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Q. Is there a linkage in the plane that traces out a circle $C$ in such a manner that the interior of the disk bounded by $C$ is never intersected by any link througout the motion?

What I mean by "a linkage in the plane" is best illustrated by the famous Peaucellier linkage, which traces out a straight line segment: (Wikipedia image.)
         WikipediaPeacellier
Some vertices are "pinned to the plane." All vertex joints are universal in the sense of allowing full $360^\circ$ motion. All links are rigid segments, which can pass over one another in a physical, layered model, e.g.,
         HowRound
          (Image from this How Round Is Your Circle web page.)


Of course it is trivial without the restriction that the links not intersect the interior of $C$: One radial link pinned to the center of $C$ suffices. And the challenge in the Peaucellier linkage was to convert the natural circular motions of linkage components into straight-line motion. Here I am seeking a vertex of the linkage to follow a natural circular motion, but with the restriction to not intersect the interior of that circle $C$. Following a subarc of $C$—say, a semicircle—without intersecting the interior of $C$, would also be quite interesting.

It may be that the 19th-century masters (Peaucellier, Lipkin, Watt, Chebyshev, et al.) did not investigate this question. But perhaps there is an easy construction I am not seeing...?


Here is a version of TMA's idea. The pantograph shown scales by $\times 2$: As joint $x$ traces arc $A$, endpoint $y$ traces arc $B$, from the exterior of $B$'s disk:
         PantographArc

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    $\begingroup$ what comes to mind is lens polishing, the surfaces of lenses for telescopes having generally had a spherical face. $\endgroup$
    – Will Jagy
    Commented Feb 8, 2014 at 1:31
  • $\begingroup$ @WillJagy: Cool connection, Will! With that hint I found several patents for lens polishing mechanisms, each impenetrable to me (although with limited time investment). $\endgroup$ Commented Feb 8, 2014 at 2:00
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    $\begingroup$ I think if you go back far enough in history there will be something understandable. Note that the ideal face of a lens with flat on the other side is actually a hyperboloid of revolution, at least for one fixed wavelength of light. For parabolic mirrors, one thing they are doing is making giant rotating baths of mercury, to get a perfect parabolic surface for large optical telescope. en.wikipedia.org/wiki/Liquid_mirror_telescope $\endgroup$
    – Will Jagy
    Commented Feb 8, 2014 at 2:16
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    $\begingroup$ You say "Following a subarc would also be interesting." But that is what Peaucellier linkage does if you move the fixed end of blue segment a bit from the other fixed join (the end of two green segments). [On the second picture you need to choose the hole on the right.] $\endgroup$ Commented Feb 8, 2014 at 5:47
  • $\begingroup$ @AntonPetrunin: You are right, just drawing an arc from its exterior is not that difficult or interesting; my misjudgement. But drawing a large portion, or all of a circle, still seems challenging. $\endgroup$ Commented Feb 8, 2014 at 21:36

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I don't have a facility with graphics; hopefully the verbal description below will work.

There is a linkage that magnifies: place a tracing stylus at point P, a marking stylus at point Q, and this linkage M will allow one to trace a figure and replicate it with a growth factor of (say) 3.

Now take M and attach it to a pivoting link with the free end fixed to P in such a way that Q traces an arc. One can get at least 120 degrees of arc this way, and arrange it so the arc is convex from the side of the combined linkage. For a full circle, a link arm has to be transported around a point and be rotated at least 180 degrees during this journey. I am confident a linkage can be designed, but have no ideas for it at present.

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    $\begingroup$ My mental picture suggests 7 links, 2 fixed pivots and six nonfixed. If one "fully extends" the linkage, one can approach 180 degrees of arc from below. I don't see how to extend this to get past 180 without crossing the interior. $\endgroup$ Commented Feb 8, 2014 at 5:09
  • $\begingroup$ Also, the linkage I picture is like that of Peaucellier, with the internal three links reversed as well as shortened, and attached to the middle of the two long links. $\endgroup$ Commented Feb 8, 2014 at 5:13
  • $\begingroup$ Very nice idea! I've illustrated a version of it, using a pantograph as the enlarging mechanism. I did not try to compute (or illustrate) the largest arc angle achievable. $\endgroup$ Commented Feb 8, 2014 at 14:42
  • $\begingroup$ Thank you for this solution, I have been looking for a mechanism like this. However, how would one go about building this to be 10x instead of 2x for example? I can not scale it for some reason, must be missing something. $\endgroup$ Commented Sep 28, 2022 at 20:45

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