I am trying to recover from design flaw on a clavichord stand I built in 1964. This Zuckermann kit (See: http://en.wikipedia.org/wiki/Wolfgang_Zuckermann) was my first furniture project. It was cut and shaped in the shop in the Chemistry Department at Washington University and assembled on the dining room table in our third floor walk-up apartment half a mile away.
It is a simple table constructed from northern catalpa (an uncle supplied it for the cost of Railway Express shipping.) It is a soft but very dimensionally stable wood. The USDA Forest Products Laboratory reports 2% radial and 5% tangential expansion. (See: http://www.woodbin.com/calcs/shrinkulator.htm) Unfortunately, I do not have a large supply of material in hand. It can be purchased but is not commonly available.
The table consists of a 14" x 47" sheet of ½" plywood with 4 tapered 24" long legs, 1.75" square at the top and 0.75" square at the bottom. The legs are tied together 8" from the floor by 0.75" x 1.5" stretchers across the short dimension and a single 0.75" x 1.5" stretcher centered on them along the long dimension. I made the error of simply butting the pieces together and pulling up the joints with one #8 woodscrew in each joint. I covered the screw holes with catalpa doweling. The endgrain of this soft wood is not up to holding a screw for very long. I made a small hole saw from a scrap of tubing which fit comfortably over the screws and bored down around the screw until I could easily pull the head through and remove the screw from the parts. I now have ~0.35" diameter holes ~0.6" deep in the legs and 0.45" deep in the stretchers and/or legs where the screws once protruded.
Most common stronger woods have at least double the expansion of catalpa. This suggests to me that simple doweling with birch dowels would invite splitting. I am thinking about creating "mortise and tenon" joints by cutting splines from the scrap catalpa I have and fitting them into mortises cut into the legs and the stretchers. The legs are ~1.20" square where the stretchers fit. Using the 1/3 rule and applying it to the legs, this suggests that the splines should be 7/16" thick. This would leave ~1/4" of wood on each side of the stretcher to support the spline. Using this same dimension, the width of the spline should be 1". Is ¼" of wood on the stretcher enough for good strength? The 5x rule (length vs. thickness) for a tenon suggests I should make the tenons 2.2" long. This exceeds the dimension of the leg at the proposed point of insertion. How deep should the mortise in the stretcher be; how long should the exposed end of the spline be? The problem is even more extreme at the joint between the shorter stretchers and the long central stretcher. Here the mortise needs to be less than the ¾" thickness of the shorter stretchers. Do I use a ½" long 7/16" x 1" tenon?
I will try to include a photo for clarification.
Thank you,
John
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