Erik Levin described a fine way of demonstrating the "humanity" of a
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
eyepiece and a 3X to 10X objective lens, which (very important !) must
focus in the plane of the object which you are measuring and twirl the
micrometers (usually with large barrels which facilitate reading to
even quarters of tenths) until one edge and then the other edge is
the crosshairs. There's no touching of the frame of the micrometer
the focus knob and on the outsides of the barrels, so the thermal
problem is minimized. And there's no "feel" at issue because there isn't
contact between the micrometers and the workpiece.
I once had to do this while seated at a huge round conference table with
client yacking away in a spirited and irrelevant conversation with about
half-a-dozen interested parties while I was checking the thread pitches
a number of broken bolts for signs of stretching (indicating an
The potential for 0.025 errors terrorized me ... Now I've just retired
age 80) so it's just a matter of remembering what I was doing a few
When I'm machining something for a press fit I usually have to rely on
feel of a small-hole gauge and then, twice over, the feel of the
in order to get the couple-of-mils interference fit that keeps 'em
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
another 40 t.p.i. feed screw, which had a short length of 50 t.p.i.
for the graduated thimble that doubles as the backlash adjuster. The
had a long life doing chucking work before I got it, so the lead screw
quite worn. I finessed the wear while making the feed-screw portion of
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
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.
on the feed screw - which eliminates the need for any setscrew to keep
the backlash adjustment in place. It's actually rather too tight,
two pairs of pliers and suitable leather cushions, to set the
I made two such feed screws; the first was a lad too loose, and they
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
George Langford in SE PA