Originally Posted by GrahamHO
The "strength" is required to prevent compression
of the core
material when a bolt securing a fitting is tightened. If the core
material is compressed the bolt cannot be tightened correctly and eventually water
will penetrate the core and in the case of plywood
, will rot
it. It requires a reasonably wide column of epoxy
filler to resist this compression
, as well as the working load. Foam will compress. Plywood
will compress. End grain balsa resists compression best but doesn't like getting wet. They all need the epoxy
treatment. Personally I use a 20 mm dia column of epoxy (using a hole saw) for a 6.5 mm bolt hole. And of course countersink the top of the hole and use a marine sealant
under the top of the bolt / fitting. In order to countersink the hole for better sealing there should be an adequate width of epoxy.
I don't think you understood my post, the prime thrust is that the bent nail trick (effort to remove core around the clearance hole, to fill with epoxy) does not really gain one anything. For a bolt that needs a 6 mm clearance hole, I simply drill a 12 mm hole right through the lay-up. Seal the bottom with tape, fill with epoxy (low viscosity to soak into the core fibres), and then redrill the 6 mm. By using a 40 mm O.D. fender
washer backing, one has the compression resistance of the epoxy (lots) and original core beyond the 12 mm dia filled hole.
RE: epoxy / core bond. No worries, that is what the epoxy is designed to do, laminated wood is totally dependant on the epoxy/wood bond.
And the actual stress on the bond is very limited, because as you have correctly indicated, the stress is actually compression not tensile.
Anyone who wishes to do the bent nail trick is certainly free to do so, but in my humble opinion, it is totally unnecessary and takes more time and effort than it is worth.
Where I do use the bent nail trick (actually just gouge balsa out with the screwdriver) is when I blind mount a fitting exposed to shear stress, like a deck
organizer. In this case, I drill a hole at least twice the size of the fastener, gouge out a bunch of core material (as far as I can reach) beyond that, wax the bolt, screw a nut and washer to the bottom, set the bolt in the hole with nut and washer at the bottom, and then pour in the epoxy. I then grind the mating surface of the organizer to key it, and bed
it with 3M5200, screwing the mounting bolts through the organizer into the now deck
captive nuts and washers.
The result is a tenacious bond that will never let go.
Beats drilling umpteen access holes through a beautiful pristine liner.