Stripped thread / Pultra P type

[Peter Mann 1Participant @petermann1]

Hi. I have a early Pultra P type lathe which I use for clock making. The brass nut on the cross slide (left to right) unit is stripped (don't ask) Question: where on earth can I get another one. A friend of mine thinks is may be a ACME type. See pictures.

[Peter Mann 1Participant @petermann1]

Pultra P type stripped thread

[John ReeseParticipant @johnreese12848]

It should not be a difficult job to make a replacement. If in doubt about the thread form, use the leadscrew as a template for grinding your threading tool. The flat at the end of the tool should be a little narrower than the flat at the root of the screw thread. That allows a little clearance at the root of the thread,

Several years ago on my first day in the machine shop the compound leadscrew stripped out of the nut. I had to find a piece of bronze big enough to make the nut then machine a new nut.

[Neil WyattModerator @neilwyatt]

Can you post a sharp close-up of the thread?

What pitch is it?

Is it just the anti-backlash section that's stripped?

Neil

[not done it yetParticipant @notdoneityet]

Bore it, bush it, and re-thread? Silver solder or loctite the bush. Your choice, but I loctited and also pinned the plain feed screw nut I am currently repairing. As it is not a ‘half-nut clamping system’ you could change the thread form to a new screw thread if you wished, so no reason to give up on that worn out nut – ie you could make it slightly less in diameter, if too tight on bush diameter and make a new threaded part for the screw.

Mine has been left in the 4 jaw, awaiting threading, due to no time in the workshop, but will have a square thread cut in it to fit the feed screw.

Someone on the forum said a square thread could not be adjusted for wear (backlash), but comprehensively failed to offer an explanation as to how any other thread form, on a simple feed nut, can be adjusted as it wears. Raglan used square feed screw threads on most of their lathes and mills. I have yet to come across any other thread form on a Raglan lathe or mill feed screw. Only the lead screw half nuts are ACME form.

Lastly, even anti-backlash nuts are no good if the screw is worn excessively in the most-used section, so do check the wear on the screw before making any changes….

[duncan webster 1Participant @duncanwebster1]

I fitted the feedscrew off what was reputed to be a Pultra cross slide to my little lathe. It is M6, not trapeziodal. Try comparing yours with a quality M6 bolt using a loupe

[Michael GilliganParticipant @michaelgilligan61133]

This may possibly be useful

’though it doesn’t specify thread-form

… as Neil said: a decent close-up of your screw-thread would be vey helpful, Peter

MichaelG.

[Howard LewisParticipant @howardlewis46836]

If you can identify the thread, and want to make a replacement nut, or a solder in sleeve, Tracy Tools may have a suitable tap.

Howard

[old martParticipant @oldmart]

Unfortunately, the wall thickness of your nuts is too thin for simply replacing the thread by bushing, I think the complete nut assembly will have to be replaced. The pictures do not enable a guess at the actual size of the thread. What is the thread pitch, there seems to be a choice of 1mm, 40 tpi or 20 tpi.

[vintage engineerParticipant @vintageengineer]

If it doesn't take a high load. You could clean the brass nut and tin it, the drill a hole in one side, fit the cross shaft and pour white metal into the hole.

This was a quite common method for making steering box female worms.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Peter Mann 1 on 22/10/2019 15:11:29:

[…] The brass nut on the cross slide (left to right) unit is stripped (don't ask) […]

.

Sorry Peter, but I do feel the need to ask :

Please tell us how you [or some previous user] managed to strip one half of an ajustable-backlash nut.

… I’m bewildered.

MichaelG.

[not done it yetParticipant @notdoneityet]

Posted by Michael Gilligan on 25/10/2019 11:31:43:.

Sorry Peter, but I do feel the need to ask :

Please tell us how you [or some previous user] managed to strip one half of an ajustable-backlash nut.

… I’m bewildered.

MichaelG.

Presumably by reducing the backlash to less than zero? Or backlash was set OK in the most worn section of the feed screw, but not at the ends of the screw…

It has likely worn the more narrow part over time, so was the likely failure mode eventually?

[not done it yetParticipant @notdoneityet]

Posted by Michael Gilligan on 25/10/2019 11:31:43:.

Sorry Peter, but I do feel the need to ask :

Please tell us how you [or some previous user] managed to strip one half of an ajustable-backlash nut.

… I’m bewildered.

MichaelG.

Presumably by reducing the backlash to less than zero? Or backlash was set OK in the most worn section of the feed screw, but not at the ends of the screw…

It has likely worn the more narrow part over time, so was the likely failure mode eventually?

[SillyOldDufferModerator @sillyoldduffer]

I adjusted the backlash on my WM280 last week and the cheap and cheerful version of the Pultra part on a Chinese lathe may be revealing. It looks like this:

It's a brass block drilled and threaded to take the lead-screw. At the rear end a slot is cut through most of the block and two small screws provided to jack the slot apart at the bottom. Once backlash is bad enough to annoy the two screws are tightened. By bending the slotted end of the block the short section of thread at the back is pushed firmly into contact with the lead-screw. Bit crude, but it takes any slack out.

After adjustment the front thread takes most of the normal working load, but the rear thread is small in comparison and it works hard when backlash is eliminated.

It took four years for backlash to increase enough on my lathe for me to fix it, but then I'm lazy! And backlash doesn't worry me much because it's easy to compensate for on the work I do by overshooting and returning.

The disadvantage of this simple mechanism is a lot of extra wear is caused if the user likes his lathe tight and constantly keeps adjusting backlash out. Even worse if the adjustment is overdone by keeping everything very tight. Most of the wear due to tight adjustment in this design will fall on the short section of thread. I can imagine a few years of enthusiastic backlash removal taking the short section of thread out.

Although better made the Pulta looks to have the same general arrangement. My money is on the cause being regular use by someone who cares enough about backlash to tweak it out frequently. And why not? The Chinese block is easily replaced and the Pultra version isn't much more complicated.

Anti-backlash gearing on radio dials is much more sophisticated. How do posh lathes eliminate backlash? Presumably not by bending brass blocks!

Dave

[Peter Mann 1Participant @petermann1]

Thanks everyone for all your replies. As I am a clock restorer rather than a clock maker, I am not used to the in's and out's of a lathe and so I went and saw a friend who does. This is the outcome. First in answer to some questions. The feed screw (is that the right term?) is 1/4" 40tpi and is badly worn. The two bronze bits are also badly worn. I was given the lathe as payment for restoring a 19century bracket clock and the feed screw always was a bit iffy. I did not strip the threads..someone else did. Anyway. My friend is going to make up a brand new feed screw and two new tapped bronze bits. thanks everyone. Peter

[Michael GilliganParticipant @michaelgilligan61133]

An excellent outcome, Peter

MichaelG.

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