It is an application of ellipse mechanism (two green sliders and a blue conrod). A grounded point traces on the conrod a curve, a portion of which has large radius. For full curve see the green one shown in
http://youtu.be/M3hlMN--gAg
Yellow workpiece is fixed to the conrod at the said portion.
The obtained profile is not absolutely circular.
Angle between two grey runways can be adjusted for various radii of the profile.
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