Quote:
Originally Posted by defjef
Gord,
Did you mean that fiber glass cannot hold threads? I believe West believes that they can.
The problem is the pitch and depth of the threads, the pilot hole and the number of threads engaged. Why wouldn't GRP have no withdrawal strength? My own experience with tapping screws in thick GRP is that the head can break off from screwing the down way more often than them spinning around and not holding.
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I expect that West Systems might be referring to
epoxy’s ability to hold a thread, not an FRG laminate. A specific West citation might help clarify their position (link, direct quote).
The conventional purpose of the
epoxy plug, that Del & others describe in sandwich (cored) assemblies, is to provide a moisture barrier*, and crush resistance for through-bolted fastenings in
compression. These plugs may very well hold a screw thread, but I’ll stand by my
advice that screws serve no structural purpose on a
boat.
* By the same token, the raw edges of cut/drilled FRG laminates should be “sealed” with epoxy.
A thicker laminate of GRP will hold a screw better than will a thinner one; but neither will hold to any significant structural specification (in blind attachment).
Perhaps the breaking screw-head was a result of binding forces (high
installation or insertion torque required), but not due to the inherent pull-out resistance of an FRG laminate/screw combination. The fibrous nature of fibreglass may contribute to both ”binding” and poor pull-out resistance.