Folding Portable Workbench – Making the Torsion Box Grid

The core of the folding portable workbench functionality — light weight combined with high stiffness — calls for the “slab” to be in reality, a slab-like torsion box.  To combine these two seemingly irreconcilable features, I constructed the torsion box from 1/4″ baltic birch plywood faces and grid, with 1/2″ baltic birch plywood perimeter.


Using a power saw and a clamped-on rip fence I cut the 1/4″ faces to size, then cut the 1/4″ grid web elements and the 1/2″ perimeter exactly the same width so they would establish a planar surface when assembled.  Once these strips were cut I clamped them together and planed them to be perfectly identical.  The ultimate strength of the bench depends on it.


For the grid itself, I simply divided the spaces inside the torsion box into the units I desired, then cut sough of the grid web elements to that number.  For example, since the torsion box top is 20″ x 60″ I used a roughly 5″ increment for the grid, meaning that I needed three long web elements and 11 short elements.


Laying out the grid to this number of units, I took the three long elements and clamped them together and marked out the spacing for the half-lap/slots that would allow for the perpendicular elements to cross over each other, then did the same thing for the eleven short elements.


Since the “joinery” at the cross-point need not be joinery at all — the two elements simply need to fit past each other, as the true strength of the beam (torsion box bench top) is established only by the character of the glue line of the web element to the faces of the box — I used my recently acquired miter box and a 1/4″ chisel to quickly chop rough openings for the intersection.


Up next – laying out the screw holes for the vise and assembling the grid.