So I bought a nice big 8" CBN wheel for sharpening my turning tools on and a few months later I found a grinder that I could fit it to.
The abrasive wheels supplied with the grinder were sub standard - another story - but the grinder itself seems decent for the price and spins smoothly with bare shafts.
I mounted the CBN wheel, spun it up and was disappointed. The level of vibration was more than I expected from a solid steel wheel, machine turned and mounted using the supplier's two matching precision turned bushes. It was worse than the CBN wheels I've seen demonstrated at shows. Enough that I started looking into returning the wheel.
Supplier's customer service started muttering about fees for returning an item more than 30 days after purchase, Not acceptable for something that smelt to me like a manufacturing defect, but the best way to answer that was to take my own measurements to prove where the problem lies.
The grinder spindles turned smoothly - checking with a dial gauge there is only 0.01mm variation at either end of the shaft when turned by hand.
The spindles are nominally 5/8" diameter aka 15.88mm - they measured in at 15.84 to 15.87mm (using a new mitutoyo digital caliper)
Bush Inner Diameters measured 15.85 to 15.87mm - I can feel the 2/100th mm clearance as a litle slop but not too bad a fit on the shaft
Bush ODs measured 31.72mm give or take a1/100th
ID of the wheel measures 31.77mm - 5/100th clearance so the bushes aren't as snug a fit in the wheel as they are to the shaft.
Lastly I measured the wheel to see if the bore has been machined in the centre of the wheel or if it is off. I can't measure accurately to the outer coated surface of the wheel but there is an inner rim that I can measure to that is not coated. Measuring at different points of the clock, I figure in the worst direction, there is 0.1mm difference in the distance from the inside of the bore to the opposite inner rim. So the bore looks to be 0.05mm offset from the real centre of the wheel. The bore is closer to the rim in the point between 2 and 3 o'clock. Is this an acceptable tolerance ? Not in my opinion. No wonder I've found complaints about the balance of these wheels since I discovered I had a problem.
Anyway, I figure that if I reinstall the wheel with the 2 o'clock direction downwards, all the slack between the shaft and bush and the bush and the wheel bore will add up on the downhill side and help counteract the offset in the bore of the CBN wheel. Maybe the vibration will reduce ?
That's exactly what happened. Remounted the wheel spins with far less vibration - more as I'd expected.
So if you are fitting a heavy CBN wheel to your grinder, there's no need to go through the rigmarole of measuring everything, just put a mark on the wheel and try mounting it a few times over with different points along the rim closest to the ground as you tighten the spindle nut. You may find a sweet spot where it runs with the least vibration.
What do you think ?
Am I being fussy ?
Do I call it a win or insist on sending the wheel & bushes back ?
The abrasive wheels supplied with the grinder were sub standard - another story - but the grinder itself seems decent for the price and spins smoothly with bare shafts.
I mounted the CBN wheel, spun it up and was disappointed. The level of vibration was more than I expected from a solid steel wheel, machine turned and mounted using the supplier's two matching precision turned bushes. It was worse than the CBN wheels I've seen demonstrated at shows. Enough that I started looking into returning the wheel.
Supplier's customer service started muttering about fees for returning an item more than 30 days after purchase, Not acceptable for something that smelt to me like a manufacturing defect, but the best way to answer that was to take my own measurements to prove where the problem lies.
The grinder spindles turned smoothly - checking with a dial gauge there is only 0.01mm variation at either end of the shaft when turned by hand.
The spindles are nominally 5/8" diameter aka 15.88mm - they measured in at 15.84 to 15.87mm (using a new mitutoyo digital caliper)
Bush Inner Diameters measured 15.85 to 15.87mm - I can feel the 2/100th mm clearance as a litle slop but not too bad a fit on the shaft
Bush ODs measured 31.72mm give or take a1/100th
ID of the wheel measures 31.77mm - 5/100th clearance so the bushes aren't as snug a fit in the wheel as they are to the shaft.
Lastly I measured the wheel to see if the bore has been machined in the centre of the wheel or if it is off. I can't measure accurately to the outer coated surface of the wheel but there is an inner rim that I can measure to that is not coated. Measuring at different points of the clock, I figure in the worst direction, there is 0.1mm difference in the distance from the inside of the bore to the opposite inner rim. So the bore looks to be 0.05mm offset from the real centre of the wheel. The bore is closer to the rim in the point between 2 and 3 o'clock. Is this an acceptable tolerance ? Not in my opinion. No wonder I've found complaints about the balance of these wheels since I discovered I had a problem.
Anyway, I figure that if I reinstall the wheel with the 2 o'clock direction downwards, all the slack between the shaft and bush and the bush and the wheel bore will add up on the downhill side and help counteract the offset in the bore of the CBN wheel. Maybe the vibration will reduce ?
That's exactly what happened. Remounted the wheel spins with far less vibration - more as I'd expected.
So if you are fitting a heavy CBN wheel to your grinder, there's no need to go through the rigmarole of measuring everything, just put a mark on the wheel and try mounting it a few times over with different points along the rim closest to the ground as you tighten the spindle nut. You may find a sweet spot where it runs with the least vibration.
What do you think ?
Am I being fussy ?
Do I call it a win or insist on sending the wheel & bushes back ?
