Continuing now with the build of the fold-up bench for the Boston Children’s Museum – I hope the jumping back and forth between projects isn’t making anyone dizzy! Previous installments in this and other threads can be found to the right of the page in the archive section.
Where I last left off, the cut out of the housed tusk tenons for the frame cross-pieces had commenced. While there are numerous ways to connect the crosspieces to the frame rails, and the original which I am replacing used some form of simple glued blind tenon, the superior joint for this form of connection is the tusk tenon. I say superior in terms of structural logic, not the usual $$$/efficiency/get it out the door matrix of consideration. This type of joint finds common application in the framing of timber floors, where it is typical to connect the main floor beams with a series of closely spaced joists, which are of smaller section. Since the main floor beams are important structural members, cutting mortises and housings into them to receive a large number of smaller floor joists can easily result in excessively weakening the beams. one solution to that would be to up-size the main beams to the point where the remaining material after all the mortising was complete would be adequate to carry the load, however this is not a wise use of material. The quick and dirty connection, if I might characterize it in that manner, is to chop housings so that the joists can simply drop vertically into the beam – this can be seen in this particular chestnut beam – this the remnant of a frame present in the house in which I live, right in the living room:
This sort of simple housed joint is very common; indeed, many brand-new frames incorporate it. Notice too the propagation of cracks for the corners of the mortises in these boxed heart beams- hah- that’s another topic so I’ll hold off on walking further down that track.
While the drop-in joist is convenient to frame, since the joists are easily installed after the rest of the frame is erected, it removes a lot of material from a crucial section of the beam. You see, any beam, oriented horizontally and loaded vertically, behaves under load very much like a steel ‘I-beam’ – an ‘I‘ beam in fact is extremely rational in configuration, material placed onlywhere it is needed. The upper surface of a beam is under compression, and the lower surface of the beam is under tension. The middle axis of the beam, halfway between upper and lower faces, is called the neutral axis (as opposed to the “Axis of Evil”), and is subject to neither compression nor tension. The best place to remove material in the beam for a housing or mortise is therefore in the section where the loads are minimal – the neutral axis is such a place. Since the top surface of a beam is under compression, chopping mortises into that portion of the timber unduly weakens it.
Now, the development of joinery for timber work has undergone a long evolution, and this is well-documented in the case of English timber framing by Cecil Hewett, now deceased. In most cases a similar process unfolds – various joints are used, and time – i.e., the observation of older buildings and how the structures performed, as well as calamitous failures in some cases, and subsequent analysis of such effects by thoughtful framers, led to incremental improvements in the joints. Eventually each joint would reach a penultimate form of maximum mechanical efficiency – by some carpenter or another. It was not necessarily the case that one carpenter’s perfection of a given joint form, through their deep understanding of the joint and insight into how to effect the highest form, would become necessarily widely-shared or understood. other carpenters may have continued using inferior forms of the joint, because they thought it was traditional, or it was what their master had taught them, etc. Once the penultimate form of joint was achieved, wherever and whenever it may have been, it was often the case afterwards that carpenters who failed to study the historic examples, or who misunderstood the logic of the form, would create a new version which was in fact inferior. Like some sort of wave, the process of joint evolution appears to rise to a pinnacle of excellence and then recedes (crashes?) to a debased form. Sometimes the lessons are lost all together, and sometimes, though the lessons are there to be seen in older buildings, if the carpenter chooses not to think about such matters then points of history are somewhat moot now aren’t they? One of the aspects to living on the east coast which I like is that there are many older wooden buildings to observe, to see what worked and what didn’t. The thing that puzzles me is when I see new buildings going up which seem to not reflect the obvious lessons available- again, another topic.
In terms of floor joist connections, the tusk tenon was the ultimate evolved form – I’ll quote Cecil Hewett directly, from his masterwork English Historic Carpentry:
“The ultimate joist end joint, believed to afford the maximum possible mechanical efficiency in cases where the neutral axis of the main joist is level with the tenons soffit. This was, apparently, developed by Master Richard Russell, and was used for the side purlins of King’s College Chapel at Cambridge in 1510~12. It is the tenon with diminished haunch, in which the sole object of the tenon is to prevent end-float. Industrial stress tests were conducted on a half-sized model of this joint…“
Those stress test observations, conducted by Dr. D. M. Brohn, are summarized a page or so later,
“This joint behaved more favorably than any of the other three in that there was virtually no rotational movement of the joint up to failure. At a jack load of 40kN there did not appear to be any movement of the joint. Towards the end of the test it appeared that the beam would fail before the joist. There was already substantial splitting along the face of the beam, and one of those splits coincided with the underside of the joist. Close to failure this split opened up and the portion of the beam below the joist was effectively spanning between the supports of the beam. This was deflecting substantially. In the end, however, the tenon broke across the weakest section, diagonally from the face of the main beam to the junction between the haunch and the top face of the tenon.“
While in the case of this bench project the Wenge is leagues strong enough that I would have no worries whatsoever about employing simple drop in crosspieces; nevertheless I choose to follow best practice – which to mean means choosing the form of joint which is most mechanically efficient for the application: in this case, the tusk tenon. The most mechanically efficient joint is also, often, the one which uses material most efficiently as well. This Wenge is precious stuff and I am building a piece to last for a long time – there’s no other way to go about it I feel than making careful choices about joinery.
The Japanese also employ this tusk tenon joint – they have a few different terms for it: kiba hozo (牙枘) “tusk tenon”, dan-gata hozo (段形枘) “stepped form tenon”, and kake-tsuki hozo (掛け付き枘) “seated penetrating tenon”, depending upon the details of how the joint is configured. This joint can be done blind, or through, housed or not, pegged or wedged. I choose to wedge it in this instance, as I feel the tenon has rather a small amount of relish for pegging, generally speaking.
So, I wonder if some readers are being kept in suspense, wondering what sort of tenon I am blathering about – so, without further ado, here’s some photos of the tusk tenons after most of the cut out had been effected:
I find the Wenge processes quite cleanly with carbide, and was able to get consistent accuracy of +/- 0.005″ with tenon thickness:
The next step was to drill some small holes at the apex of the slots where the wedges will be eventually driven, these small holes act as stress-riser eliminators, preventing the wedge’s kerf from traveling further up into the stick when the wedge is inserted:
With the tenons largely completed, I moved on to the mortises and their housings in the main frame rails. First I roughed out the mortises by drilling with a 10mm Festool brad point:
Even though the drill was virtually new and quite sharp, within a couple of holes the bit was smoking hot – this stuff is tough to drill! It took me longer than I expected to drill out the mortises. Eventually however, that stage was completed and I moved on to clean the mortises to spec. dimension using my router with edge guide:
–> go to part 5