Help!

Our Quality Manager if on indefinite leave and a calibration issue has just surfaced.

We are at the 60 day interval and the micrometers need to be calibrated before our audit next week. Our set of gage blocks includes .0625, .1250, .2500, .5000 and 1.0000 (certified to the NIST standards).

My question is: Can we put 2 gage blocks together (1.0000 + .0625), etc. in order to measure a 1-2" micrometer?

Any help would be greatly appreciated.

Sue

Sue,

Don't panic.

We put gauge blocks together, never had any problems.

Sue,

The person performing should be qualified and should know how to 'Wet' multiple blocks for the applicable check points!

Sue,

This incident should be evident of a weakness in your measurement system, shouldn't it?

Sue,

I don't wanna sound like I'm piling-on, but it does sound like a procedure (or better procedure) needs to be written for cal of mic's. But to answer you, yes you can stack standards and they are designed for this -- if the surface finish is correct and the standards clean you should be able to "rub/slide" them together so that they sort-of stick together to each other, then measure them.

The term you guys are looking for is wringing qualities of the gage blocks. Check your calibration certificate for wringability. It should be included in a statement. Worn gage blocks would affect this.

What it is is the capability of the blocks to be stacked.

Yes, it is acceptable if your blocks are capable. The better solution would be gage blocks to measure the 1 - 2" range.

Ya! That's it, wringing! Thanks Ken!

Sorry Sue for my mis-information on the stacking technique!

There was a good article on the wringing of Gage- Blocks. Feb 97 KNOW YOUR GAGE-BLOCK BAICS. Go to www.qualitymag.com

Okay, so now we have established that wringing the blocks is okay (it will add about 20 µin/wrung surface uncertainty to your measurement over using a single block). That takes care of spindle linearity.

Sue, if these are standard outside caliper-style flat-anvil micrometers, how are you going to test the micrometer anvils for parallelism and flatness? Gage blocks won't do for these tests, and you will need a monochromatic light and optical parallels. Do you have these?

Ryan

Sue,

Before we all start theorising and going into the bowels of how to measure atomic particles, can I ask a simple question……

what are you measuring with your micrometers?
:bonk: :ko:

Ryan and Geoff,

I'm just the ISO Coordinator, but I can tell you that most of the micrometers don't need to be super accurate except possibly those used at final inspection.

We make powdered metal parts that need to me miked at several stages of the operation (mold, size), but mostly just to see that the parts are within tolerance.

We are a very small company and the parts we make are not terribly intricate (bearings, sleeves, etc.) although some p/m parts require much more sophisticed measuring devices. (I worked for 19 years at such a company before being hit my the great downsizing wave);)

We do have several flange micrometers, which I think is what Ryan was referring to, and I've been told they do use blocks to calibrate them because of the generous tolerance.

Thanks so much for everyone's input - we are being audited next week to the new TS standard and I'm really holding my breath and praying for a miracle!

Sue

I would guess that the vast majority of folks who use standard outside mics are happy with readings within +/- .0001" or +/- .0002" of the standard's value, otherwise they would be using something else for the measurement. I've used and calibrated such mics from several major manufacturers (with +/- .00005" or +/- .0001" mic resolution) for over a decade and never saw a calibration instruction from any manufacturer or user mentioning the use of a "monochromatic light and optical parallels" for parallelism or flatness verification. For most folks that is probably major overkill, IMO, unless the edges only of the anvils on the mics are often used, and even then I can think of easier/cheaper ways to test for practical parallelism/flatness issues unless it were a very unusual use situation. JMO.

Mike,

First off, without a light (about $150) and a set of parallels ($168), I could shoot holes in any calibration of a micrometer, especially at ±0.0001". If you measure anything spherical, smaller than the anvil contact area, or irregularly shaped, it comes into play. I've measured "calibrated" micrometer anvils at over 0.0005" that you would not find with a gage block. Every military procedure calls for it, as does the old MIL-STD, as does the new (if it is out yet) ANSI specification. I've now worked for 3 accredited labs as well as working in a government lab, and I've yet to meet an ISO 17025 assessor that would let a lab get away without the proper check.

But I will give you that there is an alternate method of proving parallelism. You will need two calibrated spheres (spherical within 10 µin) that are of differing size by 0.012-0.013, as well as a light and an optical flat.

Overkill? Possibly for a specific application, but as an external laboratory, I have to be able to prove that an instrument meets published specifications. Flatness and parallelism are part of those specifications.

That said, these methods are those accepted and written in every standard concerning micrometers that I've ever seen. A company can omit them, if they desire, but at that point the micrometer meets a "custom specification", and no longer meets either manufacturer's specification, ANSI specifications, MIL specifications, or any other mass published specifications.

A custom specification is often fine for a company's needs, but then you have the burden of proof that it is, in fact, adequate. Quite often, the proof is more expensive than the equipment to meet the specification.

Ryan

Ryan,

Your points are well taken. As you suggest, you as a cal lab have different needs/requirements than most users. 99+% of the time we measure parts larger than the anvils, but I take a quick look at flatness/parallelism by using a small calibrated gage pin or gage block measured in different places on the anvil faces. Ideal, probably not, but suitable for my uses.

Can you point us to the procedure references you mentioned - MIL or ANSI?

I also understand where your coming from Ryan, but how many companies have tolerances of ±0.0001" on their micrometers?

Some of the products we measure have a thickness tolerance of
3mm +/-10%. Do you really expect me to calibrate our mic's and calipers to ±0.0001"? Hardly.

But then again, if you produce product with reall tight and critical tolerances then I would agree. But I also feel some companies spend foolishly when it comes to calibration services and pay for something that is not really needed.

We just went to outside calibration on equipment we used to calibrate in-house. As we are in the process of ISO 17025 accreditation, I didn't want the hassle of having to meet the calibration portion of the standard. They made a house call and did the stuff in-house. I sat there and wondered how they ever became accredited. Scary stuff!

Mike,

You can get the MIL at a very little-known site called ASSIST. It's free, and just about all of the MIL specs are there in PDF format. Go tohttp://assist2.daps.dla.mil/quicksearch/ and start looking for what you want. The one for micrometers GGG-C-105C. For flatness, it calls out either optical flats or a precision indicator with a precision reference plane (this is a very time-consuming, expensive option). For parallelism (they call it angular trueness) it calls out precision angles, feeler gages, and a precision square (a VERY expensive method, also time consuming) or autocollimator (about a $50,000 option).

The MIL specs are being replaced by ANSI specs in the US. I don't know if the micrometer spec is approved yet, but I saw a few versions of the draft. It is either ANSI/ASME B89.1.13 or ANSI/ASME B89.1.14. One was micrometers, the other was calipers, and for the life of me, I can't remember which was which. Anyway, the ANSI site is http://www.ansi.org . The problem is that all ANSI specs cost money, and are copyrighted.

There is also ISO 3611:1978, which is the international standard for micrometers, which I guess eventually we will all have to use for international trade agreements.

Oh, another site you might like to see is http://www.nssn.org , where you can look up the latest version of just about any specification worldwide.

Ryan