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 GilliganParticipant @michaelgilligan61133]

[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.

[DellParticipant @dell]

[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?

[DellParticipant @dell]

[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 ?

[DellParticipant @dell]

[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  ?

[DellParticipant @dell]

[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

[Michael GilliganParticipant @michaelgilligan61133]

[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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