Calibrating a Gage Block In-house with Calibrated Digital Calipers

O

ollie107

#1
Hi there,

I have just moved to a lab where they have recently had their calipers callibrated. Now they have sourced a rectangular piece of stainless steel (home made gage block), drawn on lines in permanent marker so that there are 3 locations of differing measurement that are always made in the same place, then carried out repeated measurements on this home made gage block with their recently calibrated calipers. This has given them consistant readings, and are now proposing to use this supposedly calibrated gage block to calibrate the same calipers in the future.

The calipers have been calibrated and have shown a measurement of uncertainty of +/- 0.0161mm. The item that we want to measure with the calipers are sieves, with the smallest diameter to measure is 4mm, and according to the procedure needs to be measured within an accuracy of 5%, which is 0.20mm. Now usually I go by the theory that anything that calibrates an item needs to have a count of at least 1/10th of the item being calibrated. However in this case, we dont need the calipers to be that accurate as they only need to be accurate to 0.20mm.
Can someone please confirm whether this method is acceptable in ISO 17205, or as I suspect we need to either need to send the block off to be calibrated.
Any comments are appreciated.:)
:thanx:
 
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Jim Wynne

Staff member
Admin
#2
Welcome to the Cove. :bigwave:

If I understand this correctly, you are calibrating calipers, then using them to verify a custom standard, and then using that standard to calibrate the calipers. The circularity of this strategy negates the traceability of the original calibration of the calipers, assuming that such traceability existed to begin with. A piece of measuring equipment can't be traceable to itself.
 
O

ollie107

#3
Many thanks for that,

O.K so if we continue to have the calipers calibrated by an external lab annually, can we use this home made gage block for intermediate checks throughout the year?
THen if we find the calipers to be out in respect to the gage block between annual calibrations, then we send the calipers to be calibtrated. What do you think? How does everyone else check that their calipers are within limits between calibrations?

:confused:
 

Jim Wynne

Staff member
Admin
#4
Many thanks for that,

O.K so if we continue to have the calipers calibrated by an external lab annually, can we use this home made gage block for intermediate checks throughout the year?
THen if we find the calipers to be out in respect to the gage block between annual calibrations, then we send the calipers to be calibtrated. What do you think? How does everyone else check that their calipers are within limits between calibrations?

:confused:
Commercial gage blocks.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#5
O.K so if we continue to have the calipers calibrated by an external lab annually, can we use this home made gage block for intermediate checks throughout the year?
It has to be calibrated traceable to a national standard. It would also have to have tolerances to be used as a gage, and they have to be either 10:1 or 4:1 (depending on your take on calibration) to the caliper.

What do you think?
Not such a good idea, unless you have a heck of a superfinishing machine. You need parallelism and surface finish on that block that it pretty extraordinary to be of much value. Plus, it needs to be made of material that will not wear and is thermally stable to be of any value.

How does everyone else check that their calipers are within limits between calibrations?
Either commercial gage blocks, as Jim mentions, or set masters
 

Hershal

Metrologist-Auditor
Staff member
Super Moderator
#6
Hi there,

I have just moved to a lab where they have recently had their calipers callibrated. Now they have sourced a rectangular piece of stainless steel (home made gage block), drawn on lines in permanent marker so that there are 3 locations of differing measurement that are always made in the same place, then carried out repeated measurements on this home made gage block with their recently calibrated calipers. This has given them consistant readings, and are now proposing to use this supposedly calibrated gage block to calibrate the same calipers in the future.

The calipers have been calibrated and have shown a measurement of uncertainty of +/- 0.0161mm. The item that we want to measure with the calipers are sieves, with the smallest diameter to measure is 4mm, and according to the procedure needs to be measured within an accuracy of 5%, which is 0.20mm. Now usually I go by the theory that anything that calibrates an item needs to have a count of at least 1/10th of the item being calibrated. However in this case, we dont need the calipers to be that accurate as they only need to be accurate to 0.20mm.
Can someone please confirm whether this method is acceptable in ISO 17205, or as I suspect we need to either need to send the block off to be calibrated.
Any comments are appreciated.:)
:thanx:
The home made gage block can be turned into a check device, used solely for a sanity check.

If the item is to be used as a gage block for calibration, then it needs to go to an accredited calibration laboratory, and the cert needs to report the uncertainties for each of the measurement points. Then make sure it is kept in a secure and clean area, and human hands never touch it again by using cloves. Then it can be used for calibration.

Hope this helps.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#7
If the item is to be used as a gage block for calibration, then it needs to go to an accredited calibration laboratory, and the cert needs to report the uncertainties for each of the measurement points. Then make sure it is kept in a secure and clean area, and human hands never touch it again by using cloves. Then it can be used for calibration.
I can't imagine they would touch it without some kind of specification, similar to the specifications used to identify grades of gage blocks. They need to know its nominal size and its specified tolerance. That tolerance needs to relate correctly to its use as a standard for that calibration (to make the excercise worth the cost). I sense that has not been prepared.
 
J

jfgunn

#8
I think the more important question should be:

Is a caliper the appropriate measuring tool to measure the diameter of the hole in a sieve?

I believe the answer to this is NO. I believe an optical comparator would be the correct measuring instrument for this purpose.

Measuring an internal diameter with flat ID jaws of a caliper does not work well.

If you are going to use this method I would buy a couple of real gage blocks for $50 - $100, then pay the $20-$30/year to have your caliper calibrated and the $20-$30 to have the few gage blocks calibrated as well. (including uncertainties of course).
 
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