John wrote:>[I don't believe that a simple micrometer like this can actually
read down to
>0.0001. The last digit is probably more of a "suggestion" than a fact. Just
>holding the mic in the heat of your fingers might make a difference - and
>Starrett certainly made mics with hard rubber insulation on the frame.]
Actually, they can do better than that, though most handheld mic's have vernier
scales that are too small to do better, and, yes, hand heat makes a difference,
especially on larger mic's. The best I have in inches is 0.00005" (50
milliionths of an inch, or 1.27 microns) and holds to about 0.0001" over the one
inch travel, but when it was new would have been better. It has a large thimble,
so it can be easily read to that resolution. I also have a 1 micron metric, but
it is only spec'd to 3 microns over 25mm travel. The vernier scale mics are the
same as the non-vernier except for the extra lines on the barrel/sleeve, and
Mitutoyo claims that you can read to 0.0001" with a 0.001" mic, as the
graduations are the appropriate width for this. I don't recall if the lines are
equivalent to 0.0001" or 0.0002" wide, but estimating to better than 0.0005" is
a bit sketchy, calibration and tracability of the eyeball and all.
The 1" round standard is useful to check for detecting common wear issues, in
particular near the zero where the most wear happens, as well as providing a
check against the zero for confirmation. The main use, IMHO, is for checking
calibration when the tool will be used to measure round parts. Yup. It makes a
difference. The tool is best calibrated a) at the dimension to be measured, and
b) using a standard of the same geometry as will be measured. Not a big deal in
general, but for the highest precision work (think a no-play, polished, slip fit
for a shaft), it makes the difference between a fine job and merely workmanlike,
or even unsatisfactory. I use gauge pins for this most of the time since they
are inexpensive and readily available to better than one micron in whatever size
you need. No, I don't work to that tolerance very often and it is not my forte,
but I like fine tools (gee, go figure) and like to keep them in top trim, and
pay for annual re-certification of my most used standards. It actually has paid
off.
Demo I have done with students: Using a large outside (caliper) micrometer with
no heat shields and a good, mirror shine, preferably carbide, anvil -- I use a
6" Tumico tubular frame, as I have one that meets the requirement and shows the
effect well due to the low mass -- measure the standard with the mic held in a
stand and temperature equilibriated. Then, hold the mic for a minute (by the
frame) and remeasure. The 6 to 9 millionths/degree F growth (on a 6"mic, a 10
degree F gain over the 8" of the frame is about 50 millionths of an inch) is
easily measurable. Then remount in the stand and let the standard warm in a
pocket for a few minutes (from 20 to 35C, or 68F to 95F). Same thing. Then
remove the spindle and sight down the bore. You are looking at a reflection of
your pupil, and is is easy to see that it is centered. Hold the mic frame by one
side and watch your pupil creep toward the other side as the frame bends.
On a 1" 0.0001/0.01mm mic, it is really not that significant on a 0.0001" mic,
and pretty much insignificant on a 0.001". *** This message was sent from a
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