The most precise way to cut an octagon is to use a mathematical formula and a special jig. The jig guides a table saw to rip off the corners of a square piece of wood, resulting in a perfect octagon. Here are the steps involved in using the formula and making and using the jig. Materials:
Two 5" x 1" x 1" pieces of hardwood
10" x 5" x 1" plywood
Drill
Screws
Framing square or other tool to measure angles
Table saw
Work piece
- To make the jig, cut one end of both of the 5" x 1" x 1" strips of hardwood at a 45-degree angle. Screw the strips of hardwood onto the plywood so that they form a 90-degree angle (figure A). Use a framing square to measure the angle, making sure it is exactly 90 degrees (figure B).
- Determine the desired length of the finished octagon (O). Cut the work piece in the shape of a perfect square, with the length of its sides (S) determined by the following formula: S = 2.414 x O. Cut the square S inches on each side.
- Calculate the rip distance (R) from the formula R = 2.914 x O. Set the rip fence at distance R. Place the jig against the rip fence so that the point where the two strips meet is against the fence (figure C).
- To use the jig, align one corner of the square work piece with the 90-degree angle on the jig (figure D). Hold the work piece on top of the jig as you send it through the table-saw blade (figure E).
- Repeat step 4 to cut off the remaining three corners of the work piece.
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