ok, some setup - I have a mill (Abene), I just bought a chinese coaxial indicator. I got a large Starrett caliper in the mail with some other stuff - it has a threaded post that is part of a larger (about 3/4 inch) protrusion that forms part of the friction hinge. The threaded post was broken off, hence too short. Thinks I, this is an opportunity to learn to use that coax indicator - I'll just mill the rest of the post off, center with the indicator and drill and tap a new hole, insert threaded rod (cut head off screw) and I'm good to to.
Well, I clamp the part to the mill table firmly, mill off the stub, center with the indicator, insert a center drill in a chuck to drill a little starting hole, and as I raise the table to the drill, I notice the hole is at least 1/8 inch away from the center. I repeat this several times, each time centering so there is less than 1 thousandth movement on the coax indicator, and each time the hole is not centered. So, I set it by eye, measure and drill - it's perfect, and all is well
EXCEPT!!! - what the heck went wrong???
well, maybe the table moved - no, I had the gibs locked. maybe the piece moved (no, it didn't)
well, mabe the indicator is bad - or maybe not.....
I tried the indicator in a 3/8 end mill holder (the shaft is 3/8) - noticed about a thousanth clearance (well, maybe 2, I didnt' measure it) and I also tried it in a drill chuck - both ended up with errors - '
maybe the cat 40 taper is bad, or dirty - clean it, it wasn't dirty.
Put 3/8 drill rod into chuck - it sweeps out a cone - waaay to much error - so there is some angle on the chuck with respect to the spindle - that's not good. put 3/8 drill rod into 3/8 end mill holder - about .100 runout at the tip - not good (note - drill rod about 5 inches long, not bent) put 3/8 drill rod into Ericksson collet holder - TIR less than .002 ---- that's more like it Put 1/2 inch drill rod into 1/2 inch end mill holder - no measurable runout - repeat, mesure .001 - good....
so, I've got at least two problems I figure, maybe 3 -
- the 3/8 end mill holder may be damaged since it seems to be oversized
- the chuck is obviously not running true so I'll need to separate it from the NMTB40 to jacobs taper adapter (it's a name brand adapter, though I don't remember the name right now) - the runout is more than the chuck should have - so ????
- maybe the indicator was defective - how to test?
And, once all this is solved, how can I hold the indicator so I am sure it's doing it's job and not finding me a hole that is offset from the center?
ok, no politics, this is real on topic stuff