Mortise and Tenon Sizing
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How do you know how thick/long/wide a tenon should be to bare a load. In episode 4 of the console table, I believe, Paul was cutting fairly thin tenons and put emphasis on not over cutting with the saw to avoid making the tenons to small (1/4″). Now it is easy for me to believe that this would indeed be too small, however where is the line between too small, just right, and too big? Is there a rule of thumb, is it experience, or does it involve a bit of maths and engineering to reach this answer? I would imagine it is somewhat of a combination of the three.
I am planing on constructing a bed frame, I have watched Paul cut mortise and tenons many times. However, I have never really understood how he arrives at specific sizes.
Our bed is, I believe, a fairly typical contemporary British posts-and-rails oak one. The rails are fixed to the posts by metallic bed fasteners, and the joints are then further enforced by bedbolts that are tightened from the inner faces of the bed rails. Apart from tightening the bolts, which is very tedious, it’s been a good construction.
(https://www.google.co.uk/search?q=bed+fasteners&tbm=isch&source=iu&ictx=1&fir=8sIgLeHJMYIDpM%253A%252CkmJtopq0rP3IiM%252C_&vet=1&usg=AI4_-kQO_2UfrIVCa9g7A2RsS1ETaDpWTA&sa=X&ved=2ahUKEwjHjPzZhf7jAhWQRRUIHfnoAZgQ9QEwAXoECAUQBg#imgrc=8sIgLeHJMYIDpM:&vet=1&spf=1565637385238)The previous bed had shallow double tenons and bedbolts, also tightened from the inside of the rails. The tenons made assembly easier and perhaps the tenon shoulders made the bed more stable side to side.
I wonder if non reinforced mortises and tenons will be strong enough; and being able to take a bed apart of course makes moving it a lot easier.
Mr. B. Hylton’s book “Illustrated Cabinetmaking: How to Design and Construct Furniture that works” (Fox Chapel Publ. 2003, 2008: ISBN 978-156523-369-0) suggests that the length of the tenon should be around ½ into the post/stile, or a bit longer if the mortise part is narrow (definition of narrow is not provided, but appears to be less than 38 mm). His claim for this suggestion is concerns about seasonal wood movements; something Mr. Hylton is quite concerned about, and perhaps less relevant for the European indoor situation.
As glue surface increases with length, a tenon apparently cannot get too long. Thus for a bed, tusk tenons with wedges should provide a good alternative to bedbolts. Nor, can a tenon be too wide/high, albeit that when its width is >10 its thickness, it should be converted to a double tenon (also makes boring for bedbolts easier).
According to Mr. Hylton, the risk for splitting the mortice stock is behind the rule of thumb on the thickness of the tenon being 1/3 of the thickness of the rail, was based on concerns for splitting the mortice piece when using hand tools. Having the piece in the vice while chopping, I imagine should reduce the risk for splitting
Sven-Olof Jansson
London, UK; Boston, MA
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