PP, Fig. 192, is a circular plate of cast iron, used to support the mold while soft; it is not shown in Fig. 193. By

the same method, only varying the outline of the sweeps, a large class of circular work may be produced, including vases, speed cones, etc. Sometimes it is necessary to cast brackets, pipes, or other projections, upon the main piece; to do this, patterns must be made of those projections, and

as many patterns as there are projections. The height at which it is required to bed in these brackets, etc., must be indicated to the molder by a small V, cut into the sweep; this will produce, as the sweep revolves, a line upon the mold. For the rest, unless simple directions can be given, the pattern maker usually visits the foundry, and assists in placing, or at least in verifying, the position of the pieces. When the mold is sufficiently hard, and before it is baked, these patterns are withdrawn.

Fig.198

A good illustration of the manner in which pattern work may be used in conjunction with sweeps, is furnished in the ordinary engine cylinder. Fig. 195 is a sectional elevation of a complete mold; Fig. 196 is a horizontal section of the same, on the line A B showing the outlet for the exhaust steam. This mold is composed of four parts that are swept or struck up—namely, SS, the seat; A B, the body; C C, the cope, and M, the main core. The latter may be struck upon a horizontal arbor, or formed in a box In addition to the parts above enumerated are the two steam port cores and the exhaust port core, all formed in core boxes. The procedure is as follows: With a board, shown in Fig. 197, the seat S S is struck up; upon this, when dried, is placed a flange of wood. It is set centrally; the seat is also carefully beveled and set by the spindle. A pattern of the slide face, with the parts in which the steam

Fig.199.

Fig.197.

and exhaust passages occur, is set in position on this flange; the top flange of wood is now added, and tem

porarily fixed to the slide face pattern, and shored up on the opposite side, so as to maintain it true and level. With the board, Fig. 198, is formed the body A B; the shape of the exterior of the mold is not important; it is left rough, but some mark must be made so as to be able, after removing it from the seat, to restore it to the position as before. When the body has dried sufficiently, the pattern flanges and slide face are withdrawn, the body being lifted from the seat for this purpose by means of bolts passing through it, and terminating in a cast annular plate at the bottom. The projecting flanges on the slide face are attached by wires and dovetails; otherwise the piece would be locked in the mold. The side print for the exhaust port is attached also by a loose wire. Fig. 199 is a board for sweeping up the cope, C C. The whole of these boards are represented as carried to the center of the spindle; allowance must, therefore, be made for the spindle and bracket. For very large cylinders, wood flanges are not used, the sweeps being made to a shape to perform the whole of the work.

CHAPTER XIV.

GEAR WHEELS AND WORM SCREW.

We now approach a class of work in which the fullest amount of care and attention on the part of the pattern maker, for the attainment of accuracy, is exceedingly desirable. Patterns for wheel work, clumsily constructed, may be positively worthless, or may at least give rise to great loss of time in the fitting shop, in correcting the defects in the castings taken from them. It is not our purpose to enter into the various methods of arriving at the proper form or curvature that is to be given to the teeth, as that is a subject quite extensive, and a study in itself. What more particularly concerns us, is the general construction of the patterns from designs furnished.

Gear wheels are of two kinds, spur and bevel; the former for transmitting motion when the shafts are parallel, and the latter to be used when the shafts are inclined to each other. When the teeth of a bevel wheel are inclined at an angle of 45° with the axis, that wheel is called a miter. Skew bevels are wheels suitable for shafts that are inclined to each other, and are not in the same plane. Pinion is a distinctive term, applied to the smaller of a pair of gear wheels, when there is a great disparity between them; or it may mean generally a small gear wheel.

Fig. 200 is a plan and section of the pattern of a spur wheel and pinion, such as is usually supplied to workmen. The plan exhibits the form of the teeth and pitch, with the size and number of arms. The sectional view shows the breadth of face, depth of hub, and ribs on the arms. In the construction of gear wheel and pinion patterns, the particular method to be adopted, as also the material to be used, will depend upon size and the service expected to

be got out of the patterns. Mahogany, dry and straight grained, is an excellent material for wheel patterns; but for large work it is too costly. In some cases the teeth are worked in mahogany and fixed to a pine body; in the majority of cases, however, pine is the only material used. The pinion may be carved out of one piece, or it may have the teeth attached to a hub; and if the latter, then the teeth may be held by dovetails, or they may be simply glued or nailed. If the pinion is so deep in proportion to its diameter as to be strong enough, and not more than 5 or 6 inches diameter over all, it may be cut from the solid. In this case, the grain of the wood must lie in the direction of the teeth. For turning the piece we must use a chuck or face plate, smaller than the pinion is at the bottom of the spaces, so as to be able to trace circles on both sides by the motion of the lathe; if such a face plate is not at our disposal, we may bore a hole in the piece to be turned, and it to it an arbor of hard wood. Having turned the pattern, trace upon it very fine circles to indicate the pitch line, the line for the roots of the teeth, and (if required) circles for the centers used in tracing certain peculiar forms of teeth. All these circles are to be traced on both sides of the pattern, and draft is to be allowed by making the circle for the roots of the teeth a little smaller on one side than on the other, and also by turning the piece slightly taper. The pinion is now to be pitched out, on one side, very accurately; this is sometimes a matter of no small difficulty, for, having passed round with the compasses a few times, the points are liable to slide into previous impressions, giving rise to error. For this reason the pattern maker does not allow the points of his compasses to fall where he intends the center of the teeth to be, until he has obtained the correct division, which is known by the com.. pass point, after having made the tour of the circle, falling exactly into the starting point. He now proceeds to