Modern Machine-Shop Practice Part 89

1286, the heel being pressed down firmly upon the tool rest. The cut is carried along the work by revolving the handle upon its axis, and from the right towards the left, at the same time that the handle is moved bodily from the left towards the right. By this combination of the two movements, if properly performed, the point of the graver will move in a line parallel to the centres of the lathe, because, while the twisting of the graver handle causes the graver point to move away from the centre of the diameter of the work, the moving of the handle bodily from left to right causes the point of the graver to approach the centre of that diameter; hence the one movement counteracts the other, producing a parallel movement, and at the same time enables the graver point to follow up the cut, using the heel as a pivotal fulcrum, and hence obviating the necessity of an inconveniently frequent moving of the heel of the tool along the rest. The most desirable range of these two movements will be very readily observed by the operator, because an excess in either of them destroys the efficacy of the heel of the graver as a fulcrum, and gives it less power to cut, and the operator has less control over the tool.

[Ill.u.s.tration: Fig. 1286.]

[Ill.u.s.tration: Fig. 1287.]

[Ill.u.s.tration: Fig. 1288.]

For finis.h.i.+ng or smoothing the work the graver is held as in Fig. 1287, the edge being brought parallel to the work surface. For bra.s.s work the top faces of the graver should be slightly bevelled in the direction shown in Fig. 1288.

The graver cuts most efficiently with the work revolving at a fast speed, or, say, at about 60 feet per minute, and for finis.h.i.+ng wrought iron or steel requires an application of water.

[Ill.u.s.tration: Fig. 1289.]

To finish work that has been operated upon by a heel tool or by a graver, the finis.h.i.+ng tool shown in Fig. 1289 may be employed. It is usually made about 5/8 or 3/4 inch wide, as the graver is employed for shorter work. It is ground so as not to let the extreme corners cut, and is used at a slow speed with water. The edge of this tool is sometimes oilstoned, causing it to cut with a clean polish. The tool is held level, brought up to the work, and a cut put on by elevating the handle end. To carry the cut forward, the tool is moved along the hand rest to nearly the amount of its width, and is brought to its cut by elevating the handle as before. When the work has been finished as near as may be with this tool, it may be finished by fine filing, the lathe running at its quickest speed; or the file may be used to show the high spots while using the finis.h.i.+ng tool.

[Ill.u.s.tration: Fig. 1290.]

For facing the ends of work the tool shown in Fig. 1290, or that shown in Fig. 1291, may be used, either of them being made from an old three-cornered file. The cutting edge at A, Fig. 1290, should be slightly curved, as shown. The point of the tool is usually brought to cut at the smallest diameter of the work, with the handle end of the tool somewhat elevated. As the cut is carried outwards the handle end of the tool is depressed, and the point correspondingly elevated. It may be used dry or with water, but the latter is necessary for finis.h.i.+ng purposes.

[Ill.u.s.tration: Fig. 1291.]

Another form of this tool is shown in Fig. 1291. It has two cutting edges A A, one of which rests on the hand rest while the other is cutting, the tool being shown in position for cutting a right-and a left-hand face, the face nearest to the work being shown in the lower view. This face should be placed against the radial face of the work, and the cut put on by turning the upper edge over towards the work while pressing the tool firmly to the lathe rest.

[Ill.u.s.tration: Fig. 1292.]

For cutting out a round corner the tool shown in Fig. 1292, employed either for roughing or smoothing purposes (water being used with it for the latter), the heel causes it to grip the hand rest firmly, and acts as a pivotal fulcrum from which the tool may be swept right and left round the curve, or a portion of it.

This tool, as in the case of all tools used upon wrought iron or steel, should not cut all round its edge simultaneously, as in that case, unless indeed it is a very narrow tool, the force placed upon it by the cut will be too great to enable the operator to hold and control it; hence the cut should be carried first on one side and then on the other, and then at the point, or else the handle end should be moved laterally, so that the point sweeps round the work. It should be brought to its cut by placing its heel close to the work, and elevating the handle end until the cutting edge meets the work.

The point or nose of the tool may obviously be made straight or square, as it is termed, to suit the work, the top rake being omitted for bra.s.s work.

[Ill.u.s.tration: Fig. 1293.]

In using this tool for cutting a groove it is better (if it be a deep groove, and imperative if it be a broad one, especially if the work be slight and apt to spring) to use a grooving tool narrower in width than the groove it is to cut, the process being shown in Fig. 1293, in which W represents a piece of work requiring the two grooves at A and B cut in it. For a narrow groove as A the tool is made about half as wide as the groove, and a cut is taken first on one side as at C, and then on the other as at D. For a wider groove three or more cuts may be made, as at E, F, G. In all cases the tool while sinking the groove is allowed to cut on the end face only; but when the groove is cut to depth, the side edges of the tool may be used to finish the sides of the groove, but the side and end edge must not cut simultaneously, or the tool will be liable to rip into the work.

[Ill.u.s.tration: Fig. 1294.]

HAND TOOLS FOR BRa.s.s WORK.--In addition to the graver as a roughing-out tool for bra.s.s work, we have the tool shown in Fig. 1294, the cutting edge being at the rounded end A. It is held firmly to the rest, which is not placed close to the work (as in the case of other tools), so as to give the tool a wide range of movement, and hence permit of the cut being carried farther along without moving its position on the rest. It may be used upon either internal or external work.

For finis.h.i.+ng bra.s.s work, tools termed sc.r.a.pers are employed.

[Ill.u.s.tration: Fig. 1295.]

Fig. 1295 represents a flat sc.r.a.per, the two end edges A and the side edges along the bevel forming the cutting edges.

[Ill.u.s.tration: Fig. 1296.]

[Ill.u.s.tration: Fig. 1297.]

[Ill.u.s.tration: Fig. 1298.]

[Ill.u.s.tration: Fig. 1299.]

In this tool the thickness of the end A is of importance, since if it be too thin it will jar or chatter. This is especially liable to occur when a broad sc.r.a.per is used, having a great length of cutting edge in operation. This may be obviated to some extent by inclining the sc.r.a.per as in Fig. 1296, which has the same effect as giving the top face negative rake, causing the tool to sc.r.a.pe rather than cut. The dividing line between the cutting and sc.r.a.ping action of a tool is found in the depth of the cut, and the presentation of the tool to the work, as well as in the shape of the tool. Suppose, for example, that we have in Fig.

1297, a piece of work W and a tool S, and the cut being light will be a sc.r.a.ping one. Now suppose that the relative positions of the size of the work and of the tool remain the same, but that the cut be deepened as in Fig. 1298, and the sc.r.a.ping action is converted into that cla.s.s of severing known as shearing, or we may reduce the depth of cut as in Fig.

1299, and the action will become a cutting one.

[Ill.u.s.tration: Fig. 1300.]

But let the depth of cut be what it may, the tool will cut and not sc.r.a.pe whenever the angle of its front face is more than 90 to the line of tool motion if the tool moves, or of work motion if the work moves to the cut. In Fig. 1300, for example, the tool is in position to cut the angle of the front face, being 110 to the direction of tool motion.

We may consider this question from another stand-point, however, inasmuch as that the tool action is a cutting one whenever the pressure of the cut is in a direction to force the tool deeper into the work, and a sc.r.a.ping one whenever this pressure tends to force the tool away from the work, a.s.suming of course that the tool has no front rake, and that the cut is light or a "mere sc.r.a.pe," as workmen say. This is ill.u.s.trated in Fig. 1301, the tool at A acting to cut, and at B to sc.r.a.pe, and the pressure of the cut upon A acting to force the tool into the work as denoted by the arrow D, while that upon B acts to force it in the direction of arrow C, or away from the work.

In addition to these distinctions between a cutting and a sc.r.a.ping action we have another, inasmuch as that if a tool is pulled or dragged to its cut its action partakes of a sc.r.a.ping one, no matter at what angle its front face may stand with relation to the work.

The end face of a flat sc.r.a.per should be at a right angle to the body of the tool, so that both edges may be equally keen, for if otherwise, as in Fig. 1302, one edge as A will be keener than the other and will be liable to jar or chatter.

[Ill.u.s.tration: Fig. 1301.]

[Ill.u.s.tration: Fig. 1302.]

The flat sc.r.a.per can be applied to all surfaces having a straight outline, whether the work is parallel or taper, providing that there is no obstruction to prevent its application to the work.

[Ill.u.s.tration: Fig. 1303.]

[Ill.u.s.tration: Fig. 1304.]

[Ill.u.s.tration: Fig. 1305.]

[Ill.u.s.tration: Fig. 1306.]

Thus, in Fig. 1303 we have a piece of work taper at _a_ and C, parallel at _e_, and with a collar at _d_, the sc.r.a.per S being shown applied to each of these sections, and it is obvious that it cannot be applied to section _a_ because the collar _d_ is in the way. This is remedied by grinding the sc.r.a.per as in Fig. 1304, enabling it to be applied to the work as in Fig. 1305. Another example of the use of a bevelled sc.r.a.per is shown in Fig. 1306, the sc.r.a.per S having its cutting edge parallel to the work and well clear of the arm H.

[Ill.u.s.tration: Fig. 1307.]

The round-nosed sc.r.a.per is used for rounding out hollow corners, or may be made to conform to any required curve or shape. It is limited in capacity, however, by an element that affects all sc.r.a.ping tools, that if too great a length of cutting edge is brought into action at one time, chattering will ensue, and to prevent this the sc.r.a.per is only made of the exact curvature of the work when it is very narrow, as at S in Fig. 1307.

For broad curves it is made of more curvature, so as to limit the length of cutting edge, as is shown in the same figure at S', and is swept round the work so as to carry the cut around the curve.

There are, however, other means employed to prevent chattering, and as these affect the flat sc.r.a.per as well as the round-nosed one, they may as well be explained with reference to the flat one.

First, then, a thin sc.r.a.per is liable to chatter, especially if used upon slight work. But the narrower the face on the end of the sc.r.a.per, the easier it is to resharpen it on the oilstone, because there is less area to oilstone. A fair thickness is about 1/20 inch; but if the sc.r.a.per was no thicker than this throughout its whole length, it would chatter violently, and it is for this reason that it is thinned at its cutting end only. Chattering is prevented in small and slight work by holding the sc.r.a.per as in Fig. 1308, applying it to the top of the work; and to reduce the acting length of cutting edge, so as to still further avoid chattering, it is sometimes held at an angle as in the top view in Fig. 1309, S being the sc.r.a.per and R the tool rest.

When the sc.r.a.per is applied to side faces, or in other cases in which a great length of cutting edge is brought into action, a piece of leather laid beneath the sc.r.a.per deadens the vibration and avoids chattering.

[Ill.u.s.tration: Fig. 1308.]

[Ill.u.s.tration: Fig. 1309.]