The Brown and Sharpe micrometer gauges form convenient and accurate tools for external measurements. They are made in various sizes and styles to measure up to 24 in., and are graduated to read English measure to thousandths and ten-thousandths of an inch; they are also made to read to hundredths of a millimetre. The decimal equivalents stamped on the frame are convenient for the immediate expression of readings in eighths, sixteenths, thirty-seconds, and sixty-fourths of an inch.

The chief mechanical principle embodied in the construction is that of a screw free to move in a fixed nut, an opening to receive the work to be measured is afforded by the backward movement of the screw, and the size of the opening is indicated by the graduations.

Referring to fig. 1, the pitch of the screw C is forty to the inch, the graduations on the barrel A, in a line parallel to the axis of the screw, are forty to the inch, and figured 0, 1, 2, etc., every fourth division. As these graduations conform to the pitch of the screw, each division equals the longitudinal distance traversed by the screw in one complete revolution, and shows that the gauge has been opened one-fortieth or twenty-five-thousaudths of an inch. This opening (between B and C) in fig. 1 is three divisions exactly, and is therefore = 3 x l-40th of an inch, or seventy-five-thousandths.

The bevelled edge of the thimble D is graduated into twenty-five equal parts, figured every fifth division, 0, 5,10, 15, 20, each division, and when coincident with the line of graduations on the barrel A, indicates that the gauge screw has made one-twenty-fifth of a revolution, and the opening of the gauge increased one-twenty-fifth of twenty-five-thousandth = one-thousandth of an inch.

Fig. 1.

Hence to read the gauge, multiply the number of divisions visible on the scale of the barrel A by 25, and add the number of divisions on the scale of the thimble D, from zero to the line coincident with the line of graduations on the hub For example, as the gauge is set in the figs. 2 and 5 there are three whole divisions visible on the barrel; multiplying this number by 25, and adding five, the number of divisions registered on the scale of the thimble, the result (3 x 25 = 75, then 75 + 5 = 80) is eighty-thousandths of an inch. After a little practice these calculations are readily made mentally.

Fro. 2.

The micrometer is shown full size at fig. 2, and measures all sizes less than an inch by thousandths of an inch; whilst the micrometer shown by fig. 3 is graduated to read to ten-thousandths of an inch. Upon the ends of the screw C, and the face of B, falls all the wear due to actual use, and in order to provide for this, B is a tightly fitting screw, which may he advanced from time to time as required. The slot for the screw-driver is shown to the left in figs. 1, 2, and 8. Thus, every gauge ought always to read accurately the dimension of the opening between B and C, no matter how constantly they are in use. The readings of ten-thousandths of an inch are obtained by means of a vernier, or series of divisions on the barrel of the gauge, as shown in fig. 4. These divisions are ten in number, and occupy the same space as nine divisions on the thimble, and for convenience in reading are figured 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 0. Accordingly, when a line on the thimble coincides with the first line of the vernier, the next two lines to the right differ from each other one-tenth of the length of a division on the thimble, the next pair of lines in order to the right differ by two-tenths, etc., as shown by the upper illustration of the two enlarged views of the graduations on barrel and thimble. Since each of the divisions thus measured are equal to one-tenth part of a division on the thimble, it follows that they are each equal to one-tenth of one-thousandth of an inch-i.e., one ten-thousandth of an inch. Hence, when the gauge is opened the thimble is turned to the left, and when a division passes a fixed point on the barrel, it shows the gauge has been opened one-thousandth of an inch. Hence, when the thimble is turned so that a line on the thimble coincides with the second line (end of the first division) of the vernier, the thimble has moved one-tenth of the length of one of its divisions, and the gauge opened one-tenth of one-thousandth, or one ten-thousandth of an inch. When a line on the thimble coincides with the third line (end of second division) of the vernier, the gauge has been opened two ten-thousandths of an inch, etc.

Fig. 3.

Fig. 4.

In the lower diagram of the vernier in fig. 4, where the fourth line marked 10 on the thimble coincides with that on the barrel marked 3, which is the third division of the vernier, shows the position which indicates three ten-thousandths of an inch. To read this gauge, note the thousandths as usual, then the number of divisions on the vernier, commencing at 0, until a line is reached with which a line on the thimble is coincident.

If the second line reached is figured 1, add one ten-thousandth, if it is figured 2, two ten-thousandths, etc. Gauges graduated to read to ten-thousandths should not be used on ordinary fine mechanical work, as in instruments of this class wear perceptibly affects the readings, which would be of comparatively slight consequence in gauges reading only to thousandths. These gauges should therefore only be used as a final test for very important accurate work, and their use requires a much more delicate sense of touch than in the case of the thousandth gauges to obtain exact readings. For such fine adjustments, these gauges are often fitted with a friction clutch for moving the screw, so that the same degree of tightness is always obtained. This is very important when measuring substances that are soft and easily indented to the extent of one ten-thousandth of an inch.

Micrometer Rolling. Mill Gauge

A micrometer gauge, which has recently been introduced for the use of sheet metal workers, is shown by fig. 5, and it is well adapted for this class of work. The gauge screw is encased and protected from dirt or injury, and means of adjustment are provided to compensate for wear. The opening in the frame is about 6 in. deep; this is a very important feature, as it enables sheet metal to be more accurately measured than would be possible with an ordinary micrometer. This great depth in the frame makes it possible to measure or gauge the metal at various points in the width of the sheet, which could not be reached with the ordinary pattern of micrometer gauge.

Fig.5.

Fig. 6.

The British standard wire gauges in three patterns are seen at figs. 6, 7, and 8-the oblong, the double circular, and the single circular respectively. The American standard wire gauge shown in fig. 9 is that gauge adopted by the American brass manufacturers, January, 1858 (see the figures given in column L of gauge table), in place of the Stubs wire gauge. This American gauge, as previously mentioned, has the decimal equivalents stamped upon the back in thousandths of an inch.

Fig. 7.

This pattern of gauge is much inferior to the micrometer type. It is not always clear which of the several standards they actually represent, whilst as often as not the faces of the openings are anything but parallel, and in consequence there is some choice left to the user to determine which place gives the actual gauge. They serve, however, as a rough guide to the thickness of the plate or the diameter of a wire; but it must be borne in mind that measurements made by it can only be approximate at the best, for it is quite impossible to determine how much larger or smaller a plate or wire is than the nearest opening that may be found in the gauge. It is to be noticed that there is no way of taking up the wear that must inevitably result after the gauge has been in use for a time.