This article describes a simple and effective method for cutting the groove for the bar into cones in cheiroballistra arms. The cones can be made with a plane as described here, or with a combination of a plane, rasp and files, as described here.
Making a grooving chisel Edit
A grooving chisel is used to make the groove for the metal bar into the wooden cone. It's not strictly required, but makes obtaining an exact fit very easy. It is composed of two parts: a wooden handle and a metal part sharpened to a chisel edge. The chisel part can be made from soft steel and is sunk to the handle. The chisel's edge must be as wide as the metal bar. Bend the metal part into an angle to allow keeping the angle between the cone and the chisel small.
Here's a picture of the grooving chisel in use:
Making the groove for the bar Edit
At the base of the cone the groove should be as deep as the metal bar you'll be using to prevent the metal from chafing the spring cord. However, at the tip the groove's depth should be considerably less, maybe ~1/2 of the thickness of the metal bar. In other words, the groove should become shallower towards the tip. The reason for this is to avoid making the tip of the cone too thin and weak. As the primary purpose of the groove is to prevent the metal bar from moving sideways, this asymmetrical groove gives us best strength with only a very minimal increase in weight.
To make the groove you need a couple of saws and the grooving chisel:
Take the saw that has the narrowest blade and saw a slot to the cone:
Use a saw with a wider blade to widen the groove:
Use a yet wider blade to widen the groove again:
Once you've run out of saws of ever increasing blade width widen the groove using the grooving chisel:
Here's a picture of the metal bar in the groove at the tip of the cone. Notice how it's sunk only partially to maximize cone integrity:
Here the metal bar is shown completely sunk into the groove at the cone's base.