Cutting Tools

The angle of clearance, or relief a, fig. i J,K &L, is an important detail of a cutting tool. It is of greater importance than an exact angle of top rake; but, given some sufficient angle of clearance, its exact amount is not of much moment. Neither need it be uniform for a given cutting edge. It may vary from say 3° to o°, or even 2o°, and under good conditions little or no practical differences will result. Actually it need never vary much from 5° to 7°. The object in giving a clearance angle is simply to prevent friction between the non-cutting face immediately adjacent to the edge and the surface of the work. The limit to this clearance is that at which insufficient support is afforded to the cutting edge.

The front, or top rake, b in fig. 1, is the angle or slope of the front, or top face, of the tool; it is varied mainly according as materials are crystalline or fibrous. In the turnings and cuttings taken off the more crystalline metals and alloys, the broken appearance of the chips is distinguished from the shavings re moved from the fibrous materials. It indicates too that extra work is put on the tool in breaking up the chips, following im mediately on their severance, and when the comminutions are very small they indicate insufficient top rake. This is a result to secure a strong tool angle, without which tools would lack the endurance required to sustain them through several hours with out regrinding.

The tool angle, c, is the angle included between top and bottom faces, and its amount, or thickness expressed in degrees, is a measure of the strength and endurance of any tool. At extremes it varies from about 15° to 85°. It is traceable in all kinds of tools, having very diverse forms.

Tools of the Chisel Type.

The grouped illustrations (figs. 2 to 6) show some of the types, but it will be understood that each is varied in dimensions, an gles and outlines to suit all the varied kinds of metals and alloys and conditions of operation. For, as every tool has to be gripped in a holder of some kind, as a slide-rest, tool-box, turret, tool holder, box, cross-slide, etc., this often determines the choice of some one form in preference to another. A broad division is that into roughing and finishing tools. Generally though not invariably the edge of the first is narrow, of the second broad, correspond ing with the deep cutting and fine traverse of the first and the shal low cutting and broad traverse of the second. The following are some of the principal forms. The round-nosed roughing tool (fig. 2) B is of straight-forward type, used for turning, planing and shaping. As the correct tool angle can only occur on the middle plane of the tool, it is usual to employ cranked tools, C, D, E, right- and left-handed, for heavy and moderately heavy duty, the direction of the cranking corresponding with that in which the tool is required to traverse. Tools for boring are cranked and many for planing (fig. 3). The slotting tools (fig. 4) embody the same principle, but their shanks are in line with the direction of cutting. Many roughing and finishing tools are of knife type H. Finishing tools have broad edges, F, G, H. They occur in straightforward and right- and left hand types. These as a rule remove less than in. in depth, while the roughing tools may cut an inch or more into the metal.

Solid Tools v. Tool-holders.

It will be observed that the foregoing are solid tools; that is, the cutting portion is forged from a solid bar of steel. This is costly when the best tool steel is used, V-tools can be slid out in their holders to operate on faces and edges situated well inward from the end of the tool-holder.

Box Tools.

In one feature the box tools of the turret lathes resemble tool-holders. The small pieces of steel used for tool hence large numbers of tools comprise points only, which are gripped in permanent holders in which they interchange. Tool

steel usually ranges from about in. to 4 in. square; most engineers' work is done with bars of from in. to i 2 in. square. It is in the smaller and medium sizes of tools that holders prove of most value. Solid tools, varying from 22 in. to 4 in. square, are used for the heaviest cutting done in the planing machine. Tool holders are not employed for very heavy work, because the heat generated would not get away fast enough from small tool points. There are scores of holders ; perhaps a dozen good approved types are in common use. They are divisible into three great groups: those in which the top rake of the tool point is embodied in the holder, and is constant ; those in which the clearance is similarly embodied; and those in which neither is provided for, but in which the tool point is ground to any angle. Charles Babbage designed the first tool-holder, and the essential type survives in several modern forms. Fig. 5 includes two forms of tool holder. In one E the tool is a bit of round steel set at an angle which gives front rake, and having the top end ground to an angle of top rake. In points are gripped in the boxes, as in tool-holders, and all the advantages which are derived from this arrangement of separating the point from its holder are thus secured (fig. 7). But in all other respects the two are dissimilar. Two or three tool-holders the other A the tool has the section of a truncated wedge, set for constant top rake, or cutting angle, and having bottom rake or clearance angle ground. This round tool is not applicable for all classes of work. It will turn plain work, and plane level faces, but will not turn or plane into corners or angles. Hence the in vention of the tool of V section, and the swivel tool-holder. The round tool-holders are made right- and left-handed, the swivel tool-holder has a universal movement. The amount of projection of the round tool points is very limited, which impairs their utility when some overhanging of the tool is necessary. The of different sizes take all the tool points used in a lathe, but a new box has to be devised in the case of almost every new job, with the exception of those the principal formation of which is the turning down of plain bars. The explanation is that, instead of a single point, several are commonly carried in a box. As com plexity increases with the number of tools, new designs and dimen sions of boxes become necessary, even though there may be family resemblances in groups. A result is that there is not, nor can there be, anything like finality in these designs. Turret work has become one of the most highly specialized departments of machine-shop practice, and the design of these boxes is already the work of specialists. More and more of the work of the common lathe is being constantly appropriated by the semi- and full automatic machines, a result to which the magazine feeds for only a scraper, but all the twist drills cut with good incisive action. An advantage possessed by all drills is that the cutting forces are balanced on each side of the centre of rotation. The same action is found in the best wood-boring bits—improved forms of the old centre-bit. But the balance is impaired if the lips are not absolutely symmetrical about the centre. This explains the castings and forgings that cannot pass through a hollow spindle have contributed to a great extent. A great deal of the efficiency of the box tools is due to the support which is afforded to the cutting edges in opposition to the stress of cutting. V-blocks are introduced in most cases as in fig. 7. In many tools a shearing operation takes place, by which the stress of cutting is lessened.