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have to be effected by driving with a hammer or a press, while others have to be “ working fits, ” suitable, say, for the revolution of a loose pulley on its shaft or of an axle in its bearings. The “ limit " or “ difference gauges ” (figs. 67 and 68) are designed for producing these working fits; that is, the plug and ring gauges differ in dimensions so that the work bored will drive tightly, or slide freely over

FIG. 68.

A, Separate caliper or snap C, Difference gauge. gauges. D, Newall adjustable limit

B, Combined internal and ex- gauge. ternal gauges. a, b, Plugs.

the work turned. These are variously sub-classified. The system which is generally accepted is embodied in the gauges by the Newall Engineering Co. These embrace farcejits, which require the application of a screw or hydraulic press; driving jits, that require less power, as that of a hammer; push fits, , in which a spindle can be thrust into its hole by hand; and running fits, such as that of shafts in bearings. Fixed gauges are made for each of these, but as this involves a heavy outlay the Newall firm have adjustable limit gau es (fig. 68, D) for external dimensions, the standard plug being useci for oles. The setting is done by screwed plugs or anvils adjusted by reference bars. In all these gauges the “ go on " and “ not go on" ends respectively are stamped on the gauge, or the equivalents of + and -.

Fixed Reference Gauges. Reference Disks and End Measuring Rods.-Shop working gauges become in time so damaged by service that they fail to measure so accurately as when new. To correct these errors reference gauges are provided, by which the inaccuracy of the worn ones is brought to the test. These are never used in the shops for actual measurement of work, but are only kept for checking the tnith of the working gauges. They include disk, stepped and end measurement gauges. The disk and the stepped are used for testing the ring gauges, the ste ped kind comprising essentially a collection of disks in one piece (fi)g. 67, D). The end measure pieces test the external gauges. The end measure standard lengths made by the Pratt & Whitney Co. are so accurate that any sizes taken at random in any numbers from i in. to 4 in., varying by sixteenths of an inch, will, when placed end to end, make up an exact length; this is a difficult test, since slight variations in the lengths of the components would add up materially when multiplied by the number of pieces. The ends are ground off with diamond dust or emery in a special machine under water, and are so true that one piece will support another by cohesive force, and this though the surfaces are less than i in. square. Movable Gauges.—This extensive grou may be regarded as compounded of the common caliper and the Whitworth measuring machine. They are required when precise dimensions have to be ascertained in whole numbers and minute fractional parts. They combine the sense of touch by contact, as in the cali ers, with the exact dimensions obtained by inspection of graduate cf) scales, either the vernier or the micrometer screw. If gauges must not vary by more than “gh-5 of an inch, which is the limit imposed by modern shop ideals, then instruments must be capable of measuring to finer dimensions than this. Hence, while the coarser classes of micrometers read directly to -fi, -5 part of an inch, the finest measure up to ~{0;5-0-.5 of an inch, about 200 times as fine as the diameter of a human hair. They range in price correspondingly from about a sovereign to £100.

The Calipers.-Common calipers (fig. 69) are adjusted over or within work, and the dimensions are taken therefrom by a rule or a gauge. They usually have no provision for minute adjustment beyond the gentle tapping of one of the legs when setting. In some forms screw adjustment is provided, and in a few instances a vernier attachment on the side of the pivot opposite to the legs. Vernier Calipers.-The vernier fitting, so named after its inventor, Pierre Vernier, in 163I, is fitted to numerous calipers and caliper rules. It is applied to calipers for engineers' use to read to11, '~m; of an inch without requiring a magnifier. The beam of the caliper is divided into inches and tenths of the inch, and each tenth into fourths and the vernier into twenty-five parts, or the beam is divided into fiftieths of an inch (fig. 70) and the vernier has 20 divisions to I9 on the rule. The caliper jaws are adapted to take both external and internal dimensions. These “beam calipers '7 are also made for metric divisions. Minor variations in design by different manufacturers are numerous.

FIG. 69.-Calipers.

A, Ordina? external type, adjusted by tapping the legs. B, Type a justed by screw in auxiliary leg. C, Scéew calipers, opened by contraction of curved spring and closed y nut.

D, Self-registering caliper, with pointer moving over quadrant. E,

F, Screw type with spring.

G, Combined internal and external for measuring chambered holes. H, Compass caliper for finding centres. J, Keyhole caliper for measuring from hole to outside of boss. Common'internal type.

FIG. 70.-Vernier Caliper.

A, Beam; B, vernier; C, fixed jaw; D, movable jaw; E, clamping head; F, abutment head, with adjusting screw a, for Fine adjustment of D.

FIG. 71.—Measuring Machine. (The Newall Engineering Co.) A, Hollow base or bed, mounted on three pointsf B, Measuring or fast headstock.

C, Movable head, or tail stock.

Spirit-level to indicate alterations in length of piece being measured due to changes in temperature, termed the indicator or comparator.

E, Measuring screw.

F, Nut for rapid adjustment of ditto. G, Knob of speed screw for slow movement of ditto. H, Dividing and measuring wh eel.

Vernier or reading bar.

a, a, Points between which contact is made. 5°