Are you sure about that 10 millionths accuracy spec? The very best CMMs can
only do about 14 millionths over that range. If that is sufficient, you
need to find someone who has a Zeiss UPMC Ultra CMM. Remember when
measuring to this level, you have to consider very carefully the pattern of
points to be collected and how the final dimension is calculated. If you
are talking about a flat or nearly flat sample with some sort of pattern,
there are optical profilometers that may do the job.
Optical machines may be fancier than required in this case. The device
to be measured is a 3/8 square bar about 2.5 inches long with some steel
balls soldered to one side. The points to be measured are the balls
themselves and their size is known.
A lab having a flat surface with a post imbedded to hold an indicator
capable of comparing at 10 millionths plus a full set of gage blocks would
be able to make such measurements. A 1x2x3 type of gage with vice like
clamping means would be needed but that should be all - hopefully.
I've found one outfit that states 70 millionths, but that's pretty far
outside the needs here. The 14 millionths you mention would certainly be
close enough and I'll add those devices to the search. Thanks--
On Jan 6, 1:56 pm, firstname.lastname@example.org wrote:
You are facing a very tough problem.
That come out to 350 nm. I have done a bit on MEMS work and an
optical microscope is going to have problems imaging that small of
detail. At that tollaerance one is going to have to specify
temprature. For example if the rod is steel one deg F of temprature
is going to change the lenth ot the bar 12 mico-inches.
It may be possible to get measurements accurate enough with the simple
set-up you describe but do not under estimate the difficulty. To measure the
apex of a ball you need a flat anvil on the indicator. But to get 10
millionths accuracy when the anvil contacts the gage block, it would have to
be in contact to better than 1 fringe optically. In other words you would
really have to wring the anvil onto the gage bock as well as the gage block
onto the bottom flat (which would have to be a glass or metal optical flat,
not a granite table). This would be tricky. The indicator anvil would also
have to be set up perfectly parallel to the bottom flat. Having a very
small flat anvil would help a lot. I know I would not trust the measurment
of one person or lab for this kind of thing
You might do better using a supermicrometer:
But while these have a repeatability of 10 microinches, the accuracy is only
There is also a method for measuring balls where you can directly compare
the ball to a gage block stack using two optical flats. It might work here
if you have a jig to hold the part vertical. The ball is placed between a
pair of optical flats on one side, and a gage block on the other side.
Measure the spacing of the fringes formed between the block and the upper
optical flat and calculate the angle of the upper flat. Then measure the
distance between the edge of the block and the apex of the ball to calculate
the height difference between the block and the ball.
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