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When gluing pieces together, I've always used an abundance of clamps and tightened them with all my might. Is it possible to tighten them too much? At some point, could I squeeze out so much glue that the joint becomes weaker?

I can see how tightening too much can cause warping or damage to the wood. Here, I'm mostly curious about the effect the tightness has on the performance of the glue.

22

Hoping to run into some more tests and calculations but this is a good start.

Yes

Clamps can be too tight and the joint could weaken or even warp, in a sense, if too much pressure is applied.

I found this out by accident using spring-style clamps to repair a break in a board. I left it out overnight for the glue to set with 3 spring clamps keeping it in place. The spring tension increased slightly during that time and it started to pull the piece away. While it still bonded, the extra tension was enough to set the break at an angle. I had to re-break and reset but the damage was done and the joint was never perfect.

I now use C-clamps for most of my gluing since I can control the amount of pressure exerted from them and I know that I can leave them alone long term and they wont move.

I would suggest using a non-spring clamp for gluing. The amount of glue is important as well as you don't want sliding to occur.

To continue on how much glue to use that would depend on your wood you have. Softer woods can be more porous and take in more glue than some of the harder woods.

Gluing SCIENCE!

A post from WoodGears.ca performs a series of experiments using some maple and varying amounts of pressure.

If you scroll to the bottom you will see the overall results. The pictures are very telling as you would see excessive pressure removed most all the glue from the joint.

The summary findings from that post.

  • Only applying glue to the mortise of a mortise and tenon joint is a bad idea
  • Excessive clamping force can weaken the joint strength by up to 20%
  • A very smooth scraped surface is just as good, if not better than a sanded surface, as long as not clamped excessively hard
  • Gaps in a joint, as long as they are filled with glue, do not adversely affect joint strength.
  • Note

    I found this article when researching this topic. According to the comments the numbers and advice referenced could be considered flawed. Just be aware if you find yourself reading this article. Read the comments as well.

    • 1
      Matthias did some tests and his results are that it can weaken a joint up to 20%. – Maxime Morin Apr 1 '15 at 13:45
    • I'm reading that article now! – Matt Apr 1 '15 at 13:54
    • 1
      Matthias makes such cool stuff :D – Daniel B. Apr 1 '15 at 23:22
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      Also interesting is that the gapped joints are the strongest, which I'd say supports the claim that glue is stronger than wood. – Daniel B. Apr 1 '15 at 23:25
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      Glue is very much not stronger than wood Daniel. A well-formed glue joint is stronger than the surrounding wood, as can be clearly demonstrated by simple tests run at home, but that's not the same thing as the glue itself being strong. – Graphus supports Monica Apr 2 '15 at 8:21
    7

    Yes clamps can be too tight but not for the reasons stated above.

    TL;DR The fear is that over-clamping will lead to a starved joint is largely baseless. In practice it is nearly impossible to do without severely damaging the wood.

    Necessary clamp pressure has been studied extensively by scientists for the timber industry, who, unlike woodworkers, can't rely on hearsay, half-truths and best guesses. They have to know how to create the strongest possible joint. And such testing has proven that very high clamp pressures are required for the strongest joint to form, beyond 1,000 pounds per square inch for some species (NOTE: this is at the glue-line, so the pressure applied by the clamp face is many thousands of pounds)

    For the home woodworker here's the take-home message: short of crushing the timber it's nearly impossible to over-clamp.

    The persistent myth that you can starve a joint by over-clamping it is based on a flawed understanding of how glue works in a wood joint and/or misinterpreting individual experiences where this appeared to happen. The actual cause however was one, or a combination of, other factors. These include:

    • too little glue applied, which leads to incomplete wetting of the opposite face
    • waiting too long before bringing the workpieces together, which also leads to incomplete wetting of the opposite face
    • not glueing freshly-worked wood surfaces — wood surfaces cut, planed or sanded can 'glaze' over time, meaning they're no longer as absorbent and therefore they can't be wet properly by the glue
    • wood surfaces not flat or smooth enough, wood must be very smooth for a good joint to form with the mating surface
    • clamps being removed before the glue had dried sufficiently

    Note: the above apply to conventional wood glues, including polyurethane adhesive, but not to epoxy.

    Lastly let me clearly state that Matthias Wandel's glue test linked to above is highly flawed and not at all scientific. He acknowledges himself that he didn't give epoxy a fair test (which he didn't) and yet he did not repeat those tests. That's the first hint that you might want to take the results with a pinch of salt.

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      This is good information. I was having an issue coming up with a reliable reference of information. Do you have a external reference for this? I think it would really round out this answer. – Matt Apr 2 '15 at 11:35
    • Unfortunately I didn't get this from just a single source I can point you to (otherwise I would have included one or more links). This is accrued from one or two industry documents that I can't link to online, as well as various posts on Woodweb (primarily from Professor Gene Wengert). – Graphus supports Monica Apr 2 '15 at 15:14
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      If you have something just for this The fear is that over-clamping will lead to a starved joint is largely baseless then I would love to see it. – Matt Apr 2 '15 at 16:30
    • "glaze over time" Is this by observation or can you direct us to a source that might explain the glazing? – Ast Pace Jul 29 '15 at 1:06
    • @ASTPace, as I say in my comment above to Matt I don't have the links for this but look for the extract of chapter 10 of Wood As An Engineering Material from the Forest Products Laboratory of the US Forest Service, You might as well look for the parent document because there's so much other great information in it. – Graphus supports Monica Jul 29 '15 at 7:22
    0

    Yes joints can be clamped too tight. Dry fit joints for good fit. Big strong clamps are band aids for poorly crafted joints.

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