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FoMRHI Comm. 16 06 Ephraim Segerman
Peg Fitting
It is about time 1 did a nuts & bolts Comm. Well, sort of. I have been updating and
expanding an NRI brochure on instrument maintenance associated with strings. Most of it is
standard procedures, and if explanations are called for, they are usually fairly standard. When
it came to pegs though, I realised that I didn't know how pegs really work, and so had to
figure it out for myself. Being trained as a physicist helped. Following is the peg-fitting
section:
Wood swells or contracts with changing humidity different amounts in different directions.
The swelling is most in the tangential direction (going around the circumference in the original
log), about half of that in the radial direction (going from the centre of the original log
outwards), and hardly at all in the grain direction (along the axis of the original log). Woods of
higher density swell or contract more for the same change of humidity than woods of lower
density, and harder woods are less compressible, so they exert stronger forces against any
hindrance when they change dimension.
Holes that are drilled and reamed in pegboxes go perpendicular to the grain direction. The
cross-section of the hole is round when just reamed, but will tend not to remain so when the
humidity changes. The dimension of the hole along the grain direction hardly changes at all,
but the direction perpendicular to it does. The difference is minimised if the other direction is
the radial one (perpendicular to the rings), but this is not necessarily an advantage.
For strength, pegs are turned so that the grain direction is along the peg shaft. When freshly
turned, the cross-section of the shaft is circular, but when the humidity changes, it tends to go
oval, with the tangential direction swelling or contracting about twice as much as the radial
direction.
Consequently, at any humidity other than that at which the pegbox hole was reamed or the peg
shaft was turned, there is a tendency to have an oval cross-section peg turning in an oval crosssection
hole. This is the norm when we use pegs, and yet they usually still work. If both the
peg and the pegbox were made of particularly hard wood, the peg cannot be made to turn
smoothly if the humidity is not just right. For pegs to work, we need some of the wood
involved to be compressible.
In medieval times, pegs were made of particularly soft hardwoods, and their sponginess made
up for any deficiencies in the shaping of the pegbox hole and the peg shaft. In the Renaissance
and early baroque, with instruments made by professional makers, pegbox holes and pegs
appear to have been more accurately made, and medium-hard hardwoods such as fruitwoods
were used for pe