ive stumbled upon an article describibng exactly what youre talking about:
"When we run the table back and forth and side to side with an indicator mounted in the spindle, we can see the tendency of the table to dip (move away from the spindle) at certain wear points. At the extremes of end to end travel on a big mill, we will also see the weight of the overhanging table cause the opposite end to rise a bit. The amount of dip can give us an indication of wear to the sliding surfaces. A few thousandths (.001-.0025") would be normal, more would be excessive. I said before that if the table passed under the indicator at a uniform angle, it would go undetected. How then is this dip possible? By uniform angle, I refer again to the example of the car driving up the ramp. If for argument's sake, someone cut several thousandths off one side of the saddle so the table angled to the floor slightly in one direction, that would cause a uniform angular displacement of the table. Wear tends not to be uniform and would only cause local displacements which would register on an indicator. "