Building the plywood form to cast the bench legs
The bolt block and bridge are made from standard framing lumber. When assembled, concrete is poured in to fill the form.
I’ll start by cutting plywood on the table saw and checking that it’s square. For these forms I used plywood as it’s what I already had around the shop. The legs are identical so one form is enough but, as I was filming and prototyping, I built two.
One from recycled fir plywood, and one from birch.
You don’t need a lot of power tools to built the forms or the bench. You can get by just fine with a circular saw, drill, and driver. You could use a circular saw, a home-made guide, and clamps for these cuts as well.
If you’ve built the garden box forms then this should be pretty straightforward.
Attach the Inset
Like the base, the inset is plywood but beveled at 30 degrees. I make one inset from birch, and the other from fir.
I lay out and mark the corner locations on the base and drill pilot holes for the screws that will attach the inset. I apply a little glue then attach the inset with screws from the underside of the base.
Next I’ll cut the three plywood sides for the form. I rip them to width then cut them to length with a mitre saw. You could use a speed square and circular saw to make these cuts instead of a miter saw.
The bolt block is made from two pieces of framing lumber. I rip the upper and lower pieces from a 2×6. I rough cut these an inch longer than needed as I will trim it to final length after the pieces are screwed together. It’s easier to align this way. I mark the location for the three bolt holes and mark the screw locations between them. I use four screws to attach these two pieces together. I clamp them, drill pilot holes, then run the screws in.
I’ll trim it to it’s final length then take this bolt block to the drill press to cut three counterbores on the outside face. I swap bits and drill three holes on the inside face to hold the bolts. I’ll talk more about the reason for the counterbore later.
The thru hole is 3/8’s diameter and the bolts fit snug with little or no play. I want the bolts to be held firmly and to not move once they are set in place.
I mark and drill pilot holes for the screws that will hold the bolt block to the base and sides. I use the drill press for this but you can freehand these.
The bridge is made from a 2×4 and holds the bolt for the stretcher.
Assembling the form parts
I cut the span and two supports to length and mark the center for the bolt. I counterbore and drill on the drill press.
All holes and counterbores could also be done using a drill guide instead of a drill press.
I mark and drill pilot holes in the supports. Then assemble the bridge with 3″ screws.
With all the parts done I’ll do a test assembly of the form to see if everything fits together as planned. I’ll use self drilling, washer head style cabinet screws for most of this. Along with some two inch and three inch wood screws.
The pilot holes really help speed this up.
Now I’ll check the fit of the bridge and attach it to the walls of the form with 2″ screws.
It doesn’t always go this well but everything fit the first time for both forms. I mark each part with a sharpie, so I can reassemble it the same way.