Calibration of Calipers, Mics, Height gauges - Recommended Gage Block Types

[Eloy Gomez]

What type of gauge blocks would be recommended for calibration of simple measuring precision tools such as calipers, micrometers, height gages, etc....? I was thinking an 81 piece grade B gauge block set?

 

[BigEd09]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

Hi Eloy,

We keep our calibration process very simple for these devices. We usually measure 3 to 4 different sizes of blocks depending upon the range of the device.

The important thing to remember is that for your calibration to be valid, the blocks that you use must be traceable to an NIST standard.

Hope this helps.

 

[DrM2u]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

What type of gauge blocks would be recommended for calibration of simple measuring precision tools such as calipers, micrometers, height gages, etc....? I was thinking an 81 piece grade B gauge block set?

Here are some things for your consideration:

1. The blocks that you will have/use must be able to cover the measurement range for all of your equipment that you will verify with them. An 81 piece set should be able to cover up to 12" if you stack the blocks together (12" calipers, height gage, etc). However you have to consider that the blocks are not perfect and the more blocks you use the more error you induce.

2. The Class of the blocks has an impact on the precision of the verification. Usually they are either PLUS or MINUS and have a certain tolerance range. The tolerance is tight enough to make the use of general purpose multiple blocks to verify a caliper perfectly feasible but might raise some doubt when verifying high precision micrometers. I prefer to use a MINUS set because in reduces the risk of 'oversizing' when stacking multiple blocks (due to impurities between the blocks and the accuracy of the blocks).

3. Consider the cost of purchasing and 'calibrating' the set. I found that it is often cheaper to replace a gage block set than to have it 'calibrated'. In my implementations I usually set the calibration intevals to 5+ years depending on the frequency of use. Another approach is to have a set for regular/daily use that you can verify with a set of high precision micrometers that you verify with a master gage block set that is traceable to a standard.

4. Consider the material the blocks are made from. Ceramic is most stable to temperature variations and resistance to wear; however it is pone to damage from impact (drop, use, etc) and cannot be calibrated (adjusted to size). Steel blocks are very good for general use and are usually cheaper. There is an option to get black oxyde blocks (black in appearance). This option provides additional protection for corrosion and wear compareed to bare steel, and it is also a good indicator when the blocks are worn (the black wears off).

5. Consider adding additional certified/calibrated blocks (4", 6" etc) to expand your verification range or increase accuracy (less blocks to stack) if needed. These can be calibrated at realy long intervals (10 yrs) due to very limited use.

Hope this helps.

 

[Hershal]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

If you are going to calibrate the other tools, I would get at least Grade 1 blocks. Certainly the 81 piece set, but if you want to go beyond 6 inches, then consider getting a set of "long blocks" also.

Make sure you take a course in uncertainty. You will have to calculate uncertainty in order to assure that your calibrations are traceable.

Make sure you have your blocks calibrated by an accredited lab. If you buy from Mitutoyo (among others) they have accreditation and can provide a certificate in accordance with the accreditation. It may cost a bit more but then you will also have the uncertainty of those blocks which you will need for your calculations.

Put the lesser used blocks on a cycle not to exceed three years, and that is only if they are used sparingly and maintained in a controlled environment, such as 20 C and less than 50% RH. Blocks used more often (e.g., the 81 piece set) should go on a 12 month cycle.

Make sure you ALWAYS use the white cloth or non-powdered latex gloves (whichever you like better), and NEVER, EVER touch the blocks with bare fingers.

When you combine blocks, known as wringing, make sure that you take that into account in your uncertainties.

Hope this helps.

 

[DrM2u]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

The suggestions posted by Hershal are great if you plan to provide true calibration services and/or have your laboratory become certified to ISO 17025. These suggestions attest to Hershal's credibility as a metrologist and metrology auditor. However, these suggestions are an overkill and go beyond the requirements of ISO 9001 and the likes for the common internal metrology lab.

There is a big difference between calibration and verification of inspection equipment; calibration implies adjustment of inspection equipment if found to be out of specified tolerances while verification implies comparison against a known reference. You need to decide where you draw the line in your case. Most companies that perform in-house verification send the equipment out for calibration or replacement if found to be out of spec.

A Grade 1 set is commonly suggested for calibration purposes and is needed for high accuracy equipment (precision micrometers and such) but a much less expensive Grade 2 would work just fine for calipers, general micrometers, height gages and such. A set of calipers has a resolution of .0005" and a good set of calipers is accurate to +/-.001". Why would you use expensive Class 1 gage blocks that are accurate to +.00005/-.00002" instead of Glass 2 that are accurate to +.00007/-.00002"? Does it make that much of a diference for you? Your call ...

Another thing is the calibration frequencies are to be established by the organization (in accordance with ISO 9001 requirements). I am not sure if ISO 17025 SPECIFIES or SUGGESTS any calibration frequencies but I do recall something along the lines that calibration services labs cannot require or impose specific calibration frequencies on their clients. The decision to increase or decrease verification/calibration frequencies should be based on previous verification results and experience. Therefore the suggestions to place the sets on no more than 3 years or 12 months cycles are just that: SUGGESSTIONS.

I am not sure what the 'white cloth' is but it is known that skin secretion from our fingers can have a corrosive/eroding effect on the bare steel of gage blocks. That does not mean NEVER to touch the blocks with bare hands, it means that you need to be aware of this fact and maintain the blocks by applying a thin layer of preserving grease on them, specially the ones that get used seldomly. The suggestion to use powder-free gloves is good if feasible; they prevent the deposit of powder and skin secretion on the blocks.

Hershal is correct about the uncertainty but fails to explain how it applies to the equipment. I am not sure about the uncertainty being required in order to provide traceability of general inspection equipment to a national or international standard (i.e. NIST) but I am sure that the word 'uncertainty' does not appear or is implied anywhere in clause 7.6 if the ISO 9001 standard. However it needs to be considered because, as a rule of thumb, you don't want the uncertainty to exceed 10% of the equipment's accuracy (i.e. +/-.0001" for calipers accurate to +/-.001"). I have no idea where the 10% came from but I venture to guess that it is based on some statistical data.

Maybe someone else can provide additional information along these lines.

 

[Hershal]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

I am not sure if ISO 17025 SPECIFIES or SUGGESTS any calibration frequencies but I do recall something along the lines that calibration services labs cannot require or impose specific calibration frequencies on their clients. The decision to increase or decrease verification/calibration frequencies should be based on previous verification results and experience. Therefore the suggestions to place the sets on no more than 3 years or 12 months cycles are just that: SUGGESSTIONS.

I am not sure what the 'white cloth' is but it is known that skin secretion from our fingers can have a corrosive/eroding effect on the bare steel of gage blocks.

Hershal is correct about the uncertainty but fails to explain how it applies to the equipment. I am not sure about the uncertainty being required in order to provide traceability of general inspection equipment to a national or international standard (i.e. NIST) but I am sure that the word 'uncertainty' does not appear or is implied anywhere in clause 7.6 if the ISO 9001 standard. However it needs to be considered because, as a rule of thumb, you don't want the uncertainty to exceed 10% of the equipment's accuracy (i.e. +/-.0001" for calipers accurate to +/-.001"). I have no idea where the 10% came from but I venture to guess that it is based on some statistical data.

Maybe someone else can provide additional information along these lines.

I will try.

ISO/IEC 17025 does not impose or even suggest calibration cycles. The document best employed for that is NCSLI's Recommended Practice RP-1. There is also an ILAC document which provides nearly the same guidance.

The white gloves are used by Metrology professionals when handling gage blocks and mass standards (weights), and various other items used to calibrate something. Gloves do exactly two things: Prevent the transfer of finger oils, and slow down (the do not prevent) the thermal transfer from taking place.

The document known as VIM (International Vocabulary of General and Specific Terms in Metrology) 1993, for traceability requires an unbroken chain of comparisons through National or international standards, with stated uncertainties at each step. VIM 2008 rephrases the requirement but does not divorce it. Put another way, no uncertainty, no traceability.

Now, if verification rather than calibration is the goal, uncertainty is not so much a requirement, but for calibration it is.

Hope this provides a bit more background, and DrM2u was correct to point out that I had not provided sufficient detail.

 

[cjssag]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

I consider myself less than a novice on the subject of calibration so please consider the source of these questions.

I had been under the impression that calibration meant verifying that an instrument was capable of making measurements with its advertised "accuracy" and that any adjustments determined to be needed during the calibration process indicated that the instrument was out of calibration. Did I get that wrong?

Also, I had been using the term verification as being synonomous with checking an instrument against, e.g., a calibrated gauge(s), to ensure its continued accuracy. Admittedly, I was never quite certain that this act wasn't calibration, too, except that it might be done on an ad hoc basis prior to using an instrument and perhaps was not recorded. How far off am I on this one?

My clients tend to be very, very small, machine shops (less than 15 employees in many cases) and - not surpisingly - do calibration only when someone forces them to and, even then, don't want to send instruments out for calibration because they have no backup, don't want to hire a traveling calibration service to do it on site because of the cost, and on it goes. So here's my final question, are you able to recommend a calibration primer: book, video, website or the like that will educate them on what to do and how to do it themselves at an affordable cost?

 

[Marc]

Re: Calibration of Calipers, Mikes, Height gauges - Recommended Gage Block Types

<snip> ... any adjustments determined to be needed during the calibration process indicated that the instrument was out of calibration. Did I get that wrong? <snip>

One often finds a measurement instrument which needs adjustment but is not technically out of calibration. The important part is to watch the trend over calibration cycles. If the instrument has to be adjusted more each cycle, even if it is within calibration limits for the instrument, it is probably indicating wear.

I'm sure others here will have a few thoughts for you on this and your other questions.

FYI: Calibration vs. Verification