MEW 219 Micrometers

In MEW 219 is an article by Bert Bishop where he explains how to check and calibrate micrometers.He states that the standard needs to have an accuracy of 0.0001" and also shows a setting standard with a plastic jacket to stop fingers heating it up.I notice that no mention was made regarding ambient temperature. If I remember correctly measuring devices, micrometers vernier calipers steel rule etc are only correct at 20 degrees centigrade so it's no use checking your micrometer on a cold and frosty morning or if it has been lying in direct sunlight.

Assuming the standard test piece has also been sitting in the shed overnight it will be at the same temp as the Mic on that cold frosty morning so no problem as you are comparing like with like.

Just remember to cool your work before you measure it if thats what floats your boat.

OK! As it was my mike in the picture, I've reset the 25-50mm mic (it was reading 24.999 on the standard) and I'll keep the (steel) standard in my sweaty hand for 5 minutes or so

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While I wait... something is running round in my workshop's roof space. I think I'll set a longworth trap tonight!

Google says a 10-degree change should make it 0.0032 mm longer.

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Well it does appear to be 0.001 to 0.002mm longer after being held for 5-10 minutes. I can't be more precise as my 'touch' isn't perfect.

As both mike and standard are steel, I don't suppose the exact temperature is critical.

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Neil

Edited By Neil Wyatt on 07/09/2014 17:17:50

I am not going into the mathematics, but as heat increases, the frame expands in a positive direction, the spindle (allbeit a shorter length than the frame) expands in the opposite direction. So, the frame counteracts the test block expansion, but the spindle reduces the measured standard. Hence 'Standard rooms' were kept at a constant known temperature. ???

BobH

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That's no fun ... You can't see the fringes

See page 28 ... Plan Parallel Optical Flats

MichaelG.

Bob,

The spindle overhang is shorter than the frame by the length of the measured object, so assuming both are steel the 'gap' between anvils will expand the same amount as a bit of steel between them.

Neil

Edited By jason udall on 07/09/2014 21:32:18

When I had to work for a living, after an incredibly long purchasing effort we acquired a Makino vertical mill, to make a single and extremely complex component. The workshop was already air conditioned to +/-2 degrees C, but where the Makino was installed, in a pocket within the workshop, we had to install a local area air con to +/-1 degree C. As part of the deal we also had to install a 24 hour, 7 day per week temp. recorder. When the part was delivered not only did the customer get a lump of metal with a serial number, but a full inspection report, and a copy of the temp. gradient when that part was being machined.

To get back on track, I seem to remember seeing, good quality steel rules with the legend "accurate at 20 degrees C", or something similar.

keep worrying

cheers

Bill

Oh dear, what a lot of fuss about nothing. My first wise old foreman showed me how to zero a micrometer, first trap a piece of clean white paper between the anvil and spindle and slowly pull it out, any dirt or oil will remain on the paper, then close the spindle on the anvil using the ratchet, if the reading is zero, then the readings taken there after will be true. Simple!

Cleaning the mike- that's what shirt collars are for, the pointy bit at the end.

Mick...Ditto

My gaffer showed me the same.. I still do it now.. 50 yrs later

George

I am a right pain in the buttocks, but on my mike, the spindle is 1.7" long before the start of the thread, so assuming the Mike is opened up to 1", there will still be 1.7" of spindle that will expand in the opposite direction, however the screwed section of the frame will counteract this expansion, so once again things are in equilibrium. All in all, a micrometer is a very clever design compared to a vernier calliper. Do digital callipers have a thermal compensation device in them ?

BobH

Life really is too short to worry about thermal expansion in a shed!

I think the answer is that the resolution of digital callipers isn't enough to worry us, but highly trained mike users whose shed is in Iceland with a glacier on one side and an erupting volcano on the other may find it an issue.

Seriously though, these things may be of little practical significance but they are (1) interesting and (2) a reminder that if we do need to work to limits of accuracy we should worry about thermal expansion and let work cool down before taking a finishing cut, for example.

Neil