In the last few days, I figured out how my angle calculation tool was incorrect, and fixed it. Now it’s correct, just not very convenient to use. The reference axes are a bit “here and there” so you really need a sense of what makes sense, in order to extract the right answer.
I then went over all of my previous calculations, and made corrections to the seat plan.
Luckily, the amount of error in an angle calculation was directly related to how far away from perpendicular the hole is. Because of this, the errors on my spindle angles were so small that they almost all rounded off in the same direction as before. The exception was with the outside posts, which had to shift their sight angle by half a degree.
Today, I used my new plan to drill test holes for the back spindles in a scrap of dimensional lumber, to see if the back matched my expectations. It was basically a complete success: I have nothing to change with these angles. The tops of the spindles form a good curve (when viewed from above), and they’re evenly spaced. As shown here, I built up the test back with some spindles I’ve already shaved, along with some sections of 1/2″ pipe. The pipe is a lot straighter, and allows better measurements. (I picked this tip up on Peter Galbert’s blog as well, unsurprisingly.)
This gave me better information than I had about the shape of the seat back. The top of the end posts here are close to where the horizontal rod will end up. It’s wider in relation to its height than I expected. Obviously, my spindles are too long; I’ll need to trim them down on both ends and/or make some new ones. I think it’ll look fine when it’s finished, but it will be a significantly smaller chair than the bow back I made previously. This is good, since that’s what I was aiming for.
Back to the plans: My original leg lines were a complete mess. I couldn’t even figure out the values I used to I calculate them, let alone the error introduced by the tool. Always show your work! They were reasonable angles, and would have made an adequate chair, but they wouldn’t have matched my original intent. From a design perspective, I don’t know whether my plans or the old angles would result in a better chair, but I’m going to try my original plans and work from there.
I’ve already shaved some spindles, and they need to dry a lot before I can use them. I’m not using a kiln right now, so I really need let them sit around for a while: hurry up and wait. Next I need to make the top rod, bend it, and let it dry as well.
This requires a steam box, so I started building one. It is small, 30″ long, becuase I plan to use it in the kitchen. We’ll see how well it ends up working in the next week or so. I also need to accurately measure the location for the rod, so I can figure out how long to make it and build a bending form.