Fig. 126.
Another form of template much used is shown in [Fig. 125]. The curves a and b are filed to the curve produced by rolling segments as before, and the holes c, d, e, are for fastening the template to an arm, such as shown in [Fig. 126], which represents a section of a wheel w, with a plug p, fitting tightly into the hub h of the wheel. This plug carries at its centre a cylindrical pin on which pivots the arm a. The template t is fastened to the arm by screws, and set so that its pitch circle coincides with the pitch circle p on the wheel, when the curves for one side of all the teeth may be marked. The template must then be turned over to mark the other side of the teeth.
The objection to this form of template is that the length of arc representing the pitch circle is too short, for it is absolutely essential that the pitch line on the template (or line representing the arc of the addendum if that be used) be greater than the width of a single tooth, because an error of the thickness of a line (in the thickness of a tooth), in the coincidence of the pitch line of the template with that of the tooth, would throw the tooth curves out to an extent altogether inadmissible where true work is essential.
Fig. 127.
To overcome this objection the template may be made to equal half the thickness of a tooth and its edge filed to represent a radial line on the wheel. But there are other objections, as, for example, that the template can only be applied to the wheel when adjusted on the arm shown in [Fig. 126], unless, indeed, a radial line be struck on every tooth of the wheel. Again, to produce the template a radial line representing the radius of the wheel must be produced, which is difficult where segments only are used to produce the curves. It is better, therefore, to form the template as shown in [Fig. 127], the projections at a b having their edges filed to coincide with the pitch circle p, so that they may be applied to a length of one arc of pitch circle at least equal to the pitch of the teeth.
The templates for the tooth curves being obtained, the wheel must be divided off on the pitch circle for the thickness of the teeth and the width of the spaces, and the templates applied to the marks or points of division to serve as guides to mark the tooth curves. Since, however, as already stated, the tooth curves are as often struck by arcs of circles as by templates, the application of such arcs and their suitability may be discussed.
Marking the Curves by Hand.
In the employment of arcs of circles several methods of finding the necessary radius are found in practice.