Spindle stress crack/fatigue

G

greenail

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So as usual I decided to take a perfectly good and working machine and mess with it. I decided to upgrade my central machinery 7x12 mini lathe to roller bearings. The back bearing pressed out easy. The front bearing was such a bear I almost quit. Unfortunately I didn't inspect the spindle closely before I pulled the bearing but after I was done I noticed stress cracks. I'm not sure if this was just a function of bad bearing puller technique or if the stress cracks were what caused the issues pulling the bearing. I really had to crank the puller screw to get the bearing off to the point where my 18mm wrench wasn't able to turn it anymore with the jaws opening up. Something tells me the puller caused the damage but the cracks are on both sides of the spindle and I don't think it was bent and I can't see any galling. It has about .2mm of runout checked in my other lathe.

Thoughts?

PXL_20201105_013800534.jpg


PXL_20201105_013815172.jpg
 
markba633csi said:
Are they actual cracks or just ugly marks? Have you priced a new spindle?
-Mark
Click to expand...
That’s what I was thinking. Looks like bad machining more than cracks. Maybe a new guy who didn’t know what he was doing or made on Friday as they used to say.
 
Those look more like "bites" made by the inner bearing race as it was slightly cocked during the removal. It is hard to keep that inner bearing race perfectly concentric with the spindle shaft while pulling.

You do have a partial remedy. You can lightly skim the face of the spindle where it meets the Chuck (when re-installed), to reduce the problem with the spindle shaft. Even doing this, you will be left with the "hop" associated with any bend in the spindle. Skimming the shaft of the spindle is NOT an option, as it is a press-fit for the bearings.

Ultimately, with that much error, you should be able to place the spindle shaft on a flat table/counter and roll it and see gaps under the shaft, and wobble in the plate where the chuck mounts without instrumentation (if it really has 0.2mm of error). If it is as bad as you say, ordering a new spindle from some place (like LittleMachineShop.com), may be your best option.
 
the spindle had very little runout before I took it out. The back bearing was a bit crunchy when I took it off. I wish I had closely inspected it before taking the bearing off...

I'm ordering another, trying to decide if I should get the 4" or the standard 3 (80mm). My other lathe has 125mm chucks and I have 2 80mm chucks for this one, no 100mm. The 3 jaw chuck it came with is actually very good.
 
I realized from a very good post here that the front bearing(next to the chuck end) was supposed to be a press fit but the other end is supposed to be a slip fit. Which it most definitely was not. Without those proper fits the bearing can never be adjusted properly, which they never were before. I know nothing of 7x.
 
Maybe this is another Moglice job for ROBRENZ. Could also be a chance to make a spindle. Probably best to just order a replacement if it is available.
 
For a few bucks take it to a machine shop and have it magna fluxed. Then you'll know for Shure
 
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