There is no common normal in the conical involute gear and the fixed chordal tooth thickness can not be measured accurately because of the coexistence of helix angle and taper.
渐开线锥形齿轮由于同时存在锥度和螺旋角，使得其固定弦齿厚无法精确测量，也没有公法线。
2
There is only approximate way to work out the normalchordal tooth thickness of worm and wormwheel at present.