Why does micrometer zero ok but read 0.001" big at 1"?

greif1 · Jul 1, 2014

I got 4 very used micrometers at an estate sale. Two seem to read ok, but the other two (one a starrett, the other something lesser known) zero ok but read 0.001" big at 1". How is this possible? The pitch on the screw can't change that much can it?

Atomkinder · Jul 1, 2014

What are you using to check it?

Edit: also, are these friction thimble, ratchet stop, "feel" stop? Could just be your hand is out of spec

Whenever I get a mechanical (vernier) mic that's new to me I disassemble it, clean the threads, check the spindle bore for damage or grime, adjust the thread tension to suit my "feel" and add just a touch of M1 oil. Then I check them every so often, even if they haven't been used, to make sure my feel is good, and that no rust is forming anywhere. Always use clean gage blocks to check along the range. Using random increments makes me feel better about accuracy, but of course the stack has tolerances too. Not likely to be an issue if the gage blocks are high quality.

DMF_TomB · Jul 1, 2014

greif1 said:
I got 4 very used micrometers at an estate sale. Two seem to read ok, but the other two (one a starrett, the other something lesser known) zero ok but read 0.001" big at 1". How is this possible? The pitch on the screw can't change that much can it?

.
clean it, then clean again, then clean it again, electronic mics just zero with button push. most mechanical micrometers 99.99999999% of the time it has dirty ends and needs a cleaning.
........ they are adjustable but if you got +.001" more than likely you got .001 of something on ends even if dirt, rust, dried coolant, etc
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if you read 1.001" then what are you measuring? 99.999% of the time what you are measuring is oversize

The real Leigh · Jul 1, 2014

What is the resolution of the mics, .001" or .0001"?

Are they electronic or mechanical? From your "very used" comment I think mechanical.

- Leigh

Spyderedge · Jul 1, 2014

Your threads are probably worn if the ends are not dirty. If your mike doesn't have carbide faces but has HSS faces, they could be burred also.

DMF_TomB · Jul 1, 2014

you either have a oversize block or you are not holding it in micrometer correct.
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it takes machinist awhile to learn to properly hold a micrometer to get repeatable readings. the larger the object the harder it is to get the proper reading. for a outside micrometer this is the smallest reading as the more you angle it in micrometer the higher the reading

greif1 · Jul 1, 2014

Aha moment! I see now that if I had a burr or dirt on the flat anvil and zeroed, then checked with my 1" Starrett disc (only making cylinder contact) I would then read oversize!

I have only lubed the screws so far, not done any real cleanup. Of the two squirrely ones, one is 0.001" reading the other 0.0001".
Thanks All!

TheKen · Jul 2, 2014

They can change that much if the threads are worn. If you have a set of gage blocks, check the micrometers at the following... .205, .410, .615, .820 & 1". This checks the thread at different thimble rotation on each reading. If those reading are not good, then you probably have thread lead error. In that case, it would need to be repaired or scrapped.

theKen

DMF_TomB · Jul 2, 2014

micrometer

greif1 said:
Aha moment! I see now that if I had a burr or dirt on the flat anvil and zeroed, then checked with my 1" Starrett disc (only making cylinder contact) I would then read oversize!

I have only lubed the screws so far, not done any real cleanup. Of the two squirrely ones, one is 0.001" reading the other 0.0001".
Thanks All!

.
normally micrometers are NOT lubricated as the screws will turn tight if oiled. if oiled then oil wiped off that sometimes works. oil takes up space and normally will cause screw to turn tight.
.
normally micrometers are set about .0001 when closed. if you got one reading 0 when closed and 1.0010 when on a 1.0000 disc and it reads more than 0.0005" off i would throw it away.
........ even a .001 reading micrometer shown be accurate to .0005" anywhere it measures. if you got many micrometers and one is different than the rest. assuming you are measuring correctly. many people are just plain bad at measuring to .0001

greif1 · Jul 2, 2014

I must disagree with the idea of not lubricating a precision thread such as is used on a micrometer.

Mitutoyo makes an oil just for that purpose: Find Mitutoyo 207000 - LUBRICANT FOR MICROMETER at Guardian Industrial Supply, a leading woman-owned distributor of industrial products.
Starrett says to put a drop on spindle and threads: http://info.starrett.com/blog/bid/1...and-care-for-your-Starrett-outside-micrometer
Tesa (Swiss) says the same 213 : Tesa Isomaster Micrometer

Bare metal to metal is going to wear much, much faster and be subject to oxidation at a much higher level.

Eric M · Jul 2, 2014

greif1 said:
I must disagree with the idea of not lubricating a precision thread such as is used on a micrometer.

Mitutoyo makes an oil just for that purpose: Find Mitutoyo 207000 - LUBRICANT FOR MICROMETER at Guardian Industrial Supply, a leading woman-owned distributor of industrial products.
Starrett says to put a drop on spindle and threads: http://info.starrett.com/blog/bid/1...and-care-for-your-Starrett-outside-micrometer
Tesa (Swiss) says the same 213 : Tesa Isomaster Micrometer

Bare metal to metal is going to wear much, much faster and be subject to oxidation at a much higher level.

I'm with you on this one. If it was a "closed system", and the oil had no place to go, you MIGHT get some tightness. Since it's not sealed, the excess oil slides out of the way. OTOH, being too generous with the oil can attract dirt and other contaminants, and that WILL cause problems with the internal threads. Just my $0.02 worth...

Atomkinder · Jul 2, 2014

Eric M said:
I'm with you on this one. If it was a "closed system", and the oil had no place to go, you MIGHT get some tightness. Since it's not sealed, the excess oil slides out of the way. OTOH, being too generous with the oil can attract dirt and other contaminants, and that WILL cause problems with the internal threads. Just my $0.02 worth...

I might add that using no lubrication is going to increase the amount of friction in the system and although the micrometer can be calibrated for some of this, eventually it will change as the threads wear together without oil, and could change. The lubricity of a minimal amount of oil would help to prevent this, IMO.

DMF_TomB · Jul 2, 2014

oil

Atomkinder said:
I might add that using no lubrication is going to increase the amount of friction in the system and although the micrometer can be calibrated for some of this, eventually it will change as the threads wear together without oil, and could change. The lubricity of a minimal amount of oil would help to prevent this, IMO.

.
as an apprentice i took my micrometer apart easily a few thousand times. each and every time i used a thin oil like 32 it would easily turn 10x harder and to turn each and every time oiled and would require exceeding the slip clutch amount.
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if i wiped oil off there is some still there. oil thinner than 32 (which is a very thin oil) is usually used for flushing thicker oils and not normally used for lubrication. thinner oil is more like mineral spirits is viscosity.
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when you take your micrometer apart at least a few thousand time then maybe you can tell me the value of oil. til you get up to 1000 reassemblies it only means to me you have never oiled your micrometer to me
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i used to take my micrometer apart at least 10 times each day at break time just to piss of the other machinist. each and every time i reassembled it was in calibration.
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anybody who says to oil their micrometer in my opinion does not know what they are talking about or is trying to sell mineral spirits as "special tool oil" at 10x normal price.. i have had unoiled micrometers well over 35 years and never ever have they not passed inspection and they work extremely well and as i often use them a few hundred times a day i see no reason ever to consider oiling them ever.
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and i dont care if they pay you $1000/hr as a machinist if you say to oil a micrometer i wouldnt pay you $5/hr as a machinist

Screwmachine · Jul 2, 2014

I oil my mics, but I might disassemble them once every 5 years, if that. They work great and calibrate. All Mits.

Atomkinder · Jul 2, 2014

DMF_TomB said:
and i dont care if they pay you $1000/hr as a machinist if you say to oil a micrometer i wouldnt pay you $5/hr as a machinist

Funny, just last night I put a drop of Starrett M1 oil on the threads of a 7-8" Mitutoyo that had been sitting in its box, never used (IE new old stock), for years. The difference in ease of turning the thimble is quite noticeable. Towards the inside range it was previously stiff enough to cause the ratchet stop to skip one or two teeth before continuing. Now it's like butter. As I said before, it has been sitting in its box with the original wax on the spindle and anvil. There was certainly a dried portion of oil in there before. If Mitutoyo does it at the factory I find it hard to believe they do not recommend it for the tool's lifetime. greif1's posted links also provide evidence of this intent.

Glad you're not my boss!

greif1 · Jul 2, 2014

Very high accuracy fine pitch screw threads like those used on grating ruling engines made by Mann and Moore Precision tool all require lubrication, sometimes even submerging the screw in an oil bath. When ruling diffraction gratings the oil film thickness between screw and nut (even though preloaded and just molecules thick!) could cause a problem of variation, so a run always started well before the actual ruling began, so the thickness (and temperature too, probably) could stabilize.

Unrelated to lubrication- but an interesting read about precision screws: http://www.practicalmachinist.com/vb/general/expiriments-leadscrew-lapping-218177/

The Davistown Museum

Oldwrench · Jul 2, 2014

Yeah, he'd have to fire me for using instrument oil on my mics. Lubrication of the Moving Parts is an old and fundamental concept you can pretty safely rely on.

Paolo_MD · Jul 2, 2014

greif1 said:
Very high accuracy fine pitch screw threads like those used on grating ruling engines made by Mann and Moore Precision tool all require lubrication, sometimes even submerging the screw in an oil bath. When ruling diffraction gratings the oil film thickness between screw and nut (even though preloaded and just molecules thick!) could cause a problem of variation, so a run always started well before the actual ruling began, so the thickness (and temperature too, probably) could stabilize...

If I recall correctly, i the Moore ruling engines the the leadscrews were immersed in a circulating oil bath in order to keep their temperature constant. I don't remember if it is true for the ruling engine, but in many cases kerosene is used instead of oil when tight tolerances and thinner lubricant film are needed.
Even precision blocks should always be oiled and the thickness of the oil film between blocks in a stack should kept under consideration when executing a measurement. The oil film is thinner and more predictable than the thickness of surface oxidation (read rust).

Paolo

PeteM · Jul 2, 2014

+1 (or 2 or 3 or 4 -- lost count) on lightly oiling micrometer threads. Where folks get in trouble is using something like WD40 that leaves a dried or oxidized film. Pure light instrument oil is what's wanted.

Eric M · Jul 2, 2014

PeteM said:
+1 (or 2 or 3 or 4 -- lost count) on lightly oiling micrometer threads. Where folks get in trouble is using something like WD40 that leaves a dried or oxidized film. Pure light instrument oil is what's wanted.

DMF - Listen to PeteM. He knows more about micrometers than most of us have forgotten!

I don't know how many micrometers he had in his collection (which has since been sold for a VERY WORTHY cause), but it was probably more than most shops...
If he says "lightly oiled", take it to the bank...
Pete - feel free to correct me if I mis-stated anything.

Why does micrometer zero ok but read 0.001" big at 1"?

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