In

Variations in detail are found in different calipers, in regard to the methods of taking up wear, and making re-adjustments so that the distance between the measuring faces shall be correct. In the instrument shown, the anvil is adjustable, to be pushed forward by a screw, and clamped by another lateral screw. the Starrett calipers the anvil is fixed, and compensation for wear is effected by the use of a thin sleeve fitting over the hub, the graduations being placed on this sleeve, which is turned slightly when required, to bring the zero line around a little. In the Slocomb tool, Fig. 213, the longitudinal adjustment is obtained by turning the nut A in the hub, the thread being of different pitch to that of the measuring screw. Backlash is absorbed by the use of a second nut B, which engages with the face of a by vee teeth, fifty-six in number, on the faces of each. To effect take-up, the measuring screw is turned out of A, and the nut B then given a portion of a revolution, to the extent of one or two of the vee teeth, which are then re-engaged. A spiral spring between A and B maintains uniform friction between the screw and nuts. The frame of the Slocomb instrument is ribbed as shown

1-8125 1-4250

3-8-.375

1-2 500

5-8 .625 3-4 .750 7-8.875 10 the. 1 .0625 3.1875 5.3128 7.4375 9 .5825 12 .6875 13.8125

15 .9375

B

in Fig. 213, and there are no decimal equivalents on it, as is the case with Fig. 211. Instead, the line of graduations on the hub is marked off on one side into fours, and on the other into fives, thus indicating eighths of an inch. The decimal equivalents under in. are marked on

VOL. VI.

Fig. 213.-Micrometer Caliper.

several hundred tons, which also has the effect of compressing the steel.

The clamping ring shown in Fig. 211 differs from that in Fig. 212 in being placed centrally; the screw is dispensed with, and the spindle clamped by a split bushing, which is closed

N

inwards by turning the knurled outer ring, the effect of the latter being to force a small roller up an inclined plane cut on the bushing, and so squeeze it inwards.

The ordinary micrometers read to 100 in., although lesser amounts can be judged pretty accurately by subdividing the graduations on the thimble by the eye. But for very fine work, a circular vernier is added on the barrel. There are ten divisions, occupying the same space as nine of the thimble divisions. After

Fig. 214.-Loose Anvil.

000

noticing that one division on the thimble has passed the line on the barrel, thus indicating a movement of in., the attention is directed to the coincidence of a thimble line with one of the vernier graduations. If it happens to be the second line, the thimble has moved one-tenth of the length of its own divisions = 1000 in.; if the third line, 1000 in., and so on. These micrometers are not used for the coarser classes of measuring, on account of the effects of wear being much more perceptible in the fine measurements taken.

When the range of micrometer calipers exceeds 1 inch, the length of movement of the measuring spindle is still limited, on account of the undesirability of giving a greater movement than an inch; two devices are then adopted according to requirements. The distance between the points is limited to sizes above 1 in. and less than 2 in., or above 2 in. and less than 3 in., and so on; or loose anvils are provided to be fitted to the frame, to give the different spans as in Fig. 214, the anvils being each secured with a nut at the end. A couple of small lock nuts placed to butt against the shoulder of the frame afford a means of giving compensation for wear. The difference in range produced by each anvil is 1 in. The usual range of these large calipers is up to 12 in., for which a horseshoe frame is preferable, with lightening holes. Metric measures are of

course taken with any of the micrometer calipers, when they are graduated suitably.

Special types of micrometers are used for particular purposes. The style of frame shown in Fig. 211 is adapted as a depth gauge by flattening its left-hand side into a foot, and making the anvil with a hole right through, to admit a small rod, which passes down into the hole to be measured, and up to the measuring spindle, which indicates the depth. Screwthread micrometers have the anvil and the spindle end shaped to embrace the sides of the thread, without actually touching at top and bottom. The anvil is recessed to a vee shape, and the spindle end forms an external vee. Some sheet-metal calipers are made with a deep gap in the frame, so that the points may be passed over some distance from the edge to get average thicknesses. Micrometers for paper, card, rubber, &c., have large discs on the anvil and spindle, to avoid the risk of

instead of utilising the barrel and thimble device already described.

The Beam Micrometers are treated under that heading.

Micrometer Gauge.-Applied to the internal, and the depth gauges which have micrometer devices embodied. The principle is the same as that in the Micrometer Caliper. In the internal gauges, the distance apart of the two end measuring points is varied by turning the thimble, and extension rods are provided to increase the range. The depth gauges, of which Fig. 215 is an example, include a foot to stand upon the work, and a rod clamped within the barrel, this rod being marked off into-in. divisions, as the micrometer screw

Fig. 216.-Internal Gauge.

has a movement of in. only. By removing the flat foot, and screwing on a plain measuring point, the instrument is converted into an ordinary internal gauge.

The ordinary two-point gauges are liable to produce inaccurate results, if placed carelessly in their holes; to avoid this difficulty the Newall Engineering Co., Ltd., construct a threepoint internal gauge, Fig. 216, which is bound to register accurately. There are three loose measuring spindles enclosed in radial arms, and the ends of these spindles are bevelled to suit the tapered end of the micrometer spindle. When the latter is moved forwards the measuring spindles are pushed outwards simultaneously, and when it is drawn backwards, coiled springs force the three spindles inwards again. Microstructure of Metals.-Much know

ledge has been gathered by the aid of the microscope respecting the minute anatomy of metals and alloys. Dr Sorby, forty years ago, was working at the preparation of sections, but the field was neglected by metallurgists until about twenty years since.

Specimens are prepared as opaque objects, and illuminated with reflected light. The chemical action produced by reagents is frequently employed to bring out certain features. The specimens are polished and mounted on slides. The fine polishing powders consist mainly of alumina, prepared from ammonia alum in various grades of division. Rough polishing is done with emery paper on rotating wooden discs. Useful magnifying powers range from 50, 140, 850, to 1,600 diameters, the latter being essential for quenched steels. Light etching to bring out the structure is done with a dilute solution of nitric acid in alcohol (1 per cent.), or with a 2 per cent. solution of

altite

ammonia nitrate on parchment, or with liquorice juice. The cementing material for attaching the specimens to the glass slides is composed of resin, beeswax, red ochre, and plaster of paris. Micro-specimens reveal the effects of heat treatment, strain, the influence of minute foreign elements, and other matters in a more or less striking way, to describe which would occupy a volume. The study of the microscopic structure of metals and alloys would not be undertaken if it were a mere recreative pursuit. It has its practical bearings, illustrating what happens under the varied conditions just named. The character of crystallisation, the dimensions or orientation of crystals, the slip or cleavage planes that separate crystals, the evidence of colour, shape, hardness, and relations between alloying elements, &c., can all be studied in the photo-micrographs. The effects of annealing are visible in the growth, and in the size of crystals; the effects of strain in elongation of crystals, and in the development of slip lines, or incipient lines of cleavage. The study has also thrown much light on the allotropic

changes of simple bodies. The effects of heat treatment are very obvious in polished and etched specimens. The illustrations on Plate XV. are from photographs kindly supplied by Professor H. C. H. Carpenter, of the University, Manchester, whose valuable work in connection with the preparation of two of the Reports to the Alloys Research Committee, of the Institution of Mechanical Engineers is well known.

Middle Part Box-See Moulding Box. Mid-Feather. The central dividing wall between the two flues in the wheel draught of a horizontal boiler.

Mid-Gear-See Link Motions.

Mil. A thousandth part of an inch, a unit used in connection with wire gauges. In the "New Imperial Standard Wire Gauge" the highest size, 70, is in. in diameter, the lowest, 50, is 10 in. or 1 mil in diameter.

1000

of work covered at once, and in the facilities which the cutters afford for milling contours, especially those that are not plane.

With regard to the first-named, it is very convenient to be able to mill a surface of several inches in width without traverse feed. In this respect milling comes particularly into rivalry with the planer, shaper, and slotter. The evil which often results is that the broad cutters spring if arbors are weak and the feed heavy. It is therefore sometimes the practice to rough deeply with a milling cutter, and finish with a single-edged tool on the planer, with a fine feed.

But when profiled work has to be done, work which combines tooling at various angles and curves, milling is unapproachable. The practice of building up gang mills affords a means

Fig. 217.-Edge Mills.

Milling. The operation of a number of rotating cutters arranged equidistantly from the axis of rotation.

Milling is a rival to the single-edged cutting tools, in which lateral feed is taken after each cut. The milling cutter usually covers the entire width of the face being tooled, and its feed is then only perpendicular thereto. The single-edged tool has no time to get cool when its cut is continuous; the edges of the milling cutter, when operating axially, are off the work during a period much longer than that which is occupied in actual cutting. But this is the smallest advantage. The principal advantages are those which result from the great breadth

width, while the cutter forming and cutter grinding machines permit of making the smaller profile cutters in the solid. profile cutters in the solid. And when irregular shapes are required, the form cutters have no rivals. A form attachment, which includes a tool slide controlled by a weight, and a tracer pin moving against the form or templet piece, is one of the departments in which milling is most valuable. The large volume of spiral work done in the universal milling machine would not be possible without the milling cutters. Neither would the large volume of gear cutting, apart from the rotary cutters. An advantage in milling is that face and edge cutters may often be successfully substituted for each other on a piece of work. Single-edged tools used