Pultra bearings

[dave-b]

As requested, here are some pics of a new old stock Pultra 10 spindle and bearings (missing the rear inner bearing- I got that wrong before ) Despite the name, this is an 8mm lathe. I have no way of knowing it's age.It came with the lathe shown. The reason this is of interest is that is finish ground, but not yet lapped-in. This means that the front bearing shell is left fat, and wedges on the shallow taper.The distance between the steep tapers is about 0.0012" as measured with a slip of cigarette paper.
As there is some conjecture about how these double conical tapers were made, I made some enquiries on Practical Machinist website and was assured by a grinding expert that the sort of measurements shown would be entirely achievable in a production environment. So, the only remaining mystery is that of lapping/fitting the bearings. I believe they were had lapped, taper on taper. I estimate about 2 microns of material removed from the shallow taper would see the steep tapers to come into contact and lapping would continue on the whole bearing surface.
(I also show some Boley bearings being lapped for renovation)

 

[Betzel]

Thanks for posting, Dave. Is there a serial number on the Pultra (or the box, if you got one)?

(sorry to be so damned curious, but...)

Q2: Is the spindle in it "pooched"?

Q3: is the nose register / collet seat (maybe other spots as well) the on the new spindle pre-cut --or left "rough/fat?"

Q4: Never tried it. How did you remove those Boley outers from their casting? Nice work!

I don't know how old this lathe is either, but looking at Tony's website, Pultra got started in the 1930's, went to war with everyone else and likely restarted after WWII, and (I think) never became a super big shop. The model 10 advert uses what appears to be 7 digit phone number, and the price list is from 1952. What's more, the 3 jaw that came with most of these is a Burnerd. Hope you got one!! I have one that is nuts-on (probably dead-nuts-on when new). Burnerd filed a US patent in 1952, got bought (by Pratt) in 1961, and changed names in 1974 and they are still great. W/O a serial number, a guess is your lathe may be mid 50's to 60's --the spindle more recent? Dunno.

Maybe just to me, but there is a difference between removing material from a "fat" spindle (yet to be fit-in) and one that has already been fit-up and used. Not saying it's impossible remove half-microns as a repair or a final fit-in, but there are no do-overs. The kung-fu needed to keep it all dead-nuts-on straight whilst removing only half-microns (however you end up doing it) is not something just anyone can do for the first time, easily, well, or outside a production environment. You may have one though, so I wish you all the best with this.

If you're repairing the original (?) or replacing the spindle (I think that's the plan?) how will you proceed?

 

[dave-b]

Betzel, there is no serial no. on the lathe. No box .The old bearings/spindle have been renovated, which is why I have not needed to use these. The bearing shells were not removed- that'just how they came, and no way of knowing the age. The new spindle collet opening and nose look finished, though there is no collet locating pin. It came with a 3 jaw chuck marked Pultra, though I think it is a generic Burnerd with a brass outer ring.( the best of their chucks is marked on the face with a "p" for precision. )
Regarding keeping the spindle in line for lapping to the bearing - it is almost impossible for it to get out of line. the taper keeps it in line, and when both bearings are lapped, they are finally lapped together when assembled in the headstock and slowly drawn together.
A couple of practical points - you will find it difficult to grip the spindle nose while lapping the front bearing. Just mount a carrier chuck with the drawbar to improve grip. For the rear bearing, place it in reverse on the spindle and grip that.
dave

 

[Betzel]

Okay, nice that the original spindle is still functional.

I see now the Boley repair is the image on the right, and still has it's outers in the headstock. I used to get motorcycle ball bearings out of their aluminum castings with low heat in an oven, but have never even thought to try to get glass-hard outer cones out of (or back into) cast iron. Have to be careful with heat. And, I imagine you could easily crack either casting or bearing with a press, on dis / re assembly, so I'll bet the OEM guys had custom jigs and the occasional accident.

To me, there's no need to run any of these "super fast" when making a standard cut, but when polishing I may speed it up (and really should use my spindle oil) but just keep my finger on it to be sure it does not get too warm. My old lathes will run faster (but not super fast) without too much heat using regular oil, but at really high speed the need for a great/tight fit and spindle oil becomes clear in about 45 seconds.

I guess I'll never know if the last hard-cone models of these lathes were made via fit-up with a couple of quick turns and some slurry (good enough for turning) or lapped to high precision in a more complex process that permitted these things to run cool and true at crazy speed --with the right oil-- if you wanted to run them like a jet engine.

 

[dave-b]

Betzel, I agree about the modest speed normally needed, even when drilling. Some of the finest work was done driven with a bow. I am pretty sure that high speeds are o.k. after suitable running-in.
The main reason for showing the Boley was to show that no rings are formed lapping taper on taper by hand, even under enlargement.
dave.

 

[Betzel]

Yep. Agreed.

The rings I have seen came from wear over a long time, and had to be caused solely by two tapers in constant (and likely sloppy) contact with each other while rotating in a slurry of old-timer oil and his contaminants. But, these rings won't come from a few turns with a mild abrasive to grey things up.

I've scraped off those rings by hand with a carbide blade, gray things up like you have and after I do the break in, everything looks real good. I use it with regular oil at slow speed and, all is well. But, if I put spindle oil in it and crank it up past 3k RPM, it will either get hot or spew oil all over everything. And, the next day, all the oil will have run out the bearings. So, I go back to synthetic 30 weight (US SAE) and run it slow. All is well again.

How they made hard cone spindles after 1968 or so has to be different. They just float in watery oil at crazy speed and won't leak at insane speed or at rest. They just weep a little, and need a spin or two in the morning to refloat from weight before putting a load back on. How do you make it like one of those?

 

[dave-b]

I was talking about the conjecture that taper-on-taper lapping causes rings. we really need someone with a new lathe to be brave enough to
open it up and look at the finish.I have a very early watchmakers drilling machine, based on a lathe headstock which still has a frosted finish, but no way of proving it is original.
personally,I think running these lathes at over 3K is crazy and unnecessary, especially with the short drive belts so popular today.
I wonder why we have so little of manufacturers recommended speeds?
dave

 

[dave-b]

Betzel, your reference of spindle performance after 1968 -- do you have evidence of this?
dave.

 

[Betzel]

Well, kinda, but it's oiled up and in storage.

Not sure exactly when it was made, but it's a G. Boley hammertone greeen WW, probably made for the American market and around the same time when the F-1 was their best machine, so maybe before 1968. It's the only plain bearing lathe I have that can scream without heat or splattering you with leaking spindle oil. The others I have are prewar, ball bearing or in various stages of pooched from my well intended repairs, but still work okay with regular oil below 1200 RPM.

Not a big fan of hammertone paint (reminds me of China!) but it was not dropped or abused. I got it with a bunch of Boley headstocks, tailstocks, beds and other stuff on eBay from an estate 10 years ago. The headstock I kept had just a yellow/brown residue of dried up mineral oil, freezing it in time. So, I just rinsed it in kerosene and put it back together. No abrasive work of any kind. The bearing surfaces were not mirrors, but they were extremely clean, not discolored from heat, grooved, ringed, or scuffed from contamination or having been run loose and/or dry, and so on (like so many others I've bought and resold).

But, no pictures. There are probably more than a few of these or better still out there, but most of what I've run across (after break-in) won't pass the scream test, they bleed out pretty quickly. You can get a small bottle of spindle oil on eBay for 5-10 pounds, but be prepared. I wear glasses.

 

[dave-b]

I have one of those Boley hammertone green headstocks. It has bronze bearigs - has yours?
I wouldn't run any lathe at screaming point, and only use abrasive if the bearings fail my "push" test.
dave.

 

[Betzel]

Really? No and wow, mine are hard steel. I've never seen a G. Boley (watchmakers lathe) done up in bronze. If anyone reading this has a G. Boley with bronze outer bearings, please chime in. I don't think (?) they are original, but I really like the idea, a lot.

The level of effort/precision needed to refinish a spindle and make a pair of outers in bronze to repair an old lathe is 100% absurd, so it's just to see if it can be done, or you can do it, etc. but think it is the best way out of being pooched, and an interesting project for skill development (way easier to scrape in) and very smooth when running. I regret selling off an American "soft" headstock.

And, I can't agree more. None of us need speed beyond occasional polishing, but it's a sure way to separate a finely finished modern spindle from an old one or one that has been repaired and now just works okay --like mine. No sense putting expensive oil into a cup just to watch it all run out while you are having lunch, so I use a dropper with a wire most of the time and I'm good for a while. I believe some modern "big boy" spindles still in production use extremely tight fitting tapers in an oil bath, and run at speeds like 75k or more. Insanely tight, but still a total loss system.

 

[dave-b]

I am sure this is an original part. While of good quality, it has a different "feel" I think it is a far eastern lathe.

 

[dave-b]

There has been a lot of conjecture about how bi-conal bearings were made. I don't think we can progress by talking. The process is well known up to the point where a final couple of microns have to be lapped off for fit. It needs somebody in the profession of heat-treating/grinding tapers to try any suggested answers. Without that, how can we resolve this?

 

[Betzel]

Yup, I don't think this will get fully resolved here. The G. Boley jig shows how the older ones were made, and I'm pretty sure they used oilstone paste for fit-in and then cut the collet register. Done. But, the more modern spindle work remains a dark art, unless someone steps up.

If perfection is needed, either for fitting up an OEM (?) replacement or "repairing" a quality used spindle that "looks bad" (thinking it can be taken back in time to be as good --or better-- than new) look for success stories. I'm not aware of any. Perhaps because we don't have an environment like the ones that made these things. If you get one that (when adjusted right) runs true under load and does not get hot at speed or leak a lot, I say use the recommended lubricant and leave it alone. If you keep the belt off when not in use, roll it before you spin it up, no grinding, etc. it'll outlive us all.

If it's in rough shape (but not fully pooched) it's worth a go if you have time and it will run fairly true, but I'd set expectations to make it useful with regular oil at lower speeds, which is what was sold before the 50's and still works fine for most of us. These have success stories in a home shop, and the timesaver idea for accelerating another break-in seems like a decent approach. I think it will probably work to fit-in a new spindle too, if the nose register can still be trued (if needed). Maybe that's the only gotcha if you're OK with just a good fit.

Removing and re-installing the outers from their castings seems risky, but Archie Perkins did it without shattering them in his book's chapter on lathe repair. Maybe that's where I heard of "oilstone paste" being used (yes, taper-on-taper) for fitting up, after his regrind, but I'm not sure. Maybe someone has it handy? It might be all we are going to get on this.

Dave, sounds as if you are happy with your Pultra now, so you're not going to make the missing part / install the new spindle?