100 Sport Aviation June 2012
HANDS ON
HINTS FOR HOMEBUILDERS
Scarf Joint
BY MIKE DANFORD, EAA 1062347
A very simple jig that took about five
minutes to design and build solved the
problem. The first step to most jigs is
deciding what the jig is going to aid. In
this case, we had a simple small back
saw. We needed something to guide the
saw until enough of a kerf was created
for the saw to guide itself.
The pictures of the jig are quite self-explanatory. First, we drew the intended
joint line on the existing cap strip. I
found a couple pieces of plywood (any
wood would work) that simply felt about
the right size. The first was laid up
against the rib, the second on top of that
and overlaying the rib, lined up with the
drawn scarf joint line. Glue, staples,
done. It’s wide enough so that one hand
WHILE WORKING ON the Standard J- 1 at
the Kermit Weeks Hangar at EAA, the
task at hand was to remove just the last
foot or so of wing rib cap strips, as the
very trailing edge of them (that attached
to the wing spar where an aileron was
located) had dry rotted. To do this correctly, we would have to either replace
the entire cap strip (more destruction
than deemed necessary) or cut off the
last foot or so and scarf on a new piece.
Using the generally accepted method,
the scarf would be 12 times the width of
the joint in length.
A scarf joint can be oriented two
ways in any given joint; we decided
on the width as that would clearly be
the easiest way of cutting the scarf on
the wing rib. The cap strips measured
1/4 by 5/8, so the scarf joint would be
12 x 5/8 = 7-1/2 inches long.
The most critical part of a scarf
joint is not necessarily the angle, but
that it fit correctly. It is always a very
shallow angle, so it’s difficult to do
with typical angle-cutting methods
(miter saws and such). In this case,
making things worse is that half of
the joint would have to be cut on the
under-cambered surface of an aircraft wing, lying on a table.
The wing of J- 1 is a very light
and old structure. Easy wood to
cut, but not very strong in its
mostly dismantled uncovered
state. We decided that power tools
were simply too risky, and the
results wouldn’t be repeatable.
Also, in this case, cutting the
joint is essentially rip-sawing,
as you’re mostly sawing with
the grain (precisely 12 times
more so than cross cutting).
Saws tend to wander more in
that case.
, but very do ods
