The answer is one word "accuracy" A vernier caliper is just that, a caliper and should not be confused with a Gauge. Many beginners (and unfortunately some who should know better) are confused by the difference and are under the impression that a modern electronic caliper is the last word in accuracy. However they are making some serious errors with this. First off the resolution of a caliper whilst useful for determining the general size of something is nowhere near the standard required for truly accurate sizing. Do not be taken in by the apparent ability of these instruments to read to two or three places of decimals, simply put they don't. There are fundamental design faults involving bending, contact area and pressures that preclude these instruments from "accurate measuring tools". In industry gauges are used. These range from adjustable units e.g. Micrometers which are designed to mitigate the worst of the drawbacks mentioned and can measure to reasonable standards of accuracy if used correctly. Through to go-nogo gauges which are unadjustable being made to exact size and to traceable standards. The way these work is that a part should fit in the Go part but not in the nogo part, if it fits neither then it is too big if it fits both it's too small. The above is a simple example and with modern methods it is possible to use lasers etc. to measure to accuracies undreamed of by Mr Vernier. Who incidentally only invented the scale not the caliper.

Does anyone practice the art of using simple internal calipers and a micrometer to measure hole size?
Your vernier caliper is all well and good but it will only give you the "max metal condition". You won't detect that a hole is big at the back edge, it is not easy to find a bell mouth hole with the vernier caliper and its unlikely that you will spot any out of roundness with the vernier caliper.
OK the spring bow caliper requires a bit more practice and feel to transfer the size to a micrometer but once mastered it can be as consistent as your typical digital caliper and more importantly tell you a lot more about the geometric shape of the hole.
Gauges, plug gauges, ring gauges, gap gauges etc. all need calibration and in some instances, setting on a regular basis to be meaningful but are really intended for use in volume production environments rather than for use in model shops or toolrooms. The toolroom is where many are made for use on the production line.

Bob D

Edited By Robert Dodds on 01/10/2014 22:57:52

Thanks Jack for purchasing the book, I do hope you will find it useful.

Regarding the gauges, I would not suggest they serve no purpose but as far as the book is concerned they are there to provide exercises in working to very close diameters and lengths.

For me, I only used the hole gauges very occasionally, that is when I need to get a hole diameter as close as I can to the gauge diameter. To do this, I set my top slide at about half a degree off of parallel with the lathes axis so that a 10 thou feed of the top slide will result in radial movement (depth of cut) of nominally 1/10th of a thou.

By this method I can take very shallow cuts as I arrive near to the required diameter for the bore, doing this until the gauge just enters the hole. Of course the tool has to be honed to a very fine edge for these depths of cut to be taken. You do also need to clean the hole completely to make the test.

This is basically the method I use in the book to arrive at very precise diameters and lengths. The following, scanned from the book, shows a round nose tool that has become magnetised and producing swarf almost like grinding dust. It shows a hole gauge being made with a chamfered leading edge to give early warning that one is approaching the required diameter and then a very slightly reduced diameter to show you are very close.

The distance gauges however I make considerable use of, typically for setting my saddle stop as can be seen by photos 1 and 2 here

Harold