Pultra model P

[Phil PParticipant @philp]

Just seen this thread on Pultra leadscrew nuts, there is a photo showing the split version.
It looks to be on the later 1750 style though, so maybe I got mixed up with which Pultra they are fitted to.
I would think it a very easy mod to carry out on a model P though.

https://www.mig-welding.co.uk/forum/threads/pultra-crosslide-leadscrew-nut.124409/

 

Phil P

[Michael Gilligan]

On Michael Gilligan Said:

The fact is that the <strong style=”font-family: Verdana, Arial, Helvetica, sans-serif; font-size: 14px;”>shape of the Whitworth thread lends itself rather well to running sweetly in a lead-screw arrangement.

Just to elaborate a little [because I consider the matter important, and this is a good place to post it] … The 55° angle and rounded crests are perfectly adequate for modest loads.

The use of square, Acme, and buttress threads was motivated by other factors … all before the days of Finite Element Analysis, but would make a worthy subject for that.

In ‘instrument’ work [microscopes, Pultras, and such] the lead-screw was made as near perfect as possible and then a slightly tight-fitting nut would be lapped to fit.

The shape of the Whitworth thread means that this can produce a sweetly-running pair that is almost sealed against the ingress of dust … because the lubricant film is only as thick as the lapping compound was !

Time, use, and abuse, ride rough-shod over such niceties of course.

 

[ steps down from soap-box ]

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

With apologies for the filthy state of it … this is a short section of lead-screw on a Pultra vertical slide that I have for restoration.

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It has a negligible level of wear, and I don’t think there can be much doubt that this is a Whitworth form thread.

MichaelG.

[mark smith 20Participant @marksmith20]

Talking of split anti backlash nuts in later 1750/ 1780 pultras.  i just relised that this must work in a similar way as if you look at the photo the central mounting boss is also threaded for the leadscrew ,so by adjusting the bronze nut in or out must take up any slack. I didnt realise both were threaded. Is the central mounting boss threaded on later pultras? There wouldnt be any need if the nut was split.

[mark smith 20Participant @marksmith20]

Ive just fiddled about on the compund slide  with the setting of the bronze nut so that any slack was gone  which strangely just involved loosening off the grub screws on the nut and tightening the two machine screws down. ( i would have thought i would have had to tighten in the grub screws so bronze nut and the thread in the central boss were pulling against each other ,very odd). I then reassembled it ,there was bad end float ,as the round nut with two holes in it (inside the dial assembly ) that requires a special tool to tighten was not tightened sufficiently .

So now even though i know the screw and nut are quite badly worn there is now no play at all. There is no back lash when rotating the dials and no  back and forth movement in the dovetail slide . very pleased.

[Phil P]

On Phil P Said:

Just seen this thread on Pultra leadscrew nuts, there is a photo showing the split version.
It looks to be on the later 1750 style though, so maybe I got mixed up with which Pultra they are fitted to.
I would think it a very easy mod to carry out on a model P though.

https://www.mig-welding.co.uk/forum/threads/pultra-crosslide-leadscrew-nut.124409/

 

Phil P

That is my original post and it was off a Pultra 17/70 however the crosslides are the same more or less across P type, 17/50 and 17/70 , some had split nuts some had single leadscrew nuts , metric or imperial.

[DellParticipant @dell]

[mark smith 20Participant @marksmith20]

Thanks for the info, but doesnt the threaded boss or spigot ( same as spigot C on your pages above ) on the model P compound slide act the same as a split nut only a couple of inches apart?

Is the spigot C on your pages threaded for the lead screw or does the leadscrew just freely pass through it?

[DellParticipant @dell]

C is totally different that is for the 15/90 a much larger lathe, fig 11 is adjustment to reduce leadscrew backlash, fig 12 is to reduce leadscrew endplay.

[mark smith 20Participant @marksmith20]

Dell i think theres some confusion as to what im trying to ask ? Im saying that on the model P both the one piece bronze nut and the spigot is threaded for the leadscrew. Is the spigot on the 1750 if it has one just a clearance hole for the screw unlike this on the model P. This image is the top slide /compound slide. So it acts like a two piece /split nut?  thanks

p.s if i am right then if you were replacing the nut /bushing or rethreading it ,wouldnt you have to deal with the threaded spigot as well?

[mark smith 20]

On mark smith 20 Said:

Dell i think theres some confusion as to what im trying to ask ? Im saying that on the model P both the one piece bronze nut and the spigot is threaded for the leadscrew. Is the spigot on the 1750 if it has one just a clearance hole for the screw unlike this on the model P. This image is the top slide /compound slide. So it acts like a two piece /split nut?  thanks

p.s if i am right then if you were replacing the nut /bushing or rethreading it ,wouldnt you have to deal with the threaded spigot as well?

Both the P type and the 17/50 & 17/70 are more or less identical apart from the handles & dials although both can be used on either, I now have a crosslide off a 17 series on my Pultra & both the leadscrew & nut are threaded can’t remember if the spigot is or not but I think it was , in the original post I bushed the nuts and tapped them but since then I have made some new ones for another crosslide, also I have seen both one & two piece nuts so logic would say the two piece nut wouldn’t have a threaded spigot but don’t know for certain, the pictures of my crosslide may help .

The last picture has the different although usual handles

[mark smith 20Participant @marksmith20]

Dell ,thanks for the replies. Nice to see more photos of pultras to compare , mine is a war finish one like in your bottom photo. Mine also has the later lever operated tailstock. It was in a terrible rusty condition when i got it but has performed well . Great machines.

I assume the spigot is removable ?

[mark smith 20]

On mark smith 20 Said:

Dell

I assume the spigot is removable ?

Yes just sits in the hole , when you take it apart watch for the ball bearings as the first one I did I thought I had lost some because there are only 9 but easily room for 10 but the manual lists 9 very odd.

[mark smith 20Participant @marksmith20]

Yes i thought there was one ball bearing missing as well have 9 but as you say room for ten.

Cross slide is proving more troublesome with  one of the the 3 slot head machine screws holding the hand wheel on ,the slot  is mangled and cant get it unscrewed at all.

[mark smith 20Participant @marksmith20]

I had to use a screw extractor to try to remove the screw , it removed the head ,so ive drill out the middle of the screw, any idea what these screws are ,they are 0.104 ” diameter . Seems to be 12 threadsover 1/4 ” so possibly 48 TPI

 

[DellParticipant @dell]

I thought The screws were BA  but maybe not all the book has is screws hand wheel 6 .

[mark smith 20Participant @marksmith20]

Possibly 6 BA  or BSW 3/32 x48  ?

[mark smith 20]

On mark smith 20 Said:

Possibly 6 BA  or BSW 3/32 x48  ?

I don’t have same otherwise I could measure, join Facebook Pultra & any watchmakers lathe group someone will know or measure for you.

[HopperParticipant @hopper]

In the absence of proper thread pitch gauges, you might get a more accurate read by screwing a screw into its threaded hole in the collar, measuring the distance from the screw head to the collar with the depth gauge on a digital caliper, then unscrewing the the screw one turn and measuring the distance again. Doing the same with four turns and dividing the result by four might give an even more accurate result. Just a thought.

Combined accurate measurement of pitch and diameter should allow you to look up on the usual thread charts what it is. Bear in mind that most commercial fasteners are made a few thou under the nominal size so they all fit together without binding.

[Michael GilliganParticipant @michaelgilligan61133]

It doesn’t guarantee anything … but do be aware that BSI ‘deprecated’ the use of the very small Whitworth threads, and recommended that BA be used for most fixings under 1/4”

MichaelG.

[Michael GilliganParticipant @michaelgilligan61133]

In the hope that this comes within the scope of ‘fair use’ quotation

here is the text from BS 93:1951

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MichaelG.

 

[mark smith 20Participant @marksmith20]

MichaelG, do you think these may be worth a try.

https://www.ebay.co.uk/itm/333469981374

[mark smith 20]

On mark smith 20 Said:

MichaelG, do you think these may be worth a try.

https://www.ebay.co.uk/itm/333469981374

I have no idea, Mark

Personally, putting myself in your place …

I would put some effort into identifying what threads are in use on your machine.

MichaelG.

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Edit: __ I think your post #759939 provides ample demonstration of the perils

… evidence of some previous ‘near enough’ substitution ?

[mark smith 20Participant @marksmith20]

Michael G, i have a feeling they may be 7/64 x 48 ,never seen any screws of that size available now.

Ive now decided they are simply 6 BA ,i found a piece of brass 6 BA rgeaded rod  and it fits perfectly and has the same diameter of the screw..

[DellParticipant @dell]

I still have it in the back of my head that they are BA do you have any BA taps?

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