As I typically turn items from about 1/4" diameter up to about 4 inches in diameter I measure the runout of my chucks by chucking a short piece of 2" Diameter TGP steel and put an indicator on the side of that bar not more than 1" from the jaws of the chuck then rotate the spindle by manually turning the large countershaft pulley,. That gives me go or no-go confidence that the chuck will provide reasonable repeatability from part to part of for a part that that's already been turned and needs some minor modification. Sometimes I will set the lathe to back-gear mode at slowest possible belt speed and snap on the switch and observe for 30 seconds or so just to satisfy myself that it turns reasonably true under power.
Most of my 3-jaw chucks are less than .005 TIR, and most of them are very old and worn chucks. I have a 6": Buck with two-piece jaws (not a Tru-Adjust) that comes in at just about .002 TIR.
I acquired a Cushman/South Bend 6" three jaw (two sets of jaws) at Cabin Fever Expo in Lebanon PA this past January that was a NOS in the original South Bend box. After disassembling and thoroughly cleaning off the Cosmoline, reassembling and rotating the backing plate through each of the three possible mounting positions the TIR comes in at less than .001.
How others measure TIR may well be different than my primitive method.
Raymond
