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)???
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/
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.
"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
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?
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".
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.
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 @
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.
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.
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
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.
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
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!
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.
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?
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
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.
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)
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.
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.