Wednesday, April 7, 2010

Screen Play (2)

The sheet of 6mm Baltic Birch ply arrived much faster than expected- I actually had it in my hands yesterday, less than 24hours after placing the order for it. Today I started drawing the frame for the tsuitate.

Here's the set of tools I used for that:


The 1.5m long Shinwa rule is a wonderful thing to have, as is the trammel set. The black mechanical pencil is a newer design from Japan, with a double clutch inside so it uses about 95% of the lead, unlike the 65% you get with the old type of pencil. We'll see how it stands up over time and the typical abuse I put them through.

In the next picture you can see the area where the legs of the frame will meet the top beam, termed a katō-kyokusen ita (lit.: curvilinear flickering flame board):


And here's how the lower ends of the legs will look:


To develop the curves I use a layout method which originates in techniques of curving the eave edge in Japanese roof work.

I've made a couple of key changes to the piece since the last posting. One of those changes involves the latticework, which I have changed from having roughly 5"x 5" openings between grill bars. The openings now have 4.625" x 7.5" dimensions, center to center. This stretching of the opening from square to rectangular form increases the visual sense of vertical compression of the piece overall and I like the look much better. The rectangles are additionally of Golden Mean proportion, which is pleasing to the eye, and they will make the front a little less busy and show more of the panel in behind.

The other change involves the frame itself, and resulted from a bit of thinking about the joint I will use to connect the legs to the top. Since it is a mitered connection, I prefer to use the locking wedged form of joint (same as on the Walnut Vanity I described in series of posts last year). Trouble is, given the form of the frame at the connection, which has an in-cut or irizumi corner inside and out, the amount of wood left for the joint is diminished from what might be considered ideal. Also, with a separate inner frame to carry the grill bars, there was a need to put a dado along the inside of the outer frame to contain the inner frame, which would further reduce the amount of wood available at the connection. And of course, the inner frame is also to be mitered at its connection too, and it was also compromised with the irizumi treatment.

To obtain a more ideal configuration for the joint, I needed to find a way to elongate the length of the miter. I didn't want to make the frame and inner frame components any wider however. I didn't want to abandon the irizumi corner treatment, of which I am quite fond.

The solution came to me yesterday while driving around running a few errands: I will make the frame in a one-piece fashion instead of two piece. That's right, instead of having an outer frame with separate inner frame, the two will be combined. That eliminates the dado between the two frames, effectively lengthens the miter, and means the wood grain between the outer frame and inner frame will automatically blend seamlessly. It also means the inner and outer frame will be the same material, so I have one less variable to obsess over. Problem solved.

The Japanese often build these frames in a very different manner to what I am planning. Unless a piece is clearly demonstrating perfection in its design and execution (and I can think of very few pieces which meet that criteria), I don't simply copy other examples. Rather, I draw inspiration and lessons from them and proceed from that point in line with my own convictions and interpretations about how to build the thing, right or wrong as they may be. Since I've had so much practice over the years learning from my own mistakes I have gotten better at spotting mistakes in other pieces.

Often the form for this flickering flame window has fairly pronounced jogs in the upper frame area, and the frame is often two piece, joined at the apex, like this example:


Personally I don't care much for the look when the jogs are a bit heavy. And the grain doesn't flow so well along the shape when it is cut in that sort of two-piece manner. Short grain breakage problems over time would not be a surprise with the above sort of design. I doubt the joint at the apex of the frame is much to write home about.

Another way I could do it would be to make the join between pieces below the turn, using some form of mortise and tenon, like this one:


I'm not too fond of that solution either for various reasons, though it is common enough to come across it, as on chair leg-crest rail connections. The form of the above window, though with strong jogs, is still quite pleasing to my eye at least. I think they look better when they have a more compressed down appearance.

I think the wedge-locking miter joint (hako shachi-sen dome) is the way to go, and with a one piece frame construction, it will be plenty strong, and from my experience with the wedge-locking miter joint, the miter stays tight through the years. As always, I'm building with the long term view in mind. That's why I don't use dowels, or biscuits ever, as they rely upon glue. This piece will not be using glue for any of the joints.

Tomorrow I will cut out the template from the ply, make some secondary templates from the first, and do the templates for the feet, and maybe chop up a bit of mahogany. It should be fun.

Thanks for dropping by the Carpentry Way today. --> on to post 3

4 comments:

  1. "To develop the curves I use a layout method which originates in techniques of curving the eave edge in Japanese roof work."

    Would you expound a little on the method? From the photo I'd guess that chords are drawn at short enough intervals to give the appearance of a smooth arc.

    To draw curves for the table legs I am working on I went with the trusty pencil-and-string to draw a 2.5m radius.
    -Mike

    PS. I am glad you keep commenting turned on for the archives, it's nice to be able to interact when my curousity gets piqued.

    ReplyDelete
  2. MS,

    hmm, well, if you look at the current post (May 12, 2002) you'll find a decently large hint there as to the graphical development.

    It's a large topic and would take a while to explain in detail. I very much appreciate the question though and please, always feel free to comment regardless of the age of the post.

    ~C

    ReplyDelete
  3. Ah, the latest post reveals the geometrical secret with a picture worth a thousand words. I did the layout exercise to see what the the error is in the connecting chords method, and it's on the order of 0.05% along half of the gazebo eave. The points don't quite follow an arc segment, but the result is sure good enough.

    Now drawing catenary curves ought to be a cinch; just hang a line down and trace its droop ;)

    ReplyDelete
  4. There is a formula for plotting catenary curves, as you may be aware.

    I try to avoid using sections of arcs, preferring other curvilinear lines. The method shown on the current post is but one of several, and each produces a slightly different curve, some flatter, some which accelerate more towards their ends.

    Glad that provided the piece of the puzzle you were looking for.

    ~C

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