at work 325.-Bevel gear. Diagram for dimensions 326.-Setting bevel-gear cutter out of centre 328.-Cutting worm wheel teeth 329.--Worm hob with relieved teeth. 330.-Construction of straight-toothed wheels 331.-Correct position of tool 332. Various positions of sliding tools. 334. (When broad-nosed), tool for finishing. 336.-Knife tools. 337.-Stiff knife tool 339.-Parting tools . 340, 341.—Groove-cutting tool 342.—Undercutting tools . 343A.-Dovetail cutting tools. 343B.-Whitworth standard screw-pitch gauge 345. 346.-Examples illustrating the various uses of gauges 346A.-Screw-cutting gauge testing drill (Wyke's patent). 347.-Tool holder 348. 349. ". 350, 351.-Swivel tool holders. 352.-Planing tools . . 353.-"Profile" steel tools 353A.-Hand reamer. 281 282 284 284 285 286 (facing) 286 290 291 292 292 293 293 294 294 295 295 296 297 298 298 299 299 300 301 302 302 302 302 303 304 304 FIG. 405A.—Solid type, portable 406.-Portable hydraulic riveter for furnace mouths 416. to speed cone 388 ,, reversing pulleys 389 bevel wheels. 389 390 390 MACHINE TOOLS AND WORKSHOP PRACTICE CHAPTER I. MEASUREMENT. THE "unit" from which our measurements of length have been taken is the "imperial" or "standard yard." "standard yard." In the strong room of the Standards Department of the Board of Trade, London, a bronze bar is kept. It is composed of copper 16, tin 2, and zinc 1 ounce avoirdupois. It is 38 in. long, and of section 1" X I". Let Fig. 1 represent the bar. At AA are inserted two golden studs. On the head of each stud a very fine line is engraved, and the distance between these fine lines when the bar is at the temperature of 62° Fahr. is one "Standard" or "British yard, and was adopted as such by Act of Parliament, passed in 1855. Forty copies were made, and one of these, of bronze, No. 11, was presented to the United States by the British Government in 1856. The yard is the unit or standard measure of length from which other Imperial measures, whether Linear, Superficial, or Solid, are ascertained. There are several copies of the standard yard in this country which have been divided into three equal divisions called feet; the foot is subdivided into twelve equal parts called inches. Further subdivisions of one inch into 8, 16, 32, or 64 parts, are termed respectively "eighths," "sixteenths," "thirty-seconds," or "sixty-fourths," each subdivision being half of the preceding. B Cylindrical standard gauges are also kept in the Standards Department of the Board of Trade, and consist of external and internal gauges from 6 inches diameter to o.1 of an inch. Measurements are taken in various ways according to the size, shape, and importance of the object measured. Standard rules are much used in making measurements. These are prepared by machines which "engrave" the fine lines to a high degree of accuracy in both British" and "Decimal systems. "Scales" have divisions representing one-half, one-quarter, one-eighth, etc., actual size. Pattern-makers' rules are a fractional part larger than standard size (in. to each foot, the amount by which some patterns are in excess of the casting required). All rules are not engraved, some have their divisional lines standing above the surface of the steel; these, however, are not used on machined, i.e. tooled work, but are used in the forge and foundry, where rigorous accuracy in absolute measurement is of less importance. The "scales are used in the drawing office, and may be made of "boxwood," "paper," or "ivory." Steel rules, " plated" to prevent oxidization, find a place there. Boxwood rules are generally 2 ft. long and hinged at the centre. The best kinds are bevelled on one edge, which, of course, brings the lines close to any surface when laid upon it. Paper scales are very convenient, as fine "needle-points" may be used upon their surface without being in any way damaged. It is, therefore, almost essential for students commencing "machine drawing to use these scales. Ivory rules are finely lined and finished; they are made two and four fold, and are exclusively used on drawings. Steel rules are made 2, 3, 4, 6, 12, 24, or 36 inches in length, and in special cases much longer. Uses of Steel and other Rules. We may transfer a measurement from a steel rule to another object, but our setting of the tool, whether dividers, compasses, calipers, or trammels, will depend upon our sense of sight; this is called "line measure." Strictly speaking, measurements thus obtained are only approximate, and, as a matter of fact, can be worked to only as such. EXAMPLE (1). Suppose eight workmen were asked to measure a rod of iron, whose actual length at first was 5 in., and each told to reduce the length by filing away one-eighth of an inch, using only a steel rule to measure with. If each workman actually filed one-eighth of an inch away, neither more nor less, the rod when finished would be 4 in. long. Such, however, would not be found to be the case when comparing the rod with a known standard 4 in. long. End and Line Measurement. EXAMPLE (2). A pulley may be required to fit tightly on a 2-in. shaft, and to be 20 in. in diameter, and 6 in. wide on the "face." The diameter and width may be satisfactorily ascertained by a steel rule, because these dimensions are not particular to 10th of an inch, so that a steel rule will answer for all practical purposes in such |