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Accurate measurement

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David Canham07/02/2018 18:13:49
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50 forum posts
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Hi people, I now have a micrometer and a Vernier calliper to use in my workshop but I have heard or read somewhere that the mic is the one to use for serious accuracy. how come? Both units are digital by the way.

David.

Mick B107/02/2018 18:50:03
2084 forum posts
121 photos

There are really 2 points of potential superiority in the mic.

Firstly there is usually a clutch or ratchet in the thimble which can deliver a consistency of pressure on the workpiece being measured, so as not to compress it, the mic spindle or pads significantly.

Secondly (and IMO more importantly) there can be a Vernier scale inscribed around the thimble which in Imperial mics measures tenths of thous.

That said, you have to decide how accurate your serious accuracy needs to be. For the engineering - full-scale and model - that I do, I've found that a good digital caliper that can measure consistently to half a thou or 0,01mm is quite adequate in more than 99% of cases. I've used my Mitutoyo Vernier mic somewhere between 2 and half-a-dozen times in the past year.

I'm glad I've got it, but I'd be surprised if I couldn't manage without.

Neil Wyatt07/02/2018 19:34:25
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18899 forum posts
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Also, the geometry of a mike is better.

A micrometer (if reasonably well made) holds its faces very accurately parallel to each other and as the closing force is applied along the measuring axis there's no tendency to tip or skew and introduce an error.

Even if you adjust the gib strip of a good quality calliper very well, it will still have a small tendency to tip and give a small error. You can get around this by closing it with thumb and forefinger on the jaws.

Neil

Jeff Dayman07/02/2018 19:34:29
2199 forum posts
45 photos

David, do you know what is meant by calibration of measuring instruments to a known standard?

Do you know what the principle of alignment is in relation to measuring tools?

If you don't know about either of those things it will be difficult for anyone to explain the relative accuracy of micrometers and calipers, and I suggest you read up online about basic metrology.

The Galyer and Shotbolt textbook "Metrology for Engineers" is still a good one and explains all the basics. Just start at the start.

Chances are for most practical model engineering many of the fine nuances in theory and the differences in these instruments will not matter, but you will benefit from a full understanding by doing some research and reading.

Oldiron07/02/2018 19:47:15
911 forum posts
40 photos

I would go with Neil' or Mick B1's simple explanations. For the model engineer or home workshop user there is simply no need to go into long winded diatribes on metrology.

regards

Simon Williams 307/02/2018 20:13:28
627 forum posts
81 photos

+1 for oldiron's keep this out of the metrology analysis approach.

For me there is something useful to be said in terms of the fact that a digital caliper might have a display that reads to hundredths of a millimetre, but that doesn't mean that it is that accurate. Accuracy has two elements here, repeatability (can you measure the same thing again and again and get the same answer) and absolute accuracy (does the answer you get - however repeatably - coincide with the "correct" answer from a standard of some kind).

The resolution of a measuring instrument is often thought of as the smallest increment the display will show - i.e. resolve. But that isn't true in the case of a digital caliper, and the resolution of the display may be smaller than the repeatability of the measurement. So I feel that the instrument resolution is a bit of a red herring when looking at calipers, and it's the repeatability which decides whether what you are making and measuring actually fits with its mating part. A micrometer is always going to win a contest on repeatability, and very likely absolute accuracy as well, unless it's really knackered or has been adjusted wrongly.

A micrometer, because of its construction and because the way it is used is less open to operator effects - gives a "better" answer on several levels - it's more repeatable and also likely to have better absolute accuracy.

Hope this helps, part of the problem is that the vocabulary of "accuracy" is loose and woolly, and we need the carefully defined concepts of the clever devils in the metrology lab' to discuss this un-ambiguously. That's not something I can do, but there are clever folk on here who can.

Best rgds Simon

David Canham07/02/2018 20:26:06
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50 forum posts
15 photos

Well that's given me some food for thought chaps, thanks very much for some really useful info on what taking a decent measurement is all about.

At the moment I am reading so many different books on loco building, boilers, injectors etc. etc. that swotting up on metrology might have to wait for a short while.

I think I need to go away and practice with both instruments and get a feel for them.

Thanks again,

David.

roy entwistle07/02/2018 20:55:43
1459 forum posts

There's a lot of things been built in the past using inside and outside calipers and a rule

Roy

Fowlers Fury07/02/2018 21:11:02
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393 forum posts
91 photos

Some wisdom above !
I find repeatability is a recurrent problem. Perhaps it's the way I take measurements yet I'm sure the current cold temperatures are a factor. Going into a cold workshop, a previously measured item at the end of a long session when everything has warmed up, invariably gives a different value especially with a digital vernier. Probably due to battery temperature rather than expansion. Most calibration certificates seem to be at 20 deg C. For the last few weeks my workspace begins at 8 deg C. (For several reasons, I'm unable to maintain the garage/workshop at a constant temp).

For a beginner, a vernier is surely the more versatile device (inside, outside, between centres + depth gauge) notwithstanding its limitations as Neil points out. However, an old but cherished Moore & Wright micrometer still gives that feeling of confidence, repeatability and spurious accuracy.


David Canham07/02/2018 21:14:08
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50 forum posts
15 photos

Well Jeff you have wetted my appetite for some more learning. I have just been on Amazon and ordered a copy of Metrology For Engineers. Good bedtime reading.

David.

vintagengineer07/02/2018 22:14:12
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468 forum posts
6 photos

Temperature has a big effect on measurement. I once made a large bronze bush to fit into a customers ali casting. It had to be a sliding fit. I sent it of to the customer who phoned me up to say it wouldn't fit. Visited customer whose workshop was about -5 degrees colder than the outside, I point this out to him and warmed the casting up to hand hot and the bush fell in!

Metal moves a lot due to temperature.

Edited By vintagengineer on 07/02/2018 22:14:38

John McNamara07/02/2018 22:14:59
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1331 forum posts
122 photos

The Vernier looses compared to the micrometer because of Abbes principal.
The error caused by the axis indication and movement being moved away from the axis of measurement.

Mitutoyo explain it well here....Also a good general reference. (Free booklet)

**LINK**

Regards
John

Oh! And i am still not happy with my wording......Read the booklet.

Edited By John McNamara on 07/02/2018 22:17:23

Edited By John McNamara on 07/02/2018 22:19:54

Edited By John McNamara on 07/02/2018 22:21:02

colin hawes08/02/2018 10:43:39
553 forum posts
18 photos

There's a couple of things to be aware of using a micrometer: the job needs to be very clean and the surface finish needs to be good enough to accept the accuracy required. Vernier callipers are inherently inaccurate for bore measurement as they have flats on the measuring face. Colin

Hopper08/02/2018 10:55:39
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5508 forum posts
137 photos

Digital "vernier" caliper for quick and easy measurement down to accuracy of a few thou (.075mm).

Micrometer for accurate measurement to .001" (.025mm) and less. Every time.

Dunno the physics and philosophy behind it, but I've been using both for 45+ years and that's been my experience.

Edited By Hopper on 08/02/2018 10:56:32

duncan webster08/02/2018 10:59:48
3710 forum posts
69 photos

I find it really easy to be 0.5mm out using metric micrometers, so I use the digi caliper to get within 0.5mm then use the mike. If I'm not looking for fabulous accuracy just use the caliper.

​Vintage Engineer's tale reminds me of a very worried production chap who rang me one morning to tell me that the very expensive stainless steel casting he'd just machined, about 1m * 2m * 0.3m, was a thou under bottom limit. Making sure the chief inspector couldn't hear I told him to measure it again in the afternoon, it then passed.

John Haine08/02/2018 11:04:14
4428 forum posts
264 photos

I have both mikes and verniers. As I've got better (well, less bad) at accurate working I have tended more to use the micrometer. This also because the digital calipers can give problems when they get dirt on the read head and the gibs get loose - easy to correct once you know what's happening but confidence sapping.

I bought a Mitutoyo (genuine) digital mike with 1 micron resolution a while back but found it very frustrating as there is non on/off switch and, not using it very often, when I took it out of its box I never knew if the reading was absolute or relative. This meant winding back to mechanical zero and re-zeroing the display, rather tedious if it was open 25mm! So I've now got a s/h Mitutoyo mechanical mike off eBay and find that pretty good and quick to use. Recently though I decided that since the digital one can't be switched off anyway it may as well sit on the bench in a stand and be used for things like measuring off telescopic bore gauges and the o/d of small cylindrical items that aren't in a chuck, taking the mechanical mike to the job for example for measuring diameter of things in the lathe.

For i/d measurement, as said above the caliper is pretty useless except to get an indication. I got a set of Mitutoyo telescopic bore gauges in a horrible greasy cracked plastic wallet for a song off a stand at an exhibition, also found a nice Starrett one which covers about 16 - 25 mm on a market stall. These measure diameter pretty well but take some practice to use them properly.

Neil Wyatt08/02/2018 11:47:08
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Moderator
18899 forum posts
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80 articles

An issue with callipers (and mikes) is making sure the faces are clean. I was in the habit of re-zeroing my verniers every time I started using them. The Mitutoyo Absolute doesn't need this but regularly reads 0.01 or 0.02mm - until I clean it properly!

So always wipe the faces before zeroing, even if they look clean!

I.M. OUTAHERE08/02/2018 19:11:48
1468 forum posts
3 photos

Just to add to what Neil says about cleaning the faces , always store them with the faces apart .

J Hancock08/02/2018 20:06:07
799 forum posts

The effect of temperature on installations will be well understood by the residents of Liege today.

The suspension bridge over the Meuse had road sections lifted 400mm at both ends by the cables shrinking in <-5deg temperatures.

FMES08/02/2018 21:44:21
607 forum posts
2 photos

Most digital calipers don't have a vernier scale.

Anyone ever seen one that has?

One manufacturer tried it a few years ago but it was not successful.

Regards

Lofty

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