Between a circular saw for cutting metal and a thin milling cutter there is no essential difference. Increase the thickness as if to produce a very wide saw, and the essential plain edge milling cutter for metal results. In its simplest form the milling cutter is a cylinder with teeth lying across its periphery, or parallel with its axis—the edge mill (fig. is), or else a disk with teeth radiating on its face, or at right angles with its axis—the end mill (fig. 16). Each is used indifferently for produc ing flat faces and edges, and for cutting grooves which are rec exceed about 6 in. in length the difficulties of hardening and grind ing render the "gang" arrangement more suitable. Thus, two, three or more similar edge mills are set end to end on an arbor, with the spiral teeth running in reverse directions, giving a broad face with balanced endlong cutting forces. From these are built up the numerous gang mills, comprising plane faces at right angles with each other, of which the straddle mills are the best known (fig. 17, A). A common element in these combinations is the key seat type B having teeth on the periphery and on both faces as in fig. 15, C, D. In a single cutter of the face type, but having teeth on back and edge also, T-slots are readily milled (fig. 16, D).
When angles, curves and profile sections are introduced, the capacity of the milling cutter is infinitely increased. The making of the cutters is also more difficult. Angular cutters (fig. 18) are used for producing the teeth of the mills themselves, for shaping the teeth of ratchet wheels, and, in combination with straight cut ters in gangs, for angular sec tions. With curves, or angles and curves in combination, taps, reamers and drills can be fluted or grooved, the teeth of wheels shaped, and in fact any outlines imparted (fig. 19).
One of the greatest advances in the practice of milling was that of making backed-off cutters. The sectional shape behind the tooth face is continued identical in form with the profile of the edge, the outline being carried back as a curve equal in radius to that of the cutting edge (fig. 20). The result is that the cutter may be sharpened on the front faces of the teeth without inter fering with the shape which will be milled, because the periphery is always constant in outline. After repeated sharpenings the teeth
would assume the form indicated by the shaded portion on two of the teeth. The limit of grinding is reached when the tooth be comes too thin and weak to stand up to its work.
The tools which operate by scraping (fig. 21) include many of the broad finishing tools of the turner in wood and metal (cf. fig. 2), and the scrape of the wood worker and the fitter. The practice of scraping surfaces true, applied to surface plates, machine slides and similar objects, was due to Sir Joseph Whitworth. It super seded the older and less accurate practice of grinding to a mutual fit. Now, with machines of precision, the practice of grinding has to a large extent displaced the more costly scraping.
Files are classed with scrapes, for, although the points are keen, there is never any front rake. Collectively there is a shearing action because the rows of teeth are cut diagonally. The sectional forms (fig. 22) and the longitu dinal forms (fig. 23) of the files are numerous, to adapt them to all classes of work. In addition, the method of cutting, and the degrees of coarseness of the teeth, vary, being single, or float cut, or double cut (fig. 24). The rasps are another group. Degrees of coarseness are designated as rough, middle cut, bastard cut, second cut, smooth, double dead smooth ; the first named is the coarsest, the last the finest. The terms are relative, since the larger a file is the coarser are its teeth, though of the same name as the teeth in a shorter file, which are finer.
These may be of cutting or non cutting types. Shears (fig. 25) have no front rake, but only a slight clearance. They generally give a slight shearing cut, because the blades do not lie parallel, but the cutting begins at one end and continues in detail to the other. But strictly the shears, like the punches, act by a severe detrusive effort ; for the punch, with its bolster (fig. 26), forms a pair of cylindrical shears. The effect of shearing is practically identical with that of punching, and like wise planing and annealing shorn edges has substantially the same influence as reamering punched holes. See also SCREW.