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Full text of "Modern Mechanical Engineering Vol-I"

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.