Art Ransom wrote:

How much clamping pressure is too much?

snip

This question was asked in "rec.woodworking" earlier this year. A "Google"
search turned up the following:

*****
LRod posted to "rec.woodworking" on Mar. 16, 2004:

Someone named Bruce posted this about a month ago:

After our discussion here, I wrote to technical support at Titebond
regarding our discussion of clamp pressure. I got a next day reply
from a very knowledgeable and helpful gentleman, Mr. Zimmerman. I'm
posting it here.

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
I am writing in response to your question about clamp pressure.
First, your calculation and understanding is correct. If you wanted
to produce 200 psi over an area 12" x 12", you would need 28,800
pounds of force. On the other hand, it is not clear whether you
often, or ever, fell short of the actual, required clamp pressure.

The actual required clamp pressure for any bond involving a wood glue
is a combination of the small amount of pressure required to squeeze
the glue into a thin, consistent layer, and the pressure necessary the
compensate for any distortion or lack of fit in the wood stock being
used. That means when the surfaces of the pieces being joined are
true, and there is no gap between the pieces when they are dry fit,
very little pressure is required. If, however, the same assembly is
being made using pieces which are bowed, twisted or ill-fitted, the
required pressure is much greater, and is largely the pressure
required to straighten the wood and pull it into position. Thus, the
actual required pressure for a bond also reflects the thickness, or
fight, of the wood involved, with much more pressure obviously
required to straighten a very thick piece of maple or oak than to
straighten a thinner piece of the same species.

In many applications, then, pressure, serves to compensate for some
lack of diligence in wood preparation. That being the case, good wood
preparation lessens the need for, or dependence on, pressure. In the
case of our literature, the high suggested pressures reflect the fact
that those individuals being addressed include those who, at least on
occasion, are trying to bond thick, poorly fitted pieces of wood, and
for those readers, the high, suggested values are, indeed, necessary.

Finally, because the bond strength produced in a joint is the result
of the entanglement of the glue particles which have been drawn into
the pores and anchored to the wood on the two sides of the joint,
there is rarely any concern for applying so much pressure that the
glue is all squeezed out.

In fact, the bond strength achieved increases as the bondline or layer
of glue becomes thinner. Given that fact, there are only two
situations in which high pressures may be counterproductive. First,
there is always a concern that the pores of wood at the bonding
surfaces not be crushed, and that is the reason that our listed
pressures are lower for the softer woods. The second situation deals
with bonds involving end grain or other open grain. There the concern
is that the open grain is prone to suck up a large amount of glue and,
if that thirst has not been quenched before clamping, that excessive
absorption of glue may result in a starved, and weak joint. Because
most bonds involve face or edge grain which is relatively straight,
that particular risk is rarely a concern.

I hope this response is helpful, and ask that you feel free to write
again or to call me at 1-800-###-#### if I can be of any further
assistance.
*****
--
Jack Novak
Buffalo, NY - USA
(Remove "SPAM" from email address to reply)