How come sometimes the obverse and reverse don't line up? I've seen a good number of coins where the obverse and reverse don't line up. Obviously the front and back are upside down from each other, but sometimes you find a coin where it appears the top or bottom die must have shifted off center a few degrees. For example, you line the obverse of the coins up in your Dansco, but when you flip the page there are one or two coins that do not line up with the rest. How common is that and when did the US Mint implement sufficient quality controls to prevent it from happening? Is there a date after which if you find an off center coin that you assume it is an error, or a counterfeit? Thanks.
Die rotation used to be very common. The shafts of the dies were round and they were held in place by some sort of set screw. Sometimes the dies were placed in the coin press out of alignment and other times the die simply rotated during use.
Assuming you are talking about rotated dies, as hobo said the dies after hubbing are steel cylinders. Now if you used a set screw pressed against a steel cylinder it wouldn't take much vibration before te die started rotating. To prevent that they grind a flat area down the side of the die body. (You can see this flat side on the state quarter dies the mint sold.) It is easier to lock the setscrew against the flat, and even if the screw loosens a bit the die still can't rotate. But these flats are ground on the die by hand. So say they are supposed to be at 6 o'clock, but the mint worker doesn't get it ground at exactly 6 o'clock. Say he grinds it at the 5:30 position instead. Now when the die is placed in the press and the set screw is tightened against the flat, the dies will be rotated 15 degrees off from where they are supposed to be. Sometimes the technician doesn't pay attention and grinds the flat well off from where he should. Sometimes the setscrew can vibrate out completely and the die can rotate. Mint tolerance allows a 15 degree rotation so as long as the flat is ground somewhere between 5:30 and 6:30 the rotation will be within tolerance. In the early mint the setscrews were simply pressed against the cylindrical body and some issues can be found with various rotations, and in some cases the dies were removed for polishing and then put back into the press with a different rotation. A good example of this is the 1807 S-276 large cent. While this coin can be found rotated to most any position most of the coins come in one of three positions because the die was removed for polishing twice and replaced with a different rotation.
