Octagon?

[Mike McDonald]

Anybody have any tricks for cutting an octagon? I'm trying to make a couple of octagonal tables. I finally tried doing some resawing and veneering. Now I need to cut the table top into an octagon.

Thanks,

[DJ Delorie]

There's some handy math here:

[Lew Hodgett]

Make a sled using the factory corner of a 3/4" sheet of plywood.

See Fred Bingham's book, Practical Yacht Joinery for details.

Makes dead nuts 45 angle.

BTDT

Lew

[DonkeyHody]

Do you want to make "pie slices" which you will glue together to make an octagon, or do you want to simply cut a solid panel into an octagon shape?

[DJ Delorie]

The problem with octagons is usually where to put that 45 degree angle cut, not how to cut it accurately.

[mac davis]

J.O.A.T. is the Octagon master, ping him and hope he's in a good mood.. Ok, hope he's in a "better than usual" mood..

mac

Please remove splinters before emailing

[dadiOH]

You mean where to cut? Find the quarter marks from the corners on each side and connect the adjacent ones.

[Chris]

A while ago I posted a similar question for a 6 sided table, got some good replies that can help you;

[Mike McDonald]

I already made the "pie slices" and have glued them to a MDF panel. Now I want to cut that panel into an octogon.

[Mike McDonald]

Yes! I can do trig and such. It's the woodworking part that's more "problematic". I already have 16 "pie slices" glued to a substrate. The slices already form a "rough" octogon. If I was a better woodworker, it would be a perfect octogon. That's the difference between theory and practice! So I need to clean up the rough octogon into a nice one.

Getting four sides nice and straight, I can do on the tablesaw. It's cutting the first two angled (45) at the correct spots that's the hard part. Once I have those two, I can again use the TS to cut the opposite sides.

[Father Haskell]

Screw the top to a sheet of plywood at 45 degrees. Run both through the saw. Rotate the top 90 degrees and repeat.

[DJ Delorie]

Draw a circle, diameter = width of table.

Draw line through center.

Draw line at 90 degrees to other line, also through center.

Two ways to draw 45s:

• use a 45 degree drafting square (bigger is better)

• bisect one 90 degree angle - same circle-making setup - two arcs centered at the intersection of the straight lines and the circle - the arcs intersect each other at two points - draw line between those two points and the centerline

Now that you have all eight lines and the big circle, draw lines perpendicular to each line and tangent to the circle. This is your octagon outline.

[Mark Ohlund]

Actually, you want to mark each side of a perfect square into *thirds* and connect the adjacent ones for a regular octagon.

~Mark.

[Bryan McCormick]

The message from Mark Ohlund contains these words:

If you can do it this way by measuring, then from maths, for a perfect octagon if you draw a square of side length 1, then measure along the two sides away from a corner a length of 0.314939, joining up the two points you have marked gives the 45 degree side of the octagon. Repeat for the other corners. The length between the two marks on one side is

0.314939.

Regards,

Bryan

[DJ Delorie]

Actually, it's 0.292893 of the width, in from each corner.

[clare at snyder.on.ca]

Mark the center. Draw a circle of a known diameter.- Devide the circle into 8 equal parts (45 degrees) Mark where the radial lines intersect the circle Join the marks with lines and cut on the lines. Make the circle diameter equal to the "point to point" dimension of the octagon. and "Bob's your uncle"

[clare at snyder.on.ca]

Easier yet, draw the 2 lines at 90 degrees. Draw a line between the intersection of these lines and the circle. Measure that line. Mark the middle of the line. draw a line through that mark and the centre of the board.

[clare at snyder.on.ca]

That's a HEXagon. 6 sides OCTAgone is 8 sides.

[DonkeyHody]

I feel your pain. Getting the angles cut perfectly enough that the pie slices all match up without any gaps is indeed a challenge. The problem is that your joint is so long that any minute error is magnified. I've tried it a couple of times and came close, but not close enough. Here's how I did it:

1. Forget trying to get all the pie slices to meet at a single point in the center. Cut 1/4" or so off the tip of each slice so you have a hole in the center.
2. Clamp ONE pie slice so that the edge to be glued is perfectly horizontal. Spread some glue on this piece. Slide its mate around on top of it until the glue begins to grab just a bit. Carefully position the second piece on top of the first by feeling the joint between your fingers. WALK AWAY for 30 minutes or more. Carefully set the glue-up aside to cure overnight.
3. Repeat step two until you have two "half octagons".
4. Put your two half octagons together dry and check the gap between them. It will need cleaning up.
5. With a tablesaw, jointer or hand plane, carefully work the joint between the two halves until it matches along its entire length. I had success by carefully shaving the edge with my tablesaw, with the outside edge of the octagon against the fence.
6. When you have the two edges mating well enough, glue them together.
7. Drill a nice round hole in the center of the octagon.
8. Plug the nice round hole with contrasting wood and sand flush.
9. You will probably have to clean up the outside edges to get everything flush.

Good Luck.

DonkeyHody "Every man is my superior in that I can learn from him." - Thomas Carlyle

[Nova]

1. Start with a square "A" x "A"

1. Draw diagonals

2. Measure from their intersection (or use a compass) to mark 1/2 "A" along the diagonal toward one corner.

1. Repeat for the three remaining corners.

2. Draw a line at a right angle to the diagonal, at the mark, to the edges of the board.

1. Repeat for the remaining three corners

2. Cut off the corners along the lines drawn in steps 5 & 6