DIAMETRAL` PITCH. This method of denoting the size of gear wheels is based upon the following theory: The diameter of the wheel at the pitch circle is supposed to be divided into as many parts as there are teeth in the wheel; then the length of one of these parts is the diametral pitch. Suppose we are given a diametral pitch of 2. To obtain the corresponding arc pitch we divide 3.1416 (the relation of the circumference to the diameter) by 2 (the diametral pitch) and 3·1416 ÷ 2 =1·57= the arc pitch in inches and decimal parts of an inch. If we are given an arc pitch to find a corresponding diametral pitch, we again simply divide 3.1416 by the given arc pitch. Example: What is the diametral pitch of a wheel whose arc pitch is 14 inches? Here 3·1416 ÷ 1·5 = 2.09 diametral pitch. Below is BROWN & SHARPE's table of circular and diametral pitches, which will be found very useful. In using diametral pitch to order wheels it is sufficient to employ two places of decimals; but where mathematical calculations are concerned it is better to use three places of decimals. TABLE AND RULE, FOR THE COMPUTATION OF THE SIZES OF TAPPING HOLES FOR The outside diameters are from Morris and Tasker's table of standard sizes. The taper used in calculating is that given by Pratt and Whitney, 1 inch to foot, and the length of threads on to § is inch, ↓ to 1 is 4, 1 to 2 is, and 2 to 4 inches is 1 inch. The rule for computing size of drills is: Subtract from the outside diameter (after allowing for the taper) the product of the pitch by 1.732, which gives the diameter at the bottom of the thread, or the size of the required drill. |