Why Can't I get my DTI to Repeat

I had an issue on a new D1-4 face plate that was kind of a non-repeatable misfit every time I mounted it. Turns out the nose taper was just a smidge small relative to the spindle nose. So even though the cam locks were pulling it inward & felt tight, it was kind of bottoming out prematurely. Clue was a tiny gap between the spindle face. Backplat mounts are kind of a tricky precision fit, it has to engage the taper with no slop but also snug to the boss flange. Not either/or but both. Once I blued the surfaces I could see what was going on. So some careful lapping using a dummy arbor brought it into tune. You might have something related to the tru-turn screws but this would supersede it so worth checking.
 
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Just thinking about possible causes intended to try an help solve the problem I have to ask,
Are you sure you are using the Dial Indicators correctly?
 
Im not saying that i know of The Right way or of The Wrong way, just asking just in case because at this point human interaction aka operator error is high on my possibilities list based on all that you have already done to diagnose the problem.
 
No offence taken. I want to learn and if I'm doing something wrong I want to know.

Agree on the camlock. I made my own backplate and even though the taper seemed to fit nice if the backplate doesn't just kiss the spindle noise forget about any repeatability.
 
Puzzling, indeed! You do seem to know what you're doing, and the setup in your photo looks OK. I'd be inclined to agree with you, that it might well be the mount. Just for fun, have you tried lightly pushing or tapping on the indicator, to see if it's mounting is solid? If the holder/mounting is weak or faulty, the problem will probably show up when you do that.

What I've done to get a solid mounting (at least for a dial indicator) is to drill holes in an aluminum plate, aligned with the screws that hold the back to the indicator. Screw on the plate in place of the normal indicator back. An extension on the side of the plate is held by the tool mounting screws of a QCTP tool holder. Align the holder vertically so that the indicator tip is on the vertical centerline of the workpiece. Use the cross slide to bring the indicator tip into good contact with the workpiece/test bar.
 
John,
Do you mean something like this? I will say three revolutions and it finally stumbled back to zero.
293244
 
Cannot be the camlock. Even if it is off, the dti would read the same amount of off each time. If you are not repeating, the only thing that can do this is the holder. I didn't read this whole thread carefully but do you have an indicator holder that fits your tool post? Something without arms. If so, mount it in that and the indicator should repeat.
 
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