Cutting out an Octagon
A Pictorial Article by Bob Hamilton
There are several ways to lay out and cut octagons, but this is the method I use. I start out with a square blank, in this case 8/4 walnut. I set a marking gauge to approximately half the width of the blank and scribe a line from two opposite edges. Usually an eyeball approximation will result in the lines being ½” or so apart. I then re-set the marking gauge to scribe a line half way between the first two, and then scribe two more lines. These lines are usually 1/16” or less apart because it is fairly easy to eyeball the mid point of a ½” gap. I then go ahead and scribe lines referenced off all four edges. The intersection of the four lines should define a square with sides 1/16” or less long. It is quite easy to set the point of a compass in the centre of a square that small.
With the centre of the blank found I use a compass to draw a circle that just touches all four sides of the square.
Photo 2: Draw circle
All four sides of a square are tangents of the largest circle that can be drawn inside the square. All eight sides of an octagon are tangents of the largest circle that can be drawn inside the octagon. Since the four sides of this square will become four of the sides of the finished octagon it follows that the other four sides of the octagon will also be tangents of this circle. That fact makes it easy to lay out the other four sides with a combination square. In practice I simply lay out one side and use a stop block on my mitre saw to ensure that all four cuts are the same.
Photo 3: Draw tangent at 45 degrees
Photo 4: Tangent drawn in
I set up my mitre saw with a zero clearance rear fence to better support the workpiece and make it easy to line up my layout marks with the kerf cut in the fence in order to set up the stop block. One important point to keep in mind is that you need to use a stop block that extends far enough out from the fence to bridge over the gap that will be there when you go to cut the last corner off.
Photo 5: Mitre saw set-up
Now butt your blank up to the stop block and fence and cut off the first corner.
Photo 6: Ready for first cut.
Once the first corner is cut off you can rotate the piece clockwise to cut off the second corner. Rotating clockwise will keep a square corner against the stop block for the first three cuts.
Photo 7: Second corner cut
Repeat for the third corner.
Photo 8: Third corner cut
On the final cut you will have a gap where the first corner was cut off. Make sure the edge against the fence is flush and the stop block is catching the other edge beyond the gap.
Photo 9: Final corner cut set-up
Once the last cut is made you should have a relatively accurate octagonal blank. It may not be perfect to the thousandth of an inch but should be close enough for all practical purposes.
Photo 10: Finished octagon
I would like to thank JimQ from the Woodturning Online forums for pointing out this easy way to lay out an octagon. It is much better and more accurate than the way I was using before.