once become a cutting tool. But its bevelled face would rub and grind on the surface of the work, producing friction and heat, and interfering with the penetrative action of the cutting edge. On the other hand, if C were tilted forwards as at E its action would approximate to that of a scrape for the time being. But the high angle of the hinder bevelled face would not afford adequate support to the cutting edge, and the latter would therefore become worn almost instantly, precisely as that of a razor or wood-working chisel would crumble away if operated on hard metal. It is obvious

therefore that the correct form for a cutting tool must depend upon a due balance being maintained between the angle of the front and of the bottom faces-" front" or "top rake," and "bottom rake" or "clearance "-considered in regard to their method of presentation to the work. Since, too, all tools used in machines are he'd rigidly in one position, differing in this respect from handoperated tools, it follows that a constant angle should be given to struments which are used for operating on a given kind of metal or alloy. It does not matter whether a tool is driven in a lathe, or a planing machine, or a sharper or a slotter; whether it is cutting on external or internal surfaces, it is always maintained in a direction perpendicularly to the point of application as in fig. 1, F, G, H, planing, turning and boring respectively. It is consistent with reason and with fact that the softer and more fibrous the metal, the keener must be the formation of the tool, and that, conversely, the harder and more crystalline the metal the more obtuse must be the cutting angles, as in the extremes of the razor and the tools fr cutting iron and steel already instanced. The three figures J. K, L show tools suitably formed for wrought iron and mild steel, fur cast iron and cast steel, and for brass respectively. Cast iron and cast steel could not be cut properly with the first, nor wrought iron and fibrous steel with the second, nor either with the third. The angles given are those which accord best with general practice, hat they are not constant, being varied by conditions, especially by lubrication and rigidity of fastenings. The profiles of the first and second tools are given mainly with the view of having material for grinding away, without the need for frequent reforging. But there are many tools which are formed quite differently when used in tool-holders and in turrets, though the same essential principles of angle are observed.

The angle of clearance, or relief, a, in fig. 1, 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 10°, or even 20, and under good conditions little or no practical differences will ruit Actually it need never vary much from 5 to 7. The object 1 giving a clearance angle is simply to prevent friction between the non-cutting face immediately adjacent to the edge and the face of the work. The limit to this clearance is that at which sufficient support is afforded to the cutting edge. These are the two facts, which if fulfilled permit of a considerable range in clearance angle. The softer the metal being cut the greater can be the Clearance; the harder the material the less clearance is permissible because the edge requires greater support.

that low in carbon, and cast iron from wrought iron. It indicates too that extra work is put on the tool in breaking up the chips, following immediately on their severance, and when the comminutions are very small they indicate insufficient top rake. This is a result that turners try to avoid when possible, or at least to minimize. Now the greater the slope of the top rake the more easily will the cuttings come away, with the minimum of break in the crystalline materials and absolutely unbroken over lengths of many feet in the fibrous ones. The breaking up, or the continuity of the cuttings, therefore affords an indication of the suitability of the amount of top rake to its work. But compromise often has to be made between the ideal and the actual. The amount of top rake has to be limited in the harder metals and alloys in order to secure a strong tool angle, without which tools would lack the endurance required to sustain them through several hours without 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. It is difficult to place some groups in the cutting category; they are on the border-line between cutting and scraping instruments.

Typical Tools.-A bare enumeration of the diverse forms in which tools of the chisel type occur is not even possible here. The grouped illustrations (figs. 2 to 6) show some of the types, but it will be understood that each is varied in dimensions, angles 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, &c., this often determines the choice of some one form in preference to another. A broad division is that into roughing and finishing

1./LJA!

FIG. 3.-Group of Planer Tools.

to avoid digging into the metal.

E, Parting or cutting-off or grooving tool.

F. V tool for grooves.

B.

Face view of roughing tool.

G,

C.

Face view of finishing tool.

D,

Right- and left-hand knife or side tools.

Right- and left-hand tools for
V-slots.

H. Ditto for T-slots.

J, Radius tool held in holder.

bar of steel. This is costly when the best tool steel is used, 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 1 in. square. It is in the smaller and medium sizes of tools that holders prove of most value. Soid tools, varying from 2 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. The best-known holders now are the Tangye, the Smith & Coventry, the Armstrong, some by Mr C, Taylor, and the Bent. The Smith & Coventry (fig. 5). used more perhaps than any other single design, includes two forms. 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 the other 4 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. The Smith & Coventry 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 invention of the tool of V-section, and the swivel toolholder. 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 V-tools can be slid out in their holders to operate on faces and edges situated to some considerable distance inwards 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 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

FIG. 5.-Group of Tool-holders.

E

A, Smith & Coventry swivelling holder. B, Holder for square steel. C, D, right- and left-hand forms of same. E, Holder for round steel. F, Holder for narrow parting-off tool. traverse of the second. The following are some of the principal forms. The round-nosed roughing tool (fig. 2) B is of straightforward 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. But the traverse of the first often exceeds an inch, while in that of the second in. is a very coarse amount of feed. Spring tools, G, used less now than formerly, are only of value for imparting a smooth finish to a surface. They are finishing tools only. Some spring tools are formed with considerable top rake, but generally they act by scraping only.

PE

FIG. 6.-Group of Chisels.
A, Paring chisel.

B, Socket chisel for heavy duty.
C, Common chipping chisel,
D, Narrow cross-cut or cape chisel.
E, Cow-mouth chisel, or gouge.
F. Straight chisel or sett.

G, Hollow chisel or sett.

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

FIG. 7.-Box Tool for Turret Lathe. (Alfred Herbert, Ltd., Coventry.) A, Cutting tool. B, Screw for adjusting radius of cut. C C. V-steadies supporting the work in opposition to A. D, Diameter of work. E, Body of holder. F. Stem which fits in the turret. respects the two are dissimilar. Two or three tool-holders 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 complexity increases with the number of tools, new designs and dimensions 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 castings and forgings that cannot pass through a hollow spindle have contributed greatly. New work is constantly being attacked in the automatic machines that was deemed impracticable a short time before; scme of the commoner jobs are produced with greater economy, while heavier castings and forgings, longer and larger bars, are tooled in the turret lathes. 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, and these not only resist the stress of the cutting, but gauge the diameter exactly.

Shearing Action-In many tools a shearing operation takes place, by which the stress of cutting is lessened. Though not very apparent, it is present in the round-nosed roughing tools, in the knife tools, in most milling cutters, as well as in all the shearing tools proper-the scissors, shears, &c.

Planes We pass by the familiar great chise group, used by woodworkers, with a brief notice. Generally the tool angles of these lie between 15° and 25°. They include the chisels proper, and the gouges in numerous shapes and proportions, used by carpenters,

iron in breaking the shaving and conferring rigidity upon the cutting iron. This rigidity is of similar value in cutting wood as in cutting metal though in a less marked degree. Drilling and Boring Tools.-Metal and timber are bored with equal facility; the tools (figs. 9 and 10) 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 examples both of scrapers and cutting tools. The common drill is 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 embodied in the best woodboring bits and augers, as the Jennings, the Gilpin and the Irwinmuch 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 necessity for the substitution of machine grinding for hand grinding of the lips, and great developments of twist drill grinding machines. Allied to the drills are the D-bits, and the reamers (fig. 11). The first-named both initiate and finish a hole;

FIG. 8.-Section through Plane,

FIG. 9.-Group of Wood-boring Bits. 4. Spoon bit. -B, Centre-bit. C, Expanding centre-bit. Gilpin or Gedge auger. E, Jennings auger. F, Irwin auger.

H

FIG. II.

A, D-bit. B, Solid reamer. C, Adjustable reamer, having six flat blades forced outward by the tapered plug. Two lock-nuts at the end fix the blades firmly after adjustment.

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 D, 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 concentricity 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. Boring tools thrust by the back centre are liable to wabble, and though they are variously coerced to prevent them from turning round, that does not check the to-and-fro wabbly