but is there any way of increasing the engagement? That might allow you to reduce the diameter of the worm by 1/8".

The drawing you're looking for is ML110_309 it's the drawing for the worm gear. It's a 18DP 14.5PA LH worm gear of 33 teeth left hand helix and 1.8334" pitch diameter.

Going by the specs on that drawing your worm should be I think 1.098" diameter 0.174" lead single start LH worm.

Edited By Pete Rimmer on 07/01/2021 22:26:36

Good advice and pretty much what I would have guessed but I can see this ending up as more expensive than the lathe itself. Maybe I might strike lucky and see a whole apron assembly for sale somewhere. Or perhaps one day I may have the skills to make these parts.

As an aside, I'm not totally convinced it is necessary to get the feed shaft working (which requires a good worm and worm wheel). A working feed shaft can power the cross feed (nice feature, but not absolutely necessary) but the separate lead screw and half nuts already work All I need is some more gears or an electronic drive to make it more usable.

Thank you Pete.

Thanks for your suggestion but I don't think so.

If this is your only lathe you could start by turning the worm gear over. That would put 90% of the load on the un-worn side. That would get your auto-feeds working at least towards the chuck but probably both directions since you'll have the worn part of the worm acting on the unworn part of the wheel, and vice-versa.

Then you're going to have to figure out the gearing to get a lead of 0.174" or 4.4196mm to make a new worm using a threading tool ground to 14.5 degrees which is ACME thread angle.

Reversing the worm... Great idea! It's definitely worth a try.

Pete,

Please excuse my questions.. I'm a novice.

You mentioned 1.098" worm diameter. I assume you got that from (0.429"R + 0.120" tooth depth) * 2 which equals 1.098" or 27.89mm

The least worn diameter of the worm measures slightly less at 27.63mm. Looking at the working drawing should we also take the specified clearance into account when calculating the worm diameter?

Yes you're right, lacking a drawing I used the figures you quote not giving any allowance for the clearance. It was more so you could compare what I think you should have with what you actually have. There would need to be a clearance allowance.

Thanks for confirming that. I took some measurements in case anyone is interested.

Looks like both original parts are cast iron. I see no reason why you couldn't make them from mild steel though.

For a permanent fix you could do a number of things:

Make a new worm and a new wheel to the original sizes.

Make a slightly larger worm and re-cut the wheel with a 1-off form cutter. Depending on the condition of the wheel it might only need cutting 10 or 20 thou deeper then you can increase the PCD of the worm the same amount.The wormwheel cutter would have to be the same OD as the worm itself and you can make it from a hardenable steel, using the 2-button method to get close to the correct form. Then you'd have to set the cutter at the helix angle, index the worm for 33 teeth and plunge-cut each tooth in turn. It's not as difficult as it sounds just a bit of work.

It really doesn't look that worn to me, it looks more like the cog isn't engaging enough of the worm

Can you get any more travel out of the engage/disengage part

Don't know if this thread might help

Edited By Ady1 on 07/01/2021 23:44:47

Yes it does look as if it's only achieving half engagement. I wonder if the stops on the rotary lever can be adjusted? Might interfere with the operation of the threading interlock, if it has one, or knowing Denford they might have used the interlock AS the stop.

Ady and Pete,

I see your point. Actually when I first tried the rotary lever I had the very feeling it wasn't engaging properly. Maybe that's why the worm became worn and not the other way round. I will take another look over the weekend. Let's hope for some warmer weather as my garage is FREEZING!

Brian

Is that cog a replacement cog? Could its teeth be too big and prevent full engagement?

The issue of unwanted disengagement in the original post makes me wonder if there is some sort of over-centering mechanism (toggle) that is not going over centre to lock it. Either there is a mechanical stop that is damaged, dirty or needs adjustment or the operator may not be putting in the necessary input to go past the toggle point.

Martin C

Martin,

I think that you and the others have raised valid points that need to be considered. It has been difficult to diagnose this problem with the apron assembled and in place as the gears are not not visible. e.g. Is it worn gears, poor meshing, or bad locking mechanism?

When the apron is removed, the feedshaft is no longer in place to hold the gears and see what is happening. What I have decided to do is to turn a short length of "dummy feedshaft" (doesn't need to be particularly accurate) to hold the gears in position when the apron is removed from the lathe. I can then turn it over by hand with the gears in full view. I should then be in a much better position to know what is happening.

This is what you call a worn traverse worm wheel:-

The replacement for my Raglan MkII shown in place - made in bronze and now working well. Previously I suffered difficulty in engaging and dis-engaging the traverse feeds. Took about 1/2 hr to 'bed in' after which I stripped it all down again, washed it out, re-oiled, and rebuilt.

The part was easy enough to make - Acme 29 deg form tool for the 'thread' - the gear cutting was 'tight' due to the proximity of the gear to the worm, but I see yours is separate, so easier to do.

Martin