Dave August wrote:

Yeah I'll jump in here.

I was taught to use all three holes on a jacobs chuck...
I agree with the 'coller gets torqued differently' theory...



I bought a new import 3 jaw lathe chuck a couple of years ago and it
came with a final inspection chart.

I noticed that in the section of that chart where the runout of a test
rod clamped in the chuck was recorded, there was one data line for
"master pinion" and another for "any pinion".

The runout written on the line for "master pinion" was less than the one
for "any pinions".

Looking at the chuck, one of the three pinion tightening holes had an
"M" stamped on the chuck body next to it.

I checked it myself after the chuck was mounted, and the inspection
chart was correct. Tightening the "M" pinion gave less runout than
tightening either of the other two pinions.

I checked another 3 jaw of mine and found a similar runout difference
when tightening different pinions, so I stamped an "M" next to the one
which gave the lowest runout.

Course now that I just finished writing this, it occurs to me that might
only hold for tightening the jaws on a piece with a specific diameter,
and that tolerances and wear on the chuck scroll may well make one of
the other pinions become the "master pinion" when smaller or larger
diameter test rods are grabbed.

Oh well, it was a nice theory anyway...and I still paint a witness mark
on the chuck and what's in it if I have to remove and replace the workpiece.

Jeff

--
Jeffry Wisnia

(W1BSV + Brass Rat '57 EE)

"Truth exists; only falsehood has to be invented."