I’ll continue on then with describing the steps taken in fitting the lower set of roofing boards to the wall plate, keta, to the lantern housing. The simple-looking jig I described in yesterday’s post serves triple duty, as it allowed me to (1) mill the tops of the short side keta noses down flat and to the precise slope, and to (2) machine the dovetail slots in their tops:
Here’s a closer view of one of the dovetail mortises:
After some paring with a less-than-sharp chisel, the slot looked like this:
The disrupted fibers on the right side at the top of the slot is in fact a thin sliver of wood, partially cracking – this is actually the corner of the tenon mortise for the housing support post, the tenon that is coming up through the keta lap joint. I had sized the length of this mortise and tenon previously, anticipating the need to put the dovetail into the top of the keta nose, so I made the mortise as deep as possible while giving me adequate depth for the dovetail slot. The deeper the slot for the dovetail the better – here in the keta nose, it is 0.25″ deep.
The third task that the MDF jig allowed me to do was the dovetail mortises in the roofing boards – this means that they are spaced precisely the same as the mortises in the keta bana. This is the jig clamped in place, squared up and ready to go:
After the slots have been routed, I used a paring block and chisel to clean them up:
And here’s the result after the two lower roof boards have been mortised and squared up:
Next it was time to make the double dovetail keys to join the roof boards to the keta bana mortises. I wanted to use something tougher and stronger than Honduran Mahogany, and looking through my various boards in the storage shed, I came upon a couple of small planks of 4/4 Bloodwood (Brosimum rubescens and other varieties of Brosimum) that my wife had bought me for a present a few years back. This wood I refer to is from Central and South America and is also often called ‘Satine’. To confuse the matter a bit, there are other species called ‘Bloodwood’ including Pterocarpus officinalis, a species local to Belize (also called ‘Dragon Blood tree’), and a couple of Australian tree species also referred to as Bloodwood, one of them being Corymbia gummifera. This Australian tree actually has a sap that looks just like syrupy blood!
The tree producing the Bloodwood I’m talking about here, Brosimum spp., also produces edible nuts called Bread Nuts which look a bit like figs but have a single large seed within them. The bark and sap (a white latex exuded by the bark) is apparently used medicinally by Indians in Northern Peru (and other locales I’m sure), and the trees are also tapped for the sap. Here’s a look at the leaves of this tree:
The tree grows to a height of 120′ or more with a trunk attaining a diameter of up to 30~40 inches. The wood is rich lustrous red, fine textured, dense and heavy. I found it planed fairly well:
It is a very hard wood and not the easiest to work – I bent teeth on my handsaw working it and the plane dulls fairly fast on the end grain cuts.
The wood takes a very high polish from planing – I wish I could find a way to capture this glassiness better with my camera:
After I had a dimensioned piece of Bloodwood to work with, I did some careful processing of the stick into a double dovetail profile using my router table. The dovetails topside and bottom are different from one another in terms of depth and width. The roof boards are only 0.375″ thick, so for that situation I made the dovetail depth about half of that and widened the dovetail somewhat from the size used in the keta bana. After the cross-section was processed, I then cut the end to slope, following up with my jury-rigged shooting board and my 54mm kanna to trim one end of each piece cleanly:
Here’s a look at the double dovetail key being slid into position in the keta bana:
And driven all the way in the fit was pretty clean and tight:
I had left the keta bana slightly long, in anticipation of the next step, which was to trim both nose and dovetail key at the same time:
After trimming, all that remains is to clean up the end grain of the keta nose and the key with a plane, which will be something to take care of later.
Then I chopped a piece off the top of the dovetail key so that the mortise in the roof board would be concealed when it was fully driven up into place:
Then, due to the fact that the keys were a pretty snug fit and couldn’t be withdrawn just by hand, I drilled a hole into the top of each key, which would allow me to drift them back out again:
The ledge I just chopped in the top of the key is clearly visible in the above photo.
Here’s how it works then to drive the key off:
The slots for these keys, as you may have noticed, extend from the keta nose up into the sloped lap surface of the other keta. The longer the key the better, in terms of strength, of course, but the other purpose of this design is that the dovetail key serves and additional role of reinforcing the lap joint from loads that may try to separate it, and thus are in a similar role to the diagonal hi-uchi pins described a few posts back. Every little bit helps, and I guess my paranoia knows no bounds too!
Next post we’ll try fitting those lower ceiling boards into place on these dovetail keys and see how that turns out – hope you’ll come back for that, post 21 in this series.