Cutting Tools

Planes.

Generally the tool angles of the chisel group lie be tween 15° and 25°. They include the chisels proper, and the gouges in numerous shapes and proportions, used by carpenters, cabinet-makers, turners, stone-masons, etc. These are mostly thrust by hand to their work, without any mechanical control. Other chisels are used percussively, as the stout mortise chisels, some of the gouges, the axes, adzes and stone-mason's tools.

The large family of planes embody chisels coerced by the me chanical control of the wooden (fig. 8) or metal stock.

Drilling and Boring Tools.

Metal and timber are bored with equal facility; the tools (figs. 9 and o) embody similar differences to the cutting tools already instanced for wood and metal. All the wood-working bits are true cutting tools, and their angles, if analysed, will be found not to differ much from those of the razor and common chisel. The drills for metal furnish ex amples both of scrapers and cutting tools. The common drill is necessity for the substitution of machine grinding for hand grind ing of the lips, and great developments of twist drill grinding ma chines. Allied to the drills are the D-bits, and the reamers (fig. I I). The first-named both initiate and finish a hole; the second are used only for smoothing and enlarging drilled holes, and for correcting holes which pass through adjacent castings or plates. The reamers remove only a mere film, and their action is that of scraping. The foregoing are examples of tools operated from one end and unsupported at the other, except in so far as they receive support within the work. One of the objectionable features of tools operated in this way is that they tend to "follow the hole," and if this is cored, or rough-drilled out of truth, there is risk of the boring tools following it to some extent at least. With the one exception of the D-bit there is no tool which can be relied on to take out a long bore with more than an approximation to con centricity throughout. Boring tools (fig. 12) held in the slide rest will spring and bend and chatter, and unless the lathe is true, or careful compensation is made for its want of truth, they will bore bigger at one end than the other.

This brings us to the large class of boring tools which are sup ported at each end by being held in bars carried between centres. There are two main varieties : in one the cutters are fixed directly in the bar (fig. 13, A to D), in the other in a head fitted on the bar (fig. 13, E), hence termed a "boring head." As lathe heads are fixed, the traverse cannot be imparted to the bars as in boring machines. The boring heads can

be traversed, or the work can be traversed by the mechanism of the lathe saddle. The latter must be done when cutters are fixed in bars. A great deal of difference exists in the details of the fittings both of bars and heads, but they are not so arbitrary as they might seem at first sight. The prin cipal differences are those due to the number of cutters used, their shapes, and their method of fastening. Bars receiving their cut ters direct include one, two or four, cutting on opposite sides, and therefore balanced. Four give better balance than two. Saws.—The saws are a natural connecting link between the chisels and the milling cutters. Saws are used for wood, metal and stone. Slabs of steel several inches in thickness are sawn tangular in cross-section.

When more than about an inch in width, surfacing cylindrical cutters are formed with spiral teeth (fig. 15, B), a device which is essential to sweetness of operation, the action being that of shear ing. These have their teeth cut on universal machines, using the dividing and spiral head and suitable change wheels, and after hardening they are sharpened on universal grinders. When cutters through as readily as, though more slowly than, timber planks. Circular and band saws are common in the smithy and the boiler and machine shops for cutting off bars, forgings and rolled sec tions. But the tooth shapes are not those used for timber, nor is the cutting speed the same. In the individual saw-teeth both cutting and scraping actions are illustrated (fig. 14). Saws which cut timber continuously with the grain, as rip, hand, band, circu lar, have incisive teeth; for though many are destitute of front rake, the method of sharpening at an angle imparts a true shear ing cut. But all cross-cutting teeth scrape only, the teeth being either of triangular or of M-form, variously modified. Teeth for metal cutting also act strictly by scraping. The pitching of the teeth is related to the nature of the material and the direction of cutting. It is coarser for timber than for metal, coarser for rip ping or sawing with the grain than for cross cutting, coarser for soft than for hard woods. The setting of teeth, or the bending over to right and left, by which the clearance is provided for the blade of the saw, is subject to similar variations. It is greatest for soft woods and least for metals, where in fact the clearance is often secured without set, by thinning the blade backwards.