Angled mortises vs angled tenons for chair construction

Rob, I am sorry for the confusion. You are correct. We will angle the back side at the bottom edge of the chairs side rail's face, the same angle as the splay of the back leg,  flowing the side of the rail out to nothing and then cut the tenon parallel with the outside of the rail and back chair leg. So yes it is a compound angle with the splay and the shoulders but with straight tenons. The stretchers have enough flex that we don't worry about the compound angle with it's tenon.
 
After reading my reply I have no idea what I was describing so here is a picture after the machine work with a small bit of hand work to trim and fit. To add another twist pictured is an upolstered over the rail back tenon.
 

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I see--you make the tenon oversize, then pare down the opposite faces to get the correct angle.  Got it--
Thanks Jeff,
Rob
 
ChuckH said:
The through M/T that is so common on the Chippendale chairs also troubled me.  Personally I find it out of place on such an elegant piece.  But that's just me - I just don't understand the need.  Last summer I had a chance to see a set of Robert Walker's dining chairs when the wife & I visited the Shirley Plantation near Charles City, Va.  While everyone else was admiring the little dog heads he carved on the ends of the chair arms, I was looking at the back of the chair.  I didn't see any through M/T's.
Click to expand...

The thru tenons are structurally superior to dry tenons.  Cutting the mortise thru allowed chair makers to produce a deeper (thus stronger) joint.  It also gave them access to the back side where wedges could be fitted to perfect the tightness of the tenon. Wedges are typical and were added at the time of construction. They are tiny, and placed about 3/16" in from the top an bottom of the tenon.  Lastly, the thru mortise allowed chairs to be repaired fairly easily. Just one more example of how our values were clearly not shared by folks from the 18th c.  Thru mortises mark the better chairs. Thru mortises are found mainly on Philadelphia and some English chairs.  They are always associated with chairs without lower stretchers.

I agree that cutting them is pain.  But this is one of those cases where those of you who do this with machines may have a harder time than I do with hand tools. In examining some Philadelphia chairs it has been observed that the mortises are slightly diamond shaped in cross section.  They may have been paired this way to allow the tenon to slip in easier.  The important part of the joint is really the fit at the front and back faces. The paring makes me wonder if the chair makers started their mortises with bored holes. Getting the angle right isn't easy and there 's no good way (that I could figure) of laying out the mortise and chopping in from both sides.  

Pretty typical to see the rear stretchers and the backs of the side rails pegged.  This is where all the stress of the chair is concentrated.  The pegs indicate either years and years of experience or a developed understanding of physics.  What is interesting to me about all of these features is how widespread they are within a region and how absent they are outside that region.  This suggests market pressures (hard to imagine in this case) or a consensus/pack mentality.

Good discussion Jeff.  You're a (glue) pot stirrer!

Adam
 
Adam,
      Nice write up, are your tenons angled or straight? If they are straight, and you angle your mortises completely thru the rear leg, do you attempt to keep the exit of the mortise in the center of the rear leg?
Thanks,
Dave
 
I use straight tenons. Tenons really have to be straight for structural reasons.  Once a peg is introduced, the problem of grain run out in the tenon becomes more acute not less so. As an engineer, I couldn't look somebody in the eye and tell them an angled tenon was going to be good enough. It may or it may not. With the angles we're talking about, there would be little continuous grain left in a tenon this long.

The location of the tenon's exit wound :) is a function of the rail thickness and the angle of the side rail (which I believe is fairly constant). I don't think any attempt was made to center it.  But I think it kinda comes out that way?  Remember that adjacent to it is the rear seat rail tenons, which are typically pegged.  I think I tried to keep my side rail tenons as close to the edge of the stock as possible to provide sufficient depth for the rear tenons and pegs. Recall that in my case, I fit the rails to the front legs, then marked the tenon angles from the built up back.  So not only did I try to push the tenon out as far as I could, the tenon was angled in relation to the stock.  (top to bottom).

Pretty sure I saw very little shoulder on the rear seat rail tenon on the chair I was copying (Deshler chair). Period tenons are also very tall (again, likely for structural reasons).  Sometimes they are so tall that there is next to no shoulder at the bottom and top.  I believe this is yet more evidence that period woodworkers knew this was a structurally critical joint and were taking specific steps to improve upon it.
 
I've made hundreds of chairs, and I think the best reason to use pins is as a clamp substitute. Draw bore pins will pull the joint up tight. Pins can be fairly slim and still do the work, so I don't think any strength is lost.
As for the other portion of the discussion, I've never taken apart a period chair that had an angled tenon, although I believe they did it. Mine are straight and 5/16 thick-Al
 
I notice that a number of posts express some surprise that these old craftsmen were able to figure out solutions to design problems that may seem to us today to require "book learning".

I have often said of my own profession, Naval Architecture, that people have been building ships that worked quite well since long before we knew how - in that case for thousands of years - because  the earliest scholarly knowledge that was published on the subject is only about 250 years old today. As recently as 100 years ago, the very best hull forms in the world were developed using hull models whittled by eye by a person with little formal education, but who had built a thousand of boats.

It seems to me that this occurs because those who actually build things get a lot of feedback which influences their next evolution, but I believe that they also develop an inate sense of physics, strength of materials, metalurgy, etc , in their day to day work.

Speaking from my own experience; as a person who built things early in life, I found that the book learning helped understand the intuition that had already developed by practice as much as it informed it.

By way of example, taking a strength of materials course would help one understand why through tenons in chair rails best have small ledges top and bottom, but it would not teach you to make them so.

Karl
 
It looks to me that if you would have flipped the rail 180 degrees, the grain may have gone almost parallel with the tenon.
 
I've never seen a New England chair with angled tenons. I think the angled tenon debate has arisen due to the simplicity of setup using modern machinery to make the mortise. I do it with cradles and angled blocks as Jeff described.
As far as pins being used, we seen pins in nearly every pre-Federal mortise and tenon situation in American furniture, so I see no reason why a chair mortise and tenon would be different. In chairs the pins serve as clamps during assembly which speeds things up considerably, as chairs are notoriously difficult to put clamps on.-Al
 
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