Tuesday, March 24, 2009

Irregular Situation II

After I had decided upon the main design details for the irregular sawhorse project, I had concluded that I wanted a means of using joinery to lock the legs to the top beam of the sawhorse. I also wanted to use a 'sacrificial' cap on the top of the beam which would take abuse from errant saw cuts and material dropped onto the beam. In previous sawhorses, I had used floating dovetail keys in one set, and the glue trick with newspaper on another, and this time I wanted to improve upon both of these methods. The glue trick worked fine, but in outdoor conditions humidity can cause the glue bond to fail prematurely - that's what happened to my sawhorses done in that manner.

In this sawhorse, I decided to form 1/2-dovetail tenons on top of each leg, which when brought into position together would protrude through the top and form a single dovetail. This dovetail would also lock the sacrificial cap into position as well.

Once I had decided upon the joinery system, the next step was to clarify the respective irregular slopes - I went with a slope of 10/3 for the short side splay, and 10/2.25 for the long side splay - through a slight trial and error process to determine how it would all come together at the top. Here's the finished cross-section view of the top:

As you can see, the legs are to be 2.5" in section, and the top beam, with cap fitted, forms a 4"x4".

I then spent some time working on the descriptive geometrical drawings necessary to determine the legs shape along with the cuts for the top and sides of the stretcher tenon shoulders. That done, I set to work on cutting out the top beam for the leg mortises.

Once the layout was complete, and the mortises knifed at their cross grain positions, I used a General International tilt-head hollow chisel mortiser, in concert with a sloped bed jig thrown together from some MDF scraps to rough out the mortises:


All of this work on the sawhorse I performed on weekends and evenings in the shop at the College of the Rockies in Kimberley B.C. (now in Cranbrook, B.C.), where I was a timber framing program instructor for a term.

When I was done with the mortiser, the result looked like this:


Here's the underside after the rough cut out:


Then I used paring blocks and chisel to clean up the inside of the mortises:


Unlike the French joiner's work practice, in Japanese joinery the housing is preferred to reinforce most connections. Without the housing, the connection is limited only to the strength of the smallest part, which is the tenon, typically at around 1/3 the thickness of the member.

I made these housings quite shallow, at only 0.25" deep. I used a router to deck the bottom of the housings, and then an MDF block with chisel to pare the shoulders:


When the cut out was complete, the mortises with housings looked like this:


The next step in the process was the processing of the legs into the irregular diamond-shape form. The diamond effect, by the way, is termed 'ku-se' in Japanese, which means 'peculiarity'. I'll detail the work undertaken on the legs in the next post in this thread.

2 comments:

  1. Interesting Chris! Looking forward to the next post. Thanks,

    Marv

    ReplyDelete
    Replies
    1. Marv,

      your comments are always appreciated, if not replied to in a timely manner!

      ~C

      Delete

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