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Construction of the strongback (pictures)

After routing one side of a 100 mm strip. Note that the right side isn't too neat... That's not important, and easier to correct after assembly

This is the way I blocked the wood strip to be able to pass the router on the right side. The left side must remain free to pass the router's guide. The right support (or stop) must be low enough to pass the bit over it. (I used a scrap piece of 4 mm plywood. The small block at the end stops the strip from sliding.

The other side of the strip - supported in much the same way. Note the 'working bench'; it's the leftover from the plywood.

After a first pass with the router. I found that it wasn't necessary to remove the paint from the strip ends.

Another strip ready for assembly.

The router generates loads of wood cuttings. Keep the surface where the router slides quite clean, else the depth of the router cut will change and it will be difficult to fit the strongback.

This is how I marked the narrow strips. Two strips are measured from one side, the other two from the other. That's why the center separation is different - EASY tolerances.

... everything cut. Now I'll have to plane them down to the correct size

My shepherd always seems to find something interesting in the pile.

The first glueing. One of the 610 mm long narrow strips glued to one of the wider (100 x 2440 mm) strips. Start glueing at the reference side of the wider strip. Not visible here is the aluminium profile - it's under the plywood (see below).

The following steps - always choose the next longer strip to continue assembly. I did use a old piece of aluminium (about 1.5 m long) to fix the strongback to, which provides a nice straight reference. I waited a couple of hours after each new strip glued.

Side view of the strongback. The unfinished side is quite noticeable (on the right).

And a view along the narrow side. Routing down to 6 mm exactly coincides with leaving two of the 5 layers of the plywood. Though it does look nice, I'm not sure glueing on the phenolic glue is mechanically sound.
Continues with mounting the forms on the strongback.
(c) John Coppens ON6JC/LW3HAZ mail