Fig. 356.
In charging the wheel with diamond dust it is necessary to use a roller shaped as in [Fig. 356], so that the axis of the roller r and wheel w shall be at a right angle, as denoted by the dotted lines. If the roller is not made to the correct cone its action will be partly a rolling and partly a sliding one, and it will strip the diamond dust from the wheel rather than force it in, the reasons for this being shown in [Figs. 57] and [58] upon the subject of bevel-wheels.
Fig. 357.
Taps for lead and similar soft metal are sometimes made with three flat sides instead of grooves. The tapping holes may in this case be made of larger diameter than the diameter of the end of the tap thread, because the metal in the hole will compress into the tap thread, and so form a full thread. Taps for other metal have also been made of half-round section. [Fig. 357] represents a tap of oval cross section, having two flutes, as shown, but it may be observed that neither half-round nor oval taps possess any points of advantage over the ordinary forms of three or four fluted taps, while the former are more troublesome and costly to manufacture.
When it is required to tap a hole very straight and true, it is sometimes the practice to provide a parallel stem to the tap, as shown in [figure] at c. This stem is made a neat working fit to the tapping hole, so that the latter serves as a guide to the tap, causing it to enter and to operate truly.
