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machinist60
02-21-2012, 10:42 PM
A wood turning friend has a grinder with two 8" dia. x 1-1/2" wide CBN wheels mounted on it. Two bushes were made to fit ~1" wheel holes and ~5/8" shaft ends. When mounted both wheels have about 0.005" side to side wobble. The washers on each side of the wheels are the standard punched kind. Perhaps turned stepped washers would hold the wheels more accurately? Comments.

I would also like to measure the CBN surface runout. I thought of wrapping a thin metal strip (probably aluminum siding material as it is the only thing I have long enough) and putting it under tension with springs so it fits tightly to the CBN surface. There is probably a better way. Suggestions welcome.

What is considered acceptable CBN surface runout?

John.

Toolguy
02-21-2012, 11:09 PM
Probably turned washers would help. For the periphery, maybe Scotch tape or clear packing tape (a little thicker).

Carld
02-21-2012, 11:09 PM
To measure the runout you need a dial indicator. I don't understand what your using the aluminum strips for. If you don't want the indicator tip to rub the wheel you could use a .001" feeler gauge between the indicator and the wheel surface and turn the wheel by hand.

justanengineer
02-21-2012, 11:15 PM
Can I ask what you will do with the measurement when you have it?

If youre trying to improve the grinder, measure the runout of the shafts without the wheels on them. If its not acceptable, fix the issue. If it is acceptable, buy better wheels, make better bushings, or simply use a properly sized wheel as appropriate.

Forrest Addy
02-22-2012, 10:21 AM
I understand your problem. You want to check the wheel's face and periphery but if you touch the wheel direstly with the indicator contact the abrasive will wear it down in a single revolution.

The CBN (or diamond) wheel cannot be casually dressed like aluminum oxide. The abrasive layer is thin and 10x as expensive as AlO2. Superabrasive wheels have to wear true in service. Mounted raw from the box on the hub the CBN wheel may have significat run-out. For initial mounting it should be trued by physically shifting it on its clearance with the hub and/or shimmed on the face to bring the grinding surface true to the spindle axis.

Back in the day, I used a strip of 0.001 to 0.003" steel feeler between wheel abrasive and the indicator. The feeler is hard and slippery and being stiff on the scale of wheel contact it spreads the indicator force widely across the wheel surface distributing wear, and protecting the indicator contact. Think of the feeler as a "condom" that protects the indicator contact.

The strip suspension should be supple as possible so the indicator reads direct from the wheel feature not bridged some distance away. I suggest suspending the feeler from the guard with a strip of tape so it tracks as the wheel is turned by hand. The force of the indicator button holds the feeler in contact. Shift the feeler often because the abrasive thins the feeler even with the light force imposed by the indicator.

Because the feeler and its suspension is so supple, the wheel can be rotated only in one direction. Reverse direction towards the suspension will shift the feeler up, unload the suspension and deliver scrambled readings or slip off the wheel completely.

gundog
02-22-2012, 11:21 AM
It seems the only measurement that maters is what it does to the material after ran through the sander. Measure a piece before and after that should give your error if one exists. True up and making better fitting shafts washers and bushings would seem to be the fix.

Mike

machinist60
02-22-2012, 12:03 PM
Thanks for the responses guys. CBN surface TIR of both wheels is 0.0017" as measured with feeler strip over box tape wrapped around the wheel (thanks for those suggestions Toolguy and Forrest Addy) protecting the indicator ball tip. This method gives a nice integration of the rough surface. Use will probably reduce this TIR, howevever the bigger issue is balance, which is out of my league. Some balancing has been done on these wheels during manufacture as evidenced by drill holes but one does not know to what level.

John.