Friday, August 21, 2009

First Light XXXV

Post 35 in this series - I hope it's not getting monotonous for anyone out there! Please check the archive at the right of the page for previous posts and, believe it or not, even other topics.

Sharp-eyed readers may have noted in my previous posts that something was different at the barge board miters. I had found through multiple re-fits of the components, that the dovetail design of the joint, perhaps due to the fact that the dovetails were only 9˚ in slope, simply didn't pull the two barge boards as tightly together at the miter as I would like. Well, to be more accurate, they held together fine until the upper ridge was installed, where, due to a myriad of parts locking together at once, the miter opened slightly on one end of the gable, no matter what I did to try to adjust it out. I realized as well that rain and sun cycles would likely conspire to only make the situation worse, and that the joinery solution I had devised was simply inadequate.

I was on the verge of beginning all over again and making another set of barge boards (hafū), but I held off and gave the issue more thought. Then I was strongly considering fitting a metal threaded rod in for a while, breaking my design goal of no metal, and using plugged mortises on the backside to conceal the pockets for the nuts. Neither solution was appealing.

Then I noticed, kind of like a veil being pulled back, that I could in fact fit a spline across the miter and peg it. There was actually room for this, and for some reason i had convinced myself earlier that there was no room. Go figure.

The pegs for this spline would land not on the hafū board proper, but could be placed so as to be on the upper portion of these one-piece boards, the portion simulating a tiled ridge. Since I was already pegging the upper ridge, I realized that pegs on that same portion of the hafū, the 'tile-ridge' portion, would look just fine. Further, they would be placed so as to flank the projecting end of the exposed ridge, thus would be somewhat obscured.

So, the problem was resolved - a crosswise spline, pegged on each side. Here's half the joint:


This new solution of course renders my first 'solution' largely moot, and I will take this lesson forward with me to the next lantern. The good news is that it did not necessitate re-making new barge boards (though I remain tempted!).

Here's a view of one of the spline mortises:


The joint slides together:


The joint prior to being drawn up with the remaining peg:


Though there is a light gap on the bottom of the miter in the photo, that comes from the way I was holding the piece in my hand on the table top - all in all, the miters look nice and tight now when all the parts of the roof are assembled, so 'mission accomplished'. I'll post up pictures of the assembled joints soon enough so you can see for yourself.

Now then, back to the making of those pesky intermediate roof ribs. When I last left off, I had fit them to the roof boards. The remaining task was to dovetail their ends so they could lock to the exposed ridge. Here's a close-up of one of those dovetails:


And here are the little piggies all lined up in a row:


A lot of work went into those little ribs, and I'm glad they came out well.

The next task was to mortise the exposed ridge for the sliding dovetails. I made a MDF jig to work with my router:


For these dovetails, I used the 14˚ bit, 8mm shank. The result of the router work was 3 mortises per side on the exposed ridge:


These mortises required a little chisel work to square them up:


I could have housed these joints as I did the connections between the ridge and the hafū , however that would have meant that I would need to fit the ridge simultaneously onto 20 backed rib/hafū top surfaces at the same time, and I thought that would be a pain in the, pardon my Scotch, arse. So, I went with barefaced dovetails, and if the ridge shrinks appreciably across its width, hopefully it will pull the ribs in with itself - otherwise there will be slight gaps at the non-humid time of year (winter season around here).

I'm not so sure, thinking more about it, that this solution would not have also worked fine for the hafū connection to the ridge as well. I think though that a cogged sort of connection between the exposed ridge and the hafū, which is what you get with the housings in this case, would be a stronger joint in terms of locking the hafū in position laterally. If I were to re-design this joint, I think I would still go with the housings, however the internal dovetails, as mentioned above, would be nixed and I would do a simple cog-lap instead. Or, possibly, combine the two ideas, using wider dovetails on the ends of the hafū and no housings. That might be the 'best' way to go. That said, narrower dovetails are to be preferred though in general, as they proportionately shrink less across their width than fatter ones, and thus remain mechanically more cohesive over time.

Weighing up the pros and cons of different joinery approaches to a given problem, factoring in the effects of shrinkage/swelling upon both joint strength and aesthetics through seasonal humidity changes and expected loading, along with ease of assembly, de-mountability, and so forth, often is cause for a fair amount of head-scratching.

I'll eave off on the fitting of ridge to those intermediate ribs, as I have taken the opportunity now, with the lantern housing free of it's roof, to commence work on the frames for the grilled openings. These frames are dadoed into the posts (hashira), sills (dodai), and plate (keta). I processed with trenches in those parts sans camera. We pick up the fabrication again at the point where I need to trim the ends of the frame pieces to fit against one another. The verticals, since they lie against the posts, which have been 'backed' to be flush with the overall prism formed by the lantern head, obey similar geometry. Though I haven't reached the point in the kō-ko-gen-hō series yet where I explain the geometry specific to splayed-post structures - - it is, to cut to the chase, the same as that on the ends of splayed sawhorse feet and tops, a cut line given by the chūkō angle.

I set up an angled shooting board with a little wedge to use my plane to trim the ends at this compound angle:


A little action shot:


After initial trimming of the end, I stand the piece up to see how it look in relation to the post it will fit against, and adjust the trim as necessary until it is parallel with both directions of post splay:


The sill and plate portions of the grill frames are parallelogram in section. Here I drop the vertical into place against the sill, and mark it's position:


Panning back a tad, here is the situation:


I follow the same structural logic with the lantern grill frames as I do with timber frames - I stand the posts on the ground (which would be on granite plinths), fit the sills between the posts, and place the plates on top of the posts.

The 'post' portions of these grill frames are connected to the sills with bridle joints (aka, 'open mortise and tenon'). Given the mix of shapes and slopes here intersecting at the joints, these are purely hand-cut affairs:


Last one for today shows the completed bridle joint (hidden) connection between post of and sill of a frame corner:


Oh yes, more to come! Go to post 36

2 comments:

  1. Hi Chris,

    This build has been fascinating! not boring in the least. I appreciate the time and detail that you've been putting into these posts. Would it be possible to post a sketch or two showing the big picture? Sometimes I find myself lost in the details and not too clear on exactly how the parts being discussed are going to interact.I know there are some good images/ diagrams in earlier posts but am not sure where. There are quite a few posts. Some of this may be due to my lack of familiarity with the terms.

    The level of craftsmanship that you are displaying is admirable. I also appreciate the amount and level of work that you are able to do with a limited set up. So often there is an attitude of needing to aquire a broad range of tools before attempting any task or project. It is refreshing to see a more basic, get it done approach.

    Please keep the posts coming!

    Regards,
    John Kissel

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  2. Hi John,

    thanks so much for your words of encouragement - to read your comment fills me with some good energy!

    Yah, I know this thread on the lantern build is getting long and it is understandable that it might get confusing for some readers, and I can well appreciate that you might be wondering how the parts being discussed are 'going to interact". All I can say is that I try to summarize every few posts or so with a 'big picture' view of the lantern at the current stage. Hopefully that will suffice. As far as the terms go, I'm afraid I must ask you to dig back through the archive to find my introductions of the specific terms and their explanations, like 'keta', 'dodai', 'hafu', 'gegyo', etc.. I hope when you do that you will find the explanations reasonably thorough.

    I don't want to repeat myself by re-explaining the terms every post, OR overdo it, as it were, with the 'lingo' (I've learned that some people are rapidly turned off by a coming across a lot of 'weird' foreign terms, or think I am 'showing off' by using such terms). So, what I try to do is generally use a mix of the English and Japanese terms in following postings, continually reconnecting the English term to the (Japanese). I ask for your patience in this regard, and if their is anything specific which you are finding confusing, don't be afraid to ask!

    As far as the tools go, I do have a decent range of hand tools to work with, and definitely like to have the 'right' tool for a job at hand, so I'm not one of those 'do it all with one chisel' sorts of enthusiasts. Obviously, if I had but one beat-up chisel made from an old leaf spring, I'd try to do the best I could, but I would not be idolizing that approach as the 'right' one to take.

    to tell the truth, I would love to have the crown jewels of the Martin stationary woodworking machine line-up glowing on my (imaginary) palatial shop floor, however such is not the situation at the current time. I have a decent sawhorse (detailed in the build thread "An Irregular Situation")and a bunch of hand tools - occasionally I use my circular saw, if only to save wear and tear on expensive handsaws. This use of predominantly hand tools on the current project is out of necessity more than any other thing. I use the appropriate tool for the work, and do the best I can under the circumstances.

    I guess one should not be limited by a set of tools or a method of work; one should be able to use a diverse range of tools to do a task. If I had a table saw, planer, and jointer avaialable, you better believe I would be using them a fair bit for the basic dimensioning tasks. I hardly find ripping sticks of wood down with a handsaw a heady piece of spiritual connection with the wood or anything like that. Heck - I have a router and I use it a fair bit, and I'm sure some are up in arms about that, decrying my 'lack of purity'. Pragmatism - now that's Japanese!

    So long as the last tool to touch the material is a blade you draw with your hand, the finish will be the one I'm after. That's the bottom line I suppose.

    ~Chris

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