Originally Posted by
3006guns
Wait! You're not done yet! If you run the spindle all the way down and it shows zero on the thimble, that does NOT mean it's automatically accurate. You need a micrometer "standard" for adjustment...a piece of ground, polished bar that is precisely 1" in length (or longer, depending on the capacity of the mic) at room temperature. Gently run the spindle down until it touches the ends of the bar and note your reading. If it zeros on 1", you're good. If not, you will need the little spanner wrench for adjustment...available Starret or a number of suppliers. Instructions for adjustment are available on line also.
Having said that, a micrometer standard is expensive so another method would be to compare your mic against one that is known to be accurate. Just measure a piece of bar stock with the first mic, then adjust yours to match. If all else fails a good machine shop with a sympathetic machinest would probably help you.
They're a wonderful, simple tool that will last for years with any care at all.