Practically speaking, I get myself tied in knots trying to think it through, but, with a 4-jaw dialing out spindle runout it seems that you'd only be able to get it true to one spot in Z. For any machining operation requiring a cut parallel to the spindle (like chambering) as you moved away from the spot you'd dialed in, your error increases. Which would be less and less true, the less runout there was to begin with, so, less runout is ALWAYS better. No?
Not exactly true. In chambering we not only want to be as accurate as we can in concentricity but also in cylindricity. Your observation of the error getting worse the further from the spindle nose is an example of being out of cylindricity.
See if you had a test bar in the spindle and it read +.0005" at some fixed point on the diameter then dialed to the end of the bar and got the exact same runout at the exact same position that would be an error of concentricity and a perfect reading of cylindricity.
Please don't take these comments as a high and mighty attitude (and not saying your did either) I was just tasked on more then one occasion to verify a test report from a machine supplier. I learned a lot from that old toolmaker. He taught me how to scrape too. So I've had first hand experience pulling out my hair trying to eliminate all the variables to be able to confirm a number.
It's a royal PITA and it enough to make you go bald and have an ulcer all at once.
