Glue joint mechanics - your opinions.

[Anonymous]

Hi i have been making triple laminated longbows for a couple of years and bamboo backed bows as well. I got the infomation on their manufacture from Pip Bickerstaff who is a master bowyer and has made well over 10,000 longbows in England.

Glueing is a hot topic in the laminated wooden bow world as the bows undergo terrific stresses when bent and shot. All the good bowyers including Pip recommend having smooth close fitting surfaces at the glue line when using Cascamite and the titebond glues and applying tight clamping pressure but a slightly roughed up surface is ok with epoxies when uding oily woods.

Out of 10,000 bows made there have been very few glue failures, testament to this method.

Steve.

 

[Digit]

I went through the same excercises, also with metal I was handed a lump of steel and made to file all faces square, but many years later, with machines available that will accomplish the same feat with a lot less pain I ask if there is any validity to it today.
Most the old timers who taught me, with great patience, to plane straight would have jumped at the chance to use the machines that I now have, trust me!

Roy.

 

[steve tighe]

It's the area that you get the glue on that's more important,the cleaner the joint the more area you can cover with the glue,the more area you can cover with the glue-the stronger the joint.

You only key up sheet materials with smooth surfaces,such as mdf,that are going to take contact adhesives.

 

[newt]

I do not sand my joints, but in Februarys THE WOODWORKER there is an article on glue joints by Keith Smith, and guess what he recommends abrading with 100 grit. He claims this gives a better bond. Who wants to do a back to back test.

 

[steve tighe]

... but in Februarys THE WOODWORKER there is an article on glue joints by Keith Smith, and guess what he recommends abrading with 100 grit. He claims this gives a better bond.

That's a safe enough claim for anyone to make in a magazine newt,the glue joint will still probably be stronger then the timber itself whether the surface's been sanded,keyed or left alone(I'm presuming we're not talking about wedged joints here...?).

How can you test something like that if the timber splits before the joint goes,anyone can try it,glue two short wide boards together & then belt it off the corner of something hard.

I suppose it probably doesn't really matter too much in the long run whether you leave the surfaces to be glued clean,or scratch them up a bit.
I've glued up timber before that's had the glued faces come straight off the saw & had no problems,but that's because the glue-joint is not the weakest part of the component.

 

[Digit]

Never having had a joint assembled with modern glues fail, I have to say that any test would only show that all are strong enough for suitably sized joints.
If the worst is strong enough one that is twice as strong is simply gilding the lily.

Roy.