Bad surface finish 6x12 surface grinder spindle disassembly questions

[JFL4066]

So after putting new bearings in the spindle, I was still seeing runout on the grinder hub taper. So this morning I took the motor spindle off the machine. Installed it on my lathe. The motor shaft end where the TEFC fan mounts I put in the 6 jaw lathe chuck. The other end I supported in my steady rest. I initially held the shaft concentric to the lathe spindle with a center in the tail stock, and then adjusted the steady rest to hold this position.
I have a video of that setup.

Then I did the grind work on the taper;

I did some test grinds after wards using different grinding wheels.
I was stepping 0.1" per pass, and very light less then 2 tenths grinding passes.
I still see some very small ripples, and will blame this on wheel balance. But I'm very happy with this improved surface finish. I touch the corner of the freshly dressed wheel to remove the sharp edge, but still see the .1" step over in the surface finish, and if any one has advice on how to correct for this, I would like to know how to improve the finish.

That "banding" in the pictures is really indicative of work piece or table not rigid IMO. Since these are Chinese quality machines you cannot assume your table surface is flat or the chuck bottom is flat. Run a DTI on both surfaces to make sure you don't have a concave or convex situation.
I would bet my milk money that the table ways are not in full contact. The table is actually moving under the wheel pressure. Up and down as well as front to back. I have had the same banding surface on my dads surface grinder with worn ways.

 

[smokey357]

From your video, wheel balance is important. A wheel balancer and dedicated arbors are on the list of items.
The bearings you used are different from mine. Your preload is Medium and your contact angle is 25°. Mine are Light preload with a 15° angle (for more radial rigidity). The 7206 BEP are 40°. Anything will be an improvement over what was in there.

 

[smokey357]

Thanks for the bearing info. While I have not seen any issue with my spindle. I did change the motor bearings a few years ago, I may change the spindle bearings some time in the future. Just because I know there are inferior bearings in there now. lol.

I'll be curious to see what bearings are currently in yours.

 

[Flyinfool]

So I still see a surface finish that is directly caused by the wheel balance. This machine just does not have enough mass to operate with an unbalanced wheel.
I did a test of the 7 wheels I have with dedicated hubs.
I have a MMA7361 analog accelerometer I got off eBay and build a amplifier, as I was not sure the oscilloscope would be able to display low millivolt output. The yellow trace is the accelerometer output directly. This has a gain of 800mV/g. The blue trace is the gain of -100, with a low pass filter. The 4th wheel produced the best surface finish, and it's not balanced, just out of the box. The 5th wheel produced the worst surface finish, and had the worst balance.

Did you try running it with no wheel to verify motor balance.

 

[metric_taper]

Did you try running it with no wheel to verify motor balance.

Yes, I did validate it was the grinding wheels.
The accelerometer data from the 7 wheels taking the RMS voltage computed by the oscilloscope are as follows;
1)........5.62mVrms
2)........7.31
3)........6.2
4)........3.3
5)........9.5
6)........8.5
7)........5.9

The ones that are out of balance the most, have a very distinctive 60Hz sound. And I can see the surface finish ripples using them.
The best grind came from wheel 4, because it had the least imbalance.
I've seen posts where owners of grinding shops say you never need to balance a 7" wheel. But this machine is a light weight, and can be thrown around with a vibrating wheel.

 

[kb58]

I admire the OP's stick-to-it-ness, but at some point, it's time to stand back and consider what's what. I mean, "real" surface grinders cost $$$$ and use bearings that all by themselves, probably cost more than the HF unit. I mean, that's got to mean something. I agree that with enough work, the HF unit can be made to work. It all comes down to the answer to the question: is this a hobby or a business?

Right before I posted this, the post above came up. Ah, so yes, and it's fairly widely known that consumables from HF are junk, and should immediately be replaced with quality parts (drills, taps, any sort of sand paper/belts, saw blades, etc, etc). So I'm still not entirely wrong

 

[JFL4066]

Point well taken kb58. If we all bought new machinery there would be no use of this forum.
Since we are mostly hobbyists, some more serious than others, we look to this forum and others for knowledge. Putting a problem on the forum for others to follow and add their knowledge is fantastic. Usually the "story" goes way beyond the initial

 

[JFL4066]

for some reason my last post got truncated... Moderator?

 

[metric_taper]

I admire the OP's stick-to-it-ness, but at some point, it's time to stand back and consider what's what. I mean, "real" surface grinders cost $$$$ and use bearings that all by themselves, probably cost more than the HF unit. I mean, that's got to mean something. I agree that with enough work, the HF unit can be made to work. It all comes down to the answer to the question: is this a hobby or a business?

Right before I posted this, the post above came up. Ah, so yes, and it's fairly widely known that consumables from HF are junk, and should immediately be replaced with quality parts (drills, taps, any sort of sand paper/belts, saw blades, etc, etc). So I'm still not entirely wrong

Hobby or business?, what is the name of this forum? It is not Practical machinist, where any posts of import tools is locked immediately. And they beat up any hobbyist for asking questions on repairing 'import junk'. And they belittle them for daring to post there. Mean bastards. And this is what I like about this forum, I don't see attacks on people.

I have this machine purchased new in 1991. Back then there was no craigs-list or FB-marketplace, where now I see old iron surface grinders, that may be clapped out junk, or not. And for half the price of what I paid new for this HF branded tool (if you read this post from the beginning you would see it has been badged to any import tool seller). Grizzly is still selling this same tool.

Look at all the Precision Matthews machine tool posts in this forum, you probably consider that import junk as well.

The consumables, i.e. grinding wheels, are all name brand from eBay sellers, and yes I'm beginning to believe they are rejects from grinding shops that got a whole box of out of balance production wheels, and became surplus. They still run ~$20 per wheel in the auctions. And getting them this way, you're left with what ever random hardness of vitrified stone is being sold. They may not be optimal for the task at hand, but I'm not a production shop, and if they were balanced, would give me the surface finish desired. If you followed this whole post from the start, you would see, I've fixed that pre-assembled kit.

If you're a production machinist, you're in the wrong forum. And I like that also, that production guys are here, and are willing to share their learned experience.

 

[JFL4066]

Good work on the vibration analysis metric_taper. Thanks for the info.

My 2 cents....
There imports, then there are imports, then there imports. lol. Germany/Switzerland, Japan, Isreal, Taiwan, China, India, etc. Price = Quality Control.

I would not knock the country of origin. They only supply what the importer/dealer orders and mandates for quality control. Problem is, the importers/wholesale buyers don't understand or know what to specify to the machine manufactures. (ie. what features to reduce quality on and what features not to, to reduce manufacturing cost). Precision Mathews seems to have it figured out for mid-line pricing.

I was an import compliance test engineer for the majority of hard goods sold in every retailer in the US.