Pultra model P

[mark smith 20Participant @marksmith20]

Dell, yes i think they are see bit i just added on to my last post. Yes i have 2,4 and 6 BA taps.

[mark smith 20]

On mark smith 20 Said:

[…]
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..

I am delighted to hear it !!

Someone innocently forcing a 55° screw into a 47.5° tapped thread does tend to do the sort of jamming that you have observed.

MichaelG.

[DellParticipant @dell]

I just spoke to someone that has the correct handle and they are 6 BA.

[david bennett 8Participant @davidbennett8]

Just a personal view

I have never been a fan of anti-backlash nuts to eliminate all play. I think that encourages more rapid wear on the leadscrew and nut, and I cannot bring myelf to simply dial in measurements without going through the motions of having backlash present. On the subject of one-piece nuts, is it not likely that if a new nut (or pair) was made, it would not benefit from splitting it until wear sets in so would be left whole ?

dave8

[david bennett 8]

On david bennett 8 Said:

Just a personal view

I have never been a fan of anti-backlash nuts to eliminate all play. I think that encourages more rapid wear on the leadscrew and nut […]

I agree whole-heartedly, Dave … constantly chasing zero backlash is self-defeating, especially as many adjustment mechanisms work by introducing some tilt into the contact-patch.

I have taken the liberty of ‘quoting’ Pultra’s statement [thanks, Dell], in the hope that it will be more widely understood.

MichaelG.

.

Note: __ that word ultimately is significant !

.

[DellParticipant @dell]

I don’t think there is such a thing as Zero backlash as anything that tight would be difficult to turn, I think the goal is just to reduce it and anyway don’t we all take backlash into account when we are using out lathes and mills .

Dell

[SillyOldDufferModerator @sillyoldduffer]

Judging by the number of times adjusting backlash comes up on the forum, there’s a fair sized minority who haven’t realised backlash is normally compensated out by the operator.    They expect backlash to be removed by some sort of mechanism.

Not a bonkers idea in that there are plenty of tools that do feature anti-backlash – like older dial-radios and gun turrets.  But the simple arrangement fitted to manual lathes is a good practical alternative: cheap, reliable, and effective.  It just requires the operator to know he has to compensate.

A DRO is the best solution when money is no object.  It reports the position of the slide, and doesn’t depend on a thread being in good condition, or the operator reading the dial correctly and remembering to compensate.

Dave

[Michael GilliganParticipant @michaelgilligan61133]

I wouldn’t for one moment argue against your comment, Dell

When I wrote:

“constantly chasing zero backlash is self-defeating”

I was thinking of it as a Unicorn

MichaelG.

[SillyOldDuffer]

On SillyOldDuffer Said:

[…] A DRO is the best solution when money is no object.  It reports the position […]

Even with the current state of technology … regrettably difficult on a machine the size of the little Pultra

🙁

MichaelG.

.

Levin and Sony made a good stab at it a few years ago … but [?]
https://levinlathe.com/sony.html

 

 

[mark smith 20Participant @marksmith20]

On the Pultra model P , the nut is one piece  but there is also the spigot  which is around 2″ away from it but also threaded for the leadscrew. How common is this arrangement ? Is it a good design? My southbend 9A just has one nut  similar to the spigot.

[mark smith 20Participant @marksmith20]

Would  this thread form be suitable for a pultra leadscrew?

I have the idea of making new screws and nut when i find time using an old  die and tap i have . Its UNS 1/4″ x 40.

I tested the split die out on a piece of 1/4 brass bar to see how clean it cuts. 60 degrees i think.

 

[duncan webster 1Participant @duncanwebster1]

<p style=”text-align: left;”>Split die won’t give consistent pitch. If that matters to you either screwcut (difficult if you only have one lathe) or buy from a reputable source.</p>

[mark smith 20Participant @marksmith20]

why wont it give a consistant pitch? Ive read they are adjustable to give high precision to match the male thread.

[duncan webster 1Participant @duncanwebster1]

They never do, don’t know why.

[david bennett 8Participant @davidbennett8]

If the pitch is inconsistent, I doubt you will notice it in a practical sense. Anyway, if it matters, get a DRO, as S.O.D. suggested. Far too many theoretical objections are thrown in the way of what is,to most of us, a practical hobby.

dave8

[david bennett 8]

On david bennett 8 Said:

[…] Anyway, if it matters, get a DRO, as S.O.D. suggested. […]

Any suggestions for a suitable DRO for machines of this size ?

MichaelG.

[david bennett 8Participant @davidbennett8]

Michael, search for Starrett Rear Plunger DRO. EDIT -that should read Back Plunger DRO 196 or 196m

dave8

[Michael GilliganParticipant @michaelgilligan61133]

Mmm …. thanks, Dave … I found references to a very nice little Analogue dial indicator.

Potentially useful, of course, but I doubt if it’s what Mr Duffer had in mind

i.e. a Digital Read Out

Are we ‘separated by a common language’ I wonder ?

MichaelG.

.

Bonus Find:

https://www.instructables.com/Cleaning-a-Starrett-196/

[mark smith 20Participant @marksmith20]

Dave 8 , it looks pretty consistant to me , so im not worried about it.  Duncan mentioned it but i can find no other  mention of inconsistant pitch anywhere using a split die on google .

But back to my question ,is there any reason not to use the UNS thread form for a lead screw if i already have the tap and die?

I have no intention of fitting a DRO.

[david bennett 8Participant @davidbennett8]

Mark, no reason at all. I once did a similar thing on a ww lathe using (from memory) a UNF lh tap and die , courtesy of a suggestion by Tracy Tools.

dave8

[mark smith 20]

On mark smith 20 Said:
[…] But back to my question ,is there any reason not to use the UNS thread form for a lead screw if i already have the tap and die? […]

I doubt the use of UNS thread form itself will make any noticeable difference to the functionality of what you can make in a home workshop environment.

Go for it … See what you can do !

The original Pultra threads will surely have been better … but you [we] don’t have the facility to make anything to that standard.

I think it worth mentioning here … The nearest modern equivalent to a Pultra is probably the Levin:

To put things in perspective, have a look at the prices on their site.

MichaelG.

[david bennett 8Participant @davidbennett8]

Michael, my error. I should have said DTI.

dave8

[Michael GilliganParticipant @michaelgilligan61133]

No problem at all, dave8

… it was an interesting little voyage of discovery anyway

[and a perfectly viable option in some use-cases]

MichaelG.

[JasonBModerator @jasonb]

Though a plunge type with digital display would be an option and at least the zero will always be the right way up.

Mention was made earlier that split dies may give a change of pitch but no actual figures given. I’m inclined to think that any errors would be quite small maybe 1% of pitch at the most, not sure what tolerances some people need to work to but that is not a lot.

lets say you want to turn a piece of nominal 3/8″ bar down to 1/4″ that is going to be around 0.125″ to come off. Now if I were doing that I would touch the tool against the work and zero my handwheel (Pultra you may just nee dto note the reading). As it is a light machine lets say you then take four cuts of half a turn of the handwheel reducing the diameter by 25thou each time. Then a lighter cut of say 7thou on the wheel or another 14 thou off diameter.

Now take a measure ment which should be in the region of 0.261″ so another 11thou to come off so wind the handwheel 5 1/2″ thou and take your final cut.

If the handwheel and thread is perfect you hit your 0.250″. If there were 1% pitch error then:

1% of 25thou pitch is 0.00025″ and 5.5/25 of that is 0.000055″ that is half a tenth of a thou or 0.0014 mm which is 1 1/2 microns. Wish I was able to measure that let alone work to it and it would change buy the time I bought it in from the workshop.

 

As to replacing the screw as your Pultra won’t screwcut and if that sort of error matters to your work then think about getting a spare screw for an imperial Sheline and modifying that to suit. I’m assuming you still want to work in imperial, if not then why not source or make a metric screw.

[mark smith 20Participant @marksmith20]

Dave8 and MichaelG , thanks i.ll  make new nuts and screws andseewhat they are like, no harm done other than wasted time if they dont work too well.

Another question when tapping  the nut what percentage of thread would be best  to aim for in this case ,75% or higher . Ive ordered 3 tapping drills from drill service Horley , which will allow between 75% to almost 100 %. I used the Guhring tapping drill calculator at :   https://guhring.com/Tech/tapdrill

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