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VOLMAN S28 (tos s28) spindle bearings

gustimusti

Plastic
Joined
Mar 25, 2021
Hi, Recently i purchased a Volman S28 lathe. It is the precendent of the TOS S28, but slightly different. Different spindle bearing setup for example. It was running well, but i had 0,016mm runout on the spindle nose and couldn't get it better with adjusting. With new bearings it should be almost zero. I also wanted to clean the bronze bearings so decided to dismantle the whole spindle. Rear bearing is still in good condition, but front bearing has suffered from lack of lubrication and/or dirty oil. This is an older type of bearing design, so I cannot find these for sale. The outer diameter is round, the inside is conical. Non splitted bronze bearing. Adjustment is possible by moving out the whole bearing. I was planning to carefully rescrape the front bearing with the front and rear bearings in place. Blue spindle taper, put in place, tighten the rear bearing. check blueprint, scrape, repeat... But looking at the scratches in the spindle taper front, maybe I should also regrind the spindle cone also? There is still space to take some material (spindle will drop deeper inside bearing). I think the spindle is case hardened, but not hard chromed.
Anyone had the same situation before in therms of spindle surface damage? What about the spindle, leave it as it as and just scrape the bronze bearing? Polish it using a flat sharping stone? I can live with 0,016mm runout, but i want to have better surface contact (scraped) so the oil distrubution is better and spindle is not heating so fast and smoother rotating. It was dragging a lot when adjusted correctly. (0.02mm radial play)

My first idea: i'm gonna use a grinded (or sraped, my machinist level) rectangular block of steel with some fine sanding/polishing paper glued/pressed on. The block will be exact in lenght so that it covers the whole lenght of the spindle cone surface. To my guess the unused parts of the surface are 'high' spots. So by spanning them with a straight surface, i'll polish the surface straight with it and i can quickly see if the cone has a slightly hollow shape due to wear and scratching. In this case the spindle can rotate a bit out of center (like 0.02mm), but it should not affect the taper significant. Just hold the block equally lightly pressed end perfect in line with the spindle. It is subjective, but also a forgiving method i would guess. I'll hope this result will be good enough because grinding between centers might give more error then there already is in the runout of the taper. I'm lucky the rear bearing is still in good condition, so alignment when scraping the front bearing should be ok.

I have another lathe, but unfortunately to short bed and to narrow bore to get spindle between centers. However I can set it up between fixed steady and chuck to slowly rotate it around.

Any advise welcome. Attached all the pictures of spindle and bearings
 

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To do this right is a big project and id venture to say the machine would not be worth that investment. That front journal is gone and needs to be ground at minimum, but probably plated. The bearing needs to be replaced or ground bigger if there's room. Keep in mind this shifts the shaft back and you need to compensate with the thrust bearing location. Rear shaft and bearing look marginal, certainly not "good". Your strategy may make it slightly better, but it could also backfire. I hope you dont have a ton of money wrapped up in this machine.
 
To do this right is a big project and id venture to say the machine would not be worth that investment. That front journal is gone and needs to be ground at minimum, but probably plated. The bearing needs to be replaced or ground bigger if there's room. Keep in mind this shifts the shaft back and you need to compensate with the thrust bearing location. Rear shaft and bearing look marginal, certainly not "good". Your strategy may make it slightly better, but it could also backfire. I hope you dont have a ton of money wrapped up in this machine.
Well, that's not good news but at least straight forward advice. It did pay some money for it, i guess triple the scrap price. I knew it was going to be a project, i've had bad used bronze bearings before, but never a damaged spindle journal. It did a lot of research on this lathe before purchase, but unfortunately there two versions. The second version has still new bearings and spindles for sale, so can be made perfect with just replacing. Unfortunately, some rare examples, without online documentation, are still the older VOLMANs.(pre communist TOS). And those are different in spindle design. So my new bearings and spindle backup is gone. I'm gonna put some effort in it, I'll see were it gets me. Worst case this lathe will be for spare parts, i know two other of these beast for sale as back up. I've also 'acces' to an old but big universal cylindrical grinder, unfortunately it has been standing unused for 15 years... Don't know if there willing to start it up and have any knowledge left over to work with it. I dont.
 
Well, that's not good news but at least straight forward advice. It did pay some money for it, i guess triple the scrap price. I knew it was going to be a project, i've had bad used bronze bearings before, but never a damaged spindle journal. It did a lot of research on this lathe before purchase, but unfortunately there two versions. The second version has still new bearings and spindles for sale, so can be made perfect with just replacing. Unfortunately, some rare examples, without online documentation, are still the older VOLMANs.(pre communist TOS). And those are different in spindle design. So my new bearings and spindle backup is gone. I'm gonna put some effort in it, I'll see were it gets me. Worst case this lathe will be for spare parts, i know two other of these beast for sale as back up. I've also 'acces' to an old but big universal cylindrical grinder, unfortunately it has been standing unused for 15 years... Don't know if there willing to start it up and have any knowledge left over to work with it. I dont.
axial displace of the spindle is not a problem to a certain distance. The bearing is designed to slide forward with the threads to compensate for wear in the front bronze bearing. If to much, i can add a shim at the rear thrust roller bearing. Anyway, thanks for the reply
 
bronzen lager voorraan draaibank tekening.png
An update: i'm sending the spindle for regrind and will make a new bronze bearing as on the original drawing. The bronze bearing had heated spots left and right (high cutting friction and no oil), and after measuring is seems to have become ovalised. So I need to make a new one anyway. But here is a thing: how do you proceed with the external diameter? I would guess there are two different options:

1) Make the OD slightly oversized, turn inner taper out.
- Scrape fit the inside taper to the newly grinded external spindle taper.
- After good scrape fit, clamp the bearing on the spindle taper, make the threads and turn/grind the OD between centers on a precise lathe?

2) Make taper, thread en OD as stated in drawing. Mount bearing in lathe and scrape fit to spindle taper.

And further, wat does NV120 mean on the threads? It looks like buttress thread, what could make sense to make sure the nuts can take a high torque on the bronze. Or is it something exotic?
 








 
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