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Just wondering about gluing things...

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Forum topic by SST posted 11-10-2007 04:59 AM 1691 views 1 time favorited 17 replies Add to Favorites Watch
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SST

790 posts in 5248 days


11-10-2007 04:59 AM

Topic tags/keywords: glue

I got to thinking the other day (doesn’t happen all that much, so I try to pay attention when it happens), and I started to wonder about glue joints and what makes them good, or not-so-good, or lousy and I realized that I’ve been gluing boards together in largely the same way as I learned to do in junior high shop class. Saw the boards, run them through a jointer, and glue ‘em up. This has served me well all these considerable years. In fact, one of my tables from that class still stands in our house and hasn’t fallen apart yet.
What got me thinking was that I was doing a bit of paint work on my car, and was roughing up the surface to paint, knowing that a rough surface is needed for a good bond, and I got to wondering if using a jointer and getting that relatively slick edge was really the best for a good bond.
It would seem to me that it might be better to (assuming your saw cuts true) leave the edges somewhat rough to get a better gluing surface, or maybe even run some 80 grit paper over the edge.
Does this sound dumb…or maybe everyone already does this and I took until now to figure it out, or…I don’t know, it just seems that the glue would adhere better to a roughened surface. -SST

-- Accuracy is not in your power tool, it's in you


17 replies so far

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Karson

35271 posts in 5454 days


#1 posted 11-10-2007 05:47 AM

Glue seems to extend into the pores of the wood. So microscopically it probably isn’t as glass smooth as you would seem.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Appomattox Virginia [email protected] ā€ 

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SPalm

5338 posts in 4936 days


#2 posted 11-10-2007 06:04 AM

Everything that I remember reading on gluing wood is that it should be a smooth surface (smoother the better). Like Karson said, the glue hooks on the fibers inside the wood. This is not true for metal (no fibers to sink in and grab). On endgrain, the fibers are really open pores and the glue has nothing to grab on to.

I have heard that if your jointer is dull, it can mash the fibers and make the glue have a hard time.

Steve

-- -- I'm no rocket surgeon

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SST

790 posts in 5248 days


#3 posted 11-10-2007 06:32 AM

Back to Karson’s comment, by roughing it slightly, wouldn’t you expose/open a lot more pores? Also, with a slight amount of roughness (I’m not talking about having really rough surfaces) would you help minimize “glue starvation” by retaining a bit more glue in the little pockets and sanding scratches?
I know this runs counter to what I (we) learned, but at some level, based on experiences in other ares, it somehow makes sense to me (although, lots of things make sense to me that seem really goofy to other folks)
-SST

-- Accuracy is not in your power tool, it's in you

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Karson

35271 posts in 5454 days


#4 posted 11-10-2007 06:45 AM

Well you want a rough surface but you also want all areas in contact with its mate. Glue works by bonding a surface with another surface.

If for example you had an 1/8” gap in many places along a board then it would not be as good a joint as a straight edge glued up.

In olden times they would create a “Sprung joint” That would usually be made with a jointer and it would leave the ends high and the middle low, maybe only a 1/16. What this would do is when you clamped the board it would put extra pressure at the ends of the board. Because, that is where is would usually come unglued, not in the middle. The wood itself continued to keep the ends pressed together, after the clamps were taken off.

-- I've been blessed with a father who liked to tinker in wood, and a wife who lets me tinker in wood. Appomattox Virginia [email protected] ā€ 

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Betsy

3394 posts in 4949 days


#5 posted 11-10-2007 07:42 AM

One other question along these lines, sorta. I’ve always learned that you should have a clean edge to glue up – another words no burn marks, saw marks, etc. But I’ve been watching a lot of videos made by folks who make their living making wood work——and there seems to be burn marks all over the place on edges. So is it a big deal to have burn marks on the edges or not. I always understood that the burn closed the pores and then the glue would not adhere and you would get failure.

-- "Our past judges our present." JFK - 1962; American Heritage Magazine

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Sawdust2

1466 posts in 5141 days


#6 posted 11-10-2007 03:16 PM

I took a course with Ian Kirby many years ago. He teaches smooth edges.

The chemical composition of the glue is stronger than the wood so a good glue joint will stay and the wood will break or tear.

He demonstrated the correctness of his theory by wetting the edge with water.On a straight edge the water will provide the cohesive force necessary to keep the wood together. If the wood is curved or ragged the water will not hold the pieces together.

He told us that we were paying him to teach us what to do. That he had done all the experiments over the years and knew what worked and what didn’t. That was his explanation of your question.

-- No piece is cut too short. It was meant for a smaller project.

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SST

790 posts in 5248 days


#7 posted 11-10-2007 04:39 PM

With all due respect for Ian Kirby, the water example doesn’t (sorry for this) hold water. I can take 2 smooth pieces of almost any material (aluminum, steel, plastic) and add a few drops of water and they stick as a function of water molecule’s cohesive nature. But we’re talking about how the glue interacts with the wood. If the glue does, indeed, enter the wood pores, then it seems to me a straight sawn edge would be preferable to a smoother jointed one because there would be more exposed pores in a (microscopically) undulating surface made by a saw blade…assuming your cuts were true.
I know this flies in the face of tradition, so maybe what I need to do in order to get this raspberry seed out of my wisdom tooth is to try it both ways and apply loads to both and measure the amount of weight to the failure point. (I guess I’m wondering if this has already been done by far more credible persons than I, or if we’ve simply always done it the other way because, like me, we all just learned it that way. Anyway, it’s always great to discuss this stuff. That’s what I like about this site. -SST

-- Accuracy is not in your power tool, it's in you

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Andy

1713 posts in 4962 days


#8 posted 11-10-2007 05:17 PM

My 2-cents worth.
Smooth is best because this gives you the most surface.A rough surface on each board means they are touching on those points only.Smooth is a relative term of course.Wood viewed under a microscope is always rough.So we are talking about an edge that looks smooth to the naked eye…. and feels smooth.Just dont over do it. A high polish or burned edge actually compresses and seal the fibers and inhibits glue absorbtion.

Side point-Remember also, that most glues do not work well as gap fillers.The best is epoxy for gaps,then yellow glue, with the polys coming in last.( check out “WOOD” mag and ” FWW”,both have recent test on all the glues.)http://www.taunton.com/finewoodworking/Materials/MaterialsPDF.aspx?id=28897

If you have a joiner,use it.It will give you a good gluing surface.But, in 30 years of making furniture I have never used a joiner.( No hate mail please! :) Joiners are great! Its just that I have a very small shop and budget and have learned to work without one)

Here is what I do:
A) I use a glue joint rip blade that gives me a very smooth surface and a tight joint.
B) I use yellow glue,Titebond 3 most of the time now because it has a longer working time, at a lower temp and is great for cutting boards and outdoor furniture.
C) I thoroughly coat both edges.
D) I then clamp using as many as I can,spacing evenly and tighten just until I get an even squeeze out and then bump them just a little more.

I have been making and selling dining tables,hutches,entertainment centers,chairs,benches,jewelry boxes,beds without any failure of a glue joint using this method. I make cutting boards regularly and that is the best test on a glue joint.Its in and out of water and drying unevenly, puting more stress on the joint than a dining table will ever see. My family have some boards so old that I have resurfaced them twice and the joints still look good.

-- If I can do it, so can you.

View Dick, & Barb Cain's profile

Dick, & Barb Cain

8693 posts in 5353 days


#9 posted 11-10-2007 05:28 PM

For all the years I’ve been woodworking, I have never sanded the edges before applying glue, & I have never had a glue joint come apart.
I think the most important thing about gluing, is having two flat surfaces, & applying the proper pressure.

-- -** You are never to old to set another goal or to dream a new dream ****************** Dick, & Barb Cain, Hibbing, MN. http://www.woodcarvingillustrated.com/gallery/member.php?uid=3627&protype=1

View dalec's profile

dalec

612 posts in 4942 days


#10 posted 11-10-2007 05:48 PM

Thanks everyone for sharing your discussion. I learn a lot just reading the various topics posted on this forum. It makes learning so much more interesting and far less frustrating.

Dalec

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Betsy

3394 posts in 4949 days


#11 posted 11-10-2007 05:58 PM

I’ve learned through this discussion as well. It’s good when we good those rasberry seeds stuck in our teeth. Because just like in school, we all had the same question and were always releived when someone brought it up. Thanks SST for being the brave soul who raised your hand in class. I’ve learned something!!

-- "Our past judges our present." JFK - 1962; American Heritage Magazine

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SST

790 posts in 5248 days


#12 posted 11-10-2007 06:03 PM

See…that’s why I love this place!

-- Accuracy is not in your power tool, it's in you

View Todd A. Clippinger's profile

Todd A. Clippinger

8901 posts in 5153 days


#13 posted 11-12-2007 12:10 AM

Your old furniture pieces are good tests for what lasts over the years.

Argyllshire had pointed out a couple of articles that I recently read too and found very interesting. I think the most interesting point made was that you could not “starve” the glue joint. I believe that was in the Wood magazine article done by Titebond.

-- Todd A. Clippinger, Montana, http://americancraftsmanworkshop.com

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SST

790 posts in 5248 days


#14 posted 11-12-2007 05:38 AM

It’s interesting to hear you say that you can’t “starve” a glue joint. I was taught to clamp glue joints VERY tightly. Probably much tighter than I currently do, and I’ve never had a joint failure either clamping that way or using less pressure. As long as the joint is true, maybe roughness, smoothness, tightness or not so tight…doesn’t really matter all that much. I guess when joints start to fail I’ll look for a better way…until then I’ll keep doin’ what I’m doin’. -SST

-- Accuracy is not in your power tool, it's in you

View Alin Dobra's profile

Alin Dobra

351 posts in 4942 days


#15 posted 11-12-2007 05:59 AM

Guys,

One thing to keep in mind is that not all glues glue in the same way. The bond made by the aliphatic resin glues (all wood glues, including Tilebond glues) is similar to the bond the lignin (the natural “glue” in wood that keeps cells together). What this means is that all is needed is for the wood should come in contact with the glue and the glue should not span more than 1/64” or so. Surprisingly, a tight fit seems unnecessary (Finewoodworking or Wood magazine had an article about joint strength and they tested loose joints as well with surprising results).

The general purpose glues, like epoxy, are “mechanical glues”. The way they work is to crawl into spaces when they are liquid and when they harden they get stuck in the small imperfections. These glues depend crucially on uneven surfaces (sanding recommended). Polyurethane (both the glue and the finish) work in the same way.

Bottom line, for wood glues, sanding or roughing out of the surfaces to be glued is a waste of time. For epoxy and polyurethane glues, it might be necessary. On wood, wood glue is as strong as epoxy and about 4 times stronger than polyurethane glues (Gorilla glue that everybody praises). Since wood glue can bridge 1/64” gaps, clamping or not clamping the joint, as long as there is enough glue to go around and it does not get out, does not matter. If you do clamp though, you get invisible glue lines, as opposed to an ugly yellowish line.

I hope this helps,
Alin

-- -- Alin Dobra, Gainesville, Florida

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