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Designing external spur gears is such a number-crunching process that designers often specify only a couple gear qualities for the manufacturer. There are times when designers would like to visually inspect a tooth shape before handing the design to production. But without special gear software, drawing an accurate tooth is a tedious process.
The cost for programs that can draw gears along with tooth modifications such as tip relief, full-root radii, or long and short-addendum profiles often exceed the gear designer's budget. However, with CAD software and the following procedure, designers can quickly and accurately "hob" a gear in much the same fashion as once was done on drawing boards with templates. The box Frequently needed gear tooth qualities, presents variables that an engineer must specify and those that must be calculated.
After drawing the gear, the designer can use it for checking clearance and mesh action with a mating gear or clearance with the gear housing. The design can generate solid gear models for determining surface area, volume, weight, and for making assemblies. The tooth profile can also be downloaded to a CNC wire-EDM machine for cutting prototype steel gears, prototype injection mold cavities, or powder-metal dies.
For this exercise, it's necessary to draw a gear with N = 18, P sub d = 16, pressure angle phi = 20deg, and with an AGMA standard rack form for coarse-pitch gears.
Calculate gear qualities first, such as standard tooth thickness
T sub std = pi/(16 X 2)
= 0.09817.
Find the pitch diameter
D = 18/16
= 1.1250 in.
Find the outer diameter
D sub o = (18+2)/16
= 1.2500 in.
Find the...