The almost innumerable variety of metal working tools and the great variety of pur poses for which they are employed make their classification into a series of general groups practically impossible within a limited space. In a general work they may be conveniently di vided into various classes according to the char acter of the work to which they are applied. Such a method of classification would group the main body of metal-working tools under foundry work, forge shop work and machine shop work, the last named including all the turning, gear-cutting and toolmaking tools and appliances. The greater number of these are treated under their special headings in the several volumes of this Encyclopedia, and as in the case of the wood-working appliances, the various kinds of machines employed in the metal-working industries will be found speci fically treated under the title METAL-WORKING MACHINERY. The larger metal-working tools are known as machine-tools, including all those machines that operate cutting tools for shaping metal, as lathes, power-drills, boring-drills, shapers, milling machines and the like.
The elementary descriptions of the various forms of small tools such as the guiding tools, the holding tools, the rasping tools, the cutting tools, the drilling tools and the striking tools already given in connection with the wood working tools will be found applicable to similar tools employed in metal working, subject, how ever, to the modifications demanded by the greater hardness of the material worked, and the greater accuracy of execution required in the finished products of some classes of metal work. In the main these modifications consist in the employment of finer and harder ma terials in the making of the tools, in the par ticular forms given to the cutting edges and in the methods by which the tools are applied in the execution of the work.
The guiding tools employed in metal work are quite similar to those already described, and comprise the various forms of rules, squares, straight-edges and calipers, all made of metal, and also the various forms of micrometer cali pers with vernier attachments which are capable of measuring dimensions as small as 1-10,000th of an inch. These calipers are divided into two general classes — the ((yoke calipers and the "beam" calipers. In the former, the outer end of the shank of the yoke contains a split-nut which is employed for making adjustments for wear. For this purpose, the nut is closed onto the screw by being advanced on the stem toward the yoke. The shell or thimble on which the graduations are marked is attached to the end of the screw and rotates with it, and moves along over the shank. A speeding arrange
ment for rapidly advancing the screw is pro vided in the form of a knurled-nut in the yoke, which is also capable of contracting a bushing over the measuring stem so as to lock it in any desired position. The measuring point and the opposing anvil are carefully ground so as to make their faces perfectly parallel with each other. These micrometers are usually provided with a screw having 40 threads to the inch, with the barrel graduated to 10ths and 40ths of an inch. By this arrangement one revolution of the screw advances the thimble one division on the barrel, equal to one-fortieth of an inch, and as the circumference of the thimble is di vided into 25 equal parts, one-twenty-fifth of one revolution of the screw advances the meas uring point one-twenty-fifth of one-fortieth, equal to one one-thousandth of an inch. By the aid of the vernier attachment applied to the barrel measurements as small as one ten-thou sandth of an inch are readily obtained.
Gauges and Indicators.— These tools are employed for indicating the sizes of wire, machine-screws, drills and plate thicknesses. Various systems of gauges are employed, in all of which the dimensions are purely arbitrary. The American or Brown and Sharp gauge was adopted to produce a gauge to overcome the irregularities in spacing of the Birmingham gauge. In this gauge the dimensions increase by regular geometrical progression, the largest dimension No. 0000 being equal to 0.46-inch and the next smaller dimension, No. 000, being obtained by multiplying 0.46 by the constant .890522, each smaller number being the prod uct of the preceding number and the constant. Gauges for indicating the gauge of wire or plates are of two forms — the angular and the notch gauges. Other forms of gauges are the "centre" gauge which is used for gauging lathe and machine centres in turning and grind ing work; the "screw-thread" gauge used for grinding threading tools; the "screw-pitch" gauge used for determining the pitch of screw threads; the "depth-gauge" used for measuring the depth of holes and recesses; the "scratch" gauge used for ruling lines parallel with the edge of a piece of work and several forms of "surface" gauges which are principally used in determining the parallelism of the surface of a piece of work with the machine table, hous ing or other plane of reference. They are also used in testing, erecting and in the setting up work on machine tools.