Well, this sure was a fun little joint! Katasage Ari is used to tie wall posts together, but I can see all kinds of potential applications.
This weeks joint was brought to us by Steven of The Twin Maples:
Stephen is really proving that hope is not lost among the young men in todays society. Check out his blog, he’s really doing a lot of neat stuff, making, asking the right questions. Most importantly, DOING.
The greatest challenge of this joint was the layout. The wall tie purlin has to be able to slot into the mortise and then drop down so that the wedge on top can be driven. For that reason I made the space at the bottom of the mortise for the wedge equal to the depth of haunch that forms the half dovetail on the wall tie beam. In the picture that Stephen put up it described this joint as being the connection at a corner post, which puzzled me, because with the depth of the half dovetail tenon it would not leave room for another mortise on the adjacent face.
The ratio of taper for my wedge was 1:12. With a mortise depth of 1.75″ that works out to a rise of a little under 3/16″. For ease of measurement I made it 3/16″ and marked out my wedge in advance.
I could have used my brace and bit to waste out the mortise, but decided to just chop it with my 1″ bench chisel. This chisel is from a twenty dollar set of five, hardly anything to write about other than to say that the sides were ground in a parallelogram shape. The chisel wants to twist quite badly when chopping down the mortise. Definitely something to check when you get a new chisel.
I also made a depth gauge for both the total mortise depth of 1-3/4″ and the 1/2″ haunch.
The mortise was cut 1/16″ deeper than the half dovetail tennon, allowing the joint to pull in tight as the top wedge is driven. The taper for the wedge was chopped in using the side layout as a rough visual guide. I almost cut it perfectly, just a few paring strokes off which were referenced with a bevel gauge.
Next I cut the haunch to depth. There’s plenty of room at this point to use a chisel, bevel down, to clean the bottom of the haunch. Now you are left with an accurate corner from which to start the half dovetail taper.
Handy little tool, this bevel gauge.
With the mortise cut it was into the home stretch to cut the half dovetail and wedge of the same width. I chamfered the back edges of the wedge in preparation for assembly.
Awesome joint! Simple and strong, I can definitely see myself using this. The wedge backs out quite readily by tapping on the post above it with a hammer. Probably the easiest joint to date to take apart. Lined up real nice and square with the wedge driven in. Oh, I forgot to mention that the end of the wedge was trimmed back about 1/4″ from the bottom of the mortise. I suppose that leaves room for the wedge to be tightened as the wall tie beams dry.
I’m ready for next week, bring it on Sebastian!