Reinforcing a Cracked Pegbox

Reinforcing a Cracked Pegbox
Jerry Pasewicz
Introduction
Throughout my training as an instrument restorer I was never very happy with the traditional
method of repairing peg box cracks. In the traditional method, half of the peg box wall thickness is
removed from the inside over the crack, and 1 or 2 pieces of cross grained maple are added to reinforce the
crack. This method is time consuming, adds a fair amount of touch-up to the job, and ultimately is
susceptible to failure due to the difference in expansion and contraction of maple cross grain as opposed to
along the grain.
The method I developed for our workshop consists of inlaying a carbon fiber ring around the
peghole inside the peg box after the crack is repaired. The carbon fiber reinforces the crack without the
need for touch-up except the blackening of the inside of the peg box. The carbon is also not susceptible to
changes in humidity.
Tooling
The custom tooling for this job consists of three basic parts. A hole saw, the one in this repair is a
12mm outside diameter, with a .5mm wall thickness, a mandrel for attaching to the hole saw, and a
matching nylon jig to make the carbon fiber ring. The nylon jig should have three parts: The inside part
matching the inside diameter of the hole saw, and the two outside halves matching the outside diameter.
Procedure
After the peg box crack is cleaned and glued, a loose fitting maple bushing is fit into the hole to be
re-enforced. The hole should not be cleaned or reamed because the bushing will later be removed, and the
original peg will be replaced. Once the bushing is in, a small hole in the center of the bushing is drilled to
match the end of the mandrel.
The hole saw is inserted into the peg box, and the mandrel is screwed into the hole saw.
At this point I put the end of the mandrel into my fourre’(bow makers hand drill) and make a doughnut
shaped hole half way through the thickness of the peg box wall. The hole saw is then taken off the mandrel
with the aid of small holes in the saw that fit a chinrest tool.
Now that the peg box is prepared, it is time to make the carbon fiber bushing. It is necessary to
make the bushing because we want the carbon fibers to run circumferentially around the ring, and I have
yet to find a carbon fiber product that does this. The carbon fiber is available in strand form as pictured.
The fiber is wrapped around the custom nylon jig 3 or 4 turns with a good epoxy, then the outside is
clamped on using a rubber band and the rings are set aside to cure.
Once the epoxy is cured, the jig is put onto the lathe and the big carbon tube is cut into separate carbon
rings.
The final steps in this repair are to glue the carbon fiber ring into the doughnut hole in the peg box, cut
down the ring when dry, and touchup the inside to make the ring invisible. Finally remove the temporary
bushing.