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.

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.

+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

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

Roy

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

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

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.

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

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

I have a Mitutoyo vernier caliper, no dials, no batteries; also a mechanical Moore and Wright micrometer; also a good quality steel rule. Somewhere there is a piece of string with knots every cubit, but that doesn't get used much. The steel rule gets used for anything that needs to be within about +/- 0.5mm, the verniers for anything that needs to be +/- 0.1mm, the micrometer for anything less than that.

cheers

Bill

The level of accuracy will depend on how the device has to function.

Slip gauges are accurate to millionths of an inch. They have to be, otherwise they would be useless as a near absolute standard of size (But only under very closely controlled conditions) For our purposes such accuracy is not needed.

For most purposes, a 6.5mm hole will suffice for a M6 bolt, to hold two pieces together..

For greater positional accuracy, you would use a fitted bolt, in a reamed hole. Or the two parts would be dowelled, with the bolts merely holding the part together.

For a guide rod, needing to be a lubricated sliding fit in its mating part, a 25 micron clearance would suffice for most purposes.

For the power cylinder of a Hot Air Engine, the piston would be a lapped fit (like the plunger and barrel of a Diesel fuel injection pump, or an Injector Needle to its Body.)

You can make things too accurately. Years ago, after a rebuild, when King George V ran out of Paddington, it ran a hot box within a few miles. Insufficient allowance had been made for the fact that a steam engine gets hot, and more so, as it runs. The old WD 2-8-0 side rods used to clank as they moved off, but quietened as everything warmed up after a few miles

An internal combustion engine will run happily when unloaded. Apply full load, and if the Piston size and shape are incorrect for the bore (which may distort as the engine heats up) and a catastrophic seizure can result.

The converse can be an engine that is difficult to start because of lack of compression, but once running and warm, performs quite well.

Horses for courses.

Howard