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 -
1. the 3/8 end mill holder may be damaged since it seems to be oversized
2. 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 ????
3. 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
--
bill
to email me, to to my web page,
www.wbnoble.com and find my email
or unscramble the following by removing spaces and correcting the obvious
spelling errors
wil lia m_b_n obl e at msn daught com
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