I tried looking for the meaning of "being off a mill" or "being of a mill" and I couldn't find any. The sentence is difficult to understand. Can you please tell me where this text is from, or add a few sentences which appear before and after this sentence?
It's so kind of you. The situation is the width of a roller causing quality issue. The reason should be the width being off a mill makes the fuse incorrectly. That's all I have in my hand. I just cannot catch the meaning of them. It must be something I do not know.
After reading Ouisch's post, the sentence is somewhat clearer. It might mean that, if the width of the roller is off by one mil (a thousandth of an inch would translate to 0.00254 cm), the fuse will, I assume, not fit.
"Being off" means the measurement is not accurate. For example, if I order a specially made cabinet for a big-screen TV, I'll supply the measurements (height, width and depth) to the cabinet maker. If I bring the cabinet home and it is half an inch too small, the TV won't fit inside. I'll call the cabinet maker and tell him "this cabinet is half an inch off."
A fuse plugs into a socket, and it must fit exactly. If the measurement is even just incorrect by one mil, it will not fit. The rollers you refer to, I believe, are the ones used to fabricate the metal plate and rows of connecting pieces used to manufacture fuses. The rollers must be of a very precise diameter in order to make fuse components of the proper size.