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263948 george@g... 2017‑11‑14 Re: Hot mic
Erik Levin described a fine way of demonstrating the "humanity" of a 
micrometer ...

There's a way around that problem: use a toolmakers microscope. One 
looks at the
object in question through a modest-magnification 'scope - usually with 
a 10X
eyepiece and a 3X to 10X objective lens, which (very important !) must 
be in
focus in the plane of the object which you are measuring and twirl the 
two
micrometers (usually with large barrels which facilitate reading to 
tenths and
even quarters of tenths) until one edge and then the other edge is 
centered in
the crosshairs. There's no touching of the frame of the micrometer 
except at
the focus knob and on the outsides of the barrels, so the thermal 
expansion
problem is minimized. And there's no "feel" at issue because there isn't 
any
contact between the micrometers and the workpiece.

I once had to do this while seated at a huge round conference table with 
my
client yacking away in a spirited and irrelevant conversation with about
half-a-dozen interested parties while I was checking the thread pitches 
of
a number of broken bolts for signs of stretching (indicating an 
overload).
The potential for 0.025 errors terrorized me ... Now I've just retired 
(at
age 80) so it's just a matter of remembering what I was doing a few 
moments
ago ...

When I'm machining something for a press fit I usually have to rely on 
the
feel of a small-hole gauge and then, twice over, the feel of the 
micrometer
in order to get the couple-of-mils interference fit that keeps 'em 
together
after applying the arbor press. A few tenths really do matter, and those
feels are important - mainly to keep them consistent.

Recently I was fixing a broken watchmakers lathe cross slide and had to 
make
another 40 t.p.i. feed screw, which had a short length of 50 t.p.i. 
threads
for the graduated thimble that doubles as the backlash adjuster. The 
lathe
had a long life doing chucking work before I got it, so the lead screw 
is
quite worn. I finessed the wear while making the feed-screw portion of 
the
piece by doing that part with the drill rod supported with a center rest
about five or six inches from the headstock where there is presumably 
less
wear, and then doing the adjustment thread up close to the headstock.

It was a pleasant surprise that the fit of the little graduated thimble
gets progressively tighter, the more thread engagement there is, because
of the pitch error in my lead screw - transferred to the 50 t.p.i. 
thread
on the feed screw - which eliminates the need for any setscrew to keep
the backlash adjustment in place. It's actually rather too tight, 
needing
two pairs of pliers and suitable leather cushions, to set the 
adjustment.

I made two such feed screws; the first was a lad too loose, and they 
both
exhibited the same tightening effect for the graduated collar.

Getting the taper out of the three-inch long feed screw was the hardest
part of the whole job, partly with the setover of the tailstock, and
partly getting the center rest adjusted. That was much easier than it
would have been to graft on a length of rod that would have been needed
to reattach the broken-off (and lost) handle.

There were also two lost gibs and an entirely missing top slide to make
from scratch. All that took a couple of months of "spare" time. Works
fine now.

George Langford in SE PA

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