In this paper calculating method of lead angle of helical gear by measuring span lengthoflength bar and base tangent length is put forward and lead angle calculating formula is deducel.
At any knots, the nearly arc length are precise for the resulting nearly arc length parametric curve, and it has the unit tangent same as that of the real arc length parametric curve.
所作近似弧长参数化曲线在插值节点处，近似弧长是精确的，并且具有与精确弧长参数曲线同方向的单位切矢。
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This paper presents two kinds of new methods for setting out circular curve: free angle and tangentlength method.