Thread: Good Wood Adhesion !!!! ????? #####

According to Mark Duginske a dressed surface is best for gluing. Contrary to what I've always been told that a rougher surface is best for gluing.

Originally Posted by funkychicken

According to Mark Duginske a dressed surface is best for gluing. Contrary to what I've always been told that a rougher surface is best for gluing.

Hi Funkychiken....

so you are saying you been roughing the surfaces for better bonding and Duginske told you otherwise...so Duginske supports the publication !!!

Anyone ever experimented with this ??? (I know i am...but i have no equipment to test strngth of bond, except my hammer )

Thanks,

George
(cyprus)

I got lost halfway through reading it, too hi-tech for me.
What I started thinking about is metal. Take 2 cubes with highly polished surfaces and rub them together. They form a bond that is almost inpossible to pry apart.
Then take 2 surfaces with very smooth laminated surfaces. The same thing happens.
It appear like the smoother the surface the stronger the bond.
I tried it with 2 wood cubes with highly polished surfaces and the same thing happened.
I read somewhere else, that the stronger (as in pressure) the clamping and the thinner the glue surface, creates the strongest bond. By thinner I do not mean starving the surface of glue but squeezing the excess glue out.
That leads me to believe that the rougher the surface, the weaker the bond.
HTH
Wolffie

Originally Posted by Duke_Nukem

Hi Funkychiken....

so you are saying you been roughing the surfaces for better bonding and Duginske told you otherwise...so Duginske supports the publication !!!

Anyone ever experimented with this ??? (I know i am...but i have no equipment to test strngth of bond, except my hammer )

Thanks,

George
(cyprus)

I've never used a rasp or hammer to rough up the timber, just a bit of 100grit

I think Wolffie's bang on the money

io think the lack of air between the surfaces in this case, causes the two surfaces to be ''sucked'' together (atmospehre pressure) I think !!!


nevetheless its quite interesting to read all of your comments...

I will have to try and simulate a few experiments:

1. rough surfaces
2. planed (hand plane) surfaces
3. planed and roughened with rasp
4. planed and roughened with 100 grit
5. planed and smotthed to a high grit 800 grit
6. planed and grit to 220..then wet surfaces and when no more roughenss exist go to 800 grit
7. planed and grit to 220..then wet surfaces and when no more roughenss exist stop and glue (hepefully i will have open fibers here)

i will try this on 5cm x 4cm x 1cm ''tiles'' same glue and hopefully same pressure (by hand)
then after set i will try and brake them appart with my hammer !!!!


not a scientific quantified method but thats all i got !!!

I will keep you posted of the outcome....

any more thoughts out there !!!!



Thanks,

George
(Cyprus)

Originally Posted by Duke_Nukem

io think the lack of air between the surfaces in this case, causes the two surfaces to be ''sucked'' together (atmospehre pressure) I think !!!


nevetheless its quite interesting to read all of your comments...

I will have to try and simulate a few experiments:

1. rough surfaces
2. planed (hand plane) surfaces
3. planed and roughened with rasp
4. planed and roughened with 100 grit
5. planed and smotthed to a high grit 800 grit
6. planed and grit to 220..then wet surfaces and when no more roughenss exist go to 800 grit
7. planed and grit to 220..then wet surfaces and when no more roughenss exist stop and glue (hepefully i will have open fibers here)

i will try this on 5cm x 4cm x 1cm ''tiles'' same glue and hopefully same pressure (by hand)
then after set i will try and brake them appart with my hammer !!!!

Sounds goods!

What I'd do is glue two long pieces together and try to break them apart by

A: Whacking them against the ground or
B: Clamping one end to a bench and placing weights on the other end (slightly scientific method)

I have been taught to get surfaces to be joined as flat and smooth as possible before gluing, and with some timbers it is also important to glue them as soon as possible after dressing the timber to prevent oil from the timber interfering with the bond. I use a hand plane or scraper to get surfaces flat and smooth.

Originally Posted by Wolffie

I got lost halfway through reading it, too hi-tech for me.
What I started thinking about is metal. Take 2 cubes with highly polished surfaces and rub them together. They form a bond that is almost inpossible to pry apart.
Then take 2 surfaces with very smooth laminated surfaces. The same thing happens.
It appear like the smoother the surface the stronger the bond.

a bit of physics, or what little I remember after all the years

when two flat highly polished metal surfaces come together you get atomic scale attraction between the two surfaces — I think of it as the two objects grow together into a single solid.

the same is not true with wood because you can't polish the surface to the extent that you get atomic scale attraction

I read somewhere else, that the stronger (as in pressure) the clamping and the thinner the glue surface, creates the strongest bond. By thinner I do not mean starving the surface of glue but squeezing the excess glue out.
That leads me to believe that the rougher the surface, the weaker the bond.

agree with a rough surface you have gaps that the glue has to fill.
These gaps represent potentioal weak spots in the joint


ian

Never was much good at physics in school, I hated the teacher but, as far as I know, laminex is not made from metal and yet it sticks together the same way???
Wolffie

Wolffie
I think it's a very similar process
atomic/molecular bonding from one surface to the other

ian

Don't go overboard with gluing.

If you spend hours prepping the surface and get it flat to within a molecule, the timber will fail before the glue.

If you prep the surface with a jointer and leave the slight scallops in place then glue, the timber will break before the glue.

If you split a log with an axe, glue it back together and clamp it, the timber will break before the glue.

If you use glue beyond its shelf life the glue will fail (have proved this with an old pot of Triton glue )

It ain't rocket science, give it a light sand before gluing and clamp it up. That's about all it really needs. Very oily woods may need other glues or preparation.

EDIT: I have read a number of instances where over-sanding to ridiculous grits actually weakened the joint. Apparently the glue had trouble adhering.

Originally Posted by Groggy

I have read a number of instances where over-sanding to ridiculous grits actually weakened the joint. Apparently the glue had trouble adhering.

Interesting, Groggy, but it's hard to see how 'proper' sanding would do that. Mike Darlow wrote in an old "Australian woodworker" how he tried to impress a client on his first woodturning job by polishing each & every spindle for a staicase with the bevel of the skew. They looked perfect, he thought, but got a torrent of abuse from the painter because they wouldn't absorb paint properly! Power sanding with worn paper might do a similar thing, perhaps?

What's already been said above about the best joint being two freshly-planed, matching surfaces is absolutely true. Toothing, roughing-up, and all the other tricks to make 'keyed' surfaces is bollocks, and will, in most cases, weaken the joint as already said.

Also true that wood & metal are not exactly equivalent in this discusssion, until the glue is applied, then molecular level forces do come into play. A proper glue bond depends on the glue 'wetting' the surfaces, & this happens best when the surfaces are cleanly and freshly cut. And the more closely apposed the wood to be joined, the stronger the glue line, because the material strength of the glue goes up as the glueline gets thinner (unless you squeeze all of the glue out!).

This isn't exactly new - it was well-researched by the Brits trying to make the wooden-bodied Mosquito bombers hold together. A bloke called J.E. Gordon has written a marvelously-readable book with lots of good technical info on the subject of glueing, among other things. It's called "The New Science of Strong Materials or Why You Don't Fall through the Floor". Highly recommended for any serious woodie.

Cheers,

Smooth surfaces are good for glueing and a strong bond does not require too much glue either.

Originally Posted by IanW

Interesting, Groggy, but it's hard to see how 'proper' sanding would do that.

I agree, but proper sanding would not include going over, say, 400 grit. In the discussions I was reading the forum members were discussing the effects that high grit sanding had on finishes. A number of posters said they had trouble with uneven absorbing in the grain if they went over (I think) 1200 grit.

Out of that conversation sprang a side discussion on sanding to very high grits before gluing. The theories were various but there were two schools of thought. The first claimed that a highly polished surface allowed the glue to exit and therefore starved the joint. The second theory was that the surface did not 'key' properly and the glue did not bond the surfaces to the depth necessary for a strong bond.

But, as you say, 'proper' sanding would not go to those lengths. A wipe with 180 - 320 before glue up seems to be more than sufficient for a good surface.


With regard to finishes, some finishers recommend to use a higher grit on end grain than the surface grain to reduce the absorption of the stain, making the stain appear more even, instead of darker on the end grain.