Note that testing the mag chuck by mounting an indicator holder to the spindle area and then traversing the table back and forth and front to back underneath it, does not say anything about how flat the chuck top surface is. If the ways are worn more at the ends (very common), then the table may quite possibly be sagging down on the ends of its travel. The middle, however, will show a very good reading, because you are only checking it at one of the points that supports the table. This spherical geometry is common to find on surface grinders and milling machines. To get a more realistic reading, put 5 blanks, perhaps the size of a coin, on the chuck, one in each corner and one in the middle. Then grind them carefully and evenly, and then measure them for thickness. Compare that with what you got from the test with the indicator attached to the spindle. Another way to do it is to take the chuck off the grinder and test it mounted on a surface plate, with any rock in the corners shimmed up. Of course, then you will need to grind it again when you reinstall it on the grinder. A REALLY good straightedge can also be useful for testing. Sometimes ways will together be "twisted" from left to right, which can show odd results at the corners of the chuck. Never assume that the ways are flat, parallel, and co-planar over their length, instead test for it.
