公法线长度是指与两个异侧齿面相切的两平行平面间的距离。对于外齿轮，指相隔若干个齿的两外侧齿面各与两平行平面中的一个平面相切，此两平行平面之间的垂直距离。对于内齿轮，指相隔若干个齿槽的两外侧齿面。
这篇文章提出了公法线长度并推导出超前角的计算公式。
In this paper base tangent length is put forward and lead angle calculating formula is deduced.
此误差可通过在公法线长度方向上加大留磨量，进行磨齿予以消除。
The error can be avoided by increasing the gear grinding allowance in length of common normal line to grind gear.
经过严密计算和实践反复验证，用公法线长度测绘法确定齿轮模数可满足要求。
After iterative and rigorous practice, it turned out that it can meet the demand when using length of common normal to measure gears modulus.
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