Modern Machine-Shop Practice Part 67

Both these guides, however, can only be applied to metal not unusually hard, and to tools rigidly held, and having their cutting edges sufficiently close to the tool point or clamp that the tool itself will not bend and spring from the pressure of the cut. The cutting speed for chilled cast-iron rolls, such, for example, as calender rolls, is but about 7 feet per minute, and the angles one to the other of the tool faces is about 75 degrees, the top face being horizontally level, and standing level with the axis of the roll.

When a tool has front rake only, the form of its cutting will depend upon the depth of its cut. With a very fine cut the cutting will come off after the manner shown in Fig. 940, while as the depth of the cut is increased, the cutting becomes a coil such as shown in Fig. 941. These coils lie closer together in proportion as the top face of the tool is given less rake, as is necessary for steel and other hard metal. Thus Fig. 940 represents a cutting from steel, the tool having front rake only, while Fig. 941 represents a cutting from a steel crank pin, the tool having side rake. The following observations apply generally to the cuttings.

[Ill.u.s.tration: Fig. 942.]

The cleaner the surface of a cutting, and the less ragged its edges are, the keener the tool has cut; thus, in Fig. 941, the raggedness shows that the tool was slightly dulled, although not sufficiently so to warrant the regrinding of the tool. Such a cutting, however, taken off wrought iron would show a tool too much dulled, or else possessing too little top rake to cut to the best advantage. In wrought iron, the tool having a keener top face, the cuttings will coil larger, and the direction in which they coil and move as they leave the tool will depend upon the shape of the tool and its height to the work.

[Ill.u.s.tration: Fig. 943.]

[Ill.u.s.tration: Fig. 944.]

In Fig. 942, for example, is a tool having front and side angle in about an equal degree, and its cutting is shown in Fig. 943, the side angle causing it to move to the right, and the front angle causing it to move towards the tool post.

The tool in Fig. 944 has side rake mainly, and the point is slightly depressed, hence its cutting would leave the work moving horizontally and towards the right hand.

[Ill.u.s.tration: Fig. 945.]

[Ill.u.s.tration: Fig. 946.]

In Fig. 945 the point of the tool is made considerably lower than the point B, and as a result the cutting would rise somewhat vertically as in Fig. 946. Indeed the heel B may be raised so as to cause the cutting to move but little to the right, but rise up almost vertically, being thrown over towards the work, and in extreme cases the cutting will rub against the surface of the work and the friction will prevent the cutting from moving to the right, hence it will roll up forming a ball, the direction of the rotation occasionally changing.

Whatever irregularities may appear in the coil of the cuttings will, if the tool is not dulled from use, arise from irregularities in the work and not from any cause attributable to the tool.

The strength of a cutting forms to a great extent a guide as to the quality of the tool, since the stronger the cutting the less it has become disintegrated, and therefore less power has been expended in removing it from the work.

The cutting speed for wrought iron should be sufficiently great that water being allowed to fall upon the work in a quick succession of drops as, say, three per second, the cuttings will leave the work so hot as to be almost unbearable in the hands, if the cut is a heavy one, as, say, reducing the work diameter 1/2 inch at a cut.

If wrought-iron cuttings break off in short pieces it may occur from black seams in the work, but if they break off short and show no tendency to coil, the tool has too little rake. If the tool gets dull too quickly and the cutting speed is not excessive, then the tool has too much clearance. If the tool edge breaks there is too much rake (providing of course that the tool has not been burnt in the forging or hardening), a fine feed will generally produce longer and closer coiled cuttings (that is of smaller diameter) than a coa.r.s.e feed, especially if the work be turned dry or without the application of water.

[Ill.u.s.tration: Fig. 947.]

Aside from these general considerations which apply to all tools, there are peculiar characteristics of particular metals; thus, for example, cast iron will admit of the tool having a greater width of cutting edge in a line with the finished surface of the metal than either steel, wrought iron, copper or bra.s.s, which renders it possible to use a finishing tool of the form shown in Fig. 947, whose breadth of cutting edge A, lying parallel with the line of feed traverse, may always exceed that for other metals, and may in the case of cast iron be increased according to the rigidity of the work, especially when held close in to the tool post.

[Ill.u.s.tration: Fig. 948.]

The corners B C may for roughing the work be rounded so as to be more durable, but for finishing cuts they should be bevelled as shown, because by this means face A can more easily be left straight than would be the case with a rounded corner. In the absence of the bevels there would be a sharp corner that would soon become dull. For finishing purposes the corners need not be so much bevelled as in figure, but may be very slightly relieved at the corners A and B, in Fig. 948, the width of the flat nose being slightly greater than the amount of feed per lathe revolution. Such tools produce the quickest and best work without chattering when the conditions are such that the work and the tool are held sufficiently rigid, and in that case may be used for the harder and tougher metals, as wrought iron and steel.

We have now to consider the height of the tool with relation to the work, which is a very important point.

[Ill.u.s.tration: Fig. 949.]

In Fig. 949, for example, let E be the washer or ring under the tool, and F therefore the fulcrum from which the tool will bend. Let the horizontal dotted line a represent the centre of the work, and it is plain that to whatever amount the tool may spring under the pressure of the cut, its motion from this spring will be in the direction of the dotted arc H, causing the tool to dip deeper into the work in proportion as the tool point is set above the work centre line A. Now the amount of tool spring will even under the most rigid conditions vary in a heavy cut with every variation in the depth of cut or in the hardness of the metal. Furthermore, as the cutting edge of the tool becomes dulled from use, its spring will increase, because the pressure required to force it to its cut becomes greater, and as a result when the conditions are such that a perceptible amount of tool spring or deflection occurs, the work will not be turned cylindrically true. Obviously the work under these conditions will be most true when the tool point is set level with the line A, pa.s.sing through the work axis.

[Ill.u.s.tration: Fig. 950.]

There are two advantages, however, in setting the tool above the work centre: first it severs the metal easier; and second, it enables the employment of more bottom rake without increasing the bottom clearance.

[Ill.u.s.tration: Fig. 951.]

Thus in Figs. 950 and 951 the diameters of the work W and the top rake of the respective tools are equal, but the tool that is set above the centre, Fig. 950, has more bottom rake but no more clearance, which occurs from the manner in which the cutting edge is presented to the work; the dotted lines represent the line of severance for each, and it is obvious that in Fig. 950, being of the shortest length for the depth of the cut will require least power to drive, because it is, as presented to the work, the sharpest wedge, as will be perceived by referring to Fig. 952, in which the tool shown in Fig. 950 is simply placed below the work centre, all other conditions as angle, &c., being equal.

From these considerations it appears that while for roughing cuts it is advantageous to set the tool above the centre, it is better where great cylindrical truth is required to set it at the centre for finishing cut.

[Ill.u.s.tration: Fig. 952.]

It may also be observed that if the lathe bed be worn it will usually be most worn at the live centre end, where it is most used, and a tool set above the centre will gradually fall as the cut proceeds towards the live centre, entering the work farther, and therefore reducing its diameter. This can be offset by setting the tailstock over, but in this case the wear of the work centres is increased, and the work will be more liable to gradually run out of true, as explained with reference to turning taper work. Sir Joseph Whitworth recommends that the tool edge be placed at the "centre" of the work, while at the same time on a line with the middle of the body of the steel. To accomplish this result it is necessary that the form of the tool be such as shown in Fig. 953, in which W represents a piece of work, R the slide rest, A the fulcrum of the tool support, the dotted line the centre of the work, and the arrow the direction in which the tool point would move from its deflection or spring. Now take the conditions shown in Fig. 954, and it will be perceived at once that the least tool deflection will have an appreciable effect in causing the tool point to advance into the work in the direction denoted by the arrow. This would impair the cylindrical truth of the work, because metals are not h.o.m.ogeneous but contain in forged metals seams and harder and softer places, and in cast metals different degrees of density, that part laying at the bottom of the mould being densest (and therefore hardest) by reason of having supported the weight of the metal above it when cooling in the mould.

[Ill.u.s.tration: Fig. 953.]

This brings us to another consideration, inasmuch as supposing the tool edge to be set level with the work centre (as in Figs. 951 and 953), the arc of deflection of the tool point will vary in its direction with relation to the work according to the vertical distance of the top of the tool rest (R in Figs. 953 and 954) from the horizontal centre of the work.

Thus the vertical distance between the point A in Fig. 953 and the work centre is less than that between A and the horizontal work centre in Fig. 954, as may be measured by prolonging the dotted lines in both figures until they pa.s.s over A, and then measuring the respective vertical distances between A and those dotted lines. It is to be noted that this distance is governed by the vertical distance of the top of the tool rest R from the work centre, but where this distance is required or desired to be reduced a strip of metal may be placed beneath the tool and between it and the slide rest.

[Ill.u.s.tration: Fig. 954.]

It will now be obvious that to produce work as nearly cylindrical as possible, the tool edge should stand as near to the slide rest as the circ.u.mstances will permit, which will hold the tool more firmly and prevent, as far as possible, its deflection or spring from the cut pressure. Both in roughing out and in finishing, this is of great importance, influencing in many cases the depth of cut the tool will carry as well as the cylindrical truth of the work.

We may now present some others of the ordinary forms of tools used in the slide rests on external or outside work, bearing in mind, however, that these are merely the princ.i.p.al forms, and that the conditions of practice require frequent changes in their forms, to suit the conditions of access to the work, &c.

Fig. 955 represents a diamond point tool much used by eastern tool makers. The sides are ground flat and the point is merely oil-stoned to take off the sharp corner. This tool is used with very fine feeds as, say, 180 work revolutions to an inch of tool traverse, taking very fine cuts, and in sharpening it the top face only is ground; hence as the height of the tool varies greatly before it is worn out, the tool elevating device must have a great range of action.

[Ill.u.s.tration: Fig. 955.]

In Fig. 956 is shown a side tool for use on wrought iron; it is bent around so that its cutting edge A may be in advance of the side of the steel, and thus permit the cutting edge to pa.s.s up into a corner. When it is bent to the left as in the figure, it is termed a right-hand side tool, and per contra when bent to the right it is a left-hand tool. The edge A must form an acute angle to edge B, so that when in a corner the point only will cut, or when the edge A meets a radial face, as in Fig.

957, the cutting edge B will be clear of the work as shown.

[Ill.u.s.tration: Fig. 956.]

If the angle of A to B is such that both those edges cut at once, the pressure due to such a broad cutting surface would cause the tool to spring or dip into the work, breaking off the tool point and perhaps forcing the work from between the lathe centres.

This tool may be fed from right to left on parallel work, or inwards and outwards on radial faces, but it produces the truest work when fed inwards on radial faces, and to the left on parallel work, while it cuts the smoothest in both cases when fed in the opposite direction.

[Ill.u.s.tration: Fig. 957.]

It is a very desirable tool on small work, since it may be used on both the stem of the work, and on the radial face, which saves the trouble of having to put in a front tool to turn the stem, and a separate tool for the radial face.

In cutting down a radial face with this tool, it is best (especially if much metal is to be cut off), if the face of the metal is hard, to carry the cut from the circ.u.mference to the centre, as shown in the plan view in Fig. 958, in which _a_ is the cutting edge of the tool, B a collar on a piece of work, _c_ the depth of the cut, and D a hard skin surface.

Thus the point of the tool cuts beneath the hard surface, which breaks away without requiring to be actually cut.

[Ill.u.s.tration: Fig. 958.]

[Ill.u.s.tration: Fig. 959.]

Fig. 959 represents a cutting off or parting tool for wrought iron, its feed being directly into the metal, as denoted by the arrow. This tool should be set exactly level with the work centre when it is desired to completely sever the work. When, however, it is used to merely cut a groove, it may be set slightly above the centre.

[Ill.u.s.tration: Fig. 960.]

[Ill.u.s.tration: Fig. 961.]