I've been searching for an answer to this question for a little while, and have found no clear results.
Before I get to the question at hand, I need to provide the appropriate background information.
At the shop I work in, we have three grinders using either aluminum oxide or ceramic blend wheels. Two of these are automatic surface grinders and the other is a CNC OD grinder. The two surface grinders have a fixed wheel speed, the CNC grinder will adjust its wheel speed to compensate for wheel diameter.
So now for the question. When is a grinding wheel spent? When is it time to change the grinding wheel? I am talking about changing the wheel because it is worn out as opposed to replacing it with a different style of wheel for a different application.
In my mind, with the types of wheels I described, I would do the following.
One the fixed wheel speed surface grinders, I would determine at what diameter the wheel begins to exhibit problems with obtaining the proper finish, burning, loading, etc..
On the CNC, with adjustable wheel speed, I would change this wheel once the maximum recommended wheel speed is reached.
Am I way off here? If so please let me know. I appreciate the help.