hello again, i just can't stop thinking about this ... i don't believe that there is backlash, especially because 1/4* means 0.554mm circumference length for the od of a 10" chuck, and that is huge, meaning that the C axis should be used only with the break on during drilling, and it would shake during contouring
vzshc=1/4*means that there is a missalignment in Y direction; there may also exist a missalignment in X direction, but this one can not be corected by using vzshc, so, for now, let's focus only on Y missalignmnet : for example, if there is 0.1mm Y radial excentricity, and drilling occurs at X50, then it will be required to use a C phase corection of 1/4*, and this sounds pretty realistic
so far so good ...
so, deviation is 1/4* +
a bit; 1/4* may be from Y missalingment, and 'a bit' may be from the fixture tir ( because the part is re-clamped ) and/or drills buckling/bending, because of wear/length/specs
kindly