Glue strength -- actual numbers?

[David]

:) I hear you George. We all have our opinions and nobody is going to change them. I'm not closed minded enough to refuse to learn "new tricks".

David

George wrote:

[David Chamberlain]

I was interested so I went down to the shop and took a 12" long X 10" wide piece of 13/16" soft maple. I ripped it in half and jointed the dges. I used Lee Valley glue and rubbed the two edges together to make sure that I had a joint that was evenly coated. I took 3 pipe clamps and used a pipe wrench on the handles. The clamps left very nice depressions in the outside of the boards.

I let the glue cure overnight and put the piece in my vise. I used a ribber mallet and beat the board until it finally broke. It did not break along the glue joint. One sample does not prove the issue but I was not able to produce a starved glue joint even with 1 clamp every 5 inches or so (1 clamp 1" in from each and and one clamp in the middle).

A more scientific experiment would involve using a hydraulic press on small (1" X 1") samples.

[max]

Soft maple is more porous that other woods. Hardwoods with little or no surface texture don't glue as well as woods like pine and mahogany. Also, 1 inch samples are not a great test because the short length of straight grain doesn't offer the board as much support and it will be more likely to split along the grain lines. WE would occasionally test glue strength by gluing up 3 or 4 strips into a top. The strips were 3 or 4 inches wide and 12 to 24 inches long. We then would put a strip at each edge raising it up off the bench and hit the center of the board until it split. Sometimes the glue joints failed if it was too cold in the shop when we glued it up. There are a lot of variables to an objective test. max

[Robert Galloway]

The starved glue joint is a myth. The important part of the glue is the part that soaks into the wood. I suppose if you could squeeze any excess glue out before it had a chance to soak in a significant amount maybe you could "starve" the joint. At a normal working pace, plenty of glue gets into the wood and the thicker the layer between the two pieces of wood, the weaker the joint. IMHO.

bob g.

David Chamberla> I was interested so I went down to the shop and took a 12" long X 10" wide

[loutent]

From all the research that I have read, the strength of the joint is not directly proportional to the clamp pressure applied. Certainly using a pipe wrench to tighten a clamp is overkill.

Normal hand pressure (or even less) is probably best if a nice thin coat of glue is applied to both surfaces.

OTOH, I have not mastered this either and learn with every new project.

Lou

[jev]

Attached is the text of message from Franklin - makers of titebond. This msg was originally posted to this forum and am sorry i can't credit the original poster.

Response from Frnklain/Titebond rep re:Calmping Pressure

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.

[mac davis]

originally posted to this forum and am sorry i can't credit the original poster.

What a pleasant way to say "Our glue will hold with little or no clamping pressure if you guys would get your shit together and make tight joints with straight wood... lol

Thanks for posting that... it is good info and a reminder to not blame the glue for my poor workmanship and to be more diligent in learning good joinery..

[George E. Cawthon]

originally posted to this forum and am sorry i can't credit the original poster.

Actually, what he said was quite different and positive not negative, "Our glue will hold if you bring the joints together, either with perfect jointing which then requires little pressure or with imperfect jointing which requires whatever pressure is needed to squeeze glue out along the entire length of the joint."

[mac davis]

That was my point, George... that it was a very positive piece...

Must be all those years in sales fermenting my brian, but I just appreciate how he stated it in such a positive manner...