"Jeff Dantzler" wrote in message
news:1073320981.674568@yasure...
Personally, I would never use a grinding wheel at more than
its rated speed. Never.

I am curious though why grinding wheels explode.

Centrifugal force?


What are the typical reasons for a wheel going (assuming
it is not being spun faster than it was meant to) ?

One fellow mentioned grinding aluminum. What happened there?

Grinding aluminum in and of itself will not cause a wheel to explode. I've
ground aluminum in a centerless grinder, even with the wrong wheel
selection (aluminum oxide when silicon carbide should have been used) for
days on end and had no difficulties with the wheel aside from having to
dress far more frequently due to wheel loading. My educated guess on a
wheel that blows up from grinding aluminum is the higher forces applied to a
wheel that didn't want to cut because of that condition. One might give
a little consideration to the idea that the loading also behaves as a wedge
and opens up a crack as the soft aluminum is driven deeper into the open
structure of a wheel by the extra pressure applied by the operator in order
to get the wheel to cut when it is fully loaded. . A harder, less friable
wheel would likely reduce the chance of that happening. Regardless of the
reason, grinding aluminum isn't a great idea, even with a silicon carbide
wheel. Belt sanders are far more forgiving, likely because the belt won't
permit deep loading.

What is the best way to get a wheel to go?

Mounting one that is known to be cracked. The forces are extreme on
wheels because of their preferred operating speeds. The slightest
defect will give the wheel a good reason to let go. Always "ring" a
wheel before mounting it, and never run one that doesn't have a distinct
ring to it. Don't just set it aside, break it so someone else won't try
to run it.

Never force a wheel on the arbor. If it won't go easily, look for the
cause. In the old days of leaded bores, it was pretty easy to scrape the
bore slightly until the wheel was an easy slip fit on the mounting arbor.
You want it to be snug enough to locate, but not be forced in any way.
That would be critical if the wheel tends to tip to a side and would be
cracked when straightened. A sloppy bore would be far better.

Mounting a wheel without the blotters. Those pieces of paper attached to
grinding wheels aren't there for advertising. Wheels that have minor
variations in their thickness can be cracked by the flanges if there's not
something to average out the clamping forces. Never run grinding wheels
without using a blotter on both faces. Use a cereal or shoe box if nothing
else. You might get away with it a thousand times, but it's asking for
trouble, and is generally not forgiving when it occurs.

Always stand to the side of any grinding wheel when you first start it,
allowing it to run for an honest minute before stepping in front of it.
That's especially important with a new wheel that has not been run since
being tested at the manufacturing facility.

Running wheels beyond their rated RPM is also a dumb trick, in spite of the
fact that they are typically run at 150% of their rated speed in testing.

Side loading on a type 1 wheel can cause a wheel to come apart, and is
likely the chief cause in many instances. If one is to do grinding that
requires pressures in that direction, there are wheels designed for the
load. A good example would be the steel backed type 6 found on carbide
tool sharpening grinders.

I would imagine that it is not a common occurance when the wheel
is not abused otherwise we'd hear about it more.

If you consider the number of wheels in proper use, you're absolutely
correct. However, it's like an airplane that goes down. Because it
doesn't happen routinely, when it does happen it's big news, and is
typically fatal to the operator. After all, in most instances the wheel
is aimed at the operator. A centerless grinder is one exception, as is a
normal reciprocating surface grinder.

I recently picked up a 3/4 HP Baldor pedastal grinder at Boeing
Surplus that looked like a wheel blew up. The cast iron side guard
is in 2 pieces and the wheel was fragmented when I got it. From what
I've read, a good way to fix the side guard (cast iron?) would be
with nickel rod--any comments?

Yes, a comment that in your instance the grinder was most likely tipped over
or was otherwise struck with something. When a wheel lets go I would
expect that you would find little of the wheel on the arbor, and the cast
iron guard would likely be more like a do-it-yourself kit, certainly not
just broken in half. I'll defer to those with welding expertise where it
comes to fixing the guard.

Jeff Dantzler
Seattle, WA

Harold