Fig. 33 shows a bell crank fastened to its shaft by means of a set screw, the same general features being noted in this as in the preceding figure. A further point is the method of expressing the distance between the faces of the principal hub and the smaller hub, "10" less 1/64" ". This method of stating a dimension is quite common among certain manufacturers, as it saves giving odd dimensions and conveys more quickly to the workman's mind what the dimension is. The other method of stating this would be "9 63/64"", which is obviously a somewhat cumbersome and odd dimension; it is easier for the machinist to read 10" on his scale and finish the distance 1/64" less, than it is to use the actual figure. This point is an instance in which the instructions furnished by the drawing to the workman are simplified for his benefit.

Method of Showing Dimensions on Detail Drawing.

Fig. 33. Method of Showing Dimensions on Detail Drawing.

Another point worth noting in this figure is that circles are dimensioned by giving the diameters in preference to the radii; this is for the benefit of the pattern maker and the machinist, who always use calipers for measuring these parts. When the radii are given, the workman is forced to multiply the radius by two in order to secure the dimension for his calipers; and it is always better to remove the chance of error on the part of the workman in the shop when making mathematical calculations. A detailed drawing should be so completely dimensioned that there will be no occasion for the workman to make any calculations himself; for, even if he is competent to do it, the responsibility for the correctness of the figures should be on the draftsman. In practically all shops the workmen are not allowed to scale the drawings in case dimensions are lacking, but are required to go to the drafting room and have further dimensions put on as required. A good detail drawing should require no such additions.