Picture this scenario.
It's 9.30am on a busy Monday morning and you have someone scheduled to cement a crown on a lower left first molar.
You pop the temporary crown off, clean the tooth and try the real crown in.
Unfortunately it doesn't fit. #%@&. What do you do now?
Many dentists will pick up a bur and start grinding on the crown trying desperately to make it fit. That is exactly the wrong thing to do from 3 viewpoints:
1. It's very high stress and lacks predictability.
You might waste 20, 30, 40 minutes in a vain attempt to get it to fit. Even if you do get it to fit, it will not be a nice job.
2. You might cement a terminally weakened crown.
The grinding thins the structure and predisposes the crown to later fracture.
3. It's incredibly bad PR.
The patient has paid big money for this crown and you are attempting to cement something that is less than prefect, something that you've had to grind the guts out of to get it to fit their tooth.
So, what's the alternative?
Personally I spend about 10 seconds trying to get something to fit. If it doesn't then it goes in the bin and I take a new impression and get a new one made.
Doing it that way means that you keep your stress levels low and deliver a quality product to your patient. It's fair for all concerned.
By the way, if your laboratory is delivering more than 1% of crowns that don't fit then you have a problem.
Either you are not taking good impressions or your laboratory has issues with quality control.