This article first appeared in Practical Engineering 1940 Vol1 No26. The contents are accurate as of 1940. This article highlights tools utilised within Precision Engineering at the time.
There is another form of micrometer for inside diameters, and it may be seen that this closely resembles the outside micrometer in general design. The bowed frame is replaced by a fixed jaw, whilst a second jaw is fitted to the end of the spindle; this is moved backward and forward by turning the thimble. It will also be noticed that the horizontal scale is "in reverse," the figures 5, 4, 3, etc., reading from the left. This type of micrometer is not very widely used, but is valuable for certain types of special boring work.
In some respects similar to the outside micrometer is the vernier gauge or vernier caliper. There is a calibrated beam with an end jaw integral with it, whilst a second jaw is attached to a slide which can be moved along the beam. Since the depth of the jaws is restricted, it will be appreciated that the vernier gauge is not suitable for measuring other than very small outside diameters: its chief use is for measuring the length of small accurate parts.
Nevertheless, some models can also be used for internal measurement due to the provision of the narrow jaw extensions. In that case there are sometimes two scales— one on each side of the beam—for external and internal measurements respectively.
The term vernier as widely used for any finely-calibrated measuring scale, but the true meaning of the term is much narrower than this. It is correctly applied only to a particular form of calibrated scale of the type developed by a certain Pierre Vernier in the seventeenth century.
The principle of the vernier is that of two scales, of which one moves over or alongside the other. On the moving scale there are a certain number of divisions the length of which totals that of a smaller number of divisions on the fixed scale. In general, the scale divisions on the beam are the same as those on the sleeve of the micrometer. That is, the inches are divided into fortieths, and every fourth of these (1/10in.) is marked from 1 to 9.
On the sliding scale there is a .6in. scale divided into 25 equal parts. Thus, the .6in. scale has one more division than has the same length on the fixed scale (25 as to 24). The difference in length between the divisions on the two scales is therefore 1/25 of 1/40in.., or 1/1,000in. In this respect, the general arrangement is similar to that on the micrometer.
If we start off with the zero marks on the two scales exactly in line, and then move the slide to the right so that its second division lines up with the second division on the fixed scale, we shall have moved the slide 2/1,.000in. Similarly, if it is moved so that the third divisions coincide, the scale will have been moved 3/1,000in. This principle holds good throughout the movement of the scale —to which is attached the moving jaw.
It is now possible to see exactly how the vernier gauge is used. If, for example, the jaws are opened until the zero on the vernier scale is in line with the 5 line on the fixed scale, the opening will be .5in. Suppose now that the slide is moved two more small divisions (fortieths of an inch), the distance between the jaws will be .5 plus .05 (twice .025) in. At this point the zero mark on the vernier will be directly opposite the second small division past the 5. And if we now move the vernier scale still further to the right, using the screw adjustment until the eighth line on the vernier scale is in line with one of the fines on the fixed scale, the additional opening will be .008 (eight thousandths) in. Thus, the final setting of the vernier will show that the opening between the jaws is .5, plus .05, plus .008in. By adding these together we know that the setting is .558in.
If a vernier is not available it is suggested that the reader should draw two scales similar to those on the actual instrument and practise setting them to various positions. A little experience will soon make him familiar with the use of this gauge. Rather than draw the scales to full size it would be better to make them at least twice full size so that they are more conveniently handled.
There is a wide range of vernier gauges, all of which are similar apart from their size. They vary from about 2in. to several feet, but a 6in. instrument will meet most requirements. If there is not a separate scale for inside measurements it will be clear that the total width of the two projecting jaws must be added to the reading when taking inside measurements; this dimension would probably be stamped on the instrument.
Having seen the principle of the vernier gauge it is possible to understand the working of the micrometer which is designed for measuring in ten-thousandths. The micrometer is similar in construction but there is an additional scale round the sleeve. There are ten divisions on this scale, and their total length is exactly equal to that of nine divisions on the scale round the end of the thimble. Thus, the difference in length of the divisions is one-tenth of that of the divisions on the thimble; as each of the latter is equivalent to 1/1,000in., this difference is equivalent to 1/10,000in. (.0001in. as a decimal). In taking the reading, therefore, the thimble is turned until one of the lines on its scale is directly opposite one of those on the vernier scale set off round the hub.
This will better be understood by making reference to where the reading shown is .4697in. This is made up as follows: The thimble is turned to 4 on the sleeve scale (that is .4in.); it is then further unscrewed two more small divisions—fortieths—-which is an additional .05in.; the thimble is then turned until the 19th division on its scale is opposite the datum or zero line on the sleeve (adding .019in.); finally, the thimble is turned still further in an anti-clockwise direction until the 7 line on the vernier scale is opposite a line on the thimble scale— that gives an additional .0007in.
Adding these readings together we get: .4, plus .05, plus .019, plus .0007, which gives us the total of .4697in.
So small are the divisions on the vernier that it is generally desirable to employ a magnifying glass when reading them. This applies chiefly to the ten-thousandths micrometer, but the method is often recommended in connection with the ordinary vernier gauge.
Another use of the micrometer principle is for a depth gauge. It is in many respects similar to inside a micrometer, and there are interchangeable spindles for dealing with holes of different depths.