Surprising Fine Woodworking Joint Strength Results

there is no way that’s right! if it was then we would see splines in all of our antiques. those guys knew what they were doing. i will never believe that a spline is as strong as a mortise and tenon.

They do say that seasonal wood movement will tear the miter apart but does that mean you should use it for mdf? I dunno it all seems weird to me, and the butt tenon being weaker than just a butt joint… Can’t…. compute….

-- I am always doing that which I can not do, in order that I may learn how to do it. - Pablo Picasso

now that just doesn’t make sense. kinda like the ole Greek method of science. sit and think. like if a tree has green leaves anything with green leaves it a tree. kinda like they just said glue is stronger than the wood so doesn’t matter the joint.

” kinda like they just said glue is stronger than the wood so doesn’t matter the joint.”
^^^Well, in truth: Modern glue IS stronger than the wood. GF glue generally has a shear strength stronger than that of hard maple….and thats strong!

If you take any two pieces of wood and glue them long-grain to long-grain with regular wood glue, then put one side in a vice and try and break the joint the board will break before the glue joint fails.

I still havent seen the article, but I’m eager to take a look when it comes in.

Ryan

-- If you can't set a good example, at least serve as a horrible warning... www.rarewoodcreations.com

Very interesting article with some interesting results. The numbers are there and it’s not like they tested only one example of each joint. The butt joint being stronger than the cope and stick was a big surprise. I would have liked to have a little more explanation as to why these results happened.

-- Working at Woodworking http://www.VillageLaneFurniture.com

I haven’t seen the new article but here is an older one .
There was a demo on youtube a couple years back suggesting that the glue was the stronger component in most non structural joints.
Here’s a test done by the folks at wood magazine
Bob

-- A mind, like a home, is furnished by its owner

I haven’t got the new issue yet, but I look forward to reading the article when it does come.

there is no way that’s right! if it was then we would see splines in all of our antiques. those guys knew what they were doing.

Yeah, but those guys didn’t have the glue we have today, so you can’t really use that logic.

-- Charlie M. "Woodworking - patience = firewood"

thats very true charlie. didn’t even think of that. but if you think about it the glue joint will most likely fail or weaken sometime. if your glue fails or you get a weak joint i’d rather a dovetail or a mortise and tenon over a biscuit or spline or any of that stuff any day.

^^TWW: The likelyhood of modern glues failing is pretty much zero. Unlike the glues of old (hide, fish, etc) modern glues are synthetic….waiting for modern glues to degrade or weaken is kinda like waiting for a plastic bottle to decompose in the landfill :)

Anyway, Heres the reason the splined miter is so strong…..and yes, stronger than M&T :)

I’ll use a simple stick as an example:

With a M&T joint, the failure point is at the shoulder of the tennon….across the grain. Kind of like breaking a stick across your knee. Not that difficult to do, as wood fibers will tear and fracture perpinicular to long grain easily.

Now…take that same stick, and instead of breaking it laterally over your knee, grab ahold of both ends and pull the stick apart (tear it in half)...you can’t do it. That’s because the shear strength on wood is extremely high (maple is something like 10,000 lbs shear strength)

With a splined miter, you are essentially stringing that “stick” across the joint, and in order to get it to fail you will have to tear the “stick” in half. The miter itself is weak (end grain to end grain) but by putting that long-grain spline across the joint at 45 degrees, you make it super strong.

Ryan

-- If you can't set a good example, at least serve as a horrible warning... www.rarewoodcreations.com

Chris Schwartz sure will be sad to see that the pegged M&T is weaker than just your garden variety M&T :)

-- I am always doing that which I can not do, in order that I may learn how to do it. - Pablo Picasso

I have made my living for a long time analyzing stress in various components. After looking at the FWW article, I don’t see any inconsistencies. Wood, in general, is amost 20 times stronger with the grain that across. If you look at the FWW article, most of the failures are across the grain. The longer the segment of long grain sticking into the mating part, the higher the load that was carried. This would seem to validate the thru tenons from the Arts & Crafts period.

From the pictures and descriptions, I suspect that the domino joint was not correctly glued. Other than that, the relative strength of the joints seems consistent with both theory and my experience.

Back to tenon length for a minute, the cross grain strength is quite low. The real resistance comes from the bending resistance offered by the portion of the rail “attached” to the stile. This bending resistance is proportional to the third power of the length. Thus, if you increase the length of the tenon from ½” to 1” the strength goes up by a factor of 8!

As far as the tenon is concerned, the strength is directly related to the cross sectional area. The more the better!

The bottom line is that stub tenons and cope-and-stick have proved to be durable in cabinet construction despite having low numbers compared to other joint types. On the other hand, where large stresses are anticipated as in chairs, large long tenons are the way to go.

Map

-- measure once, cut once, swear, start over

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I recently made a basket out 1/4” thick by3” wide joined with glued miters and no splines. I had it glued up but I didn’t like the scrolled design, so I tried to tear it apart by hand and couldn’t do it. I then stomped on it and it broke, but not at the miter joints.

Knowing these things allows us to take advantage of these fantastic glues without obsessing about strength and all the extra work that entails.

-- Mike, American in Norway