Q: As a carpenter, I occasionally need to create elliptical shapes for things such as door transoms. I've tried the nails-and-string method, but driving nails isn't always an option. Is there another method I could try?
A: Mike Patterson, owner of Patterson Builders and Remodelers, in Gaithersburg, Md., responds: I can never seem to remember the nails-and-string method, so I use a technique that I learned back in Cub Scouts that uses a series of straight lines to make an elliptical shape.
I first establish the height and width of the elliptical shape I need (which for a door transom with an elliptical arch would actually be just half of an ellipse). I lay out the elliptical shape on a piece of cardboard, plywood, or whatever works for the size I need, typically drawing only half of the arch, and then reversing that piece to trace the other half.
To create the ellipse, I start by drawing an "L" shape, where the vertical leg equals the height of the elliptical arch, and the horizontal leg represents half of the ellipse's width. Next, I divide each of those lines into the same number of segments. The more segments I use, the more accurate the shape will be (see illustration, below). Then I number the segments "1" through whatever number of segments I've decided to use. On the horizontal leg, the numbering begins at the mark closest to the intersection of the two lines. On the vertical leg, though, I start the numbering at the mark farthest from the intersection.
I connect the same numbers on the two legs—1 on the vertical leg to 1 on the horizontal leg, 2 to 2, and so on—with straight lines. Joining the points creates something that looks like a spider web, with the straight lines intersecting and overlapping to form short straight segments of the ellipse's circumference. With a pencil, I fair the segments into an elliptical curve, and then cut out the shape. I use this pattern to mark one side of the arch and simply flip it over to mark the other side.