The former had to be at least 85cm in height, which already includes some tolerance, to created the curved back form, this is quite a large distance to cover, and the former is large and quite expensive therefore i could only budget one attempt at getting this right, and didn’t want any warping in the form due to the gaps between ribs being too large. I decided that nine ribs, would create a suitable distance between each, that it wouldn’t suck the flexi ply in, although because the vacuum force is spread over such a large area, this might not be anything to worry about. Using 16 mm ply pieces that had been machine cut on the CNC, meant that the accuracy of the former would be high.
16 mm x 9 = 144 mm
850 mm – 144 mm = 706 mm
706 mm / 8 (no. of gaps between ribs) = 88.25 mm
= 8.8 cm per gap approx
Pieces of 2×1 pine blanks were cut to the length of 8.8cm on the band saw, then placed on top the a layer of the CNC’d ply. The pine pieces were marked, taken off and a pilot hole drilled from the other side of the ply piece. The pine spacers were screwed using 40 mm screws from the underside of the ply piece, this process was repeated on each ply piece, alternating the placement of the spacers per layer of ply. Once layer was completed the another layer was placed on top, allowing the spacers from the layer below to be screwed in and attached to the layer above.
Once the nine layer were completed and the structure deemed solid, the flexi-ply (cross grain) was stapled using a compressed air staple gun, in linear fashion, vertically and horizontally, into the flat edges of the ply ribs, to pull the flexi-ply crisply in to follow the edge of the former.