Fig. 37 shows a piece designed to receive a threaded rod at one end and to clamp rigidly to a shaft by means of a bolt at the other. This detail, simple as it appears, is awkward to make, on account of the bolt boss being at an angle with the principal center lines. The lower view is a cross section, because, if the ordinary elevation were shown, it would have a series of ellipses showing the bolt boss projections. It is always desirable to avoid oblique projections of circular shapes on account of the difficulty of drawing same; moreover, the ellipses produced do not show the construction as plainly as a straight projection. The method adopted in such cases is to throw off a straight projection at the same angle as the part in question makes with the principal center lines. In the present instance it is necessary to show that the boss for the bolt is 3/8" radius, and that the boss is centrally located with the hub, which is readily done by the straight projection thrown off. In the cross-sectional view, the lines representing the thread appear to the eye sloped in the wrong direction, or as though the thread were left-hand. A moment's thought, however, will convince the student that, since the section taken is through the middle of the hole, we are merely looking at the back side of the hole, and that the threads of a right handed screw on the back side must necessarily slope in the direction as shown. In the case of the thread on the bolt for the clamping hub, shown dotted, the lines of the thread appear right-handed to the eye, it being universal practice in the case of dotted threads to show the side only next the eye. If the threads on the back side of the bolt were also shown, they would slope in the other direction, crossing the other lines, and to draw them in would obviously cause confusion.

Fig. 37. Detail Drawing with a projection Thrown off at an Angle with the principal Centre Lines.