20 degrees vs 14.5 degrees pressure angle for gears

It's not as simple as simply choosing the correct cutter. It is not possible to cut proper straight tooth bevel gears using involute cutters. However, there are two approximate methods.

Straight tooth bevel gears are normally designed using the diametrical pitch on the outer edge. An approximation to the tooth form is created in three passes, one normal pass and a two passes offset slightly in either direction. The involute cutter will correctly form the curve on the outer end, but the curve at the inner end is incorrect. This can be corrected with a file, or by running in with a mating gear. For a straight tooth bevel gear the equivalent number of teeth, used to select the cutter number, is the actual number of teeth divided by the cosine of the pitch cone angle. This tends to make the teeth shape more rack like. The problem with this method is that the involute cutter needs to have the correct form for the selected DP, but needs to be narrower than standard so it will pass through the small end. Cutters of this type used to be available commercially, but are now obsolete and pretty much impossible to source.

The alternative method, getting around the problem of a special cutter, is the parallel depth method. This still requires three passes of the cutter, but the gear is designed using the diametrical pitch at the inner edge of the gear. So a standard involute cutter can be used. The problem with this method, for an existing application, is that the diametrical pitch at the outer edge is unlikely to be a standard value. In which case the gears will not fit without a lot of faffing about with DP and tooth counts. Even then some changes to the design will be needed.

Andrew