The engineer has the responsibility for the design of an involute gear set, and the tools that are used to cut the teeth in the gear blanks. If general, if you scan down the summary sheet, the diametral pitch (DP), pressure angle (PA) and base pitch (BP) are the same on the information sheet for the pinion, gear and cutter. However, this may not be the case. The only number that must be the same is the base pitch (BP). The DP and the PA can be different. The most frequent use of pitch and pressure angle changes in the cutter is to increase the root radius in small numbers of teeth. The operation is known as “short leading.”
Base Pitch = Circular Pitch x Cosine of the pressure angle (PA) Figure 1
Base Pitch (BP) equals the arc distance along the circumference of the base circle between two adjacent profiles running in the same direction. A constant Base Pitch (BP) can be delivered by changing the circular pitch and cosine of the pressure angle. Changing the number of teeth can also change the circular pitch. Table 1, Table 2
When using alternate cutter, check the results of the cutting of the outside diameter and root angle. Another benefit of base pitch is an index. Prototype gear manufactures and small gear shops use the Base Pitch (BP) as a method to control inventory. Base Pitch (BP) is the one value that cannot be changed between the gears in a mesh and the cutting tools.