Digital Calipers - Can you rank these from the measurements?

On larger calipers of repute, they will give you a formula to calculate the error due to temp of the part being measured and the error of the instrument itself at different intervals or lengths. We call calipers very nearly there's. If you are within 0.02mm that is about right. If a 100 mm gauge block shows to be either longer than 100.01 or shorter than 99.99 mm , either the temp range is way off, the instrument is junk or you are not operating it correctly. Some come with a sprung slider that allows the user to use a fairly constant pressure when taking measurements and setting the reference zero. A ball bearing race is a good easy way of checking for outside comparison dimensions and inside diameter comparisons. Of course if you are after precision, you would not be using a caliper anyway. I have had a really good run from Mitutoyo digital calipers and also with the cheaper Insize brand 0-200 caliper that brought a couple of years ago. The Insize come with a test certificate and it conforms well to the gauge block tests I have done with the instrument. The very cheap calipers I brought quite a few years ago, are just that cheap and are ok for wood work where you do not need better than 0.1 from size. You can use a piece low stretch fishing line, wrap it around the part 10 times is a lot more accurate on straight diameters and using a rule or tape measure to get the total line length.

Neil

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Dave,

I have a Mitutoyo CD-6"B which is quite old, but well-cared-for.

I also have a set of 025.10000 Tesa optical flats **LINK**

http://www.tesatechnology.com/en-gb/products/optical-flats-with-two-parallel-faces-p111.htm?redirect=1&c=fr#.V-uSvet4WrU

No.1 in this set is specified at 12mm [ish]

The calliper consistently reads 12.00mm

Q.E.D.

MichaelG.

Suffice to say for now that the Mitutoyo Absolute reads 25.00 consistently on my test bar.

Neil

I think Mitutoyo specify repeatability and accuracy. From memory accuracy is +/- 0.02 and repeatability 0.01mm. The repeatability is probably the usual digital +/- 1 count which is unavoidable. The 0.02 will in part relate to the length of the scale / accuracy of the encoder.

Really in that respect that's little different to good vernier calliper, they read to 0.001 and can roughly indicate over and under.

From 10 to 30C the over all length change of a 200mm calliper is around 40um / 1 part in 5000 but they may use some alloy that has a lower coefficient than that.

Temperature in old money circa  0.0016".

John

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Edited By Ajohnw on 28/09/2016 13:18:18

It's interesting to see the anthropic principle applied to measurement again. It's easy to read an imperial vernier scale. King Henry's thumb again and a mm being too small.

Parallax? I read them square on - when I used to use them. I have switched back a vernier height gauge now so will be using them more. It lacks a flip over magnifier but a loupe or glass is easy to use.

John

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Interesting reading.

Total agreement with controlling temperature of "standard" and of the measuring device, and the importance of consistency in pressure applied /feel.

(Otherwise, why do large companies spend a lot of money on temperature and humidity controlled rooms for checking the accuracy of their measuring instruments?)

My practice with any measuring device is to wipe the jaws/anvils and check that Zero IS the reading, before taking each measurement. With callipers, I try to measure away from the tip, to minimise any effect of deflection. (My measuring kit, even the cheap LIDL digital calliper, is probably more accurate than my handiwork). I just look for consistency, and expect more from a dial or vernier calliper, and a reputable micrometer than from a DRO.

Strangely, this week, the DRO on the tailstock of my lathe began behaving erratically. Removing the battey and refitting after a delay did not work. Stripping and cleaning the slideways was no more effective, especially when it "froze"on 000.0. Having nothing to lose, I plucked up courage and removed the PCB and the Buttons. On the Zero area of the PCB was one speck of dirt. Once wiped off, and with everything reassembled, it seems to work satisfactorily, again.

Being a fully paid up Luddite, DROs are convenient, but mechanical devices inspire more confidence!

Howard

QED? Certainly not. I'm afraid you'll have to lend me your Mitutoyo and Optical Flats so that I can re-run the tests under the same experimental conditions in my secret underground laboratory.

Obviously you can trust me to return your desirable goodies. Eventually ...

Dave

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Nice try, Dave

MichaelG.