Let me run this by y'all...octagonal table

[Phil Crow]

I was, er, commissioned by a friend to build a card table about 5 feet across. 8 sides, each about 18 inches (broke out the ol' cosine calculator for that one) long, made of tubafores squared up. My plan at this point is to gather scrap from my jobsite for a couple of weeks and make (8) 18"x30" panels, then cut them into accurate triangles and spline joint them together to make the table top. Then, mortise in some chip trays and cupholders with the router, slap some poly on it and call it a day. Simple enough, but I see some potential pitfalls and in the interest of safety, I figured I'd get some opinions.

I can build a crosscut sled to precisely cut a 22.5 degree angle (see

but that sled is only for a small piece. The pieces I want to cut are 30" wide, and without any extension tables on my table saw, I am wondering how to build a sled to cut a piece 30" wide. I do have an outfeed table (my workbench) so maybe a looong sled to hang off the infeed side of the saw?

I don't think I'll need any biscuits or anything to join the pieces for the individual panels, but I'll need something extra between the triangles, right? I figured that something to be splines about

1/2"x2". The problem there is that I have a wobble dado (don't ask--came with the saw) that is, in my opinion, dubious at best, dangerous at worst. Cutting a half-inch by one mortise seems like a lot of wood to hog out with a wobble dado.

This project will be stretching my skills a bit, and the finished product doesn't have to look great, but better is always better. Any help would be appreciated.

Thanks,

-Phil Crow

[firstjois]

While waiting for someone who can help you I thought I'd toss in this oddity: while looking at some old furniture I saw a gaming table that had a second "table" underneath with dividers between the each section that separated the section that would belong to each individual playing at the table. These sections looked big enough for a glass of some kind of drink and a play to lay down your cards or tiles where they wouldn't be seen by others. Seems like you could put your snack there, too, instead of having everything all over the table or on trays that stuck out all over the place. Nope, I have no idea of the construction, I was just too impressed with how cool it looked.

Josie

[Andy Dingley]

Make two half tables, then joint the edges, then join them into a table. Hopefully this won't do anything, but if there has been any inaccuracy in the angles it'll avoid awkward gaps in the table. If you're doing an ocatagon or a dodecagon, it might even be worth truing the quarters to 90°.

For tubafours, I'd probably lay it out with a pencil, clamp a guide down and cut it with a handheld saw.

Sounds easy enough, if you have a saw table with double slots on one side. Just make sure the runners are long enough to allow the sled to pass right across the blade without coming adrift.

If your table has a slot on each side, then you can either make a double runner sled (which will have to be huge) or rely on a single slot to guide it.

If it's a spline slot, saw it out with a plain blade. Cut the side walls, then a couple of cuts in the middle. The rest will come out easily with a chisel.

You could also make three sawcuts, then take 1/4" out at a time with the wobble saw. As it's only a shallow slot, then it's no big deal (although a single pass cut is more than I'd want to do with a wobbler)

[Robert Bonomi]

Is it supposed to be 60" from the middle of one flat across to the other side, or 60" from corner to corner?

For a 60" corner-to-corner dimension, my CAD program says 22-15/16" for the side dimension.

For a 60" mid-side-to-mid-side dimension, my CAD program says 64.94" corner- to-corner, and 24.85" for each of the 8 sides.

I only get about 43-1/2" mid-side-to-mid-side, and 47" corner-to-corner, given an 18" side length.

Suggest you double-check the dimensions, before starting construction.

I'd do it the 'lazy' way.

Make the top in fourths -- *not* eighths.

you can get 2 'fourths' out of a 4'x4' section of ply. or the entire top out of a full sheet.

Construction is especially easy, if you can go a little bigger than 60". Using 48"x48" squares will result in a 68" table. if you really want

60", use trim to 42-1/2" square before proceeding further

Now, make a precise corner-to-corner cut of each square.

Make sure that the 4 resulting pieces are _exactly_ the same size. trim as needed.

Next, measure from the 'mid-point' on the diagonal, to the opposite corner. and lay out that same measure down each of the sides, starting from that same corner.

Rip at that point, parallel to the other side of the square corner. Two such cuts for each of the 4 pieces.

Note: These cuts are all to exactly the same dimension. Set the saw fence *once*, and make _all_ the cuts.

Note: if the measurement is a bit off, you end up with one set of four sides that are a little longer/shorter than the other set of four. Each side in each set of four _is_ exactly the same size, and they alternate as you go around the table. It still 'looks good', even if it isn't a perfect octagon. (This kind of design I _like_ -- screw it up and it still looks good.:)

Lastly, lay out the 4 pieces, with the square corners touching in the middle.

Voila! an octagon table top.

BTW, the 'lazy' way to support/stabilize the top, is to make another set of pieces -- made exactly the same way as the first ones, but starting with squares that are about a foot smaller. Line them up 'center on center', But with the edges of the 4 pieces turned 45 degrees. Now the joint in one layer is in the middle of the solid piece in the other layer.

[Bill Stock]

I made a couple of octagonal planters recently, so I've got some experience.

I know you want to get the wood cheap, but cutting all those pieces and getting them to line up is going to be a bitch. I probably stating the obvious here, but an octagon is just a square with the corners cut off. I believe baltic birch comes in 5*5 sheets, so that would give you your 60" across. For an octagon 60" across the sides would be 24.85" (Width /2.414). So measure 17 9/16" from the corners and draw a 45 to the opposite side of your sheet. You'll end up with the four triangles to cut off your sheet. Double check your 8 sides to make sure they all measure the same.

[JGS]

[Phil Crow]

D'oh! You know, you're exactly right, as my mockup told me yesterday!

Using one half-sheet of plywood, does this leave me with four 45-45-90 triangles with one four foot side?

If I have the triangles, am I cutting a little triangle off the corner of the big (4' side) triangle? I just tried it with a piece of paper, and I think that's what you mean.

You see, that's why I keep coming back here. The primary purpose of the thing was to do it for free, but in light of the fact that I can make THAT table top in an hour may force me to tell my buddy to apply crowbar to wallet. Either way, I'll post pics when we get it finished.

Thanks.

-Phil Crow

[Robert Bonomi]

You get 2 triangles out of 1 4'x4' piece; each of the pieces has _two_ 4' sides. And a long-side diagonal that is about 68". (You cut on only one of the diagonals of each 4x4 piece.)

That's why you need _2_ 4'x4' squares -- to get all 4 'fourths'. :)

Yup. the cuts are parallel to the long side.

Think of a stop-sign. With a horizontal and a vertical center-line drawn on. Extend those centerlines out well past the edge of the sign. And extend the 'diagonal' edges of the sign until they meet those extended centerlines.

It should now be _real_ obvious how the design works.

HAH! Wait til you get the bill for my 'architectural services'.

As you probably noticed, there are only three 'critical' features to the design: 1) The 90 degree corners do have to be exactly 90 degrees. Luckily you _can_, in general, trust the 'factory' corner on full sheets of ply or other sheet goods for that. 2) The edges that join to make those 90 degree corners must be straight. Again, one can generally trust the factory edge for this. 3) When you trim off those 'triangle' corners from the big triangle pieces, you have to have to cut to the _same_ width for all the pieces. So you set the fence *once* to make all the cuts, and you get that match 'automatically'. As it happens, _none_ of the measurements/cuts *you* make has to be 'precise' to any dimensional measurement. Everything _works_, even if you are 'a bit' off. This is the *true* measure of a 'lazy' design -- even if you build it sloppy, things still fit right.

[Phil Crow]

Snip

[Phil Crow]

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Can I buy ya a couple of beers instead?

Thanks, Rob, et al.

-Phil Crow