Friday, January 17, 2014

A note on plywood

A few years back, I made some 12 foot box beams for a long glue up. Construction was straightforward: top and bottom had grooves for the webbing, (the vertical part of the I-beam cross section) and the whole got glued and screwed. 

For those beams, I used regular, paint grade, birch faced ply. And what I found was that there was a huge variation in thickness. The ply wouldn't always fit into the grooves. 

To compensate, I rabbeted the edges of the webbing on a router table by running them between the fence and bit. The result was a consistent thickness, and a rabbet that was nonexistent in some places, and over 1/16" deep in others. That over-thickness was what had been keeping the ply from fitting into the grooves during the dry fit. Once thicknessed, everything went together fine.

The grooves in the top and bottom were of consistent depth, which meant that there would be a variation in the remaining thickness, translating into a variation in flatness of the surface. On a 12 foot span, 1/16" or more isn't a big deal. But the project I'm working on now (Henceforth, Project X, until design trademarking is worked out) will require tighter tolerances.

These box beams are only 5' long. And this time I used good Baltic birch. (As opposed to the home center equivalent, which usually has overlapped laminations, and flaky surface veneers.) No variation in thickness, and the web fits cleanly and consistently into the grooves.

I'm much happier with these beams. Project X will have some funky 3-way miter joints that transition into compound curves. That means I need accurately flat blanks glued up to work with the jigs. The process has been interesting to engineer.

More on that soon, along with the write up on the hay rake assembly.

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