This article discussed how one can make elliptically tillered cones for the cheiroballistra. The process is quite a bit more difficult than the ones used to make linearly tapering cones:
However, as discussed in the Cheiroballistra arms article, elliptical tiller is required in the cheiroballistra cones unless dimensions of the arms are arbitrarily increased.
In this article elm is used, but any sufficiently strong hardwood should work fine.
The process Edit
Initial splitting Edit
The initial splitting is discussed in the Splitting wood for cones article.
Fine splitting Edit
After initial splitting the cones look like this:
Here the growth rings and the back of the cone are not aligned, and we need to fix this. One way to achieve this is to align a heavy knife parallel to the topmost layer of soft spring growth and use a maller or similar to beat the knife slowly throught the wood:
Planing into a rectangle Edit
Next plane the cone into a rectangle. You can also plane the back of the cone if a lot of the strong summer growth still remains on top of the softer spring growth. The end result should have the back of the cone and the topmost growth ring fairly well aligned:
Digging out the spring growth Edit
Next we need to "dig out" the spring growth. In most woods the spring growth is significantly more porous and softer than the strong summer growth. In addition to porousness the spring growth is often of a different color which makes it easier to detect. To gauge how much summer growth you still have to dig out have a look at the growth rings at the sides of the cone:
Carefully remove wood first using a draw-knife, then continue with a rasp. Eventually you will start revealing spots of spring growth:
At this point start treading carefully so that you don't accidentally cut through the relatively weak layer of spring growth and possibly cut the wood fibers of the summer growth just below it. Use a cabinet scraper to clean up the rasp marks every now and then.
Once the entire summer growth ring is visible carefully remove it with a cabinet scraper or a file. The result should look like this:
As can be seen, the back of the cone is quite round. This is not a problem if you're making the cross-section of the cones more or less round. However, for cones that are (heavily) rounded rectangles decrowning is required.
Decrowning is a term used by traditional bowmakers to describe the removal of the "crown" in a bow's limb. The idea is to increase the durability of the bow's limb by removing the narrow, overstressed part at the bow's back. We can do the same for the cheiroballistra arm after we've dug through the soft summer growth.
The best tool for decrowning is probably the cabinet scraper. Push the tool gently from the base of the cone towards the tip, so that very thin layers of wood are removed along the entire length of the cone. The end result will look about this:
At this point make the groove for metal bar as described in the Cutting the grooves to the cones article.
The tillering process can be made a lot easier and precise with help from modern technology. Print the outline of the arms in scale (or about so) and cut out the outline. Rinse this narrow strip of paper well with water and carefully squeeze out excess water. Apply carpenter's glue on one side of the cone and place the wet paper on top. Carefully adjust the paper so that the ends and the top of the outline are in correct position. If the cone is slightly curved, you can carefully bend the paper to align it perfectly. Repeat the process on the other side of the cone. The end result should look like this (here without the groove for the bar, though):
Once the glue and paper is dry, cut the cone to correct length, then remove wood from the bottom of the cone until the cone's outline closely matches that on the paper. When almost done, it is helpful to file the cone's edge upwards in an angle; the point is to see exactly when all of the outline on paper is filed away and then, at the end, file down the crowned part in that runs down the length of the cone.
At this point you can width-taper the cones linearly and finally round the corners as described in the Planing the cones (alternative method) article. Just make sure not to make the arm's cross-section entirely round - it is not only unnecessary, but puts unnecessary stress on the surface of the wood on the back (=where the bar is) and belly. A heavily rounded rectangle works equally well and is more durable.
The next step is to assemble all the parts of the arm.