Grinding wheel speed

[Mr T]

Hio

A student asked me yesterday "If burning the tool is a problem with dry grinding wheels why not just run them slower?". I couldn't come up with an good answer to that, although I am sure there is a good reason to do with optional cutting speed. Any suggestions?

Chris

 

[CHJ]

Burning a tool on a grinding wheel is due to poor cutting and resultant excessive friction (Blunt Grit and too much applied pressure) not the speed of the wheel.
It's just as important to keep a wheel sharp as any other cutting tool, some of the more modern wheels with ceramic substrate (red & Blue wheels) and associated bonding materials have a degree of self sharpening as the grit structure breaks off in a multitude of small sharp facets in use continually presenting sharp edges rather than rounding off (blunting) as is the case with aluminium oxide. **

Obviously a faster Blunt surface will produce the friction heat quicker, but this is not the fault of the speed but the fault of the operator in not keeping it sharp.
A slower surface speed may give the impression of less burning in a given time if blunt but material removal will be slower if sharp, for some operators this may be an advantage as it will be more forgiving of tool positioning and give more time to ***** what is happening with material removal.

In a commercial environment wheels are often selected with differing bonding agents to provide a degree of grit shedding as it blunts to reduce the friction and need for sharpening (dressing)

 

[Mr T]

Thanks for the response CHJ. My grinding wheel is aluminium oxide and is dressed regularly with a diamond dresser. In a teaching environment the main issue is operator error, which could be mitigated by a slower speed, hence the question. I suppose the answer is that slower speed would give slower cutting which is one of the main advantages of dry grinding.

I prefer to use a whetstone grinder myself and only have the dry grinder to demonstrate a possible cheaper grinding alternatives.

 

[Cheshirechappie]

There's no reason why you shouldn't run a grinding wheel slower, except that the metal removal rate will be lower. Think of the old hand-crank grinders - you'd be doing well to get 750 rpm out of one of those; they still work, but slower. As CHJ says, you still have to keep the wheel in good order - a glazed wheel will generate more heat than a clean one whether it's in a bench grinder or a hand-crank.

The wheel speeds specified by grinding wheel manufacturers are a compromise between highest possible work rate, and safety - too high a rotational speed induces unsafe stresses in the periphery of the wheel which could lead to premature (and dramatic) failure. Lower rotational speed, lower peripheral stresses. The maximum RPM specification could be exceeded a bit, but you'd be starting to push your luck. Stick within it, and the chances of a wheel in good condition failing are very low indeed.

 

[Spindle]

The maximum RPM specification could be exceeded a bit, but you'd be starting to push your luck. Stick within it, and the chances of a wheel in good condition failing are very low indeed.

Hi

This is one of the most irresponsible statements I have read on this forum - to condone exceeding the safe working speed of a grinding wheel is stupidity in itself. Safe working speeds are exactly that, a speed at which the grinding wheel is designed to operate safely - operation at higher speeds falls outwith the design of the wheel.

Please consider retracting this statement.

Rant over Mick

 

[Cheshirechappie]

The maximum RPM specification could be exceeded a bit, but you'd be starting to push your luck. Stick within it, and the chances of a wheel in good condition failing are very low indeed.

Hi

This is one of the most irresponsible statements I have read on this forum - to condone exceeding the safe working speed of a grinding wheel is stupidity in itself. Safe working speeds are exactly that, a speed at which the grinding wheel is designed to operate safely - operation at higher speeds falls outwith the design of the wheel.

Please consider retracting this statement.

Rant over Mick

I will not retract that statement.

Mick - with the greatest respect - read again what was written. It isn't untrue - you'd be starting to push your luck if you exceeded the safe recommended maximum rotational speed of a grinding wheel. If you stay within it, you'll be fine.

A grinding wheel is most unlikely to fail at slightly above max. recommended. The more you exceed it, the more the chances of failure increase.

Just to be absolutely clear - I DO NOT ADVOCATE EXCEEDING THE MANUFACTURER'S RECOMMENDED MAXIMUM ROTATIONAL SPEED. I just described what would happen if you did.

 

[woodbrains]

Hello,

Many grinding machine manufacturers make half speed grinders for this reason, (1500 as opposed to 3000 RPM) dry grinding HSS is more forgiving than that of HCS, where a minor hesitation will result in a blued edge and drawn temper. I think high speed grinding is best in a machine shop and the half speed variety in a woodshop. 1500 RPM is plenty fast enough for woodworking tools and might just save an edge from operator error.

Mike.