Post by David C. HurdChris,
Of equal importance, often more so, to clamping pressure is
surface preparation. With well prepared surfaces and glues like
Titebond it is difficult to go wrong unless you clamp with so much
pressure that you squeeze all the glue out and starve the joint. You
want wood to wood. Planed surfaces are better than sanded ones, but
sanding works just fine too. Pieces, such as bridges and braces that
go onto curved tops need to be carefully arched to match the curve. I
seldom use any steel clamps with my work and find that wood clamps,
like go bar sticks and cam clamps, usually can't get enough pressure
to starve joints. Just remember that epoxies, to a certain extent
hide glue, poly glues and the like need less pressure as they require
more glue left in the joint that the Titebond types whose bonding
power interacts molecularly with the wood. In general using Titebond
requires a slightly more developed degree of craftsmanship to make
really good joints.
Bob Gleason
Bob, all due respect, but I think you've got some of the properties of Hide
glue mixed up with those of Titebond.
I agree that Hide glue requires less clamping pressure but that's because
once set, it will do it own clamping...*It* is the glue that has molecular
interactions with the wood, not Titebond which act like every other PVA on
the market (it appears the nomenclature of Aliphatic Resin (AR) is a
marketing ploy to differentiate yellow from white PVA...go figure). Hide
glue does require extremely well mated surfaces (it has little to no filler
properties) but, as mentioned it does not need to be clamped as tightly as a
PVA joint. Once set and undisturbed, it will bring the pieces closer as the
glue molecules shrink. So you simply need enough pressure to keep the pieces
from moving in relation to each other (hence the effectiveness of cam clamps
and go bars)...for instance, on a cleat, you can simply hold it on the crack
with your fingers, moving it back and forth to squeeze out the excess until
it cools and starts to gel...it then requires *no* clamping whatsoever and,
so long as it is left undisturbed, in 24 hours it will be extremelly
snug...in fact, as time goes on and the moisture continues to leave the glue
molecules, they will continue to shrink bringing the pieces closer yet (very
effective for fixing cracks...plus it really seeps into the smallest
opening...some refer to this phenomenon as the HHG death grip. A caution
here is that with HHG, the molecules are formed when the glue gels, if the
pieces are moved once it has gelled, but before it has gained strength, the
molecular walls will be broken and the joint will eventually fail...so once
it gels, it is critical that the surfaces are not moved until the glue has
completely set...in other words, HHG has a definitely limited assembly time
(this bench time can be extended somewhat by heating the surfaces before
applying the glue, but once it turns from transparent to cloudy, your time
is up).
On the other hand Titebond does not have this limitation and so, for many,
it is considered more convenient and friendly. Like most other PVA and EVA
glues, it dries in microscopic strings configurations, leaving gaps between
the strings, so it is not quite as tight as an HHG joint. Finally, while PVA
and HHG joints can be dissassembled with heat and water, epoxies,
particularly the two part stuff is pretty irreversible and, IMHO, not
recommended at all for stringed instrument construction.
In case anyone is interested, here's a relevant link:
http://www.taunton.com/finewoodworking/pages/w00010.asp
Cheers, CS
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As such, they carry as much weight as a feather in a snow storm.
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