Tuesday, September 22, 2009

First Light XLVIII

Post number 48. Previous posts in this thread, along with other topics, are archived in the sidebar to the right of the page.

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Today I will show the final joinery task with this lantern project, namely the locking pins which tie together the lower level of hijiki, through the pillow blocks, and into the sills of the lantern housing, or dodai.

To connect the central Lignum Vitae draw bar, which ties the parts, to the lower tier of hijiki, I decided that I could employ what I will call an interference pin, rather than a conventional peg through the pin. Given that the pin is only .375" in size, an appropriately-sized peg would be too small to mortise, with my tools, and given the awesome strength of the Lignum Vitae, the toughest wood on the planet, I thought I could get away with it. Time will tell of course.

First I mortised for the interference pegs, which will also be made out of Lignum Vitae:


As you can see, these peg mortises are rectangular, rather than square, only 0.1875" x 0.25" in size.

The completed mortises, all four of them, in the two hijiki parts involved here:


Here's a pin slid into place, showing the near half-interference of the pin in the mortise - well, about 0.125" actually:


I used a chisel to mark out the mortise walls on the pin, then sawed and chopped the waste material out from the pin, leaving a 0.1875" wide trench through the pin. There was about 0.375" of relish on the pin below the trench cut, which I estimate to be adequate.

Here is a look at after the peg is installed though the central pin, but before the dodai is slid and pressed down on top of the pillow block:


Normally I try to avoid displaying too much of the joinery mechanism, and given that these hijiki are somewhat buried and obscured by the 45˚ oriented hijiki adjacent to them, the peg is fairly discreet -I cut them flush for that very reason.

Now then, here comes the tricky bit: the connection at the top of the pin, at the dodai level. At this location, the lantern housing grills sit, and again, given the small size of the through pin, pegging is not really an option. I decided to adapt a type of joint used in high class Japanese residential construction - a joint that connects a sliding door upper track to a stub post. This is a variant on a cross-sliding lock dovetail, shino sashi ari tsugi, a joint not found in Western carpentry as far as I can tell.

This joint is better explained in pictures. First I made a jig in MDF, from which I would reference my router:


The result of the initial cut-out is a crosswise trench, cutting partially across the Lignum Vitae pin, and angled at 14˚:


This cut creates a half dovetail on top of the Lignum Vitae pin.

All four locations now mortised:


Then I used a 6mm chisel to square up the oblique trenches:


Then it was time to take the dodai off of the supporting parts as I needed to deal with both the pins and their slot mortises separately:


For the dodai slots, I adjusted the router fence to cut the slot 0.01" wider, in towards the direction of the pin:


And for the pins, I deepened the shoulder cut a little with a chisel:


Here then are the sliding fixing pins, shachi-sen, which I made out of Bloodwood - they have a 14˚slope machined into their upper surface and are precisely sized for thickness to the slots in which they fit:


So how does this joint work? Well, first I reassembled the parts, and placed the shachi-sen into their respective slots in the dodai:


Then I use a drift to push the shachi-sen sideways, where they run into the half dovetail on the top of the Lignum Vitae through-pin -- given that there is some 0.01" of interference at that meeting point, the pressure of the sliding pin causes the dovetail to lock up and draw upward slightly:


The sliding pin fully into position -I leave a little room on the opposite side of the trench so that the sliding pin can be drifted back out when necessary:


All four locations now complete:


I designed this joint so that with the lantern grilles in place, the mechanism of the sliding pin is completely covered over and hence protected from the weather:


All that remains now is to fit the glass into the grilles, and chamfer/finish plane a few parts, followed by final assembly. That will form the subject of the last two posts in this build thread. The end is nigh! Stay tuned for post 49

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