It now was time to move on to the lay out and cut out of the upper tier of beams that lay atop the lantern head posts. Since these beams are akin to the wall plates, I shall call them by that name, which in Japanese is keta, '桁'. This character, or kanji, is the combination of the identifying radical for tree, '木' - here horizontally compressed on the left side of the character - and the element '行' on the right. The element '行' derives from a pictograph of an intersection of roads:
Thus, '桁' is literally the wood which intersects, a reference to beams/girders. The character also refers to a rack for hanging kimono, and an abacus bar, which leads to a secondary meaning as a digit/numerical placeholder. For example, a three-digit number would be a mi-keta (三桁). Anyway, perhaps I digress....
I began my work on the keta by cutting them out of a larger block of mahogany with a handsaw:
Besides the four sticks I would need for the keta, I was also cutting out the ridgepole, or muna-gi (棟木) at the same time, for a total of five pieces this round:
That saw work was followed by a round of jointing, squaring and dimensioning with my Funahiro 70mm smoothing plane, proceeding in stages to near-finish planing:
Here then are the four pieces of keta ready for layout:
Now, the corners of the keta are to connect to one another with mitered laps, as per usual in this project, which also means that I would have decent mechanical attachment to the supporting posts by pegging the crossing points to the post tenons. This is exactly the same sort of connection, with diagonally-oriented tenons and pegs, as I had done for the lower connection of posts to dodai (described previously in this thread). In the case of the keta however, the pegging would only have decent 'bite' in terms of holding the lower keta in each lap. There was some locking of material in he upper keta as well, but not, it must be said, with a whole lot of wood. To contract, in the case of the connection to the dodai, the upper crossing members are to be additionally pinned down to the supporting beams and hijiki below (not yet done or described in this build thread), so I could feel confident about the overall strength of the assembly in that location. In the case of the keta-post connection however, there was no other direct connection except the post tenon. Further, the post tenon was about as long as I felt I could make it without running into short grain problems, and roof boards would lay atop the connection, so there was no option to run the tenon long and peg at the upper half of the lap.
For those readers unsure of what I mean by 'short grain' aka, 'grain runout', follow this link for a basic explanation from a luthier's point of view.
I needed to reinforce the connection at the lap and post tenon, my concern being that if the lantern roof was subject to rotational forces, the keta could be forced apart at the connections, the upper keta coming adrift from the lower. My solution to this problem was to use a Japanese diagonal splining technique called hi-uchi-sen. Hi-uchi refers to a 'flint' and sen refers to 'pin' or 'spline', thus it is literally a flint pin. Flints are struck on the diagonal to create a spark, and these sorts of reinforcing splines run diagonally, are themselves tapered wedges in form and lock up the joint by being driven against one another, hence the name (this is my guess at least). I might add that a diagonal corner beam, running across between dodai (or keta) is termed a hi-uchi-bari.
The layout for these hi-uchi splines is somewhat straightforward, being similar to a hip rafter, however that has not stopped me from laying out the mortises incorrectly on a number of occasions in the past! So, I was methodical and cautious in my layout. The nature of these pins and how they work will become clearer I expect as the photos proceed, in this and subsequent postings.
The hi-uchi pins I'm doing here are regular, in that they run on a 45˚ in plan view, however unlike a simple 45˚ brace, these slope as they go, thus making for somewhat tricky mortising. After completing the layout for the mortises, I grabbed a piece of scrap and made up a jig to help guide my drill-mortising:
Then, onward with the drill-out, using a bit that was .0625" (1/16") under size:
Happily, the exit points came out decently:
Here's a look down one of the rabbit holes:
I drilled out all the hi-uchi sen mortises, then began the cut out of the mitered laps. After that, I had a tidy pile of keta looking most appetizing on the dining table
The reader might notice that the long side keta are taller in section than the short side pieces, the reason for which will become clear in time as I post more photos of the build-up. The difference in heights of the respective pieces added a slight complexity to the layout, leading to more than one double-check before I started cutting.
This then is the basic arrangement of the keta:
Then it was time for the chisel work to define the eight hi-uchi sen mortises, a task which occupied me for three hours or thereabouts:
A near-completed mortise looks like this:
Final trimming of these mortises will occur after I have made up the sen and fit them, a task which I have yet to tackle.
Here's a look at the short side keta, mortises and laps now cut out:
The small square mortises in the center of each piece is for a peg to fix the stub posts, yet to be cut, which will support the ridgepole. Here's a view of the upper faces: