Joining An Octagon

When you're cutting miters for an octagonal (eight-sided) frame the chance for error increases dramatically. Each piece for the frame requires two cuts at 22'A° — for a total of 16 mitered cuts at this angle. If your setting is off just '/i-degree. the combined gap would be about Vi".

If the joints are off, you could recut all the miters, and probably wind up even farther off in the other direction. Instead, some minor adjustments can be made during assembly. First, 1 glue pieces together to form two halves of the octagon. It would be nice if the four open ends of these two halves fit together perfectly. But that rarely happens.

As long as the gap between these two halves is not more than Yh", corrections can be made for a perfect fit. I use the jig shown in Fig. 1.

First I rip a clean edge on a piece of plywood. Then I lay the assembled half-oetagon face down and tack the plywood to it. (This means the nail holes will be on the back side of the octagon.)

The key thing here is the placement of the octagonal half on the plywood. If the miters were cut perfectly, all four corners of the miters would rest on the very edge of the plywood. But since they're probably off. you want to position the assembly so the two long corners extend beyond the edge of the plywood, and the two short corners are right on the edge, see Fig. 2.

Place the jig on the saw to make a trim cut. The blade should just barely skim along the edge of the plywood — trimming off the long corner. This insures that you're changing the original angle (and the depth of the spline groove) as little as possible.

Repeat the same procedure for the other half-octagon, and you should wind up with a perfect fit.

FIGURE 1

Plywood Joinery

ALIGN FLUSH WtTH PLYWOOD

tack half-octagon to plywood _-

from bottom

FIGURE 1

FIGURE 2

ALIGN FLUSH WtTH PLYWOOD

tack half-octagon to plywood _-

from bottom

Turned Fruit Bowl

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