Octagonalise all the things

Or the Anarchist’s Saw Bench… part 2

We’ve been taking a much needed vacation in Italy for the past week, which involved introducing the Apprentice to great pasta, and also her first trip on a plane, so the saw benches are not yet finished. Before we flew out I did manage to progress the legs for the first saw bench however. To be honest, I’m not sure I have the right mindset for production work and even though I’m building a pair of saw benches I decided to work up the legs for one bench at a time. This approach means that when I come to making the legs for the second bench I should hopefully be able to apply any lessons learned, or avoid pitfalls identified, from the first bench.

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The legs for these saw benches have a tapered octagon cross section, which is at its widest point at the floor. The top end of the legs terminates in a conical tenon. There are two ways to establish a tapered octagon – you can either plane a straight octagon and then taper it, or you can plane a tapered square cross section and then octagonalise it. Charles H. Hayward wrote that octagonalising a tapered square is the best way to approach this task, and who I am I to argue?

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Laying out the octagons using simple geometry

The stock for my legs was slightly oversized, and I laid out the octagons at each end of the legs taking this into account. Laying out the octagons sounds a little like proper maths at first, but using a good compass and some old-as-the-hills geometry means that there is no need to remember angles or do anything funky with numbers. Simply scribe a circle at the end of each leg for which the diameter corresponds with the width of the octagon. Use that circle to draw a square, and then set your compass so that with one point at the corner of the square the other point scribes an arc that passes through the centre point of the circle. Do that from every corner of the square, and each point where the arcs hit the sides of the square is a corner of the octagon. Which sounds complex, but only takes 5 minutes to sink in even if you are as numerically challenged as I am.

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With the octagons laid out at both ends of every leg I planed the legs to a tapered square cross section, removing most of the waste with my No.5 jack plane before truing everything up with the No.8 jointer.

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Octagonalisation in progress

Octagonalising the tapered square is then simply a matter of removing the corners from the square cross section, working to the layout lines on the end grain of each leg. Although you could mark the facets of each octagon along the sides of the legs, that adds an additional step to the work and I’m not sure whether it would really make planing to shape any easier. To be honest, no one is going to measure each face of the octagon to check that they are all perfectly equal, so if the legs look good they are good (with apologies to the legendary Joe Meek). You can make a wedged jig to hold the legs while octagonalising them, but I found that cinching each leg between two bench dogs worked just as well.

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One batch of octagonalised legs.

Again, I used my No.5 to remove most of the waste, truing up with the No.8 just before I hit my layout lines. I found that takling a heavy cut with the jack was beneficial, particularly for the first couple of strokes at each corner, as this removed the aris of the workpiece and left a ledge of sufficient width on which to balance the plane accurately.

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Lovely thick ribbons off the No.5 jack plane

4 thoughts on “Octagonalise all the things

  1. Pingback: In praise of the humble octagon | Over the Wireless

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