Modern Machine-Shop Practice Part 68

When the tool is very narrow at _c_, Fig. 960, or long as in Fig. 961, it may be strengthened by being deepened, the bottom B projecting below the level of the tool steel, which will prevent undue spring and the chattering to which this tool is liable.

[Ill.u.s.tration: Fig. 962.]

[Ill.u.s.tration: Fig. 963.]

To enable the sides of the tool to clear the groove it cuts, the width at _c_ should slightly exceed that at D, and the thickness along the top _a_ should slightly exceed that at the bottom B.

When the tool is used to cut a wide groove as, say, 3/8-inch wide, in a small lathe, it is necessary to carry down two cuts, making the tool about 1/4 inch wide at _c_, which is a convenient size, affording sufficient strength for ordinary uses.

[Ill.u.s.tration: Fig. 964.]

[Ill.u.s.tration: Fig. 965.]

When used on wrought iron the top face may, with advantage, be given top rake as in Fig. 962, which on account of causing the tool to cut easier, will reduce the spring of the work W in the direction of arrow A. For bra.s.s work, however, the top should be ground in an opposite direction, as in Figs. 963 and 964, which will enable it to cut smoother and with less liability to rip into the metal, especially if the tool requires to be held far out from the tool post. To capacitate the tool to cut a groove close up to a shoulder, it should be forged to the shape shown in Fig. 965. As it is very subject to spring, it should not, unless the conditions are such as to give rigidity to both the work and the tool, be set above the work centres.

[Ill.u.s.tration: Fig. 966.]

When a grooving or parting tool is to be used close up to the lathe dog, its cutting end may be bent at an angle, as in Fig. 966, so that it may be adjusted on the lathe rest, so that the work driver will not strike against the slide rest.

[Ill.u.s.tration: Fig. 967.]

In Figs. 967, 968, and 969, are represented the facing tool, side tool, or knife tool, as it is promiscuously termed, which is sometimes made thicker at the bottom as in Fig. 969. It is mainly used for squaring up side faces, as upon the ends of work or the sides of heads or collars. A is the cutting edge which may be ground so as to cut at and near the end, for large work in which it is necessary to feed the tool in with the cross slide, or to cut along its full length for small work in which the longitudinal feed is used. To facilitate the grinding, the bottom may be cut away, as at B in Fig. 968.

[Ill.u.s.tration: Fig. 968.]

[Ill.u.s.tration: Fig. 969.]

In some practice the bottom B, Fig. 969, of the tool, is made thicker than the top A, which is, however, unnecessary, unless for heavy cuts, for which the tool would be otherwise unsuitable on account of weakness.

For all ordinary facing purposes, it should be made of equal thickness, which will reduce the area to be ground in sharpening the tool.

[Ill.u.s.tration: Fig. 970.]

[Ill.u.s.tration: Fig. 971.]

On small work the edge A A should be ground straight, and set at a right angle to the work, so that it may face off the whole surface at once, but for work of large diameter it should be ground and set as in Figs.

970 and 971, so that it will cut deepest at the end E, enabling it to carry a finis.h.i.+ng cut from the circ.u.mference to the centre, by feeding it with the cross-feed screw.

[Ill.u.s.tration: Fig. 972.]

The cutting edge should be level with the centre of the work, the angle of the top face D being about 35 degrees in the direction of the arrow C for wrought iron, and level if used for bra.s.s. When this tool is to be used for a face close to the work driver it should be bent at an angle as in Fig. 972, so as to enable the driver to clear the slide rest, and when used for countersunk head bolts, it may be bent at an angle as in Fig. 973, so that when it is once set to give the head the correct degree of taper, it will turn successive heads to the correct taper without requiring each head to be fitted to its place.

[Ill.u.s.tration: Fig. 973.]

In Fig. 974 is shown the spring tool which is employed to finish smoothly round corners or sweeps, which it will do to better advantage than any other slide rest tool, because it is capable of carrying a larger amount of cutting edge in simultaneous operation. This property is due to the shape of the tool, the bend or curve serving as a spring to enable the tool to bend rather than dig into the work.

[Ill.u.s.tration: Fig. 974.]

This form of tool is sometimes objected to on the ground that it does not turn true, but this is not the case if the tool is properly formed and placed at the correct height with relation to the work. In the first place the top face should, even on wrought iron, have but very little top rake, and indeed none at all if held far out from the tool post, while for bra.s.s, negative top rake may be employed to advantage. The height of the cutting edge B should be level with the top of the tool steel as denoted by the dotted line in the figure, and in no case should it stand above that level. The cutting edge should be placed about level with the horizontal centre of the work, but in no case above it.

It is from this error that the tool is frequently condemned, because if placed above, the broad cutting edge causes the tool to spring slightly and dig into the metal, whereas when placed at the middle of the height of the work the spring will not have that effect, as already explained when referring to front tools. Furthermore, the spring of the tool (from inequalities in the texture or from seams in the metal) will be in a line so nearly coincident with the work surface that the latter will be practically true, and from the smoothness and the evenness of the curve this tool will produce a much better work than any other tool, unless indeed the curve be of a very small radius, as, say, about 1/4 inch only, in which case a hand tool such as shown in Fig. 1292 may be employed; spring tools are intended to finish only, and not to rough out the work.

[Ill.u.s.tration: Fig. 975.]

The curves, as B in Fig. 974 for a round corner and C for a bead, should be carefully and smoothly finished to the required curve and the top face only ground to sharpen the tool, so as to maintain the curve as nearly as possible; but if the curve is a very large one, the tool will require to be a part of the curve only, and must be operated by the slide rest around the curve.

For finis.h.i.+ng the curves or round corners in cast-iron work the spring tool is especially advantageous, as it will produce a polished clean surface of exquisite finish if used with water, and the cutting speed is exceedingly slow, as about 7 feet per minute.

LATHE SLIDE REST TOOLS FOR BRa.s.s WORK.

Nearly all the tools used in the slide rest upon iron work may be employed upon bra.s.s work, but the top faces should not have rake, that is to say, they should have their top faces lying in the same plane as the bottom plane of the tool steel which rests on the slide rest. For if the top face is too keen it rips rather than cuts the bra.s.s, giving it a patchy, mottled appearance.

[Ill.u.s.tration: Fig. 976.]

Fig. 975 represents a front tool for bra.s.s, which is used for carrying cuts along outside work or for facing purposes, corresponding, so far as its use is concerned, to the diamond point or front tool for iron. The top face of this tool must in no case be given rake of any kind, as that would cause it to tear rather than to cut the metal, and also to chatter. The point A should be slightly rounded and the width at B and depth at C must be regulated to suit the depth of cut taken, the rule being that slightness in either of these directions causes the tool to chatter. When held far out from the tool post or under other conditions in which the tool cannot be rigidly held, the top face should be ground away towards the end, thus depressing the point A, after the manner shown with reference to the cutting-off tool for bra.s.s in Fig. 963. The manner in which the cuttings come off bra.s.s work when a front tool is used, depends upon the hardness of the bra.s.s and the speed at which the tool cuts.

[Ill.u.s.tration: Fig. 977.]

[Ill.u.s.tration: Fig. 978.]

In the harder kinds of bra.s.s, such as that termed gun metal, composition, or bell metal, the cuttings will fly off the tool in short angular grains, such as indicated in Fig. 976, travelling a yard or two after leaving the tool if a fairly quick cutting speed is used. But if the cutting speed is too slow the cuttings will come off slowly and fly but a few inches. In the softer kinds of bra.s.s, such as yellow bra.s.s, the cuttings are longer and inclined to form short curls, which will, if cut at a high speed, fly a few inches only after leaving the tool.

In Fig. 977 is shown a right-hand side tool for bra.s.s work. It is used to carry cuts along short work, and to carry facing cuts at the same time, thus avoiding the necessity to move the position of the tool to enable it to carry a facing cut, as would be necessary if a front tool for bra.s.s were used. It is peculiarly adapted, therefore, for bra.s.s bolts, or other short work having a head or collar to be faced especially; hence, it may be traversed to its cut in either direction without requiring to be moved in the tool post. It may also be used to advantage for boring purposes. It will be found that this tool will cut smoother and will be less liable to chatter if its top face is ground slightly down towards the point and if it be not forged too slight either in depth or across B. Its clearance on the side is given by forging it to the diamond shape shown in the sectional view. To make the tool a left-handed one it must be bent to the right, the clearance being in any case on the inside of the curve.

[Ill.u.s.tration: Fig. 979.]

The forms of single-pointed slide rest tools employed to cut [V]-threads in the lathe are shown in Fig. 978, which represents a tool for external, and Fig. 979, which represents one for an internal [V]-thread, the latter being a tool ground to accurate shape and secured in a holder by the set screw S.

It is obvious that a Whitworth thread might be cut with a single-pointed tool such as shown in Fig. 980, the corner at B being rounded to cut the rounded tops of the thread. It is more usual, however, to employ a chaser set in the tool point in the same manner as a single-pointed tool, or in a holder fixed in the tool post. When a single-pointed tool is employed to cut a thread, the angles of its sides are not the same as the angle of the thread it produces, which occurs because the tool must have clearance to enable it to cut. In Fig. 981, for example, is a single-pointed tool without any clearance, and, as a result, it cannot enter the work to cut it. In Fig. 982 the tool is shown with clearance, and, as a result, the angle of the cutting edge is not the same angle as the sides of the tool are, because the top face is not at a right angle to the sides of the tool. It is obvious that the angle of the sides of the tool must be taken along the dotted line in Fig. 982.

[Ill.u.s.tration: Fig. 980.]

It follows then that a tool whose sides are at a given angle will cut a different angle of thread for every variation in the amount of clearance. But whatever the amount of clearance may be, the tool will produce correct results providing that the gauge to which the tool is ground is held level, as in Fig. 983 at A, and not at an angle as at B.

The tool, however, must be set at the correct height with relation to the work, and its top surface must point to the work axis to produce correct results.

[Ill.u.s.tration: Fig. 981.]

[Ill.u.s.tration: Fig. 982.]

Suppose, for example, that in Fig. 984 A is a piece of work, its horizontal centre being represented by the dotted line C, and its centre of revolution being at C. Now suppose D is a screw-cutting tool cutting a depth of thread denoted by E. G is another lathe tool having teeth of the same form and angle as D, but lifted above the horizontal centre of the work. The depth of thread cut by G is denoted by F, which is shallower, though it will be seen that the point of G has entered the work to the same depth or distance (of the tool point) as D has. It is obvious, however, that for any fixed height, a tool suitable to cut any required depth or angle can be made, but it would be difficult to gauge when the tool stood at its proper height.

[Ill.u.s.tration: Fig. 983.]

To facilitate setting the height of the tool, a gauge such as shown in Fig. 985 may be used, the height of the line A from the base equalling the height or distance between the top surface of the cross slides and the axial line of the lathe centres. If the lathe, however, have an elevating slide rest, the rest must be set level before applying the gauge. Or in place of using the gauge, the tool stool or tool holder, as the case may be, may be made of such height that when level in the tool post its top face points to the axis of the lathe centre, the tool being sharpened on the angles and not ground on the top face.

[Ill.u.s.tration: Fig. 984.]

[Ill.u.s.tration: Fig. 985.]