Calibrating Micrometers

As a chap that was spent most of his working life in the industrial instumentation and measurment sector. It always amazes me when topic like this raises it's head Model engineering workshop forums, In my workshop whilst using my lathes and milling machines I more than happy if I can machine to better than 0.001. I realize this is my own work and not work for others

Is that metric or imperial Peter?

A small anecdote

At my second job in a general machine shop there were some very old, rather crude looking and horribly painted micrometers long out of daily use up on a window shelf. Obviously bought at a sale by the owner when he first started out they were sorry looking things and never used.

When a new guy started - ie myself - one guy would take one of these mics to his point in the workshop and another would be at the other end of the workshop - "Anyone got the 6" mic" he would shout out . The other would acknowledge and the reply "Throw it over here then Dave" would see said mic winging it's way through the air down the workshop and obviously miscatched to clatter across the floor before being casually picked and taken to his machine to a "Cheers mate" response. The look on faces new to this including my own must have been a picture.

No for some reason we never did bother calibrating these mics

Tug

That would be nanometers

If the OP is near Nottingham. He is welcome to pop round and check against my slip gauges.

Regards.

David.

Thank you, David, for the offer. Unfortunately, I am down south in Buckinghamshire in Aylesbury.

My machining abilities are not that great so using micrometers for comparative measurements will be the most likely use. However, one reason why I bought them was to measure things like crankshaft journals for car engines. Again, I appreciate that there are tolerances and ensuring that the micrometers are at least are at 0 when reading 0 is all that I am looking for. The advice given will enable me to achieve this.

Thank you.

James.

Not really, but it's revealing to compare the three micrometers by measuring some ordinary blocks in various combinations. Setting Zero to read zero is usually a simple adjustment, but second-hand micrometers are liable to be worn or damaged beyond hope of achieving their original specification. For instance, the thread is likely to be worn over the range it was most used over, so a 4" micrometer might be accurate from 0" to 2", but carp from 2" to 4". Another unfixable problem is a bent frame due to dropping.

A short length of bright mild steel strip sawn into blocks makes a reasonable test set. Although BMS isn't particularly accurate, it's consistently the same size over short lengths, which is all that's needed to confirm micrometers get the same answer. If all three micrometers are in close agreement, hurrah! If they disagree with each other then at least two are duds, possibly all of them.

Does the accuracy of a micrometer matter? Good question.

If accuracy is actually important then buying an uncalibrated second-hand micrometer is daft. The build quality and original specification of the very best make was only valid when the item was new. Since then history has been at it. There's a huge difference between a little used carefully kept instrument and the same model, clapped out by heavy use, tossed into a toolbox and first applied as a G-Clamp in 1955. The proud owner doesn't know what he has unless the instrument is calibrated. And even then he's unlikely to use it at the correct temperature.

Fortunately most home workshops achieve fit by comparing rather than measuring. We rarely measure critical dimensions, email them to a friend, and expect him to make a fitting part by return. Instead, our other tools are compared with the micrometer and the actual dimension is almost irrelevant. Not necessary to know a shaft is precisely 0.7499" only that a bush bored to fit will slide on. A digital caliper works well as a comparator and old-fashioned spring calipers are remarkably effective too. Micrometers used as comparators don't need to be calibrated or in good condition. Nor do they need to be properly calibrated for most ordinary work like telling the difference between 10mm and ⅜" diameter brass rod.

If I were to mill a steel block to a random size in the range 24 to 25mm by 15 to 16mm by 9 to 10mm and ask a 100 members to measure it, I would expect hundreds of different answers! They'd all be nearly right, but scattered around the true value. Worse if the block were about 1250 mm long - too big for most instruments. I don't have any means of accurately measuring such a block myself, but we could all produce a close copy by comparison. Copying within 0.02mm (a thou) is fairly easy. Better than that gets exponentially more difficult, but it can be done. The pros probably wouldn't do it that way because trustworthy calibrated equipment gets the job done much faster and more reliably. If only high-end measuring was affordable...

Dave

Lots of topic drift here!. Michael is dead right as usual.

Andrew.

Can you 'calibrate' a micrometer? Surely the thread pitch is determinative, so you can only set zero or maximum. Testing will show damage, twist or stiffness - at which point the thing either passes, or fails and is thrown away.

.

Yes ...

Calibration doesn’t necessarily mean proving it’s right, or making it right.

It usually means assessing and documenting how wrong it is !!

MichaelG.

.

Edit: Here is an excellent example by Renishaw :

https://resources.renishaw.com/en/download/(03fa15e2b2614c5ea4a59272ec72ad9f)

Edited By Michael Gilligan on 13/11/2020 13:58:07

A micrometer that can go seamlessly from 0 to 4". Not all that common I think, but try this for size Mike

I realise the term can be widely used, but in my book this is an inapprpriate substitute (fancy word) for set and test. My experience is that calibration adjusts the inputs and outputs to be correctly correlated throughout a range, but this isn't possible for a micrometer, save the setting at only one distance.

.

That is your choice, John

I have no further comment, except to say that I am happy to be guided by NPL

”Calibration is a process in which an instrument or piece of equipment's accuracy is compared with a recognised standard.”

Ref. https://www.npl.co.uk/calibration

MichaelG.

Edited By Michael Gilligan on 13/11/2020 14:34:14

I recognise the range meaning, but not that it's the only correct definition of 'calibration', or that calibration is being misused in this thread. Michael's definition is my understanding too, particularly where he says calibration doesn't include making it right!

Perfectly acceptable to adjust for scale error rather than fix an instrument, though not sensible for a working micrometer. When a micrometer fails calibration, safer to get rid of it rather than expect the user to apply corrections from a table or graph. Much too easy to confuse a good micrometer with the inaccurate one. If accuracy is needed, sell the dud on ebay or bin it.

By the by 'Zeroing' isn't just the act resetting a micrometer barrel or pressing the zero button on a digital caliper. The word is also used to mean calibration in the widest sense.

Dave

Hi, or just use it as a mini G-clamp for that delicate work.

Regards Nick.

My vernier Height Gauge failed its Calibration check, and was condemned. It had a total error of 0.002" over 18"

For the very limited range that it is used in my workshop, I can live with an error of that magnitude, so saved it from the skip! Being a vernier, with my eyesight, and a magnifier, I can just about claim to accurate in reading it to a thou!

Don't like the thought of selling an instrument known to be inaccurate, unless the deficiency is declared.

Either BIB IT, or use the barrel for measurements over a very limited range, say a Saddle stop?

Howard

Offered without comment ... This, from Mitutoyo :

**LINK** : https://youtu.be/NrLLmwi7-vE

MichaelG.

My Mitutoyo 0-25mm micrometer is now in my home shop for approximate sized parts. It has errors upto 0.01mm in places in the area's that it has been used the most over the years. But at closed it is zero, and still measures correct at the end of it's total travel of 26mm is still correct. But the variations happen at the 8mm to 16mm range. Some is under by 5um(1/2 division) while others are as much as 0.01 when compared to known standards or known sizes.

So what looks like exactly 12mm is actully 11.99 mm so makes for a nice slide fit for a 12mm bearing. No good of it really needed to be 12mm.

Neil

So here is what Mitutoyo think.

https://www.youtube.com/watch?v=NV1VToAqfXI&ab_channel=MitutoyoAmericaCorporation

and here is how they think you should calibrate a micrometer.

Pick the best measuring instrument you have. Say its a 1" inch mic. Zero it by closing down in the approved manner and adjust to read same. This is now defined as your own personal 1 inch standard. Irrespective of what other standards exist. You may now extend the range to your other instruments by measuring and stacking bearing journals, parallels, ball bearings and turned guages etc. If you are really keen you can note down the variations.

Rule 1. Don't chase imagined problems, wait until they are real and demonstrable.

regards Martin