For those of you who don't get abpw:
Rough sketch of chair leg and rail (legs angled 7 degrees from vertical)
I know what I _think_, but which "floating tenon" joint to YOU _know_ to be stronger and why?
B is stronger because there is enough material strength in the mortise of the rail.
slightly stronger if you use a real tenon. For a chair this could be significant.
Chris
While I tend to agree on the surface, that's actually been proven to not make much difference, if any, and certainly a "real" _angled_ tenon could be much less strong depending upon the grain of the particular piece.
The jury is still out ... :)
Actually, with "A" there appears to be more "material" for "strength" at the top of the rail/stretcher, where you (not "you" personally, but a non-engineer for sure) would think it would do the most good:
hmmm.... ;)
"Not much difference" isn't the same as "no difference". Your point about the angled "real" tenon is well-taken.
There's only one solution. Build a few of each, test to destruction, and write it up for FWW.
Chris
I'm to lazy to do it for you but if you calculate the section modulus of each joint, you'll find "B" is stronger.
S = M/Z
S = Stress M = Bending Moment Z = Section Modulus
Z = I/d
I = Moment of Inertia d = Distance from centroid to outermost fiber.
I (for a retangle) = bh^3/12 = 1/2ad^2
b = base h = height a = area d = distance from center of area to center of bending moment.
Any decent strength text will give you the formula for a circle.
(Dredged from my memory from all those strength of materials classes taken so long ago.)
Have fun.
Lew
in the stretcher is short-grained and prone to splitting.
"B" does not have that flaw. "B" is stronger.
The quote was "not much difference, if any" ... and a lot of folks, like the beadlock folks, David Marks, and Lon Schleining (sp) of FWW, to name a few, seem ready to swear to/by the "if any" part.
The chair was designed a hundred years ago ... it's time now to suss out the possibility of new/different joinery methods for a "production" run that will hopefully last the same length of time.
I tried to cancel that post, but I guess it got through anyway. The racking resistance with shoulders will be fine (possibly better), as long as the tenon doesn't pull out or the shoulders fail.
"A" has the advantage that the mortices are at right angles to the ends, making it simpler to mill.
If you're looking at a "production" run, it really might be worth testing both designs to failure.
Chris
post hoc, ergo propter hoc? ... a 7 degree difference in grain direction would not be uncommon in any angled tenon.
But I tend to agree with you ... grain direction appears to be a key factor.
Bingo ... and therein lies the reason for the question. :)
Yep ... a limited production run, in a space limited shop (but with plenty between the ears, some would say), by a time limited wooddorker, wanting to test the limits of angled "loose tenon" joinery.
Actually, both are doable, and while I've already cracked the "B" methodology with a homemade jig, "A" is easier, and much, much more "precise" (at least without that JDS Multi-router I keep threatening to buy).
... and in joinery, precision often trumps even a tiny bit sloppy, with regard to strength.
Thanks for the input, Chris.
I don't think so. With B the rail would be clamped vertically and the mortise made vertically, the angle is irrelevant. To do A you've have to hold the rail at an angle. The leg is maybe harder depending how the mortise is made. Not very difficult to tilt the table on a drill press.
And I cast another vote for B being stronger. You've got more uninterrupted long grain on the end of the rail.
-Leuf
If you precut the rail/stretcher end to the required angle (7 degrees in this case), they (leg and rail) are both easy, 90 degree plunges.
"B" is trickier, less precise IME, and requires a much longer bit.
With a router you mean? How does one keep a router on the tiny surface of the end of the rail?
-Leuf
He has a cool jig to clamp to the rail that holds the router.
of "B", in what I'd call a double secret probation - uhh, wait a minute - a "Double Secret Wedged Tenon".
Usually a wedged tenon is a show joint but you can do a secret one by undercutting the edges of your mortises by the seven degrees described to form a kind of keystone shaped hollow.
Cut your wedges to fill that seven degree void once they bottom out on the mortise. Cut the appropriate slots in the tenon. Butter the tenon and the wedges up and insert the wedges just barely. Drive the loose tenon home to form a dovetail within the keystoned void. Prep the other end of the tenon the same way and clamp her up.
You'll wind up with opposing dovetails.
Dat sumbitch be strong.
Regards,
Tom Watson
tjwatson1ATcomcastDOTnet (real email)
I reckon ... excellent, vivid description. Thanks, Tom.
My credentials: degree in mechanical engineering with a specialization in strength of materials. Formerly a licensed professional engineer in the state of Illinois. Based on the rough drawing, if you made 10 of these joints each way and tested them to failure, I'm not sure you could measure a statiscally-meaningful difference between the two. Now, I haven't spent all night thinking about it, so if someone can make a persuasive argument to the contrary, I'm open. So with that said, offhand I'd go with the one you find easier to machine.
todd
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