A cheval is done on a frog when a section has suffered major damage or is worn beyond the ability for the frog to function properly. I have updated my process for this type of repair since reading an article by Kari Azure from Triangle Strings.
The damage to this frog extends a considerable amount into the frog. Additionally the thumb protrusion has been worn/filed down very thin exacerbating the damage. Historically, a cheval is performed by planing away original material leaving a flat surface to glue new ebony to. A rounded cheval will maximize the surface area for gluing while minimizing the amount of original material that will need to be removed. This process will help feather the joint to blend the two pieces of wood together.
Making a Sketch
Before beginning a repair like this, a tracing and sketch are made of the existing frog. Critical dimensions are noted and will be used for reference when refitting the heal plate and liner.
Dismantling the Frog
The heel plate, liner, and pins are removed prior to working on the frog. A soldering iron works well to break the bond. Often by heating the pins with the tip of the soldering iron, they will pop right out like the wack-a-mole game.
Preparing the Surface
If part of the thumb protrusion was to be kept intact, dental compound would have been used support the fragile area. The frog is then set up in the mill will a ball burr. A rounded surface will maximize the surface area for gluing while minimizing the amount of original material that will need to be removed.
Preparing New Wood
A piece of ebony is selected to match the color and grain structure of the frog. The ebony is turned in the lathe to match the diameter of the ball cutter. A good orientation is determined to match the grain direction and medullary rays. Witness marks are made on the dowel and some additional fitting is completed by hand to ensure a perfect joint.
Once a good fit is established. The dowel is cut close to length and glued to the frog.
Thus begins the process of shaping the new material. The channels will need to be made to fit the existing silver heal plate and liner. It is extremely important that the frog aligns up perfectly with head once fit back to the stick. All the metal parts will need to be reglued and pinned in place. A new hole is drilled out for the eyelet and a new one is fit and adjusted.
The frog is lightly polished and the bow reassembled. This repaired bow will now function properly and serve the musician for many more years. I share some thoughts about the value of repaired bows in the article: The Future of Restored Bows.