Micrometer calibration woes in Seattle

So we're having major issues with our micromter calibration service here in Seattle. We never had issues with our outside mic's for decades until we started doing coast guard work, which requires our mics to be certified. So we paid big money for the little stickers, and since then every single mic has had issues holding tolerance. Right out the gate, each outside mic reads .0005" big over the standard. Doesn't matter if you use the Mitutoyo clicker on the end, or just go by feel. We have a dozen machinists, and we're all getting the same results. Some of the mics are off more than .015", so they must not be tightening them properly as well. We have the same guys working here before and after the calibration was done.

I called the company and they apparently calibrate them at a temperature much greater than the ambient (unheated) temperature of our shop, which would account for a few tenths, but not 5 tenths. They must also use higher pressure than is required. They even said some of our standards were too long and ground them down! I can't imagine how a steel rod would GROW in length when there are no dings on the tips, and the tips areconstantly being polished with hardened micrometer anvils.

In any case, we aren't happy. I'm tired of figuring the .0005" into my calculations when machining and the other mics loosening up is unacceptable. I'm wondering what service other shops use for calibration. We're in Seattle, so it's nice to use somebody local but if I have to ship them out then I will.

It would also be nice if they did repair work. I use long island indicator repair for that (and they're awesome) but they're on the other side of the us.

Any input is appreciated!

I have heard of plenty of people who reset the zero on there mics after they come back from cal ,calibration is not just about the zero point anyway ,the anvils should have been checked for flat and parallel and the pitch error of the screw and probably other things I can't think of at this minute.

Grinding the end of your standards sounds a bit off ,even if they have grown( which they can) I think the normal thing to do is put the ACTUAL length on the calibration cert to use as a reference.If they have reduced the length surely it would be by lapping ,not grinding.

sable said:

I have heard of plenty of people who reset the zero on there mics...

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How do you "reset zero"???

Zero is zero. It means that the anvil faces are touching.

It does not mean tightening the thimble with a 12" crescent wrench.

If you're consistently reading .0005" oversize with multiple instruments and multiple operators, I would question the cal house.
Ask to see their CoC's for the instruments they used to certify your tools.

- Leigh

sable said:

If they have reduced the length surely it would be by lapping ,not grinding.

Click to expand...

Probably. My point is only that I'm highly suspicious of a standard that has grown. Perhaps I need to tour the calibration company's shop and procedures if I use them again.

sable said:

I have heard of plenty of people who reset the zero on there mics after they come back from cal ,calibration is not just about the zero point anyway ,the anvils should have been checked for flat and parallel and the pitch error of the screw and probably other things I can't think of at this minute.

Click to expand...

Yea, I hate to do that though. It's just insane to pay for the certs and then have to readjust everything myself every time. I've got parts to machine and don't want to fool around with this stuff. I'd rather look into getting my own labels made and sticking them on myself, or sending the mics to somebody else who will adjust them properly. I purchased some new Brown and Sharpe, swiss made outside micrometers to put in my home shop, and they're all dead nuts right from the factory. Whereas the shop sets of Mitutoyo's, Starrett's, etc we paid to be calibrated measure +.0005" on the same standard. Plus all the issues of some of them loosening up.

So if anybody can recommend a calibration service that they're happy with please chime in.

The real Leigh said:

How do you "reset zero"???

Zero is zero. It means that the anvil faces are touching.

It does not mean tightening the thimble with a 12" crescent wrench.

If you're consistently reading .0005" oversize with multiple instruments and multiple operators, I would question the cal house.
Ask to see their CoC's for the instruments they used to certify your tools.

- Leigh

Click to expand...

We did send a couple back to be re-checked, and they came back the same way. I asked about calibrating at the temperatures in which we actually use the instruments, but they would not. To be certified they need to be at 70ish or so (I don't remember the exact number). But again, the temp difference from 50 to 70 will not affect the mic 5 tenths in my exerience.

We run a job shop, and for most jobs 5 tenths here or there isn't a big deal. But there are some that matter, and we have had issues. If a mic is consistently off it's annoying, but you can work with it. But when the setting slips altogether into several thousands of it's a big problem. I did some field inspections last week with the mic which was off .015", which caused some major problems for the big heavy parts that were supposed to fit! Part of that was my fault for not bringing the standard with me to the job site, but just the same I'd prefer to have reliable inspection equipment.

Bread said:

Probably. My point is only that I'm highly suspicious of a standard that has grown. Perhaps I need to tour the calibration company's shop and procedures if I use them again.

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If that cal lab made ANY changes to the standards, they voided the CoC, and therefore cannot certify your tools.

- Leigh

Bread said:

I asked about calibrating at the temperatures in which we actually use the instruments, but they would not. To be certified they need to be at 70ish or so (I don't remember the exact number).

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They must calibrate your equipment at the same temperature at which their standards were calibrated.
Deviating from that value requires a lot of calculations and fallderall that they probably don't want to do (time is money).

They would also need to set up a room with special climate control at the desired temperature.
They could not install their regular cal equipment there, so it would have to be a whole new lab.

Bread said:

If a mic is consistently off it's annoying, but you can work with it. But when the setting slips altogether into several thousands of it's a big problem. I did some field inspections last week with the mic which was off .015", which caused some major problems for the big heavy parts that were supposed to fit! Part of that was my fault for not bringing the standard with me to the job site, but just the same I'd prefer to have reliable inspection equipment.

Click to expand...

I don't understand the error you described.

If a measurement is off consistently by .015" at every point, then your zero is set incorrectly.

Thermal problems are proportional, not linear, so a .015" error in 1 inch would be a .030" error in 2 inches, etc.
This is the result of thermal expansion/contraction due to working temp differing from calibration temp.

- Leigh

The real Leigh said:

How do you "reset zero"???

Zero is zero. It means that the anvil faces are touching.

It does not mean tightening the thimble with a 12" crescent wrench.

If you're consistently reading .0005" oversize with multiple instruments and multiple operators, I would question the cal house.
Ask to see their CoC's for the instruments they used to certify your tools.

- Leigh

Click to expand...

I wasn't saying that it was right to have to move the zero ,just that people say they do , sometimes it's probably BS a bit like the guy that once told me that he regrinds every new drill he gets (by hand ) because they never cut quite so good . I dare say that not everyone in every calibration lab does their job to perfection ,the same as any other line of work and that is probably the case here.

sable said:

I dare say that not everyone in every calibration lab does their job to perfection ...

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They're supposed to, 100% of the time.

I guarantee that when I worked in a cal lab, every cert I wrote was dead accurate within the limits of our standards.
The same was true for the other cal techs in the shop.

- Leigh

The real Leigh said:

How do you "reset zero"???

Zero is zero. It means that the anvil faces are touching.

It does not mean tightening the thimble with a 12" crescent wrench.

If you're consistently reading .0005" oversize with multiple instruments and multiple operators, I would question the cal house.
Ask to see their CoC's for the instruments they used to certify your tools.

- Leigh

Click to expand...

Apart from the "like" I gave you I've never heard of a calibration faciity that adjusted the instruments sent for calibration as I believe the OP thinks is happening. Any adjustment to an instrument after the certiicate is issued would IMO require a new calibration.

Is it me that's misunderstanding something?. Is the subject a "repair shop" or a calibration lab.?

Gordon B. Clarke said:

Apart from the "like" I gave you I've never heard of a calibration faciity that adjusted the instruments sent for calibration as I believe the OP thinks is happening. Any adjustment to an instrument after the certiicate is issued would IMO require a new calibration.

Is it me that's misunderstanding something?. Is the subject a "repair shop" or a calibration lab.?

Click to expand...

Sable's original comment in post #2 of this thread was:

sable said:

I have heard of plenty of people who reset the zero on there[sp] mics...

Click to expand...

which seems to be talking about end users changing the zero setting (however they might do that).

Then my reply in post #6 to Bread was:

The real Leigh said:

Bread said:

Probably. My point is only that I'm highly suspicious of a standard that has grown. Perhaps I need to tour the calibration company's shop and procedures if I use them again.

Click to expand...

If that cal lab made ANY changes to the standards, they voided the CoC, and therefore cannot certify your tools.

Click to expand...

So it appears the discussion herein relates to both the cal lab and the end user, without adequately differentiating the two situations.

- Leigh

Your problem is very strange.
5 tenths off is kind of big to me but maybe these are 10" mics. and it's a percent of size thing.
.015 is what the heck territory. How do you even do this?

First of course is to know your own checking standards.
I should mention round ones are always suspect and don't work well but they do make bad faces show up.

People are often too quick to grab that little wrench that comes with a mic and "fix" it without thinking a lot about "why".

Something on your end or your sources end is crazy wrong.
Just for giggles, does a brand new, never used mic check OK?

There must be other cal houses nearby that could confirm measurements of your standards for you.
T'is a whole set of rules here for such a house and they should darn well be able to trace to NIST and know the max error spread they can provide you with at calibration.
On the other hand when gage calib got some focus I did see some people jumping in to make the money by placing the nifty stickers that really did not "get" this world.

There is no way for any here to know if the nutty numbers are yours or the cal lab's.
Welcome to the topsy-turvy world that QC people and have to live in.

You are at the entrance to the rabbit hole. While you need some answers are you sure you want to dive in?
Try a second source, hopefully it will work and you can go on making parts.
Bob

Gordon B. Clarke said:

Is it me that's misunderstanding something?. Is the subject a "repair shop" or a calibration lab.?

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The most important aspect is the cal lab. I'm fishing for recommendations in that area. I just mentioned that it would be nice if they could also do repair. I replace dial bezels and simple things like that myself, but anything deeper I'd rather send it out. I've used long island indicator repair and been very happy, but it's nice to support local business when at all possible.

The first time we used the local cal shop, they did not 'need' to make any corrections to our standards. We have sets of both domed, and flat faced inside mic standards. But when we got the outside mics back (individual mics measuring range 0-24") they all measured +.0005" on the exact same untouched standards. So, that's the issue. How is it possible they are using OUR standard and getting a zero reading? Either they keep their shop at the same temp as death valley, and/or they use way more finger pressure than we do. They certainly don't use the built in clickers because those don't bring the reading to zero either.

The brand new Brown and Sharpe mics I bought were dead on using the same standards. This set is 599-181-901, which is one of the best. Mechanical Outside Micrometer Sets | MSCDirect.com

In any event, I just want to use a different calibration shop. I know the mics are wrong, and we have all been making parts based off the standards for probably a couple years now, simply compensating for the 5 tenths. I'm tired of it and don't want to readust them all myself. I want somebody else to do it since I'm already paying for it

The setting bars with the micrometers will be made from similar steel to the micrometers. If the micrometers read 0 against them at 68°F, they'll also read 0 against them at 50°F to within a gnat's whisker. Conversely, the lab should measure and state the lengths of any setting rods in the set. May well charge for it, but it should still be done so you can use them.

Do you have a set of Jo blocks that you can use as an independent check of the micrometers and the setting bars? Again, steel is preferable to carbide because it'll match better at different temperatures.

This all assumes that the parts you're making and measuring are steel. Measuring a 24" brass part at 50°F with a steel micrometer calibrated at 68°F won't give an answer that's meaningful at any other temperature without some maths .

I'd expect a simple standard like a gauge block or rod to follow the expansion coefficient very closely. There's no such guarantee with a more complicated assembly like a micrometer, vernier or whatever. Things can warp and any two dissimilar materials that are screwed together can form a bimetal strip, magnifying displacements tremendously. It's not just a matter of linear coefficient. Now, I ran into this with sensitive optical equipment, so maybe at the .001" level you don't notice this stuff.

It's actually very hard for a cal lab to certify something outside the normal temperature that all their own stuff is certified at. This holds for the electronic calibrations I sometimes do, and I'd probably refuse anything more than a couple degrees different than 20C.

The idea that they altered a standard is so bizarre and offensive to me that I'd never use them again. I'd want dimensions, corrections and history, nothing more. The discrepancies described are so large that I'd be very suspicious. Maybe send a few pieces to a lab far away as a reality check.

so these are all mechanical mics, yes?
i will note with electronic mics it is common and sometimes required to set zero against a standard and I routinely do so - no wrench involved

when you measure gauge blocks and compare them to your mic standards, do you get the same answers?

bryan_machine said:

i will note with electronic mics it is common and sometimes required to set zero against a standard and I routinely do so - no wrench involved

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Different application... one that is not easily duplicated with mechanical mics.
Setting an arbitrary zero on a reference enables you to read the dimensional error of the part directly.

bryan_machine said:

when you measure gauge blocks and compare them to your mic standards, do you get the same answers?

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Gauge blocks are two or three orders of magnitude more accurate than mic standards.

- Leigh

If I was in a shop or company where micrometers etc. were sent out for a calibration certificate then I think I'd do a quick check of them first rather than just snding them out "willy nilly". If a mechanical measuring instrument was "off" (for example a micrometer not at "zero", and remembering that "zero" on a 1 - 2" micrometer isn't as easy as on a 0 - 1" one) then I'd adjust it to what I thought was the correct reading before sending it. From the calibration report I'd want to see how accurate it is at other steps as a micrometer shouldn't be checked at "regular" steps/intervals as in not 0.1, 0.2, 0.3 etc.

How about taking your standard and your mic that is off after calibration to the cal lab and ask for an explanation? Personal, face to face.

There seems to be something wrong, but it is not obvious on which side. Show them how you do the check, let you show how they do.


Nick

Gordon B. Clarke said:

If I was in a shop or company where micrometers etc. were sent out for a calibration certificate then I think I'd do a quick check of them first rather than just s[e]nding them out "willy nilly". If a mechanical measuring instrument was "off" (for example a micrometer not at "zero", and remembering that "zero" on a 1 - 2" micrometer isn't as easy as on a 0 - 1" one) then I'd adjust it to what I thought was the correct reading before sending it.

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By doing so you would invalidate one of the major purposes of the calibration process, which is to evaluate the stability of the measuring tool over time.

This is easier to envision in the case of electronic equipment, which can have predictable aging characteristics over time.
But the same principle applies to any measuring tool.

Obviously this does not apply in the case of tools for which the zero-point is adjusted routinely during use.

- Leigh