以下摘自网络:
(ANSI花键标准中对form circle的说明)Form Circle is the circle which defines the deepest points of involute form control of the tooth profile. This circle along with the tooth tip circle (or start of chamfer circle) determines the limits of tooth profile requiring control. It is located near the major circle on the internal spline and near the minor circle on the external spline.
渐开线终止圆：渐开线终止圆是用于控制齿廓上渐开线的极限距离的圆。渐开线终止圆与齿顶圆（或者修缘线的起点所在的圆）共同限制了所要求控制的渐开线齿廓的范围。渐开线终止圆的位置靠近内花键的大圆（齿根圆）或者外花键的小圆（齿根圆）。
译者注：这段话的意思是：只有在渐开线终止圆与齿顶圆（或者修缘线的起点所在的圆）之间的花键的齿形为渐开线齿形，其余部分不一定按照渐开线成形。
译者注2：在《GB/T 3478.1-1995 圆柱直齿渐开线花键》中，将Form Cycle定义为两个圆：内花键的渐开线终止圆和外花键的渐开线起始圆。而在本标准中，将这两个圆统称为Form Cycle，译者将Form Cycle翻译为渐开线终止圆。