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  Working with the Ellipse  
By Mike Sloggatt

When working with circles--of ANY kind, always remember


An ellipse is defined by the Major Axis (or total run), and the Minor Axis (or total rise)
The Semi-Major Axis is half the Major Axis
The Semi-Minor Axis is half the Minor Axis

The Semi-Major Axis plays a critical role in shaping an ellipse.

The shape of an ellipse is determined by the Semi-Major Axis (half the length of the run or span).
First, determine the rise of the ellipse desired.
Next, using the length of the Semi-Major Axis, swung from the top of the ellipse, strike two intersection points along the Major Axis.

These are the Focal Points.

Use a thin wire to draw an ellipse. Single-strand picture-framing wire works best.

1. Drive a screw or nail into each end of the Major Axis.
2. Stretch wire from screw to screw, tightening the wire securely on each screw.
3. Loosen the screws and insert them at the Focal Points.
4. Pull the slack up to the top of the ellipse and use a pencil to scribe the shape.


The next time you have to cut a hole in the roof for a piece of pipe...especially if the hole needs to be finish-carpentry quality, use the string method to layout the ellipse.

Click here for a Sketchup Drawing & Tutorial

Using a string isn't the only way to layout or cut an ellipse. Here's the trammel arm I use, with my router, for cutting elliptical shapes.
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