Spindle Plain Bearing Journal Concentricity

I'm having a South Bend 16" plain bearing spindle reworked with hard chrome to repair a damaged front journal surface. Two questions to consider:

1. What is the max concentricity deviation between the new journal and the original factory journal that plain bearings should be able to tolerate?
2. How far undersized could the new journal be finished and the bearing expanders still take up the slack?

For reference, the South Bend factory spec on the journal in question is 2.8735" to 2.8740", ground and superfinished to 2-4 microinches.

Thanks,

Tom

Really hard questions Tom, I don’t have a clue of the answers to those questions, but one thing just struck me...Of course, I’m a noob so I may have not thought this through fully...

While those shops are reworking the concentricity of the bearing journals, have you been keeping an eye on the internal bore as well? I was thinking if you ever turn between centers or use a collet, that would have to match the outside concentricity or you’ll have good performance with a chuck but a wobble with a collet or center.
Actually, as I write this, I wonder if you also have to worry about the spindle taper concentricity although maybe that comes out in the wash with a machined in place chuck adapter?
Maybe the grinding house knows this or have a way to guarantee all surfaces are concentric just by the technique or methodology they use. I assume in the factory, all surfaces are ground in the same setup.

This is a very interesting topic for sure. Your labor of love continues.


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Barry, in this case, there is only one non-original surface, the one that is being chromed and reground. The original factory 60-degree grinding centers are still present on the ID at each end of the spindle.

Assuming the grinding shop does a good job dialing in (lapping, actually) their grinding centers relative to the other surfaces, the internal nose taper will be at least as good as it was from South Bend.

Okay got it, thanks for clarifying Tom.


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thomasutley said:

Barry, in this case, there is only one non-original surface, the one that is being chromed and reground. The original factory 60-degree grinding centers are still present on the ID at each end of the spindle.

Assuming the grinding shop does a good job dialing in (lapping, actually) their grinding centers relative to the other surfaces, the internal nose taper will be at least as good as it was from South Bend.

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Id get both journals chromed and ground. That way they can grind both journals in the same set up guarantying concentricity.

MCritchley said:

Id get both journals chromed and ground. That way they can grind both journals in the same set up guarantying concentricity.

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That thought did cross my mind. The downside is having both bearing journals ground perfectly concentric but off-axis from the rest of the part won't do me any favors on the nose tapers with collets or chucks.

Then when get everything back together check the spindle axial and radial bore runout.
tom

Have you contacted Ted to see if he has any for sale? He sold me a good used 16" spindle for a very fair price. I bought it because the first thread for the chuck was damaged but in the end I decided to keep the one I bought from Ted as a spare and re-use the one I had.

If he has one I would bet it would be a cheaper solution than chroming/grinding.

As an alternative, have you contacted a spindle repair company to see if they can do it? I'm sure they know what specs to hold it to.

Don't know if they do these types of spindles, but I use:

Dave Marshick - Precision Spindle Repair Services Co. Inc - (248) 544-0100 (in Michigan)

markz528 said:

Have you contacted Ted to see if he has any for sale?

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Looks like Ted is all out of this configuration. It’s an L00 to replace the original threaded nose spindle. Unfortunately it needed some work before I can install it.

The backstory is a local Tucson shop did a good job on the hard chrome repair but left the new journal about 0.0005” out of concentric working to their typical shop tolerances. Too much, in my opinion, for an oil film that’s only 0.00035” thick at the low end of the SBL bearing clearance spec.

It’s at a reputable shop in Phoenix now for evaluation ahead of regrinding. This shop is accustomed to tighter tolerance work.

i'm confused: if the journal on the main bearing is tilted slightly relative to the rear bearing, (or it is axially off by half a thousandths), you still have about what, a foot between the two bearings?

So its not optimal but their mistake doesn't use up but a fraction of the bearing clearance.

johansen said:

i'm confused: if the journal on the main bearing is tilted slightly relative to the rear bearing, (or it is axially off by half a thousandths), you still have about what, a foot between the two bearings?

So its not optimal but their mistake doesn't use up but a fraction of the bearing clearance.

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Closer to two feet than one on a 16”, actually, which helps your argument.

I can’t dispute your assertion. This is Spidey Sense engineering on my part. I don’t see an oil film having anywhere near the breakthrough pressure with a tilted journal surface even if it’s not a hard interference at both ends.

Out of curiosity I measured the runout on the end of my 16" spindle (in collet hole) - assembled in the machine. Depending where I measured it was 0.0001" to just under 0.0002". So 0.0005" concentricity does not look too good.

markz528 said:

Out of curiosity I measured the runout on the end of my 16" spindle (in collet hole) - assembled in the machine. Depending where I measured it was 0.0001" to just under 0.0002". So 0.0005" concentricity does not look too good.

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Agreed! My threaded spindle has similar runout. Half a thou would be a disaster even if the bearings were set loose enough to survive the abuse.

thomasutley said:

That thought did cross my mind. The downside is having both bearing journals ground perfectly concentric but off-axis from the rest of the part won't do me any favors on the nose tapers with collets or chucks.

Click to expand...

Seems like grinding od and connecting I’d to match leaves it to the pros. Sending one and not the other likely would not be a matched set like how it came from the factory.

Good that you have the original 60 degrees and true to original true concentric diameter. I trust there are no pits or dings on them.

We used to always where we could true up the face of anything and concentric a lot in four jaw chucks and reorder a center 60 degree . It does not take much distortion on that 60 degree on the part or the live center into that 60 degree under pressure. The live center should be primo surface and bearing condition. Also the taper of the live center must not have any dings nor even the tapered I’d of your tail stock inside should be smoothe. You can dykem your tapered surfaces put them together male and femal and look to see after removal that there may be dykem that rubs off at a point. Then polis or Stone it in that one spot- the high point.

Spinit said:

Seems like grinding od and connecting I’d to match leaves it to the pros. Sending one and not the other likely would not be a matched set like how it came from the factory.

Good that you have the original 60 degrees and true to original true concentric diameter. I trust there are no pits or dings on them.

We used to always where we could true up the face of anything and concentric a lot in four jaw chucks and reorder a center 60 degree . It does not take much distortion on that 60 degree on the part or the live center into that 60 degree under pressure. The live center should be primo surface and bearing condition. Also the taper of the live center must not have any dings nor even the tapered I’d of your tail stock inside should be smoothe. You can dykem your tapered surfaces put them together male and femal and look to see after removal that there may be dykem that rubs off at a point. Then polis or Stone it in that one spot- the high point.

Click to expand...

The shop I picked to do the final grinding (repair to the repair) lapped their grinding centers to the factory 60 degree centers on the part. They finished the work and I'm picking up the part this week. The new surface is now within 0.0001" TIR of the second journal. In fact, all exterior surfaces now run within 0.0001" of true. Because they did not touch any interior taper surfaces beyond lapping the grinding centers, I'm assuming the nose taper is also within a tenth of true given my experience with other SBL Co. spindles.

The downside is the part is now 0.0012" undersize after fixing the first shop's grinding results, which were out of concentric by 0.0005" and slightly egg-shaped. I do not believe being this much under the factory diameter spec will be an issue for me given SBL's split bearing/expander design. Worst case, the spindle nose points down a few arc-seconds and I have to scrape the far end under the headstock to trim it back up. That's work I know how to do.

Precision grinding on hard chrome, as you said above, is best left to pros who are set up for it.

Once I get the spindle swap completed, I'll add all the expensive lessons learned to the machine restoration log I've been keeping on this forum. While I'm pleased to have a good spindle finally, this was not one of my better financial decisions. Lots of 20/20 hindsight on this one.

Great to hear! I personally do not think you will have an issue with the oversized clearance.