Pultra model P

[mark smith 20Participant @marksmith20]

Ive been using a pultra model p several times a week  for years but ive decided to give it a service. I restored it years ago . The runout at the nose is still basically zero. Im having problems with the cross and compound slides probably wear on the screws and nut. Both move back and forth around 1mm or so when i grab it in my hand . I assume the screws are 40 TPI and 1/4″ , hand  dial goes to 24. Is there any where to buy replacements or would i have to make screw and nut myself?? Never cut such fine threads before or would you use a die?

Also in the photo of the underneath, the strip of metal that seats against the bed seems to be off,i.e its not perpendicular to the cross slide.Which means i have to move the crosslide off the 0 degree mark to turn anything straight. How is this held on i only see two sticking out pins ,maybe one pin has bent but the strip is around 1mm out of allignment

Anyone?

Thanks

[bernard towersParticipant @bernardtowers37738]

I wouldn’t think that any degree scales are to be relied on for that sort of accuracy just use a dowel in a collet and a dti, same as you would if setting for taper with your sine tables.

[DellParticipant @dell]

I use a P type and restore Pultra lathes, the guide on the bottom of the crosslide is normally fixed by two pins and a bolt into a T nut, as for the leadscrew nut I have sleeved the original nut and silver soldered then re threaded with tap from Tracy tools and I have also made new nuts.

Pultra lead screw

[DellParticipant @dell]

Most Pultra’s are very good quality & accurate even though they are 80 ish years old this is how little my P type has.

[bernard towersParticipant @bernardtowers37738]

Cant see you being a customer for a mini lathe Dell!!!

[bernard towers]

On bernard towers Said:

Cant see you being a customer for a mini lathe Dell!!!

Definitely notBernard I also have Myford but it has a bit of runout .

[mark smith 20Participant @marksmith20]

Bernard, yes i know they arent usually accurate but its a little annoying on an otherwise pretty accurate little lathe.

Dell, would you do anything about the out of line guide ?? Once i notice these things i usually get an itch to fix them.

Another thing is why do they only have one guide strip to seat on the bed??

What is the thread form of the leadscrews? I suspect mines 40 tpi x 1/4″.

On the link you posted you said your dials go up to 25 , mine go to 24 , which if i divide 1000 thousands by 24 gives 41.66 tpi  . I dont know if this is right or not???

[JasonBModerator @jasonb]

Is it 24 divisions or more likely 24 numbers plus 0 which is 25 divisions eg 1 thou per division = 25thou which is also 40tpi

[mark smith 20Participant @marksmith20]

Jason no the dial goes to 23 then 0 ??

Ignore comment ,your right goes 19/21/23  ,24 isnt marked then 0

 

[JasonBModerator @jasonb]

40tpi it is then, the shorter half marks are 0.0005″

Good as an accurate lathe may be they need to be driven correctly so knowing what size cut you are pputting on is quite important if you expect accurate work to come off them.

[John HaineParticipant @johnhaine32865]

Is there a missing tapered gib?

[mark smith 20Participant @marksmith20]

John no it just has two straight gibs of brass or bronze.

[mark smith]

On mark smith 

Dell, would you do anything about the out of line guide ?? Once i notice these things i usually get an itch to fix them.

Another thing is why do they only have one guide strip to seat on the bed??

 

Yes if was me I would correct the guide , I had to make one not so long ago , if you notice a lot of plain turning lathes only have one guide probably for ease & quickness of fitting.

the thread is 1/4” X 40tpi.

check everything is tight at the front end of the crosslide & compound because they can come loose because 1mm seems a lot.

[JasonBModerator @jasonb]

Does sound like there is some end float in there somewhere as 1mm is almost 2 turns of the screw which would mean it’s stripped!

[mark smith 20Participant @marksmith20]

Jason ive dismantled the compound slide firstly and the leadscrew is moving around 0.45mm back and forth in the nut. So less than 1mm but feels like it.

Dell but what is the thread form , minature acme or what?

[mark smith 20]

On mark smith 20 Said:

[…] Dell but what is the thread form , minature acme or what?

 

From here it looks like Whitworth … but it could be “what”

Do you not have a magnifying glass ?

MichaelG.

[Phil PParticipant @philp]

I wonder if that solid leadscrew nut is original ?
The ones on my model P from memory were split into two halves with adjusting screws to take out the backlash.

You slackened off the two cheese head screws, then tightened a couple of grub screws to push the two halves apart along the thread axis, then re-tightened the cheese head screws.

Phil P

[JasonBModerator @jasonb]

Whit or ME, both 55deg. Will be easy to buy a 40tpi ME from Tracey make sur eyou get the hand right, probably left hand.

[mark smith 20Participant @marksmith20]

Phil p , its one piece i think cant see any split but will look tommorrow , though it does have grub screws in the other corners.

Michael ive got a digital microscope but will have to wait til tomorrow for a photo.

Jason,thanks im sure its right hand .

 

[mark smith 20Participant @marksmith20]

Michael G here a photo, they look pretty worn to me.

[Michael GilliganParticipant @michaelgilligan61133]

Thanks Mark

… still looks like a perfectly reasonable Whitworth thread-form to me.

MichaelG.

.

https://en.wikipedia.org/wiki/British_Standard_Whitworth#/media/File:Whitworth_Thread.svg

.

https://www.rolledthreads.com/a-quick-guide-to-acme-threads/

 

Go compare.

[mark smith 20]

On mark smith 20 Said:

Dell but what is the thread form , minature acme or what?

 

The thread is ME here is the link from the original post

1/4″ X 40 TPI ME

[mark smith 20]

On mark smith 20 Said:

Jason ive dismantled the compound slide firstly and the leadscrew is moving around 0.45mm back and forth in the nut. So less than 1mm but feels like it.

Better to express the backlash in terms of dial graduations, not guestimates.   1mm, if true, means something is seriously wrong.   Before rushing to fix this, be aware that rather high-levels of backlash are acceptable in a lathe because the operator can easily compensate for it.   Backlash is perhaps best tolerated until the amount of compensation becomes annoying; that’s because adjusting the nut to reduce it wears the nut out.

Judging by the lead-screw photo, my guess is that the nut either needs adjusting, or is worn out and should be replaced.  A good photo of the inside of the nut would help decide.

Dell but what is the thread form , minature acme or what?

Dell says the thread is 40tpi Model Engineer and I believe him, though it might seem an odd choice at first.    ME is a Whitworth form thread that doesn’t follow the BSW and BSF standards in terms of diameter.    Nor does ME have to!  In this example, BSW and BSF both apply to nuts and bolts, not lead-screws.   It happens that ME standardised 40TPI on several diameters, one of which I guess happens to be a good choice for the Pultra.   In theory ACME, being designed to transmit power, is a better choice for lead-screws, but the Pultra is a small precision lathe, not a metal muncher.  40TPI ME is a good compromise I think.

Dave

[SillyOldDuffer]

On SillyOldDuffer Said:

Dell says the thread is 40tpi Model Engineer and I believe him, though it might seem an odd choice at first.    ME is a Whitworth form thread that doesn’t follow the BSW and BSF standards in terms of diameter.    Nor does ME have to!  In this example, BSW and BSF both apply to nuts and bolts, not lead-screws. […]

Already clearly noted by making reference to Whitworth thread form

The fact is that the shape of the Whitworth thread lends itself rather well to running sweetly in a lead-screw arrangement.

MichaelG.

[mark smith 20Participant @marksmith20]

Michael G ,thanks.

Dave, heres some photos of the inside of the nut. to me it looks like the thread trough is pretty wide compared to the peak .

[Author]

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