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This has been bugging me because it seems counter-intuitive. Why do some tables have long, wide tenons? I'm talking about tables that have, for example, 3" wide, 1/2" thick, 1-1/2" long tenons on the aprons. Is that really necessary? Dining room chairs are very often joined with two 3/8" dowels at each joint with depths of 1" on either side of the joint. That seems sufficient to hold 280 pound Uncle George and he no doubt weighs more than all the dishes and food on the table.

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doweling is quick and dirty (anything to get the product out) and they're really all that strong and they fail over time due to expansion/contraction of the wood surrounding the dowels......
table legs act as levers against the apron joinery hence the better joinery...
got cheater bar...
how ever you can find leg/apron joints that are doweled...
 
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Discussion Starter #3 (Edited)
how ever you can find leg/apron joints that are doweled...
Yeah, I've seen some of those too.

Colin Knecht, who has a Youtube channel on woodworking, did did a comparative strength test on various kinds of joinery. It ranged from butt joints to lap joints and included pocket screws, biscuits, dowels and tenons. The biscuits were not suitable for much except face frames. But what surprised me was that dowels and tenons were not that much different strength-wise and during one set of tests both the dowels and tenons actually broke the material he was joining, not the joint.

But I think he made a measurement error...it appeared to me that the lap joint failed at about 800 pounds, not 700 as he stated.

I'd guess you are correct about seasonal movement eventually loosening dowel joints but why would not tenons also loosen?

Don't know if you've seen this but here's the video of his test:

 

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Yeah, I've seen some of those too.

Colin Knecht, who has a Youtube channel on woodworking, did did a comparative strength test on various kinds of joinery. It ranged from butt joints to lap joints and included pocket screws, biscuits, dowels and tenons. The biscuits were not suitable for much except face frames. But what surprised me was that dowels and tenons were not that much different strength-wise and during one set of tests bothe the dowels and tenons actually broke the material he was joining, not the joint.

I'd guess you are correct about seasonal movement eventually loosening dowel joints but why would not tenons also loosen?

Don't know if you've seen this but here's the video of his test:

Jointery tests
All the data I see on dowel joints test against "young" joints, not one that's been through several seasonal variances in humidity.
I thought R. Bruce Hoadley put this one to bed 30 years ago in Fine Woodworking...
As he says, "IIf good dowel joints aren’t the oldest joints ever made, loose ones must be."
Preview - The Dowel Joint - Fine Woodworking Article

tests run on "fresh" joints. And the tests are "racking" stress only.
And I am skeptical of anyone who says, "500 pounds of pressure" Force is measured in pounds, pressure in pounds per square inch, and torque in foot-pounds (or the metric counterparts).

"For every complex problem there is an answer that is clear, simple, and wrong." (H.L. Mencken)

Joint strength is a complex problem....
There are variables in the material

Variations in wood species, moisture, density, propensity to split, wood movement indices, etc. etc.
Lumber vs. engineered wood
Variations in wood axes - Anisotropic – Orthotropic properties of radial, tangential, and axial planes (Hooke's Law)
Fit / gaps (is a "weaker joint" that fits well better than a "stronger joint" that is ill-fitting?)
Surrounding wood failure (even if the "joint" A is 10 times as strong as another type of joint B, if the wood fails first on both, it's a moot point)
Depth of penetration of joint from one component to another

Another of the complexities are the types of stress. I identified 6 kinds o stress on a joint --

shear
compression
tension
racking
cleavage
peel.

Did you ever see a dovetail on a chair leg or a half-lap on a drawer?

There are also different ways the wood is joined

side-grain to side-grain (at 0 or 90, or some other angle)
end-grain to end-grain
end-grain to side-grain
end-grain to face-grain

Joints also have multiple attributes

Speed to make
Appearance
Cost, especially if new tooling or special per-unit materials involved
Strength

And there are multiple stress-failure considerations

Soft fail (gradual loosening) vs. hard fail (fails with a crash)
flexibility vs rigidity
Repairable vs. non-repairable (the "shear pin" factor)
moisture fluctuations over time, differential contraction's effects on the joint
Dynamic (sudden) vs. static (continuous) stress

Most every test is done on a "fresh joint" not one that's gone through multiple wet-dry seasonal cycles. (one of the dowel jig vendors that claims they're the strongest joint specifies materials & their dimensions, type of test, and other considerations to tip the scale.
Unless you have end-grain to end-grain (a very uncommon joint in practice), the wood will be moving in different directions and tend to break the glue joint in at least one side of the joint
You have minimal glue surface (on a round dowel) and most of that will be end-grain to side grain
You have limited penetration distance if you are using dowel pins (usually max at 3/4 to 1" each side)

And the big question is, "Is it strong enough?" You have different strength requirements for a chair and a picture frame.
 

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My first thought aligned with stick. The joints tested where fresh joints. This, by coincidence, was how, long ago when I was an apprentice, we tested our own joints when we were learning how to do them. and why. We learned how to make them... then we destroyed them to see the effects. When I later had apprentices, I had them do the same.

PhilP and I had the same job title, different names for the same job. Me, finish carpenter, him Joiner. Just from my experience:

Biscuit, dowel, loose tenon are all sub-types of loose tenon arch-type joints. They strive to be an easier version of a Mortise and Tenon joint, where the long grain of each side are glued together to try to join the grain.

In a Biscuit Joint, the Biscuit wafer's grain is already compromised. But is a quick and easy way to join thin stock together, such as picture frames or door/window casing. Biscuits consistently will fail at the wafer. But that joint is stronger that a butt joint.

Dowels are stronger and last a little longer. The dowel pin has a better grain structure. But on destruction, the shear force perpendicular to the dowel (from any side) will push the round of the dowel to try to separate/spit the grain of the sides of the joint. It is better than a biscuit but less area/strength than a loose tenon. Once you get to this level, then you also get into pinning. where a tenon can be pinned to help prevent it from separating.

A loose tenon has 2 rounded sides, 2 flat sides. On destruction, the shear force perpendicular to the rounded sides will push the round of the loose tenon to try to separate/spit the grain of the sides of the adjoining joint. It is better than a dowel, but less less area/strength than a mortise and tenon.

Mortise and tenon have the tenon made out the end of one side of the joint, so no compromise of grain (continuation of). Flat sides and more area.

Then spline type joints, which basically align along the same physics and typing. There is some crossover. You have loose spline and spline. Each sub-type under that has further sub-types. You join grain from sides to try to get them to join together in strength. Post and Beam construction is M&T. Destruction is hard. You really have to destroy the wood of one side to destroy it.

To me, a pocket screw, if used without a loose tenon, it is just a reinforced butt joint. I use pocket screws in cabinetry, but as reinforcement to a loose tenon or spline joint. Pocket screws are not a new concept. Kreg did not invent them. Kraig just had good marketing and a better constructed jig.

Over time, with shrinkage and contraction, if the glue and pieces of the joint do not have the same factors, the joint will fail in a different way, from separation. . Each step up, takes longer to fail. If the joint is reinforced by fasteners, wedges or pins, it will last longer, because it helps prevent separation. If you use the wrong adhesive, where the adhesive is too much difference, from the wood, then the wood around it breaks down and fails.

But, don't beat me up. That is just what I was taught, and what I've experienced since the mid-70's. That is how it's been taught before me, since, at least the 1600's.
 

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Discussion Starter #6
I "get" Stick's comment about dowels loosening over time. We have a dining room set my wife inherited from her parents and I've had to knock some of the chairs apart and re-glue the dowels. But, like I said, they seem to hold the mythical 280 pound "Uncle George" OK after they've been repaired. And it's hard to ignore the similar results of the dowel and mortise & tenon joints back-to-back in the video.
 

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Interesting lessons provided by Stick and Chuck. My choice for furniture would by tenons due to the surface area provided. All joints will degrade over time (my knees are a good example) so plants turning out the junk probably think that either the piece will be in the dump before it falls apart or at a minimum their warranty will have expired.
 

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The picture from DaninVan is exactly what concerns me about a lot of what I build. Things have a limit as to what they should be expected to withstand.
 

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Keith; joking aside, did you notice the absolute confidence that the table dancers have in the ability of the furniture to take it?
Why are they so confident? Where did they learn that a table can take hundreds of pounds of weight? Seriously.
And apparently it can...
 

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It has nothing to do with confidence Dan, it has to do with stupid and that is the hardest thing of all to fix.
 

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Yes, but, but, but...
Stupid aside, the tables apparently can take that kind of punishment (without collapsing). A testament to centuries of trial and error in woodworking.
We often have discussions here re different ways of doing stuff. Sometimes they arise from a relatively inexperienced woodworker wanting to reinvent the wheel, ignoring the previously mentioned centuries of acquired knowledge and tested experience.
It's great that folks want to think things through for themselves, but they ignore the wealth of inherited knowledge at their own peril and expense.
That's the whole point of apprenticeships.
My background is in building construction, not cabinetmaking. I've learned so much from being 'tutored' by you guys over the past few years; I owe you a huge 'Thankyou!!!'
 

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It's for when that elephant in the room, or the 800 Lb gorilla in the room, decides to sit down.
 
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