Thread: Power to weight ratio - grinders and cbn wheels

when i was doing my research on CBN wheels some time ago, a general guide is atleast 1hp (~750watts).

The main issue with an underpowered machine is the time it takes to get the motor up to speed, the longer it takes the more wear on the motor and thus causes potential issues. One can easily get around this by giving it a good spin prior to hitting the power button.

Originally Posted by Gary H

My main concern is the wear on the bearings if my ol' trusty 280W six incher has to swing that extra weight. I have searched a whole heap of threads relating to CBN wheels, but cannot find any regarding the power to weight so maybe it's not as important as I think. Any help will be much appreciated, especially from ones who have "been there, done that".

If the wheel and the motor are well balanced and if the bearings are OK to start with, the effect on bearings should be a minor concern. The main issue with grinders is in their acceleration phase, and in their load phase ie while sharpening (or grinding). Once they are up to speed relatively little power is needed to keep them going until they need to actually grind something.

The acceleration phase is where the weight of the wheel has to be accelerated up to speed by the torque of the motor.
The load phase is when the grinding or sharpening generates a "back torque" which slows the wheel down which the motor then has to work hard to overcome.

In general the motors on grinders are matched to their wheel size to enable the motor to accelerate a (right size) wheel up to speed in a reasonable time. If the wheel is too heavy for the motor the motor will take longer and draw more current so has to work harder (hotter). Long term this can result in cooking the motor.

Same with the back torque. A large radius (not necessarily heavier) wheel is more easily slowed down by applying a tool tip needing to be sharpened. Thus the motor has to work harder (and hence hotter) to maintain wheel speed (ie keep the wheel in synch with the Mains Frequency). Long term again, heavy applications of workpieces to the wheel may also cook the motor.

A small benefit of a heavier wheel is that it requires more back torque (heavier tool tip application) to slow down so this is less of an issue for light touch ups so will only really come into play if you wanted to say reshape a cutting tip where a heavier application is required.

So in general the wheel size should match the motor size. A 6" wheel is nominally best driven with at least ~300W motor while an 8" wheel needs at least about 600W.
Bear in mind this is output power which lower than input power by abut 25-30%. Most cheap grinders only quote input power.

I have 2 HD 3P 1HP grinders and one SP with a 680W output motor, and have my CBN wheel on on of the 3P grinders - I use the CBN wheel mainly for touching up and reshaping MW lathe tips which are much smaller than wood turning chisels.

Thanks Tonzeyd and BobL. Your advice is much appreciated and now I know what to look for. I thought it was a bit funny that only one of many websites quoted the weights of CBN wheels (and they ARE heavy). And I only found one site where the output as well as the input was quoted. That was for Abbott & Ashby grinders which showed the output was two thirds of the input for each of their grinders - I'm impressed by their honesty. I only intend to use the CBN wheel for HSS turning chisel touch ups, so very little pressure will be applied and I am happy to go with a 6" wheel if and when I find one with reasonable power. A lot of advertisers show a 330w (input) grinder as Heavy Duty , but now I know better, so thank you!

BobL: Another question - With almost all current ads showing just the Input power, do you know if this is a recent trend or always been that way. I am interested in getting hold of an older grinder (GMF maybe) and am wondering if the earlier ones may have been labelled with the Output power (I wish!). You're probably too young to know the answer

Cheers, Gary H