Ruling line from edge curves twist check

Method for checking for twist along a ruling line between two curves, such as a ruling line intended for a developable surface:

  1. Draw a line tangent to one curve at the end of the ruling line. I use Line with Tangent and then PointOnCurve options. The lengths of the line is not important.

  2. Create a planar surface from the ruling line and tangent line. I use Sweep 1 or ExtrudeCrvAlongCrv.

  3. Repeat steps 1 and 2 with the second curve at the other end of the ruling line to create a second planar surface…

  4. Measure the angle between the planar surfaces. I use Angle with the TwoObject option. If the angle is zero (or 180) then there is not twist.

Example file: RLC 01.3dm (54.8 KB)

Methods for checking twist along the ruling lines of a ruled surface are discussed in Verifying developable surfaces

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Hi David - Projecting both tangent lines to a CPlane normal to the ruling line and then Angle seems like a less painful process to get that.


That looks more “painful” to me. Your method requires the additional step of setting the CPlane normal to the ruling line, and substitute two uses of ProjectToCPlane for two uses of Sweep1 or ExtrudeCrvAlongCrv. Both methods require creating the two tangent lines and measuring an angle. My preference is not to mess with changing CPlanes unless needed.

OK, well, either way, in case it helps speed things along, here’s a python that does it all in one go - seems to work so far here. (1.4 KB)

To use the Python script use RunPythonScript, or a macro:

_-RunPythonScript "Full path to py file inside double-quotes"


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Thanks. I’ll try it.