On pinions with few teeth, the tip of a generating rack or hob cuts into the lower flank and removes part of the involute it just formed, leaving a weak, waisted root. With a 20 degree standard full-depth rack it starts below about 17 teeth. Undercut shortens the active profile, cuts contact ratio and weakens the tooth, and is avoided by positive profile shift, a higher pressure angle or more teeth.
Ten-tooth pinion with no shift, of course it's undercut, you can see the waist at the bottom of every tooth.
Seventeen teeth is the textbook limit at 20 degrees, but designers often accept 14 with a little undercut or add shift instead.
Also heard as
- tooth undercut
- undercutting
Related terms
Profile shift
Moving the cutter in or out from the standard position, making teeth fatter or thinner without changing pitch.
Pressure angle
The angle between the line of tooth contact force and the tangent to the pitch circle, usually 20 degrees.
Pinion
The smaller of two meshing gears, usually the driver in a speed reducer.
Contact ratio
The average number of tooth pairs in contact at once, which largely sets how smoothly and quietly a pair runs.
Same word, different worlds
A recess deliberately machined below finished size so the coating can fill it to full thickness.
Welding, Brazing & NDEA notch the arc melts into the base metal beside the weld toe that the weld metal never fills back in.
GeosyntheticsDigging out soft or unsuitable subgrade below grade and replacing it with better fill.