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Lonnie Bird Q&A Section |
Octagonal patio table
Some day I do hope to be able to take one or more of your classes. In
the meantime I offered my wife the opportunity of letting me build her
a 50" round segmented octagonal patio table. I have mitered all
the segments, measured and remeasured, and used a CAD drawing template, but still when
I fit the last segment it is too long and some joints not quite perfect. I
know that the error is very small and accumulative, but do you have a methodology
(book or article to recommend) that doesn't require CNC technology to help reduce
such error accumulation or isolate a guilty segment or two? The project
is a lot of fun and I'd like to build a few more for friends and family, but
I'm not certain how to correct this problem. It seems like the more technology
we buy and use the more traditional methods are lost to us.
- Clark
Plymouth, MA
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Lonnie
replies:
Polygons can be a bit difficult; I always begin by determining
the angle that is required by dividing 180 by the number
of segments in the polygon. For example, 180 divided by
8 = 22.5 degrees. Next, I determine which tool to use to make the angled cut. To construct a hollow octagonal column I use a
chamfer router bit, Amana Tool no.49410. This bit creates an accurate 22.5 degree
bevel along the edges stock up to 13/16" in thickness. To bevel the edges of
thicker stock I use the tablesaw and tilt the blade to 22.5 degrees.
Constructing a large octagonal table top requires a 22.5 degree miter on both
ends of each of the eight segments. As you've pointed out, even a small error
in the angle is accumulative and will prevent the joints from fitting. Rather than trust the angle scale on the tablesaw I've found that the best solution
is to test the setup with inexpensive wood such as pine or poplar.
For example, in the attached photo I beveled the edge of a poplar strip, cut
the strip into eight pieces and joined them with masking tape. The assembled
octagon reveals any error in the angle. And, if so, which direction.
Also, when constructing a octagonal table top if just one piece is slightly longer
or shorter than the others the joints will not fit, even if the angle is correct.
To ensure that all pieces are equal in length I position each piece against a
stop when making the cut.
And I would certainly enjoy having you in a class. We post our new schedule each
year in September.
View Octogon Illustration
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