The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

Chapter 12 – Scarfing 115
Figure 12-11 Planing narrow plywood strips with a block
plane.
possibility of aggregate error is high, we recommend
against clamping a number of wide scarf joints in a
single operation.
To prepare plywood scarf joints for bonding, first line
up the bevels to check them for mating accuracy. This
requires a flat working surface—either a floor or a
bench that is large enough to accommodate the two
pieces being joined. You will need a steel straightedge
about 1' (300mm) long to lay across the face of the
scarf joint to help sight it from above. Mate the two
beveled surfaces by pushing them into each other,
tapping them a fraction of an inch at a time. Between
taps, press the bevels from the top with your hand,
holding both plywood panels flush with the underlying
surface. <strong>The</strong>n place the straightedge across the joint; if
the bevels are meeting correctly with no overlap, the
straightedge should be touching both surfaces evenly.
2"
2"
Bench
2"
If there is an overlap of the bevels, the straightedge will
rock slightly on the featheredge and stand high on one
side or the other of the panels. If there is a gap between
the beveled surfaces, simply sighting the edge of the
scarf should indicate this. Occasionally there may be
an advantage in tapping the panels together until the
straightedge indicates that a slight override exists
between the bevel surfaces. <strong>The</strong>n, draw the two panels
from each other in very small increments until the
straightedge again reads that a flat surface exists
between the two panels. With long scarf joints, line
up the bevel carefully on one edge and then insert a
temporary staple. <strong>The</strong>n line up the opposite edge in
similar fashion and insert a staple in it. If you have
machined the bevels straight on both panels, the mating
between the two in the center portion of the panel
should follow the accuracy that is developed at either
end. When the panels are in perfect alignment, make
surmarks across the faces of both panels so that when
you apply adhesive, you can reassemble them in the
exact position. Make the surmarks across the scarf joint
edge at approximately a 45° angle. Make at least two
surmarks, one at each edge of the panel, and insert as
many as you need in between.
Apply the adhesive to the beveled plywood faces using
either one of the methods described for use with
lumber. Lumber bevels are composed entirely of endgrain
wood, but at least one third and perhaps as much
as two thirds of the joint face of plywood will be flat
grain. This will not absorb the large amounts of epoxy
that end grain does, so you can apply less epoxy and
still be sure of a sound bond between panels.
Clamps should be placed
as close as possible to
the scarf.
Figure 12-12 Stacking 1 ⁄4" thick panels to cut several 8-to-1 scarf bevels simultaneously. <strong>The</strong> setback of each panel is based on
the panel thickness and bevel ratio.
1 ⁄4"
2"