Bad surface finish 6x12 surface grinder spindle disassembly questions

[Janderso]

Cheap grinding wheels will also do that, due to out of balance.

John,
What brand is good quality?
How much should I expect to pay for a good quality 7” x 1/2” wheel?

 

[Janderso]

I had to go see what my SG is reading.Just for a reference, it looks like my taper is within .0002” and when I push/pull, maybe .0001”.
This is after a five minute warm up.
Hmm, I couldn’t load it. I’ll try a YouTube link.

 

[extropic]

@metric_taper

I suggest that, after you get the bearing off the fan end, you mount the shaft between centers and survey the whole thing.
I'm wondering if you'll be able to identify a bend or bow that might be corrected by "straightening".
What I mean by survey is to Sharpie some "clocking" marks (at least 4) on the largest OD and take indicator readings at intervals all along the shaft and armature. Record the readings in a table that shows not only the magnitude of the deviation, but also the clocking.

My point is that the whole rotating assembly should be dynamically balanced and having a "straight" assembly is a good start.

You mentioned that you were looking for 7206 bearings with shields on one side. If you're looking for individual bearings (not a matched pair), why not get them as the more common double shielded and remove a shield as required? YMMV.

 

[metric_taper]

That was my next thing to do. I just got 3 new hubs off ebay. And don't know if they run a "swash plate" wobble.

I tested all 4 of the hubs. All produce axial runout, that at the end of a 7" wheel is pretty big.
This first video is of the one that came with the machine. Note the wobble to the nut that came with the machine, it's the threads on the spindle that appear to done with a hand held die, and the operator didn't do a good job keeping it square. At least that's what I think happened.
View attachment OEM grinding hub runout.mp4

 

[metric_taper]

@metric_taper

I suggest that, after you get the bearing off the fan end, you mount the shaft between centers and survey the whole thing.
I'm wondering if you'll be able to identify a bend or bow that might be corrected by "straightening".
What I mean by survey is to Sharpie some "clocking" marks (at least 4) on the largest OD and take indicator readings at intervals all along the shaft and armature. Record the readings in a table that shows not only the magnitude of the deviation, but also the clocking.

My point is that the whole rotating assembly should be dynamically balanced and having a "straight" assembly is a good start.

You mentioned that you were looking for 7206 bearings with shields on one side. If you're looking for individual bearings (not a matched pair), why not get them as the more common double shielded and remove a shield as required? YMMV.

Thanks, that seems like a good approach to seeing what I have.
I want the bearings as a matched pair, otherwise I would do exactly that. But can't find any on eBay that have any shield on either side. I can afford $100 on a pair (ABEC7), but don't want to order new as I'm seeing $600 on up in prices.
Seeing the wack marks on the aluminum squirrel cage ends of the rotor, makes me think this was dropped during production. I hope it's not bent, but just poorly machined, and I can correct it.
I'm thinking using a 6204 bearing on the fan end of the shaft, to replace the 6303 there now, and machine a sleeve to press on. I believe both bearings must be running without runout and mostly parallel to the whole shaft.

 

[Lo-Fi]

It's worth watching as he explains many aspects of spindle work in exquisite detail. The section on scraping or lapping the nut to correct it from bending the spindle if it's out of square is particularly enlightening.

A few thoughts...

I'd not put it beyond the realms of possibility that your spindle was finish machined in situ in its own bearings. Such setups are not built to be disassembled without subsequent work. If this is the case, you will find that the journals may well not be concentric as they will have "corrected out" inaccuracies of cheaper non precision bearings, which they are if the markings are to be believed. Subsequent wear may well have knocked the entire assembly out, it simply wasn't done well in manufacture, or at some point a bearing has shifted slightly.

All conjecture and offered as food for thought.

What is the preload mechanism in your machine, out of interest?

 

[metric_taper]

I tested all 4 of the hubs. All produce axial runout, that at the end of a 7" wheel is pretty big.
This first video is of the one that came with the machine. Note the wobble to the nut that came with the machine, it's the threads on the spindle that appear to done with a hand held die, and the operator didn't do a good job keeping it square. At least that's what I think happened.

Here is an eBay hub: Better runout, but still not good.
View attachment grinding hub eBay#2.mp4

 

[Flyinfool]

Now have you tried clocking the hub to different positions to see if you can reduce the runout?

 

[extropic]

I would not buy any "used" bearings for a grinder spindle, without a money back guarantee.
And then, only if I was willing to waste the time to receive, check and return the bearings.
Why would someone be selling a used, matched pair of angular contact bearings?
Easier to find a full set of hens teeth.

Not sure how to proceed until reviewing the results of a survey.

 

[metric_taper]

Now have you tried clocking the hub to different positions to see if you can reduce the runout?

I worked with the original hub, and could get it down to 4 tenths. But that is not a solution I would ever consider. Everything needs to run true, as well the hubs.