How to calibrate large calipers?

We have a few digital and vernier calipers that are 24, 36, 40, and even 80 inches wide at maximum. I'm wondering how to calibrate the inner and outer jaws for something that big? We have a few micrometer standards, the largest is 20 inches, but that only works for the OD Jaws.

I was thinking of asking our machining department to manufacture a large checking fixture that I could verify with CMM.
Only issue is something that large is likely to be out of round, but I could specify locations to put the jaws I guess.

Thanks

Why not contact companies that make and calibrate large tools like that, either for purchasing or how to do it. Bear in mind a calibration standard that large would have to held at a rock solid temperature

Tom

I'll probably catch a rash of shit for saying this, and I'm not 100% comfortable with it my self, but here goes. Do you really need to check something like a vernier (or even a digital caliper?) at the extreme limit of it's range? Any error from wear will be on the contact surfaces themselves or show up as slop in the slide, so if they check out on smaller standards, I don't see how they could then suddenly be off further up on their range. Unless, I suppose, if they were used extensively in one area, the rail may be more worn in that one spot, but checking at only one point near their limit would not show that any way.
If they are quality, brand name calipers, I would be comfortable assuming they were checked for error over their entire range at the factory. If they still check out good on a smaller standard, I'd say they are good all over. Of course certifying that is a different thing and should probably be done by professionals as already noted.

Piper3T said:

I was thinking of asking our machining department to manufacture a large checking fixture that I could verify with CMM.
Only issue is something that large is likely to be out of round, but I could specify locations to put the jaws I guess.

Click to expand...

I vote for sending it to someone who does this for a living if you want it done right. If you have a CMM large enough to check a "fixture" large enough to check the calipers, why not just use the CMM to check the calipers? Not exactly NIST traceable, but better than a fixture.

Piper3T said:

We have a few digital and vernier calipers that are 24, 36, 40, and even 80 inches wide at maximum. I'm wondering how to calibrate the inner and outer jaws for something that big? We have a few micrometer standards, the largest is 20 inches, but that only works for the OD Jaws.

I was thinking of asking our machining department to manufacture a large checking fixture that I could verify with CMM.
Only issue is something that large is likely to be out of round, but I could specify locations to put the jaws I guess.

Thanks

Click to expand...

Why don't you use the CMM to verify them directly? Set a distance on the caliper and measure it with the CMM. Given that they are only good to 0.001" you can even measure them with a probe on a large enough VMC. They are most likely either ok or way out ...

dee
;-D

Temperature control (including operator hand warming) and uniform slide tightness are important with the larger calipers. I'd check the moving jaw gib for play at least every 10" or thereabouts.

Given that you really shouldn't use calipers for measurements closer than +/- .0015 or so, the standards used for larger micrometers should be fine, I've bought longer ones off eBay. And sure, use the CMM for testing, you can even check the beams for straightness.

I have a 40" mitotoyo vernier caliper. It's never been dropped or abused. It's never occured to me to get it calibrated or even checked. Whatever mits did at the factory to verify it's accuracy is good enough for me. (I'm with Derek Smalls on this)

For the stuff I do it could be off +/-.005 and it wouldn't matter. the customer wouldn't know or likely care.

I have a customer who may require me to calibrate my tools, in which case I'll send it out for calibration, which I'd expect it to pass.

Thank you for the suggestions. Never thought to use the CMM directly.
Our current QA calibration procedures are to calibrate calipers every 90 days. This was written before we started making larger and larger parts and was overlooked. I was hoping there was a simpler solution. I think what I'll do is keep small calipers on 90 days but large ones on one year (used less frequently) and send them out. I'll probably non-inventory the 80 inch until we need it. Maybe I'll use the CMM for that one when we do.

Piper3T said:

Thank you for the suggestions. Never thought to use the CMM directly.
Our current QA calibration procedures are to calibrate calipers every 90 days. This was written before we started making larger and larger parts and was overlooked. I was hoping there was a simpler solution. I think what I'll do is keep small calipers on 90 days but large ones on one year (used less frequently) and send them out. I'll probably non-inventory the 80 inch until we need it. Maybe I'll use the CMM for that one when we do.

Click to expand...

That is one of the most rational, reasonable, and logical things I've ever heard from a QC person. You'll do well in this field.

Have you checked into buying some larger "standards"?

I have stds (and mics) up to at least 36", and on a feature the other day that I needed to hold tighter than a nuns c on the lathe - somewhere around ??? 18-22" maybe ??? I used both the digi calipers as well as the mics.

I don't trust big mics as you cannot feel them. Or should I re-word it to say that you can feel them less and less as they get bigger and bigger. The calipers - not so much - BUT you can't wrap around a 30" shaft with the calipers. This feature was reachable from the face, so the calipers fit fine.

So I dug out a std in the range that I was working and checked both units. Charted both as far as "reads .001 big or .0015 small". (Keep in mind that large mics will cover a 4" range generally with replaceable anvils. I used both tools as best that I could, and wrote down their readings. When I got to where both units agreed on size - it's good.

But the 24" digi calipers was off about .001 from the std that I was using compared to ZERO.


I have not had good experiences with calibration services in the past, and this was just confirmed/updated again this last week where two rings for the same size were AT LEAST .001, and more like .002+ in disagree-ance with each other. I will take the std, or in this case - the set guage ANY DAY over a sticker on a guage!
We did have the set guage in this case, and one was right on, and one was way loose.

Could you just order up a std for a size in the range that you will be needing on a job coming up - as you go? IDK how available 40"-80" stds are, nor what one might cost?

From the std tho - you should be able to work your way backwards. In my case - when I doo big bores - I doo use inside mics, but I use them mostly in the way that I would use snap guages. (But have more confidence in the mics) I always go back to the big calipers and/or mics to confirm size. Sometimes they read right on, other times they may be out .0015, depending on cleanliness of the kit, wear, and how well you (I) wrung the spacers all together upon assy. I just will not count on an interchangeable tool to be right-on at any time. It all needs checked.

Also - the 75" std will not fluctuate with temp nearly like a 75" ID ring guage would. And it fits in the cabinet much easier - and with only one person, as opposed to a forktruck tossing it on the top shelf of the pallet racking since it's too big to be anywhere w/o risking getting hit or dropped. (Boy - tha'd be a bad day eh?) But then - maybe you would want the ring to move with the temp if you are boring a ring in a non-climate controlled room? I like to let criticals set overnight sometimes.


Also - if you were to send it out to be certified (I highly doubt that they would ever "calibrate" it - as that would be a factory setting - whether digi or vernier?) at what dimmension would you have them "set" it to? It's prolly not 100% the same everywhere (as my 24" above showed), so if you have it "set and cert'd" at 60", would that be close enough when you were dooing work at 45 or 75"?


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Think Snow Eh!
Ox

i have used the attachment bar they are sold 6 feet long with pins every 6" you clamp a 6" digital caliper too.
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i just zero to Starrett ruler scale 4 foot or 6 foot, etc. basically using a standard. i would only trust the ruler scale to .001 per foot so 6 footer could be .006" off
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you measure longer distances and repeatability often is +/-.004", obvious heat from hands, instrument bending, trying to find minimum or max reading. not easy.
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you want better put on large cnc mill use .0001 indicator in mag base and zero DRO and then move to other end zero (sometimes clamp flat block on so something to zero off of) and read DRO
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only a shallow bore indicator gage made in longer length resting on part will measure .001 or less set to a gage bar. without something to rest on very hard to get repeatability. we have different length pins made as often you need a specific length pin or jaws. they are tapered so only measure on ends. when a foot or less you can measure .0001" but longer than that you be doing good to repeat .001" and you will see readings change the longer you hold in your hands. some use gloves or rags to insulate