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INVOLUTE OF A CURVE, OF A DEVELOPABLE SURFACE


Notion studied by Monge in 1771.

 
If M0 is the current point on (G0), the current point M of an involute of this curve is the locus of the points ;
Cartesian parametrization: .
Radius of curvature: .

I) Involutes of a curve.

The involutes of a curve (G0) are the trajectories described by the points of a line rolling without slipping on this curve.

Therefore, they trace, on the tangent developable (S0) of (G0), the orthogonal trajectories to these tangents; they also form an equivalence class of parallel curves.
They are also the curves for which the initial curve is an evolute.

When the surface (S0) is developed on a plane, the 3D involutes become the 2D involutes of the image of (G0) on this plane.

An involute is planar iff the initial curve is a helix. In this case, all the involutes are planar.
 

II) Involutes of a developable surface.

The involutes of a developable surface (S0) are the trajectories of a point on a plane rolling without slipping on the surface (S0); therefore, they are the curves for which the initial surface is the polar developable. They present a cuspidal point on (S0), with a tangent that is orthogonal to (S0); and they form an equivalence class of parallel curves.

When (S0) is a cylinder, the involutes are the involutes of the planar sections perpendicular to the axis.

When (S0) is a cone, the involutes are traced on spheres centered on the vertex of the cone: they are the loci of a point on a circle centered on the vertex of the cone rolling without slipping on an orthogonal trajectory of the generatrices of the cone.
 
In the case of a cone of revolution, they are spherical helices.

Problem: how to derive the parametrization of the involutes of (S0) from the parametrization of its cuspidal edge  ????
 
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© Robert FERRÉOL  2018