method of fixing the teeth. This, however, is really a small matter when dealing with pinions; and, therefore, bevel pinions usually have their teeth attached by dovetails, excepting those of small size. If it is decided to use dovetails, we proceed as follows: The body of the pinion has been turned and divided, and the perpendiculars all finely drawn in. Cut out of thin wood a piece of the size which the dovetails are intended to be, which is such that a small margin of tooth may be left on each side; set the piece on the rim, at a distance from a perpendicular equal to the margin allowed; set it by the square, shown in Fig. 207, as the dovetail must have such a taper that its sides may both tend towards the point X, before alluded to, namely, the intersection of the axes of the shafts. This will be the case if, when one side of the dovetail template has been set square, the other is square also. By this template, lines for all the dovetails are scribed on the face; the depth is laid off on the drawing by lines tending toward X; and from this the depth of each end of the recess may be gaged on the pattern. No curvature is given to the bottom of this; it is pared out flat with the chisel. The dovetails are now fitted, and left projecting above the face; they are driven moderately tight; the projecting parts are then turned off level with the rim.

We have now to go through the same process as before described for making and attaching teeth. When the glue is well set, each should be knocked out, numbered, and the dovetail bradded. Fig. 208 is a section and half plan of a bevel wheel. In the latter the shape of the teeth is not shown, but merely their thickness at the pitch line; in the sectional view, a few teeth are laid out in profile upon arcs struck from the centers, A and B, which are the points of intersection of perpendiculars from the ends of the teeth (at the pitch line) and the center line. In the section on one side is shown a series of rectangles, numbered from 1

to 5; these represent the segments of which the rim is composed. It is true that they might be made more nearly to approximate to the shape of the rim by sawing them to a bevel, but a machine suitable for this is not in every shop; and when it is considered that the segments themselves are

Fig.203.

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usually not more than inch in thickness, it will be seen ៖ that the additional complication counterbalances the saving in lumber and time in turning. If, however, the wheel is very large, or where thick segments are employed, we may

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advantageously saw the segments to a bevel. The method described for turning the bevel pinion is exactly suitable for the wheel; the arms will be checked together, but need not be built into the rim, unless we desire an exceptionally strong pattern; the obliquity of the rim enables us to get a good purchase, by means of screws through the end of each arm into it. Care must be taken to have the ends of the arms each to bear properly on the rim; otherwise the rim will be thrown out of true in screwing.

It will be remembered that, in treating upon the spur wheel, we had, in forming the box for shaping the teeth, simply to draw out on each end the natural size of the tooth, that is, if we except a slight diminution towards one end for draught; but the conical form of a bevel wheel gives a little extra trouble. In Fig. 208 the tooth proper is of the length of the face of the wheel, as seen in section. Now all lines bounding the teeth must converge to the point X; so if we take F F to represent the length of the box, we must strike out upon the large end an enlarged, and upon the small end a diminished tooth; then by planing to these lines we shall have formed such a box that any piece shaped in the gap formed in it, will be of the proper size and shape for a tooth. It would confuse our engraving too much were we to attempt to show the enlarged and diminished tooth on the ends of the box; but the principle is easily understood, as we have but to follow out whatever method has been adopted on the drawing for producing the tooth curves.

THE WORM, OR ENDLESS SCREW.

A worm pattern, when cut by hand, involves a slow and tedious operation; and even with the utmost care we can scarcely expect to produce an article so perfect as it would be if cut in a screw-cutting lathe. But however well

adapted the screw-cutting lathe may be for producing good screws in metal, it will not be found to give such good results when wood is the material to be operated upon; this may be accounted for by reason of the high speed required to make a clean job with wood in a lathe, which is altogether incompatible with the working of the gearing necessary for cutting screws, at least of such fast pitches as are usually required for worms. Besides, special tools must be made for use in the lathe, conforming to the shape of the tooth; for a worm is really one long tooth wound about a cylinder. There are a few other minor difficulties attending the cutting of a wooden worm in a screw-cutting lathe; and when all are considered, it is doubtful if there is much gain over the old-time hand method. We will, however, describe both:

Let Fig. 209 represent the complete pattern. To make it in either way, take two pieces, cach to form one half of

Fig. 209.

the pattern; peg and screw them together at the ends, an excess of stuff being allowed at each end for the accommodation of such screws or dogs, if the latter are more convenient, as Turn the piece down to

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they might be in a large pattern.
the size over the top of the thread, after
which the prints, P P, are turned.
posing it to be determined to cut the thread
in a lathe, we must have ready a few tools
adapted for the work; the first of which is
the parting tool, very similar to a parting
tool for brass, Fig. 210-namely, flat and

Fig.210.

level on the cutting face, but with a great deal more bottom rake, as strength is not so much an object, and the

tool is more easily sharpened. We have also in addition a little projection, like the point of a penknife, formed by filing away the steel in the center; these points are to cut the fibres of the wood, the severed portion being scraped away by the flat part of the tool. We must not forget to give a side rake to the tool corresponding to the pitch we have to cut; and the width of the tool is to be a shade narrower than the space in the worm at the narrowest part, which is generally at the root of the tooth. Having suitably adjusted the change wheels to the pitch required, we drive down the parting tool until the leading points are on a level with what is to be the bottom of the spaces; a parting tool without cutting points is now adjusted, and the space made of the required depth. We now have cut a worm with a square thread; and it remains to finish to the required form of tooth. To do this, some have essayed a tool such as shown in Fig. 211; but this will not work, for the reason that it is end wood which we have to cut. Were we cutting across the grain-as, for instance, in making the groove with the parting tool— then the one shown in Fig. 211, which is nothing but a scraper, would act very well. The tool shown in plan and section, Fig. 212, has a keen edge imparted to it by piercing a hole through the steel and filing to a bevel; it must Fig. 212. then be nicely oilstoned. The only ob

Fig. 211.

jection to this tool is the difficulty of sharpening it. We ought not to suffer both sides of the tool to cut at once; in fact, the tool itself should not be made quite so wide as the space it has to finish. Furthermore, if the pattern is very large,

it will be necessary to have two tools for finishing-one to cut from the pitch line inwards, and the other to complete