MEW 219 Micrometers

[Eric CoxParticipant @ericcox50497]

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.

[Eric CoxParticipant @ericcox50497]

[jason udallParticipant @jasonudall57142]

Lets assume the gauge and mic are steel…coeff expansion 0.12 percent for 100 Degrees kelvin or C

So for say 50 degrees…cold frosty to sweat box.. 0.06 per cent…
Notional 25.4 mm gauge…thats a difference of 0.015 mm

[jason udallParticipant @jasonudall57142]

But since both mic and gauge have the same temperature. ..thus 25.4 will still read 25.4
As a check of the mic its fair .
As a check against absolute ( whatever that is) not so good…..
As error against absolute…at 45 C thats +25 so 0.0075 mm error

But its not really that simple. .since the mic is made of a frame..subject to expansion and a moving bit also subject to expansion..but heres the kicker…the second part is not constant length…so error would depend on “reading ” of the mic..

[JasonBModerator @jasonb]

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.

[Neil WyattModerator @neilwyatt]

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

…

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.

…

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.

…

Neil

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

[Speedy Builder5Participant @speedybuilder5]

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

[Michael GilliganParticipant @michaelgilligan61133]

Posted by jason udall on 07/09/2014 15:55:12:
Lets assume the gauge and mic are steel…

.

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

See page 28 … Plan Parallel Optical Flats

MichaelG.

[Neil WyattModerator @neilwyatt]

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

[jason udallParticipant @jasonudall57142]

Lets imagine a G clamp.
Rather than a mic…
The moving bit remains the same length but the amount within the frame varies…so if the 1.5″ frame expands say 1 thou the half inch of moving bit will expand 0.33 thou leaving 0.7 thou difference….

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

[jason udallParticipant @jasonudall57142]

But the 1″ bar will too have expanded if at the same temp…

There is a reason why calibrations are defined at a fixed temperature…

[jason udallParticipant @jasonudall57142]

But the 1″ bar will too have expanded if at the same temp…

There is a reason why calibrations are defined at a fixed temperature…

[jason udallParticipant @jasonudall57142]

Some cnc machines ( hardinge gt sp I know did ) have in effect a reference mechanism and a set of offsets “hidden” from the user to automatically compensate for thermals….
The machine held 0.1 micron all shift…from frosty start to end of day.
The mics while not at 20C…were at blood heat ..a fairly well defined reference….

[Bill PudneyParticipant @billpudney37759]

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

[mickParticipant @mick65121]

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!

[jason udallParticipant @jasonudall57142]

Mick..
Be happy.
An AWARENESS of the effects and magnitude is good but most of the time shouldn’t worry one.

[Gordon WParticipant @gordonw]

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

[mechman48Participant @mechman48]

Mick…Ditto

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

George

[Speedy Builder5Participant @speedybuilder5]

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

[mickParticipant @mick65121]

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

[Neil WyattModerator @neilwyatt]

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

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