In this special class of pattern work, the greatest accuracy and care must be taken, not only in building up the rim of the wheel, but in fitting and placing on the rim the blocks, out of which the teeth are to be formed, and most of all in laying out the teeth regularly and, accurately on the tooth blocks. A pattern for a gear wheel, whose teeth are carelessly made is almost worthless, the time lost in chipping and filing, for the purpose of correction, being too great to allow the use of such a pattern.
This cast pattern is turned up and placed in the milling machine where the teeth are cut and spaced with accuracy and to the exact form of tooth required. This metal pattern is used without draft.
This method of making gear patterns, however, is expensive, and is used only when many wheels are to be cast of the same size and number of teeth from the same pattern, and, as in the case of pulleys, the wooden pattern is still used for all special sizes of gears.
For these wooden patterns we shall now give a few hints as to the best methods of construction. As the form of the tooth used by the draftsman will play no part in the construction of the pattern, we think it would be out of place here to enter into a discussion of the relative merits of the single curve, double curve, or other form of tooth.
The single curve or involute tooth, however, has the great advantage of being the only form of gear which can be run at varying distances of axes, and transmit an unvarying velocity and amount of power. The common contention that two gears will crowd harder on their bearings when the single curve, or involute form is used has not been proven in actual practice. The practical methods for obtaining the curves for either the involute or for the epicycloidal tooth, the two forms in most common use. are taken up in Mechanical Drawing.
In the illustrations here given the single curve form of tooth is used.
In the construction of gear-wheel patterns, the methods employed in making and fastening the tooth, or the blocks out of which the teeth are to be formed, to the rim of the wheel, varies greatly. It was formerly the custom to dovetail the tooth into the rim of the wheel as shown in Fig. 236 This was the case especially when the teeth were large, as in 2 pitch or larger.
This is, however, an unnecessary expense and a waste of time, and in addition, the cutting of the dovetails and the driving home of the dovetailed tooth, often have the effect of distorting the rim to some extent. A better, or at least a more Economical method is to fit the tooth blocks as shown in Fig. 237, which for strength and durability is found to be in no way inferior to dovetailing, and the saving of labor and time is very great.
In this method we have always the advantage of a smooth, clean fillet at the root of each tooth, and having the grain of the wood, not only for the fillets, but also on the whole depth circle, run in the same direction as the grain of the wood which forms the tooth. This means a smoother pattern, more easily moulded, and a better casting.
In the former method, Fig. 236, it is almost impossible to form a fillet on each side of the tooth, as it runs off to a thin feather edge which continually splinters and chips off; still further, the bottom of the tooth space, that is, the whole depth circle is the rim of the wheel, composed of layers of segments with changing grain which will not mould so smoothly as in the second method.
The blocks for the teeth should always be cut in strips two or three bet in length, in order to thoroughly season the wood, at least so far as it is possible to do so, while other parts of the wheel are being constructed.
Only straight-grained wood should be used for teeth. The segments for building up the rim should be cut out next, then the arms put together and shaped as required. It is a good plan to fasten the arms central to the face plate of the lathe, and to turn out a recess, Say 1/16 inch or 3/32 inch deep to receive the hubs as shown in Fig. 238. This makes a strouger connection and does away with the trouble of fitting and connecting the hub. with the thin feather edge of the hub fillet, to the surface of the web of the arms. The same method is of great adtantage when fitting the-hsite of pulleys and other wheels. The arms mussi be put together receive these ends are now cut into this half rim, and the arms fitted and glued in place but not bo tightly as to strain the rim and cause it to spring after it is removed from the chuck. The remaining courses for the rim are now fitted and glued on, and the rim turned and finished to the required size and shape.
The face should be glue-sized to prepare it for the blocks which are to form the teeth of the gear.
After sizing and removing the raised grain of the wood, the periphery of the wheel must be spaced for the required number of teeth. With a try square and very sharp awl draw lines through the points obtained by the spacing as shown in Fig. 240. Should the teeth be of moderate size, say 3 pitch or less, the tooth blocks should be glued on so as to meet each other on the rim of the wheel as shown in Fig. 241.