The Gougeon Brothers on Boat Construction - WEST SYSTEM Epoxy

136 Core Boatbuilding Techniques
Any drill will do: however, the spade bit will give you more
major diameter surface area for a given hole depth because
of the square cut.
<strong>The</strong> smaller diameter pilot hole gives the fastener bite into
the blocking holding the fastener and fitting until the epoxy
cures. You can grind the long tip on the spade bit to avoid
drilling through the blocking.
Figure 14-7 Drilling the pilot hole and oversized hole.
use two bits. Drill about three-quarters of the desired
length with an oversize bit and then drill out the
bottom with a bit which is smaller than the screw. In
either case, keep drill tips sharp. In our testing, we
noticed that wood fibers that are cut cleanly absorb
Oversized Hole
Pilot Hole
3/4
1/4
X x + 1/4"
Figure 14-8 <strong>The</strong> oversized hole increases exposed substrate
area and creates an epoxy annulus around the fastener.
Figure 14-9 Bond threaded rods or studs into the substrate
as an alternative to bolts or screws for easily removable
hardware. This is a useful variation for engine, motor, or
machine installations. You can eliminate the pilot hole by
adding another nut to the bottom of the threaded rod for
alignment.
epoxy better than fibers that have been burnished by
dull drills. Burnished samples also failed more
frequently than clean-cut ones.
Since you fill oversize holes with epoxy before inserting
fasteners, it’s easiest to work on near-horizontal surfaces,
with gravity in your favor. After drilling a hole, remove
all wood chips and fill the hole to the top with epoxy.
Allow the epoxy to soak into the wood for 5 to 10
minutes. This waiting period is extremely important
because a significant portion of the epoxy may be
absorbed by the wood, especially in smaller holes. If
you insert the fastener too soon, the wood may continue
to drain epoxy away from the screw. If, after waiting,
you notice a significant reduction in the epoxy supply,
add more before inserting the fastener.
In most situations, the fastener will take up a good deal
of the volume of the hole and epoxy will be forced onto
the surface as you insert the fastener. To minimize this
problem, remove some epoxy from the hole with a
syringe just before installing the fastener. You want
some excess to flow out since this will tell you that the
hole is amply supplied with epoxy, so don’t withdraw
too much. A little experience will teach you how much
you can remove without jeopardizing the wood/fastener
bond.
Voids in oversize holes detract from the strength of the
bonded fasteners and should therefore be avoided.
Careful monitoring of epoxy absorption after the initial
5- to 10-minute waiting period will help prevent the