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4

Use a bore gauge to verify a 3" micrometer.

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woodchucker

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#1
I bought a 3" starrett mic a few weeks ago. It reads to the 10th. It did not come with a standard. And I don't have a 2" standard.
I started looking on ebay, and saw some nice Mitutoyo standards. Was about to buy them when I thought, hey, I have a bore gauge (Lufkin).
And I thought, I could setup the bore gauge with a 2" setup zero it, and then calibrate the mic. My 2" mic agreed with the reading.
Well , maybe it's not as accurate, maybe I can't trust the 10ths. But I believe it to be a solid setup.
What do you think?
 

Profkanz

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#2
Could be done.
But I think you will find it to be pretty iffy setting an adjustable with an adjustable.
There are good reasons that fixed standards are used for this. Most likely speed and ease of use.
 

Chipper5783

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#4
Make yourself a "standard". It does not need to be 2.0000" long. In fact any where between 2" - 3" will be fine, so long as you have a way of accurately determining the length (call a friend?) - label it and keep it with the micrometer.

Per above comments, you could use most any quality piece of tooling. I'd bet even a cheap 123 block would be good enough, especially if you could verify a location on it (call that friend?) as 2.000x, record the location and the 4th significant digit after the decimal.
 

woodchucker

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#5
Most likely speed and ease of use.
That's what I'm thinking. My 2" mic reads 2" on the bore gauge, but it is difficult to hold a bore gauge w/o a lock and make sure it's set.
The gage blocks that Ken recommends is a good idea. Then I get dual use. Even if the set is off by 10ths. My work is not at the 10th, not for money at least.
 

4gsr

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#6
Generally, your bore gage or inside mic is checked against your OD mics.

When you refer to a bore gage, first thing that comes to mind are the 3-point ID measuring instrument that generally reads in tenths. These generally require checking against a calibrated setting ring. No way of using this type of instrument to verify the calibration if a OD mic.

The other type of bore gage is the dial type which is set to a calibrated setting ring or can be set using a mic, but cannot be used to check at mic to.

Overall, you really need to get the proper mic standard to calibrate your mic to, or invest into a decent set of gage blocks. Most used ones are decent for H-M shops to use. I have several used sets I've bought over the years. Even though, you can't rung them together anymore, they are still accurate for checking your mics to, most of the time. I do have a good set of Mitutoyo square gage blocks that are in excellent shape I've used in comparison to some of the "really" used gage blocks using mics and I cannot detect any differences in readings. Again mics can only detect measurements of "tenths", where gage blocks, with proper equipment, controlled environment, and expertise, can detect measurements of "millionths" of an inch. That would be splitting a human hair about 100 times!

In my real world I work in, If I get things within .001" of an inch, I'm doing good!

If you find the mic to be off a little using your bore mic to check against, don't adjust the mic. Your mic is more than likely dead accuracy compared to the bore mic. Just my two cents worth. I don't claim to be an expert at this stuff, just lots of experience dealing with precision measuring tools and QC people that are supposed to know how to properly calibrate your measuring tools. Save that discussion for another time.

Maybe some day we can get Tony to speak on this. He's our residence expert on Quality in my opinion.

Ken
 
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