Micrometer Calibration - Seeking actual Calibration Procedure for 0-1" Micrometer

Q

Qualqueen

#11
Re: Micometer calibration - Seeking actual calibration procedure of a 0-1" micrometer

Be sure to check the flatness of the anvils for valid calibration - either using gage balls or optical flats. It is one of the most overlooked aspects of micrometer calibration in in-house labs.
This is a very valid point. Does anyone have a procedure for checking with optical flats? :cfingers: I know it pertains to the number of bands.
 
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bobdoering

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#12
I do know that the Edmunds Scientific Optical catalog has a nice overview of the general use of optical flats (may also be on their website), but I do not have a procedure specifically to use them for micrometers (or other measuring surfaces of hand gages!) Other optical flat/monochromatic light suppliers have good info, too.

For those that have never used optical flats - you need the optical flats and a monochromatic light! I have been places where they had flats, but no light - and no idea how to use them!
 
J

John Nabors - 2009

#14
This is from mcnallyinstitute.com. It refers to measuring mechanical seal faces rather than micrometer anvil faces, but it might still be edifying:



There are several ways you could measure flatness :
  • You could place a straight edge on the surface and look for daylight between the straight edge and the sample. As you would guess this method is not accurate enough for our purposes.
  • You could place "machinist's bluing dye" on a know flat, rub the sample piece against it and look for transfer of the dye. Again this method would not be accurate enough for our purposes.
  • You could read the flatness by using an optical flat and a monochromatic light source, and this is the method that is used by all of us in the sealing industry.
To understand this last method of measurement you only have to know that it is a characteristic of light that when two lights of the same wave length interfere with each other, the light disappears and the reflecting piece goes black. When you discuss visible light, color and wave length mean the same thing, so to make the measurement we use :
  • A monochromatic or single wave length light source. Any color (wave length) could be used, but most companies use a pink color that comes off a helium gas light source. This color has a wave length of just about 0,6 microns (0.000023 inches).
  • You will also need a precision ground and polished clear glass of optical quality that has been lapped flat on at least one side.
The optical flat is placed on the piece to be measured. The monochromatic light is aimed at the piece and this light reflects off of the piece back through the optical flat causing interference light bands. If the distance between the optical flat and the piece we are measuring is one half the wave length of helium, or an even multiple of the number, the band will show black. This is referred to as a helium light band and because it is one half the wave length of helium it measures 0,3 microns or 0.0000116 inches.

So call one helium band roughly 10 millionths of an inch. Hope this helps.

Regards -John
 
J

John Nabors - 2009

#15
For those that have never used optical flats - you need the optical flats and a monochromatic light! I have been places where they had flats, but no light - and no idea how to use them!
Lord only knows how many Q-labs out there have one of those B&S micrometer checking sets (you know the ones, in the wooden box with the 10 gage blocks stepped .105" apart and the two optical flats) that have no monochromatic light, but still think that they are really calibrating flatness on their micrometer anvils. I wonder what it is that they see?
 

bobdoering

Stop X-bar/R Madness!!
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#16
Lord only knows how many Q-labs out there have one of those B&S micrometer checking sets (you know the ones, in the wooden box with the 10 gage blocks stepped .105" apart and the two optical flats) that have no monochromatic light, but still think that they are really calibrating flatness on their micrometer anvils. I wonder what it is that they see?
I know! I have seen that!
 
J

John Nabors - 2009

#17
If I may step outside the box for just a minute..

I once calibrated a micrometer using optical flats and He light, then, just out of pure darn curiousity, clamped the mic into a Zeiss CMM with a scanning head and checked flatness. Guess what? I got about .00003" by both methods.
 
C

chriswhitehead

#20
dbulak: Not meant to hijack your original thread, apologies.

JStain1,
We dont have that procedure 33K6-4-15-1. We custom developed internal procedure for our micrometers.
Just curious about one point; while measuring master between between the anvils,Is there any mention about "Number ratchet rotations"?
I was told by a senior metrologist from the old school as 2 clicks. About right?
Govind.
I've been trawling all avenues for micrometer calibrations. Opinion is devided on the number of clicks needed. I've seen in writting the need for two or three clicks, never one. If I have a play I would say I do indeed need two or three clicks. In my procedure I'm writing I've stated two quicks, later changed it to three, then put it back to two. I feel two is enough from my trials on the real thing.
 
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