Modern Machine-Shop Practice Part 102

This process may, in many cases, be artificially a.s.sisted. Suppose, for example, a washer is too large in its bore; it should have its hole and part of its radial faces filled with fire-clay, as shown in Fig. 1439, in which A is the washer and B B the clay, _c_ _c_ being pieces of wire to hold the fire-clay and prevent its falling off. The washer should be heated to a clear red and plunged in the water D D, which will cool and shrink the exterior and exposed metal in advance of the interior, which will compress to accommodate the contraction of the outer metal, hence the hole will be reduced. This operation may be repeated until the hole be entirely closed.

[Ill.u.s.tration: Fig. 1440.]

[Ill.u.s.tration: Fig. 1441.]

Another method of closing such a piece as an eye of large diameter compared to its section, is shown in Fig. 1440; first dipping the heated eye at A and holding it there till cold and then slowly lowering it into the water, which would close the diameter across C, and, after reheating, dipping at D till cold, and then slowly immersing, which would close the eye across E. To shrink a square ring, the whole ring would require to be heated and a side of the square dipped, as shown in Fig. 1441, until quite cold, and then immersed slowly for about an inch, the operation being performed with a separate heating for each side.

Connecting rod straps, wheel-tires, and a large variety of work may be refitted by this process, but in each case the outside diameter will be reduced.

CHAPTER XV.--MEASURING TOOLS.

[Ill.u.s.tration: Fig. 1442.]

[Ill.u.s.tration: Fig. 1443.]

For what may be termed the length measurements of lathe work it is obvious that caliper gauges, such as shown in Fig. 1402, may be employed. Since, however, these length measurements rarely require to be so accurate as the diametrical measurements, the ordinary lineal rule is very commonly employed in work not done under the standard gauge system.

It is obvious, however, that when a number of pieces are to be turned to corresponding lengths, a strip of sheet iron, or of iron rod made to the required length, may be employed; a piece of sheet iron filed to have the necessary steps being used where there are several steps in the work; but if the lineal measuring rule is used, and more than one measurement of length is to be taken, some one point, as one end of the work, should be taken wherefrom to measure all the other distances.

Suppose, for example, that Fig. 1442 represents a crank pin requiring to have its end collar 1/4 inch thick, the part A 2 inches long, part B 3 inches long, collar C 1/2 inch thick, and the part D 7 inches long. If the length of each piece were taken separately and independently of the others, any errors of measurement would multiply; whereas, if some one point be taken as a point wherefrom to measure all the other distances, error is less liable to occur, while at the same time an error in one measurement would not affect the correctness of the others. In the case of the crank pin shown, the collar C would be the best point wherefrom to take all the other measurements. First, it would require to be made to its proper thickness, and the lengths of B, A, and the end collar should be measured from its nearest radial face. The length of D should then be measured from the same radial face, the thickness of the collar being added to the required length of D, or D may be measured from the nearest radial face of C, providing C be of its exact proper thickness.

In measuring the length of the taper part D, a correct measurement will not be obtained by laying the rule along its surface, because that surface does not lie parallel to its axis, hence it is necessary to apply the measuring rule, as shown in Fig. 1443, in which S is a straight-edge held firmly against the radial face of the crank pin (the radial face being of course turned true), and R is the measuring rule placed true with the axial line of the crank pin. Whenever the diameters of the lengths to be measured vary, this mode of measuring must be employed. On small work, or on short distances requiring to be very exact, a gauge such as shown in Fig. 1444 at A may be employed, which will not only give more correct results, but because it is more convenient, as it can be conveniently held or tried to the work with one hand while the other hand is applied to the feed screw handle to withdraw the cutting tool at the proper moment, and to the feed nut to unlock it and stop the feed.

[Ill.u.s.tration: Fig. 1444.]

[Ill.u.s.tration: Fig. 1445.]

On long work a wooden strip is the best, especially if the work has varying diameters and a number of pieces of work require to be made exactly alike. In Fig. 1445 S represents the wooden strip, and W the work. The strip is marked across by lines representing the distances apart the shoulders of the work require to be; thus the lines A, B, C, D, E, F, G, represent the distances apart of the radial faces _a_, _b_, _c_, _d_, _e_, _f_, _g_, on the work, and these lines will be in the same plane as the shoulders if the latter are turned to correct lengths.

To compare the radial faces with the lines, a straight-edge must be held to each successive shoulder (as already described) that is of smaller diameter than the largest radial face on the work.

If the wooden strip be made the full length of the work the dog or clamp driving the work will require to be removed every time the wooden gauge is applied, and since the work must be turned end for end in the lathe to be finished, it would be as well to let the length of the wood gauge terminate before reaching the work driver, as, say, midway between E and F.

When a lineal distance is marked by lines, and this distance is to be transferred to another piece of work and marked thereon by lines, the operation may be performed, for short distances or radii, by the common compa.s.ses employed to mark circles, but for greater distances where compa.s.ses would be c.u.mbersome, the trammels are employed.

Fig. 1446 represents a pair of trammels made entirely of metal, and therefore suitable for machinists' use, in which the points require to be pressed to the work to mark the lines. A A represents a bar of square steel; or for very long trammels wood may be used. B represents a head fastened tightly to one end, and through B pa.s.ses the leg or pointer C, which is thus adjustable as to its projecting distance, as C can be fastened in any position by the thumb-screw D. The head E is made to a good sliding fit upon the bottom and two side faces of A A; but at the top there is sufficient s.p.a.ce to admit a spring, which pa.s.ses through E.

F is the leg screwed into E, which is locked in position by the thumb-screw G. The head E is thus adjustable along the whole length of the bar or rod A A. The object of the spring is as follows:--If the head E were made to fit the bar A A closely on all four sides, the burrs raised upon the top side of the rod A A by the end of the thumb-screw G would be likely to impede its easy motion. Then again, when the sliding head E has worn a trifle loose upon the bar A A, and is loosened for adjustment, it would be liable to hang on one side, and only to right itself when the screw G brought it to a proper bearing upon the under side of the bar A A, and thus tightening the head E would alter the adjustment of the point. The spring, however, always keeps the lower face of the square hole through E bearing evenly against the corresponding face of the bar, so that tightening the screw G does not affect the adjustment, and, furthermore, the end of the set-screw, bearing against the spring instead of against the top of the rod, prevents the latter from getting burred.

[Ill.u.s.tration: Fig. 1446.]

The flat place at I I is to prevent the burrs raised by the thumb-screw end from preventing the easy sliding of leg C through B.

[Ill.u.s.tration: Fig. 1447.]

In some cases a gib is employed, as shown at A in Fig. 1447, instead of a spring, the advantage being that it is less liable to come out of place when moving the head along the bar.

The trammels should always be tried to the work in the same relative position as that in which they were set, otherwise the deflection of the bar may vitiate the correctness of the measurement; thus, if the rod or bar stood vertical when the points were adjusted for distance to set them to the required distance, it should also stand vertical upon the work when applied to transfer that distance, otherwise the deflection of the bar from its own weight will affect the correctness of the operation. Again, when applied to the work the latter should be suspended as nearly as convenient in the same position as the work will occupy when erected to its place.

Thus, suppose the trammels be set to the crank pin centres of a locomotive, then the bar will stand horizontally. Now the side rod, or coupling rod, as it may be more properly termed, should be stood on edge and should rest on its ends, because its bearings wherever it will rest when on the engine are at the ends; thus the deflection of the trammel rod will be in the same direction when applied to the work as it was when applied to the engine, and the deflection of the coupling rod will be in the same direction when tried by the trammel as when on the engine. The importance of this may be understood when it is mentioned that if the coupling rod be a long one, resting it on its side and supporting it in the middle instead of at its ends will cause a difference of 1/50th inch in its length.

[Ill.u.s.tration: Fig. 1448.]

Another lineal measuring gauge employed in the machine shop is shown in Fig. 1448. It is employed to measure the distance between two faces, and therefore in place of inside calipers, in cases where from the extreme distance to be measured it would require the use of inside calipers too large to be conveniently handled. Its application is more general upon planing machine work than any other, although it is frequently used by the lathe hand or turner, and by the vice hand and erector. It consists of two legs A and B, held together by the screws C D, which screw into nuts. These nuts should have a shoulder fitting into the slots in both legs, so as to form a guide to the legs. The screws are set up so as to just bind both legs together but leaving them free enough to move under a slight friction. The gauge is then set to length by lightly striking the ends E, and when adjusted the screws C D are screwed firmly home.

The ends E are rounded somewhat, as is shown, to prevent them from swelling or burring by reason of the blows given to adjust them.

For striking circles we have the compa.s.ses or dividers, which are made in various forms.

[Ill.u.s.tration: Fig. 1449.]

Thus, Fig. 1449 represents a pair of spring dividers, the bow spring at the head acting to keep the points apart, and the screw and nut being employed to close and to adjust them.

[Ill.u.s.tration: Fig. 1450.]

Another form is shown in Fig. 1450, the legs being operated by a right and left-hand screw, which may be locked in position by the set-screw shown.

For very small circles the fork scriber shown in Fig. 1451 is an excellent tool, since it may be used with great pressure so as to cut a deep line in the surface of the work. This tool is much used by boiler makers, but is a very useful one for the machinist for a variety of marking purposes, which will be described with reference to vice work.

For larger work we have the compa.s.ses, a common form of which is shown in Fig. 1452, in which the leg A is slotted to receive the arc piece C, which has a threaded stem pa.s.sing through E, and is provided with a nut at B; at D is a spring which holds the face of the nut B firmly against the leg E; at A is a thumb-screw for securing the leg to the arm C. The thumb-screw A being loosened, the compa.s.s legs may be rudely adjusted for distance apart, and A is then tightened. The adjustment is finally made by operating the nut B, which, on account of its fine thread, enables a very fine adjustment to be easily made.

[Ill.u.s.tration: Fig. 1451.]

[Ill.u.s.tration: Fig. 1452.]

[Ill.u.s.tration: Fig. 1453.]

It is often very convenient to be able to set one leg of a pair of dividers to be longer than the other, for which purpose a socket B, Fig.

1453, is provided, being pierced to receive a movable piece A, and split so that by means of a set-screw C the movable piece A may be gripped or released at pleasure.

[Ill.u.s.tration: Fig. 1454.]

For finding the centres of bodies or for testing the truth of a centre already marked, the compa.s.s calipers shown in Fig. 1454, are employed.

It is composed of one leg similar to the leg of a pair of compa.s.ses, while the other is formed the same as the leg of an inside caliper. The uses of the compa.s.s calipers are manifold, the princ.i.p.al being ill.u.s.trated as follows:--

[Ill.u.s.tration: Fig. 1455.]

Let it be required to find the centre of a rectangular block, and they are applied as in Fig. 1455, the curved leg being rested against the edge and a mark being made with the compa.s.s leg. This being done from all four sides of the work gives the centre of the piece.

[Ill.u.s.tration: Fig. 1456.]

In the case of a hole its bore must be plugged and the compa.s.s calipers applied as in Fig. 1456.

[Ill.u.s.tration: Fig. 1457.]

For marking a line true with the axial line of a cylindrical body, we have the instrument W in Fig. 1457, which is shown applied to a shaft S.

The two angles of the instrument are at a right angle one to another, so that when placed on a cylindrical body the contact will cause the edge of W to be parallel with the axis of the shaft. The edge is bevelled, as shown, so that the lines of division of inches and parts may come close to the work surface, and a scriber may be used to mark a line of the required length. A scriber is a piece of steel wire having a hardened sharp point wherewith to draw lines.

On account of the instrument W finding its princ.i.p.al application in marking key seats upon shafts, it is termed the "key-seat rule."

[Ill.u.s.tration: Fig. 1458.]