[pianotech] Epoxy was first pinblock

[Jim Ialeggio]

Terry wrote:
<But my question remains unanswered: from a structural standpoint, what 
would be the problem with filling a wide gap the entire length of the 
flange to pinblock joint? I don't think there would be a problem.

Terry, to address, but perhaps not definitively answer your specific 
question. My experience of epoxy, both in my previous field of 
architectural historic conservation, and in the piano realm is that, the 
substance has some unique and valuable qualities, but it is not the cure 
all panacea many assume it out to be. It, like any substance, natural or 
man-made, has limitations.

In the 80's when the historic conservation crowd took on epoxy with 
gusto, is was used sometimes to create large structural gaps, in 
structural and decorative timbers. Five years down the road, the large 
gaps exhibited a huge failure rate. The protocols were adjusted to use 
it as an adhesive between reasonably fit "Dutchmen" with no more than 
1/4" glue lines, and preferably less. The problem was that the expansion 
rates of the epoxy and its surrounding materials were incompatible. In a 
large gap the relative inflexibility of epoxy was simply too hard and 
brittle to withstand the push and pull of wood.

Also, epoxy continues to harden as it ages, becoming more brittle than 
even a 30 day complete cure would suggest. It continues to crosslink. 
How long, I don't know, but even the most flexible epoxies, turned into 
brittle masses over a period of years. So assuming your flange/block 
joint depended entirely on the epoxy for its fit, would this aging 
brittle material be able to take the load you are wondering 
about...maybe and maybe not???

It seems, from what I've seen, that the situation where epoxy performs 
most reliably is when it is used as a component of a composite. The 
"pretty good wood to flange fit" with epoxy consolidating the wood 
bearing surface a little bit, and the epoxy effectively spreading the 
load to the entire wooden bearing face, makes long term sense to me. It 
uses the substance as a component of a composite. That's why I use it in 
this fashion.

I have had, even very recently, epoxy failures. These failures which 
never made it into a piano, but only as test joints. These failures, and 
all my previous experience with the stuff leads me to use it conservatively.

Jim Ialeggio

-- 
Jim Ialeggio	
jim at grandpianosolutions.com
978 425-9026
Shirley Center, MA