Dial thickness gage calibration

If the dial thickness gage has flat anvils, as most of them do, you will want to make sure they lie parallel when closed. Clean the anvils with alcohol as needed. The easy way to check this is to hold them up to a light and look for gaps. If there are no obvious gaps then set the dial to zero. At this point, insert a calibrated gage block between the anvils and check the dial reading several times. You should not be off by more than one graduation. If it's off, then make sure the block is lying flat and properly seated. Also make sure that the anvils—and the gage block—are very, very clean. It stands to reason that if the anvils are not parallel, you'll get a different reading on one side of the anvils. Use the gage block to test for this possibility.
If your thickness gage has a relatively short range, then one or two different size gage blocks should suffice for calibration. Choose one for the middle range and one for the far range. Of course, if you only use the thickness gage for some specific measurement, then choose a gage block with approximately the same dimension.