8o PATTERN-MAKING prints must be cored over, and the mould and cores be coveted with a plain top. Fig. 49 shows the provision made for the rim. A short length of sweep has a half-arm boss with its half print, covered with a block print. Into the impression made by this the core, rammed in the box (fig. 50), is set. Taper is given, as shown, to the sides of print and core. The central boss that corresponds with this type of wheel is shown in fig. 51. This is rammed in a parted box distinct from the rim, having the joints separated for the insertion of the arms, and is dried. The boss mould is centred relatively to the rim, and levelled before the arms are inserted. These are then covered with the top half of the boss mould. 3. GEAR-WHEEL PATTERNS Although this department of work has been deeply invaded by the insistent demand for cut gears, a very large volume remains. Wheels with cut teeth are expensive, and they are not usually found in common machines, such as ordinary cranes, contractors' machinery, and the like. Another important fact which favours the retention of cast gears is that the patterns now made are far superior to those of some years ago. A high grade of work- manship has been demanded and met, partly due to the employment of machines for cutting pattern teeth, and partly to the fact that firms make these for the trade, the pioneers being Messrs. Ernest M. Brown & Co. of Huddersfield. And Wadkin & Co. of Leicester have revolutionized the methods of some shops by the introduction of the " Mechanical Wood-worker " in core-box work, and in the teeth of gear-wheel patterns. In the general shops these patterns are the speciality of one or two only of the hands. Tooth Forms.—It is essential that the teeth of all wheels of the same pitch shall be made to a correct contour, so as to secure a rolling contact as far as may be and a uniform velocity-ratio. In cycloidal or double-curved teeth this is secured by making the diameter of the rolling circle, to be rolled on the pitch circle, equal to the radius of the smallest wheel of the series. This gives radial flanks for the smallest or basic pinion, and undercut flanks for those below that size. This is embodied in an odontograph scale. For involute or single-curved teeth, which have been largely substituted for cycloidal, the basis is the rack, having teeth with straight, sloping flanks. The point of contact of the teeth lies on the line passing through the point of contact of the pitch circles and tangential to the base circles. In the cycloids, curves are generated from the pitch circle; in the involutes, the pitch circles have but an arbitrary relation to the base circles. This explains why correct tooth contact occurs whether the ideal pitch cylinders are or are not in contact, and why, by increasing the addendum in small pinions, under- cut of the teeth can be avoided. The circular pitch is most generally used for pattern gears, but the diametral is commonly associated with the involutes.