# Grinding wheel peripheral speed question

• posted
Hi
I have a Silicon Carbide wheel that *was* 250mm in diameter rated at
2480 rpm it is now 195mm in diameter and I am thinking about using it
on my bench grinder that is rated at 2850 rpm.
I have calculated that the peripheral speed at 195mm diameter and 2850
rpm is lower than when it was 250mm and used at 2480 rpm.
Am I correct in assuming that it is safe to use or is there a hidden
problem that may break the space/time continuum (or my body)???
Thanks
Tim
• posted
In addition to peripheral speed, perhaps centrifugal acceleration (cue physics jihad) is also a factor.
Lessee, speed=r(omega), but accel. = (omega)^2 / r, omega in radians/ sec.
Rotational speed increase is by a factor of 1.15, squared this is 1.32. Of course, the wheel no longer has to support (retain) the more outer parts of the wheel which have been worn away. But I don't think I'd stick my face in front of it.
Dave
• posted
"TMN" wrote: (clip) Am I correct in assuming that it is safe to use or is there a hidden problem (clip) ^^^^^^^^^^^^^^^ The RPM limit is set by the manufacturer to limit centrifugal force to a safe level. (You certainly don't want to burst the wheel.) But centrifugal force is NOT proportional to peripheral speed. It is proportional to RPM^2xR. Peripheral speed is proportional to RPMxR, so centrifugal force turns out to be proportional to peripheral speed x RPM. If you keep the same peripheral speed by increasing RPM, you are INCREASING centrifugal force.
The part I don't know how to deal with is that centrifugal force is the total of the forces on all the layers of the wheel. As the wheel gets smaller, this may cancel the effect I have derived above. Anyone able to offer help here?
• posted
On the r - omega squared argument the conditions are about 3 percent "worse" now. I can't get my mind around the stress analysis at the moment, but I would also wonder whether a wheel that has been used long enough to wear this much might have accumulated "damage", i.e. microscopic cracking,
At constant speed, the fall in "r omega squared" with wear reduces the stresses which help to offset "damage".
• posted
Stolen from the internet
So since the tensile strength is proportional to the square of both the Radius and the Spin Speed, it seems that radius times the rpm ought to remain a constant. ie if the radius is 10% less then the rpm can be 10% higher.
Dan
• posted
Actually, if the radius is 10% less (9/10 of original), then the rpm could be 11.111...% higher (10/9 of original) without affecting the result, e.g. radius 100mm @ 2000 rpm gives the same result in that formula as radius 90mm @ 2222.22 rpm.
• posted
Hey, what's the worst that could happen?
Murphy was an optimist.
Steve
• posted
Help from a technical aspect, no. I'm no mathematician, and not a physicist. Help regards running a grinding wheel faster that has had the diameter reduced? Absolutely. The surface speed dictates the safe operational speed of grinding wheels. It is common practice to run small wheels faster to regain cutting efficiency.
The hardness of a wheel revolves around surface speed. As you slow the speed down by attrition, the wheel behaves softer and softer, to a point where it loses it's ability to perform without shedding the abrasive prematurely. All that one must do to restore performance is to increase surface speed to acceptable limits. For vitrified wheels, it's fairly common to accept 6,000 sfpm as a safe, but top end speed.
Harold
• posted
I typically stand to the side of grinding/wire wheels. jic. :)
• posted
According to Harold and Susan Vordos :
[ ... ]
[ ... ]
But always stand out of the path of debris the first time you spin it faster than before -- and I believe that some suggest ten minutes at speed to be sure before you start to use it.
Enjoy, DoN.
• posted
Ten minutes is a bit much----a wheel will usually come apart immediately----30/60 seconds should be fairly safe. Neither of us talked about "ringing" the wheel, which is likely redundant, but who knows who's reading?
Place the wheel, *any wheel that is to be mounted*, on the shank of a screwdriver, or other object, and tap it with the handle of another. If the wheel doesn't ring (like a bell), but has a dull thunk kind of sound instead, the wheel should be broken and discarded. Do not simply discard it ------there's always a chance some poor unknowing dope could mount it and spool it up. If you've ever seen what an exploding wheel is capable of doing, you'll understand why I offer this advice.
Harold
• posted
thanks for all the input - the wheel is a excellent quality wheel that was discarded because of the reduced diameter - it was given to me by an old (master ???) machinist who collected these discarded wheels and used them at home.
It is fascinating to speak to someone like this who poured babbitt bearings 3ft in diameter for marine engines - turned tire molds on a lathe with a 12ft swing and played cards at lunch time with other machinists *in* the cylinders of a triple expansion marine steam engine.
Tim
• posted
Ill back you up there Harold, Ive seen first hand what an exploding wheel can do - and its not pleasant! Even an exploding polishing disk on a 5" grinder can leave welts.
I also back you up regarding destroying damaged things. I threw out a toaster last week that for no known reason was constantly tripping my earth leakage device. I didnt just throw it in the bin... I cut off the power cable, put it on the ground, drove over it with my car, then threw it in the bin. Same goes for anything that i discard at work, especially for electrical faults. I know people will bring things home from the dump because i do it myself. if something is really a hazzard, i will crush it before disposing. Ditto with materials; I insist that all my staff mark hardened steel with paint before putting it in to the scrap bin ie; bisalloy 450. Ive lost a few punches and damaged some shear blades that way!
Shaun
Shaun
• posted
Ring it, mount it between blotters and a decent set of flanges, set the grinder in a corner and plug it in to run from a safe distance away. If it doesn't blow up in the first few minutes you will be fine. If you have employees don't do it, OSHA will bite a chunk from your ass for exceeding MOS ratings.
• posted
I think you all should stop looking at this from a physics or scientific point of view. In stead look at it from an economic point of view. What is the cost of a new or proper wheel versus the cost of an ER visit or even the co-pay, or worse?
Chuck P.
• posted
But there will be no cost of an ER visit if the physics are correct. The truth is that all the grinding wheels are made the same. The smaller ones can be used at higher rpm and are labeled that way. The larger ones can not be used at as high an rpm and are labeled accordingly. If you make a smaller wheel from a bigger wheel, it is now a smaller wheel and can be run at a higher rpm.
Do you really think that they use some better way of constructing smaller wheels?
Dan
• posted
From my position, it IS looking at it from an economic point of view, and science and physics do not contradict the position. I see no profit in discarding a wheel that has ample service life left when it can be run faster to compensate for loss of surface speed. The only problem with the idea is that few grinders offer the ability to change speeds, so it is usually not a consideration.
Pay close attention to the post by dcaster. REAL close attention.
*There is no risk* in speeding up a wheel that is worn in size, not as long as you don't exceed the design surface speed. The risk returns to the risk of a new wheel that is run at the rated speed. Why should it not? That's all we're talking about. Wheels blow up occasionally-----even nice, new, crisp wheels. Airplanes fall out of the sky, and boats sink. There are risks with everything. Don't break the rules and you're as safe as you can hope to be.
Harold
• posted
Just curious - how much is a new wheel? - if your using it at home for hobby tool grinding, how long would a new wheel last? - ie, while its an interesting subject, is it worth the time and effort.....and thats assuming the "old" wheels are fault free........balanced,no cracks, - are they hydroscopic?(dont know - Harold?)
Most of the wheels at school are well worn through regular dressing, they are used to teach tool grinding so get a LOT of dressing (to teach how its done) - but a bigger dia wheel will be easier to grind on, at least with 1/2 inch tool steel. With 1/4 inch steel, doesnt seem to worry them - a function (I think) of the curve of a big dia wheel versus a little one.....
And maybe the lifetime of experience of the master machinist will subconsciously be aware of any potential for disaster rather than the "hope this werks" attitude of the inexperienced....
(yeh, bit of a rant, interesting to argue about. Harold says its OK, will take him at his word. Would be totally presumptuous for me to argue with him)
Andrew VK3BFA.
• posted
Good new wheels are not cheap. My out of date J & L catalog lists a Norton 8 inch by 1 inch at \$83. But will last a long time. I have a 8 inch wheel that must be at least twenty years old. But now it is a six inch wheel.
I also have a bunch of 14 inch cut off wheel that I got used that are about 9 inches in diameter. I use them in my 9 inch angle grinder. The nine inch grinder turns faster than the rpm listed on the cut off wheels so I would never put a 14 inch cut off wheel on it.
If your old wheel is cracked, it is unsafe at any speed.
Dan
• posted
I have a book published by Norton, the grinding wheel makers, that is a basic instruction manual covering many different types of grinding. The book in part covers the peripheral speed of grinding wheels. How it is important to have the correct speed for the wheel and the work being done with it. On machines with a variable speed spindle the wheel RPM is sometimes increased as the wheel wears to maintain the proper peripheral speed of the wheel. This of course depends on the wheel, the machine, and the work being done. ERS

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