Recently I was working on a SketchUp model of Thomas Throop’s Oval Side Table for plans. This table has an elliptical top and shelf; a fairly basic shape that is easy enough to model in SketchUp although it might not be all that obvious. Although there are some extensions that will draw them for you I usually just use the Circle and Scale tools to do this. But when I need to show the ellipse in a plan I like to include the foci with dimensions to make it easy to layout in the shop. If you know the distance between the foci nails driven into the pattern board, a loop of string and a pencil will then make quick work of drawing the ellipse. The trick is figuring out where the foci need to be. You could calculate the locations but there are easier ways. In this video I demonstrate doing it with an Ellipse plugin from Sketchucation.
It’s worth noting that the length of the string from foci to edge to foci is the same as the major dimension of the ellipse.
You could locate the foci by construction with the Arc tool, too. Draw arcs from the middle of the side (there should be a vertex there) with the radius equal to half the major length. The arcs will intersect the centerline at the foci.
Give it a try and see how you get on.
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