Octagon Picnic Table

[mike wacker]

I'm making an attempt to build a new one of these. Built one 15 years or so ago and really like the step thru style. This time I am trying to "band" the top with a 5 inch plus or minus wide "ring". I have ALOT of scrap 2x6 and 2x8 Southern yellow pine to work out how to do this right.

My first attempt consisted of dressing enough of this down to 1.25 inches to clean it up, including jointing, thickness planing, and then running thru the table saw to have pretty consistant product to work with.

I then cut 8 pieces 21 1/2 long pieces long point to long point using my chop saw set at the 22.5 degree stop. I'm using a 20 year old Makita 14 inch none beveling chop saw. This was to produce parts to form an octagon very close to 52 inches flat to flat. I then pocket screwed 4 pieces together to form halves of the octagon. Then came the test. I aligned the two halves to join them. The result is less than acceptable, even for a picnic table. The long points touched and the short points had approximately 1/8 inch gaps.

So any hints or reference materials out there to help me figure out how to improve these cuts so I get a better fit?

[Jason Beam]

Mike,

Just take a block plane or a sanding block to those long points on the halves you already have glued up till they match. I picked that trick up in a segmented woodturning tip somewhere. If you're only an 8th off, the slight change in angle will not likely cause you any trouble, really.

But ... if you wanna start over, the long points touching first means your angles are just a little too accute - probably closer to 22 than 22.5. But it's SLIGHT. With peices the sizes you're working with and the magnification of error you're getting with four joints, it means you'd have to add 1/64" to the short end of our pieces. I'd just sand/plane the halves so they meet up, personally.

....

If you wanna really rube-goldberg it: Lay two pieces of sandpaper on a nice big flat surface - stick 'em down with duct tape or what have you. Then take your already assembled half and stand it up on edge to get the two ends of the outer pieces to be co-planer. Silly exercise, but incredibly precise way (it's how the segmented turners do it, i've seen).

[Tom Veatch]

My first thought is that IF the chords are all identical lengths and widths, the angles are slightly off. It wouldn't take much angular error on each piece to yield the gap you describe.

From the way you describe the assembly sequence, I assume the 3 miter joints on each half are snugged up tight. Would loosening those joints before joining the halves distribute the gap such that it would be acceptable (~ 1/32 at each joint)?

Other than that, I can only suggest trimming the miter symmetrically such that the point to point dimensions are reduced about 1/32 inch (1/64 on each end).

Assuming uniform board length, width and miter angle,

Inside Length = Outside Length - 2 x (width x tan 22.5)

So if the board width is 5 1/2", outside length is 21 1/2", and miter angle is consistenly 22.5 degrees, the inside length should be:

21.5 - 2 x (5.5 x tan 22.5) = 21.5 - 2 x 5.5 x 0.41421 = 16.94365 = ~16 15/16 (+ a hair)

My guess is that it's a little shorter which means your actual miter angle is slightly greater than 22.5

[Lee Schierer]

I agree completely with the other assements that your angle is off a couple of tenths. What I do when making a octogon is to cut the pieces intentionally 1/4-1/2" too long. Then I do a trial fit with a band clamp around the outside. If the joints come out perfect, I just trim the pieces to length. If they are not perfect then I make slight adjustments and trim and fit until I get a perfect fit at all joints. Then I make one more set of cuts to get to the final length. The result is perfect joints.