Unfortunately, one of the more common major repairs to a bow is repairing a head that has broken away from the shaft of the stick. This type of break across the head can be caused by many things, but most commonly is caused by dropping the bow on a hard surface.
There are several different techniques that can be used to repair a broken head, but the one that is most commonly used and is highly successful is the spline repair. This repair is very successful in repairing a broken head without any affect on the playing qualities of the bow. The spline head repair is also sometimes referred to as a laminated head repair, as the technique involves inserting a piece of wood at an angle to the grain in the head (effectively laminating the wood in the head). The inserted spline and the large gluing area result in a very strong repair.
This bow was broken across the head in a manner where the break lined up with the bottom of the stick. Fortunately, it was a clean break, without any wood missing and with minimal splintering of the wood.
The first step in repairing the head is to glue it back in place. Realigning the head in exactly the original location along the broken grain lines can sometimes be challenging if the wood was splintered when the bow broke. My preference in glue for this repair is either an industrial strength cyanoacrylate or a thin epoxy-type glue.
Once the head is glued on and allowed time to dry, the next step is to cut out the channel that the spline will be fit into. I do this on my bench top machinist's lathe, made by the Sherline Company. The cutting saw is a circular two inch diameter jeweler's saw.
I hold the stick securely in a block that has a padded groove, and use a series of shims below the block to get the exact height and angle of cut that is needed.
The bow is held in place with clamps and the shims allow adjustments to the height and angle of cut.
When the bow is placed in the block, I make sure that the front of the block supports the back of the bow head to prevent any movement caused by the stick flexing while the spline channel is cut.
Before I line up the head of the bow for cutting, I move the milling table forward of the saw blade, and advance it to the maximum depth that I want the cut to be. I make a note of this measurement and use the handwheel calibrations to make sure that I do not exceed this depth when cutting.
The bow head clamped up, measuring for depth.
Once the stick is straight and correctly angled in the holding block, it needs to be aligned with the saw in order to make a cut down the crest of the head.
When I cut the spline, I first make a small cut, then back the bow away from the blade to make sure that the cut is centered down the crest of the bow head.
It is very important that time be spent in correctly aligning the bow for cutting the spline channel. The set up time is always far greater than the time to cut the channel for the spline. If the bow is not lined up or angled correctly, the spline will be crooked in the head of the bow.
This video shows the cutting the channel for the spline. In cutting, I don't normally stop the saw, but did so here to reposition the camera.
The channel for the spline
The next step is to make the spline. I use pernambuco for this, but I have seen other woods used successfully as well as fiber and carbon fiber.
The diameter of the circular saw that I use is 2", so I made a template from a business card that has the same diameter. I trace around the template to establish the same shape on the piece that will become the spline.
The template, the spline, and the bow head.
I use a plane and a file to work the thickness of the spline down to the same width as the channel. I want a tight fit with no gaps, but if the spline is too tight against the walls of the channel, it could hydraulically push the glue out when it is inserted.
I want the grain in the spline to be approximately 45 degrees to the grain in the head of the bow.
Testing the fit of the spline in the channel before gluing.
I mark the outline of the bow on the spline and score it with a knife point to aid in gluing. I also make a mark on one side to make sure that I get the spline in the head of the bow the same way each time.
The spline glued into the head of the bow.
Initial trimming of the spline is done with a knife.
Secondary trimming of the spline is done with fine files.
Final working down of the spline is done with progressively finer grits of abrasive paper.
The finished spline after sealing with a coat of varnish. I do not make any attempt to color match and conceal the head spline.