Regards,

Peter G. Shaw

I have a friend who is a retired tool room engineer and who has a micrometer with a vernier scale thus reading to “tenths”. On comparing readings from my micrometers (no vernier scales) and estimating in between graduations, I was very gratified to discover that I could estimate almost as accurately as my friends “tenths” scale micrometer was reading. My friend said that this was quite normal in that the eye/brain combination is capable of splitting small divisions quite accurately.

Regards,

Peter G. Shaw.

Oh dear, the formatting hasn’t worked!

Sorry, you’ll just have to transcribe them onto paper or a spreadsheet.

Regards,

Peter G. Shaw

As I said, the standards, accurate at 20°C, and the measuring instruments were left overnight in a warmish room and the tests done in the same room with a temperature of 20.9°C at the start and 20.7°C at the end. All tests were done at least twice. The measuring instruments are:

Imperial Micrometer, resolution 0.001″ – Mic 1
Metric micrometer, resolution 0.01mm – Mic 2
Vernier caliper, marked INOX (but this may just be another name for stainless steel), resolution 0.05mm – INOX
Draper dial caliper, resolution 0.02mm – Draper
Rolson digital caliper, resolution 0.01mm, accuracy 0.02mm
(0-100mm), 0.03 (100-150mm), repeatability 0.01mm – Rolson
Workzone digital caliper, resolution 0.01mm, – Workzone
Note the limited specification of the Workzone caliper.

The results are shown: Device name – reading 1/reading 2

Mic 1 – 0.9843/0.9843″ (Equivalent to 25.00122mm)
Mic 2 – 25.004/25.005
INOX – 25.00/25.00, 50.00/50.00, 75.00/75.00, 100.00/100.00, 125.00/125.05, 150.00/150.00
Draper – 25.045/25.045, 50.085/50.085, 75.087/75.082, 100.140/100.100, 125.095/125.085, 150.185/150.100
Rolson – 24.98/24.98, 49.98/49.98, 74.98/74.99, 99.99/99.98, 125.02/124.99, 149.99/150.05
Workzone – 25.03/25.04, 50.05/50.03, 75.08/75.05, 100.10/100.04, 125.00/125.00, 150.00/150.00

Comments:
The two micrometers and the Draper dial vernier were estimated by splitting the graduations.
The INOX vernier is difficult to determine which graduation to use. I suspect that some of these readings are actually between 0.00 to 0.05mm higher.
I suspect that the slight differences on the purely mechanical devices are down to varying pressure when measuring. I suspect the same will apply to the digital units, however the Rolson repeatability seems very good, albeit consistently low. Maybe I am applying too much pressure.
The Workzone device does not seem particularly consistent to me.

Well, after all that, I don’t know what to do. It does seem to me, that as an amateur making stuff for myself, that at any one time,I need to use one device only to the exclusion of the others. This does mean that the idea, as I originally intended, of using the Workzone caliper to make a digital height guage is a non-starter. This does also make me wonder just how accurate are all those lathes and milling machines fitted with digital readouts.

Regards,

Peter G. Shaw

ps. I’ve tried to set the readings as in a table. At the time of writing, I do not know how it will appear. If it’s a mess, well, sorry, I don’t know what else to do.

As a result, I have bought the Mitutoyo 25mm, 50mm & 75mm micrometer standards and intend to do some tests. The standards are supposedly accurate to within certain limits at a specific temperature (20°C or 25°?) so what I intend to do is allow all the equipment to reach the same temperature by leaving them in the same room for a few hours, and then to tabulate results at 25mm, 50mm, 75mm, 100mm(25 + 75), 125mm(50 + 75), and possibly 150mm (by using all three devices). This should then give me some idea of
the various discrepancies, and, with luck, their linearities.

There are problems of course in that the three higher readings will not be aa accurate as the lower readings due to the tolerances of the standards being added (or subtracted in which case a combination may be more accurate).

Obviously, for one-off work, then using just one item should result in good work, hopefully, the above test will determine which device to use.

Regards,

Peter G. Shaw

Precision/resolution is not the same as accuracy.

For example, the digital calipers for sell at “Little Machine Shop” have a “0.0005″ (0.01 mm) resolution” but only a “0.001″ (0.02 mm) accuracy”. That means when you see .6015 it could really be anywhere from .6005 to .6025.

http://littlemachineshop.com/products/product_view.php?ProductID=2874&category=

I have promised a set of secondhand slip gauges in the future and they do come up on ebay from time to time.

Finally from my understanding (and reading some tool suppliers catalogues) the need to have slip gauges calibrated is very expensive so whilst the professionals would insist on the calibration, the majority of model engineers would not.

Again thank you for the views of a proffesional. Mike