Does anyone know the possible failure modes of overspeeding the rated RPM of your spindle?
One possible mode of failure which comes to mind is:
overheating -> cooked bearing grease -> no bearing grease -> foul language
I am talking about two scenarios:
1) Spindle rated @ 3500RPM, but used at 6000
2) Spindle rated @ 6000RPM, but used at 8000
First clue will be heat. With prolonged use, the bearings will get hotter, and noisier, and friction and heat and noise will continue to increase with further use. Basically, shortened lifetime. How much depends on the specific bearings, VERY much on how they're set up, lubrication, or lack thereof, etc. It will not spontaneously explode, if that's what you're concerned about. The failure will be slow, and not the least bit dramatic, unless you ignore the warning signs for a very long time. Even then, it's final act will be to sieze up.