Fig. 39 shows a gear with a split hub, the bolts through the hub being for the purpose of tightly clamping same to the shaft. This is an illustration of the conventional method of showing a gear with standard proportions of teeth. If the drawing were made exactly as the gear would look, it would be necessary to spend a large amount of time inking in the outlines of the 72 teeth around the circumference of the gear; instead of doing this, the pitch line of the gear is shown, and circles drawn indicating the top and bottom of the teeth. The pitch diameter and outside diameter are given, and the proper depth of tooth cut to be made by the gear cutter; nothing more is necessary.
Fig. 39. Detail of Gear with Split Hub.
The information which enables the workman to choose the proper cutter is given in the title; thus, "Cut 72 Teeth - 1 Diametral Pitch - Involute." The workman will receive from the tool room a standard involute cutter marked for 72 teeth, 1 diametral pitch. After placing same on the arbor of the gear-cutting machine, he will drop the cutter into the gear blank to the depth called for by the drawing; if this is accurately done, it will give the desired standard thickness and clearance of the teeth, and the automatic indexing of the machine will produce the 72 teeth, accurately spaced. If the gear were a cast gear - in which case the pattern maker would have to lay out the teeth and finish them one by one on the pattern - it would be necessary to draw one or two teeth on the circumference of the gear, and give sufficient figures to enable him to lay out the teeth either by his dividers or by a template. In neither case, however, should the detail draftsman take the time to draw the teeth in any great number around the circumference of the gear; this is a very common thing for the young draftsman to do, and nothing shows his greenness more. He should bear in mind that when the proper instruction has been conveyed by his drawing, any more time spent on it is wasteful.
Fig. 40. Detail of Gear with Solid Hub.
The cross section of the gear is shown to enable the necessary figures to be placed upon the rim and hub. In the right-hand view, only one-half the complete circle of the gear is shown; nothing would be gained by showing the other half, therefore it would be a waste of time and space on the drawing to make it.
Attention is called to the method of indicating the dimensions of the arms, the breadth and thickness of the oval being indicated as follows: "6¾" X 2¼""; this is a "short cut" which will usually answer, but it does not actually show the section of the arm. It would be better actually to make a cross section, as shown in the lower portion of the right-hand view, giving the actual radii for the section, as otherwise the pattern maker might make an arm more or less blunt on the ends than the draftsman intended.