By employing a pressure angle larger than standard, it is possible to use fewer pinion teeth, and

Question:

By employing a pressure angle larger than standard, it is possible to use fewer pinion teeth, and hence obtain smaller gears without undercutting during machining. If the gears are spur gears, what is the smallest possible pressure angle φt that can be obtained without undercutting for a 9-tooth pinion to mesh with a rack?