EXPLODING 20"x8" WHEEL

[BMccarthy81]

I've had an incident at my office, we were running a 20" by 8" ceramic grinding wheel on a #2 cincinnati grinder, and the wheel, for lack of a better word, exploded. The wheel was from a new company, and was to be used on a trial basis. It wasn't too soft or too hard, it was very much like a green norton wheel. As far as we could tell, before the wheel was placed into the machine, there were no visible cracks on the OD or on the ID. It was placed in the machine for well over a week, ran wonderful parts, regularly dressed, and had coolant running on it.
The day the wheel cracked and "exploded", it had ran parts during the morning. The wheel ran for a time afterwards, meaning that it wasn't grinding parts or had coolant running on it for a good 3-4 hours. The operator then turned the coolant back on, began his setup and walked away from the machine to check on something. He said the sound the wheel made sounded like a shotgun blast. The wheel was in chunks that ranged from grit and pebbles, to grapefruits. The top of the machine was encased by tough plastic, for a mist collector, and without a few people would have been greatly hurt.
My question is what could have been the cause of this? The company that I contacted, I won't say the name for bad press, stated that the wheels crack if they are over tightened. We've used many wheels before, and we never had a wheel explode in our machine. We have seen many different cracks in many wheels that were caused by shipping or being dropped, and we are very meticulous about the wheels that we recieve.
What could be a cause of this situation? Any input is greatly appreciated.

 

[aamoroso]

Any defect in the wheel itself, including a crack from over tightening would cause that. It is sort of odd that it was spinning for 3-4 hours and cracked when the operator began setting up and walked away. It sounds to me like someone may have made a mistake. It is a good possibility the setup operator may have started moving the head, or released a lock and the head was drifting into the table while he was away.

Fill what's empty. Empty what's full. And scratch where it itches.

 

[steveh]

We had a similar incident on a GC 500 centerless machine with a 24 inch wheel a few months back. This parts running thru the machine had been rattling so I asked the team leader to dress it. It blew up on the first dress pass (coolant on the diamond) just as it came to the exit side of the machine. Fortunately all the fragments went straight down (400 pounds of fragments) and no one was hurt. There was, however, about $30,000 damage to the machine.
We noticed that the flange on the exit side of the wheel was undersized. Never did figure out how this flange came to be used (but then, we have a number of spindles and do wheel changes often).
We shipped the pieces to Norton along with our observations after the Norton rep came in next morning. They did a very thorough job, in fact, Norton has a 'wheel jury' to analyze such occurrences. Their conclusion was that the flange was too small and put excessive pressure on the wheel (which was almost worn down to the change point).
Anyway, I would suggest that you contact the wheel manufacturer and see if they offer a similar service.

 

[laundry]

Hi BMccarthy81
I had a grinding wheel explode on me years ago,with look I had just started up the grinding wheel,and it blew apart
I found that the wrong size flanges had been fitted,but they hade been made in house by one of our engineers.
my conclusoin was that over time the wheel had mooved off centre,but as the wheel was dressed it was put back in centre.The wheel is now out of balance. As with your wheel it had run for some days without any problems.
regards.

Barry.

 

[BMccarthy81]

The wheel did break straight down, and we were lucky it caused minimal damage. The machine is modified so that parts are fed into the wheel on a table driven by a cnc controler. It is not uncommon for a wheel to running with no parts on in our machines.
The wheel was kept in balance by an automatic electronic wheel balancer from SBS Dynamic Balance System, and as far as we new it was running smoothly.

 

[griffengm]

BMccarthy81,
Given the timing you describe (during or right after a set up) I suspect something, a tool or bolt etc, lying loose and falling into the wheel. It'll do it every time.

Griffy