Modern Machine-Shop Practice Part 147

An excellent mallet for the machinist's use, for driving finished work without damaging it, is formed of raw hide secured in a metal eye that receives the handle. Or for the same purpose a lead hammer is used, being especially serviceable for setting work in machines.

What is known as pening, or paning, consists of hammering the skin of metal to stretch it on the side that is hammered. It may be employed either to bend or to straighten. Suppose, for example, we have a piece of metal that is bent to a half circle, and if we take a light hammer and hammer it on the concave side and all over its surface the piece will straighten out to an amount depending on the amount of pening. Or if he hammers the convex side the piece will bend to a smaller circle.

The principle involved is, that if one side of a piece is elongated and the other remains of its original length, the only shape it can a.s.sume to accommodate or permit the elongation is that of a curve of which the convex side is the longest. It follows, therefore, that the hammer blows must in pening be sufficiently light to condense or stretch the metal on one side only of the metal, and not forcible enough to effect it all through.

In order to accomplish this stretching as rapidly as possible it is necessary to use a light hammer, with sufficient force to be expended in condensing the metal at its surface, and to so form the hammer that it shall expend its force upon the work with a dead blow, that is, with as little rebound as possible. These results are best accomplished with a ball pened hammer, such as shown in Fig. 2108 and weighing about 1/2 lb.

The blows should fall dead; that is, the hammer should fall, to a great extent, by its own weight, the number rather than the force of the blows being depended upon; hence, the hammer marks will not be deep. This is of especial importance when pening has to be performed upon finished work, because, if the marks sink deeply, proportionately more grinding or filing is required to efface them; and for this reason the force of the blows should be as near equal as possible. Another and a more important reason, however, is that the effect of the pening does not penetrate deeply; and if much of the pened surface is removed, the effects of the pening will be also removed. The work should not be rested upon metal, but upon wood.

[Ill.u.s.tration: Fig. 2142.]

[Ill.u.s.tration: Fig. 2143.]

The following are examples of pening. Fig. 2142 represents a shaft bent as shown, the arms being too wide at A, which may be corrected by pening at B. If the error was in the arms themselves and not in the stem, the side faces of the arms would require to be pened. Thus in Fig. 2143 the distance A is too short, and the pening must be at B C.

[Ill.u.s.tration: Fig. 2144.]

Fig. 2144 represents a strap requiring to be closed across A, the pening being at C or D. But as pening at D would bend the crown and unpair the bed of the bra.s.ses, it is preferable to pene at C. In either case the jaws will close as denoted by the dotted lines.

Fig. 2145 represents another common form of connecting rod strap, and in this case the pening may be most quickly and effectively done at the crown as denoted by the dots; and as this would alter the inside curve, the bra.s.s or box fitting into it must be refitted. In case the pening should be overdone it is better to modify it by filing away some of the pened surface.

Cast iron is more rapidly affected by pening than either wrought iron or steel. One of the most useful applications of pening is in the case of moulding patterns, which in time may become warped from the rapping of the pattern in the mould, and this warping may be corrected by judicious pening, or suppose that a number of plates, such as represented in Fig.

2146, having been cast, it is found that the ends of the tongues A B curl up when cooling in the mould, then the tongues may be pened as at C D, throwing them down to the requisite amount, and thus moulding the pattern to accommodate the curling in cooling.

[Ill.u.s.tration: Fig. 2145.]

[Ill.u.s.tration: Fig. 2146.]

The riveting usually performed by the machinist is generally upon cold metal. The blows in this case should fall dead and the riveting be performed with a view to stretching the metal uniformly and evenly over the surface to be riveted.

[Ill.u.s.tration: Fig. 2147.]

[Ill.u.s.tration: Fig. 2148.]

An excellent example of cold riveting is given in the crank pin P in Figs. 2147 and 2148. C is the crank (both being shown in section). The end of the pin should be recessed as shown at A, so that it may be the more readily riveted outward to fill the countersink shown in the crank at B, B. The crank-pin is rested upon a piece of copper D interposed between it and the iron block E to prevent damage to the finished face of the crank-pin.

The riveting blows should be given with a ball-faced hammer, and delivered with a view to stretch the whole end face of the crank-pin evenly. Otherwise the riveted surface will be apt to split as shown.

This usually occurs from not riveting the area at and near the circ.u.mference sufficiently, although it may occur from riveting that part of the area too much. The line of travel of the hammer should not be directly vertical, but somewhat lateral in a direction from the centre towards the circ.u.mference. If the countersink is a deep one, it is desirable to leave the crank-pin sufficiently too long, so that after the riveting has proceeded some time the surface of the metal which has become condensed and crystallized from the direct impact of the hammer blows, may be chipped away, leaving a surface that is swollen by the riveting without being so much condensed. This enables a much greater spreading of the metal without splitting it.

If in this cla.s.s of work the riveted piece (as the crank-pin) is not driven in very tight before riveting, the riveting blows will be apt to jar the pin back. Hence, it is necessary to occasionally drive the pin home. The riveting should proceed equally all over, as if one side be riveted in advance of the other it tends to throw the pin out of true.

When, however, the riveting begins to bed the pin, four equidistant places may be riveted home in advance so as to bring the pin home and hold it firmly.

[Ill.u.s.tration: Fig. 2149.]

[Ill.u.s.tration: Fig. 2150.]

[Ill.u.s.tration: Fig. 2151.]

THE CHISEL.--The machinist's cold chisel is made from the two forms of steel shown in Figs. 2149, 2150, and 2151, and of these the former is preferable because it has two broad flats diametrally opposite and these form a guide to the eye in holding the chisel on the grindstone, and aid in grinding the facets that form the cutting edge true. Furthermore, as the cutting edge is in the same plane as these flats they serve as a guide to denote when the chisel edge lies parallel to the work surface, which is necessary to produce true and smooth chipping.

The width of the chisel may be made greater, as in Figs. 2152 and 2153, for bra.s.s or cast-iron work than for wrought iron or steel for the following reasons. On account of the toughness and hardness of wrought iron and steel the full force of a 1-3/4 lb. hammer, having a handle 13 inches long, may be used on a chisel about 7/8 inch wide without danger of causing the metal to break out below the chipping line, but if such a chisel be used with full force blows upon cast iron or bra.s.s the metal is apt to break out in front of the chisel, the line of fracture often pa.s.sing below the level it is intended to chip down to. Hence if a narrow chisel is used lighter blows must be delivered. But by using a broader chisel the force of the blow is distributed over a longer length of cutting edge, and full force blows may be used without danger of breaking out the metal.

[Ill.u.s.tration: Fig. 2152.]

[Ill.u.s.tration: Fig. 2153.]

[Ill.u.s.tration: Fig. 2154.]

[Ill.u.s.tration: Fig. 2155.]

[Ill.u.s.tration: Fig. 2156.]

[Ill.u.s.tration: Fig. 2157.]

[Ill.u.s.tration: Fig. 2158.]

The cutting end of the chisel should be kept thin, as in that case it cuts both easier and smoother. The total length of a chisel should not when new exceed 8 inches, for if made longer it is not suitable for heavy or smooth chipping, as it will bend and spring under heavy blows, and cannot be held steadily. The forged part should not exceed about 2-1/2 or 3 inches in length, as a long taper greatly conduces to springiness, whereas solidity is of great importance both to rapid and smooth work. The facets forming the cutting edge should be straight in their widths, as at B in Fig. 2154, and not rounded as at A, so that the face next to the work may form a guide in holding the chisel at the proper angle to maintain the depth of the cut. This angle depends upon the nature of the material to be cut; the facets forming an angle one to the other of about 65 for cast steel and about 50 for gun metal or bra.s.s. The more acute these angles the nearer the body of the chisel lies parallel with the work and the more effective the hammer blows.

Thus in Fig. 2155 chisel C is the position of the chisel for wrought iron, and position D is for steel. The angles should always be made, therefore, as acute as the hardness of the material will permit. If they are too acute the cutting edge will be apt to bend in its length, while if not sufficiently acute they will not cut keen enough; hence the object is to make them as acute as possible without causing the cutting edge to bend in its length. For copper and other soft metals the angle may be about 30 or 35, the chisel end being kept thin so that it may not become wedged between the work and the chipping, which will bend but little, and is, therefore, apt to grip the wedge end of the chisel. The cutting edge should be slightly rounded in its length, which will strengthen it and also enable a fine finishing clip to be taken off, as in Fig. 2156, the width of the chip not extending fully across the chisel width so that the corners are not under duty and are not, therefore, liable to break, or dig in and prevent smooth chipping. In some practice the edge is made straight in its length, as shown in Fig.

2149, which is permissible in heavy chipping when a cape chisel has been used, but in any event an edge rounded in its length is preferable. If the edge is hollow in its length, as shown in Fig. 2157, and magnified in Fig. 2158, the chip acts as a wedge to force the corners outwards as denoted by the arrows, causing them to break under a heavy cut, and, furthermore, a smooth cut cannot be taken when the corners of the chisel meet the work surface.

If the facets are ground under on one side, those on the other, as in Fig. 2159, the edge will not be parallel with the flats of the chisel, so that in holding it the flats will not form a guide to determine when the edge lies parallel to the work surface as it should do. The edge should also be at a right angle to the length of chisel, as denoted by the lines, as in Fig. 2160, for if not at a right angle the chisel will be apt to move sideways after each blow, and cannot be held steadily.

[Ill.u.s.tration: Fig. 2159.]

[Ill.u.s.tration: Fig. 2160.]

The chisel should be held as close to its head as possible, so that the hand will steady the head as much as possible, and should be pushed forward firmly and steadily to its cut, which will greatly facilitate rapid and smooth chipping, and for wrought iron and copper it is found better to occasionally moisten the chisel with oil or water, the former being preferable.

[Ill.u.s.tration: Fig. 2161.]

Messrs Tangye, of Birmingham, have introduced the employment of chisel holders, such as shown in Fig. 2161, the object being to fit to each holder a number of short pieces of steel for chisels so that a number can be ground or forged at one time; obviously chisels of different shapes require different forms of handle.

[Ill.u.s.tration: Fig. 2162.]

[Ill.u.s.tration: Fig. 2163.]

[Ill.u.s.tration: Fig. 2164.]

When a heavy cut is to be taken the cape (Fig. 2162) chisel is used, first to carry through grooves or channels, such as shown in Fig. 2176 at A, B, and C, the distance apart of these grooves being slightly less than the width of the flat chisel, whose cut is shown partly carried across at D in the figure. The width of a cape chisel should gradually decrease from A to B in Fig. 2163, so that its side will be free in the groove it cuts, and the chisel head will be free to be moved sideways, and the direction of the groove may be governed thereby. If the chisel end be made parallel, as at C in Fig. 2164, it will have no play in the groove and the head cannot be moved; hence if the groove is started out of line, as it is apt to be, it will continue so.

[Ill.u.s.tration: Fig. 2165.]

[Ill.u.s.tration: Fig. 2166.]

[Ill.u.s.tration: Fig. 2167.]

[Ill.u.s.tration: Fig. 2168.]

The round-nosed chisel, Figs. 2165 and 2166, may be straight from H nearly to the point G, but should be bevelled at and near G, so that the chisel head may be raised or lowered to govern the depth of the cut. Its round nose should also be wider than the metal higher up, so that the chisel head may be moved sideways to govern the direction of the cut as in the cape chisel. The cow mouth chisel, Figs. 2167 and 2168, should be bevelled from G to the point to enable the governing of the depth of the cut, and should be of greater curvature than the corner it is to cut out, so that its corners cannot wedge in the work.

[Ill.u.s.tration: Fig. 2169.]