Crankshaft grinding is not an automated process and requires a large amount of skill from the machine operator to get the job done right. Many engines have detected vibrations in them during operation, which is caused by machinists not doing the proper work needed to keep the shaft balanced once it is installed and in use.
Improper grinding and measurements can mess up the internals of the crank once it is in operation by the grind being off by even 1000th of an inch. This will cause additional wear and tear on your crank and rods, causing even more damage over time.
When doing crankshaft grinding, you need to make sure that it is properly centered and properly angled to get each throw exact. Each pair of throws for the rods will require the entire crank to be repositioned a precise distance from the center line of the crank. The operator needs to know what the crank goes to so that they don't apply improper cuts to the shaft, causing issues in the system once it has been reinstalled.
Aligning the Front and Rear of the Crank
The other aspect that comes into the process during crankshaft grinding is keeping the front and rear ends of the crank on an even scale to prevent rattling from it being off balance. In the event that this is not maintained properly, the entire shaft will put unneeded stress on your rods while it is in operation, causing the engine to vibrate until something breaks.
Avoid Over Grinding
It is a good idea to find out if the crank has been in a shop for crankshaft grinding in the past. This is going to effect the end product in a very serious way, as only so much material can be removed before it renders the part useless, so before thinking that your crank is bent or warped, you will want to check a Chilton's and find out the exact measurements of the part. If the measurements are off, then it is likely that it has been ground in the past. If the measurement is dead on, there is likely nothing wrong to begin with.