I searched for "prior art" for cutting compound tapers like these, but I couldn't find anything that did what I needed. This is the idea I came up with. There must be simpler ways to do this, but once I started down this path, I had to see it through. Fortunately, it worked quite well.
The heart of the jig is a little indexing cog and matching cradle. I machined the pieces on a CNC, but this could also be done with a hole saw and careful drilling. The notches on the cog are spaced such that the resulting diamond will have a 72 degree angle on opposing acute corners.
The cradle gets glued to the jig base, slightly angled to desired taper. A removable arm has it's own tooth to lock in to the top of the cog to clamp the cog firmly when cutting.
The other end of the jig has a block with a removable dowel pivot. The hole is at the same height off the base as the center hole in the cog at the other end. Together, they form a rotisserie for the spike being cut.
Each end of the blank is center drilled.
The cog is screwed temporarily to one end of a blank. I marked the center of the two narrow lobes to help align the cog at the appropriate angle. The screw is tightened firmly to prevent any rotation of the blank while it's being cut.
A benefit of this approach is that nothing indexes off the sides of the blank, which means it doesn't have to be surfaced ahead of time.
One edge cut.
To position for the next cut, the clamping arm is moved out of the way and the cog lifted and rotated to the next notch and clamped back in place.
After the final cut of the first spike.
This resulted in very consistent pieces.
The one operator-error was that I knew I was pushing the envelope with one of the dimensions of the blanks (too narrow). The result was that some of the spikes had incomplete diamond cross sections at the thick end. At first I thought it was tear-out, but when I looked more closely, I found it was an exposed edge of the blank. Nothing a little sanding won't conceal.
A spike colony.
Overall, I'm very pleased with how the spikes turned out. If I were doing it again, I'd be more careful with the blank sizing, and possibly I'd tweak the jig a bit. But it worked as well as I could have hoped.
Wow, that is a neat solution! I was thinking of a regular tapering jig but with the squashed profile, your solution is perfect. Did you expect or have any issues with your indexing "knob" spinning/slipping on the base of the spikes?
Thanks, Splinter. I did keep an eye out for any slippage, but with the screw tightened firmly, the rough plywood against the cherry end grain didn't budge at all. Also, I went slow with the cut to reduce rotational forces.
