A. An eccentric lathe works on the same principles as an ellipsograph(also called a trammel).
The farthest point (x) the pencil can reach up and down is governed by the pivot riding in the horizontal track.
The farthest it can reach left or right (y) depends on the setting of the pivot in the vertical track.
The path of the pencil between these extremes is governed by both pivots to produce one-quarter of the ellipse.
B. If the arm is locked in place and the base plate turned, a pencil held at a point above the
base plate will draw an ellipse on the plate. This is the basic geometry of an eccentric lathe, but it requires some ingenuity to transmit the power.
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