Calibration of micrometer by gauge block

N

nick1980

#1
I have audit next week. Pls help!

I want to use a gauge block to calibrate a micrometer.

My product's min width 0.002mm.

My micrometer can show to 4 digit, i.e. 0.0005mm, 0.0010, 0.0015, etc.

A standard 0.002mm gauge block that is measured by external lab. The lab. report shows it is 0.00205mm.

A consultant told me I can't use it the gauge block to calibrate micrometer because it is out of tolerance.

His explanation as below:
min spec=0.002mm
micrometer's resolution=0.0005 (4 digit at least)
micrometer's tolerance=0.0005/2=0.0002mm (at least)
gauge block's resolution from lab report=0.00005mm (5 digit at least)
gauge block's accepted tolerance=0.00005/2=0.00002mm (at least)

He said the gauge block is out of tolerance as 0.00210-0.002=0.00005mm, out of 0.00002 acc tol.

--------------------------------------------------------------------
I don't understand his calculation. It's so difficult to pass the calibration as he said above.

By 1/2 accuracy rule (best is 1/10), I suppose

min spec=0.002mm
micrometer's resolution=0.0005 (4 digit at least)
micrometer's tolerance=0.0020/2=0.0010mm (at least)
gauge block's resolution from lab report=0.0005mm (4 digit at least)
gauge block's accepted tolerance=0.001/2=0.0005mm (at least)

Am I correct?

Can you all explain how to define the internal MMD and external standard tolerance?
 
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Q

qualeety

#2
inadequate measuring instrument

Huh? 1/2 accuracy rule? Can you quote me the source? The minimum should be 1/10, not 1/2. (If i am wrong, I would like someone to correct me on this)

You product's min width 0.002mm....means your meansuring instrument should have the resolution of maximum 10% of the tolerance width or 0.0002mm

Since your micrometer can show the resolution of 0.0005mm, it is inadequate.

To answer your questions

Similarly, the gage block should be maximum 10% of the resolution of the instrument you are using. So, as long as the accuracy of the gage block is less than or equal to 0.00005mm (10% of your micromenter resolution of 0.0005mm), it should be ok. Thus, you can use your gage block to calibrate the micrometer.

Furthermore, regarding your statement "micrometer's resolution=0.0005 (4 digit at least)", it is not a matter of number of digits but resolution. 0.0005 and 0.0001 are completely different, athough both are four digits.

I hope I answered your question. If you need more info, please refer to AIAG Measurement System Analysis or other metrology books.
 
J

jane_ackerman - 2010

#3
nick1980 said:
I have audit next week. Pls help!
A standard 0.002mm gauge block that is measured by external lab. The lab. report shows it is 0.00205mm.
First, I'm guessing that meant to say that you have a 1.002 gage block?
Whatever the case, it all depends on what "grade" of blocks you have.
I have a set of grade 1 (00 by new standards) and the accuracy requirements used by the company I send them to (an A2LA accredited lab) are as follows:

Thru .500 mm = +-/ .00010
Thru 10.0 mm = +/- .00007
Thru 25.0 mm = +/- .00007
Thru 50.0 mm = +/- .00010
Thru 75.0 mm = +/- .00012
Thru 100.0 mm = +/- .00015

One of our other plants has a grade 0 set and their accuracy requirements are as follows:

Thru .500 mm = +-/ .00014
Thru 10.0 mm = +/- .00012
Thru 25.0 mm = +/- .00014
Thru 50.0 mm = +/- .00020
Thru 75.0 mm = +/- .00025
Thru 100.0 mm = +/-.00030

They (our accredited calibration house) faxed me a copy of the tolerance requirements for all grades within a matter of 2 minutes... I'm sure yours will do the same with a simple phone call.

Hope this helps :)
 
N

nick1980

#4
I want to know the tolerance decision concept. I suppose the step as below:

1. Decide the min spec. of product part, 0.002mm.
2. Decide the resolution of internal equipment, 10 % of min spec.= 0.002/10=+/-0.0002mm. (Max. is 10% of resolution. How about min. accepted %, 25%, 33% or 50%?)
3. Decide the gage block's resolution from lab report, 10% of resolution of internal equipment=0.00002mm.
4. Decide the accepted tolerance of gage block, 10% of 0.00002mm, +/-0.000002mm. (I'm not sure)---> (but it seems very difficult to have such accurate equipment in external lab.)

Can you all tell me the steps is correct or not?
Also, is the above as the correct step for choosing appropriate and capable equipment?
Most concern and confusing to me is the acceptance criteria of micrometer and the gage block. (As Jane said, the tolerance can be found from the manufacturer of my grade 1 gage block. However, my consultant said I use a wrong gage block without sufficient accuracy.)

-------------------------------------------------
From the consultant's comment below, I don't understand his decision concept. He uses 10% of min. spec. to determine the micrometer's resolution.

However, do you all agree his decision concept of micrometer's tolerance 0.0020/2=0.0010 (At least).

min spec=0.002mm
micrometer's resolution=0.0005 (4 digit at least)
micrometer's tolerance=0.0020/2=0.0010mm (at least)
gauge block's resolution from lab report=0.0005mm (4 digit at least)
gauge block's accepted tolerance=0.001/2=0.0005mm (at least)
 
J

jane_ackerman - 2010

#5
Original statement: “He said the gauge block is out of tolerance as 0.00210-0.002=0.00005mm, out of 0.00002 acc tol.”
New statement: “However, my consultant said I use a wrong gage block without sufficient accuracy”

“Out of tolerance” and “insufficient accuracy” are 2 totally different issues.
Which are you asking about?

As far as gage accuracy:
The auditor cannot determine the accuracy of the gage based on resolution. The manufacturer will state this in the owners manual and/or catalog and it varies from being equal to the resolution, if not worse (I have yet to find one with an accuracy better than the resolution… but that doesn’t mean that one does not exist).
Mitutoyo Examples:
293-330 ( 0-1” mics ) Resolution .00005” Accuracy +/- .00005”
406-721-30 ( 0-1” mics ) Resolution .00005” Accuracy +/- .00015”

As far as a block being out of tolerance:
I have actually covered myself in the past (good reason trying to stay within the 10% rule) when I had a gage block exceed calibration tolerance.
I replaced the block, but noted my investigation on the cert (the amount of tolerance exceeded on the gage block would not have impacted the calibration results or affected product quality because the micrometer cannot read to that small of a resolution).
IE: My 1.17 gage block was actually 1.1703. This made no difference on a micrometer that has a resolution of .001

See pages 42 & 43 of the MSA Manual for the source of the following information:

When referring to the 10% rule, the MSA Manual only calls this “a general rule of thumb”

When talking about Reference Standards, it states (shortened, not quoted):
Higher order of discrimination
Lower measurement system error
Calibrated with traceability to NIST
(nothing about the 10% rule)

Additionally, the trend has shifted from using product specification to using process variation to determine gage discrimination.
 
B

brahmaiah

#6
I have audit next week. Pls help!

I want to use a gauge block to calibrate a micrometer.

My product's min width 0.002mm.

My micrometer can show to 4 digit, i.e. 0.0005mm, 0.0010, 0.0015, etc.

A standard 0.002mm gauge block that is measured by external lab. The lab. report shows it is 0.00205mm.

A consultant told me I can't use it the gauge block to calibrate micrometer because it is out of tolerance.

His explanation as below:
min spec=0.002mm
micrometer's resolution=0.0005 (4 digit at least)
micrometer's tolerance=0.0005/2=0.0002mm (at least)
gauge block's resolution from lab report=0.00005mm (5 digit at least)
gauge block's accepted tolerance=0.00005/2=0.00002mm (at least)

He said the gauge block is out of tolerance as 0.00210-0.002=0.00005mm, out of 0.00002 acc tol.

--------------------------------------------------------------------
I don't understand his calculation. It's so difficult to pass the calibration as he said above.

By 1/2 accuracy rule (best is 1/10), I suppose

min spec=0.002mm
micrometer's resolution=0.0005 (4 digit at least)
micrometer's tolerance=0.0020/2=0.0010mm (at least)
gauge block's resolution from lab report=0.0005mm (4 digit at least)
gauge block's accepted tolerance=0.001/2=0.0005mm (at least)

Am I correct?

Can you all explain how to define the internal MMD and external standard tolerance?
Using gage blocks to calibrate a Micrometer is a wrong method.You have to use cylindrical master plugs.Gage blocks cannot contact the worn out central portion of the micrometer.
If your concern is only the upcoming audit.The auditors only will check if you are following the documented procedure/work instruction.You have to document a work instruction for calibration and match it with your practice.Auditors are not required to be experts in every thing.
V.J.Brahmaiah
 
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