Following is a method I’ve made up for making contour mats used in vacuum bagging/sandwich construction. This mat is for a single concave that tapers shallow to deep, nose to tail. It’s made from a 20mm sheet of VH (28kg/m3) EPS cut with a hot wire.
First I’ve made 2 timber strips that taper from about 15mm to 5mm over the length of the foam sheet. These are taped down to a flat bench evenly spaced apart (same width as the foam).
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I then place a sheet of MDF (and or ply) over the tapered strips, I’ve also got 2 x 3mm thick MDF “rails” taped down to this sheets, they’re spaced apart the same distance as the mat width.
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Lay in the EPS between the rails with another sheet of ply on top.
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Place weights on top of this to flex the EPS, get a straight edge to check that the top ply doesn’t flex too low so it fouls the hotwire.
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If everything looks OK, load up the remaining length of the sheet so its pressed firmly on the bench (I have a timber strip under the bricks). At this point I’ve placed additional weight to spread out the concave in the middle of the board
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Hotwire down the length of the sheet using the 2 side rails, take the weights and top sheet off…
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This one turned out a little too peaked in the middle, but I’ll sand it down a bit.Having a wider plank or half width sheet of ply would’ve helped smooth out the curve.
Trim length and outline as required, then onto your rocker jig with a sheet of 3mm MDF to protect the foam and smooth out any bumps.
I’ve also used this sort of process the put deck camber in when hot wire cutting board blanks.
Hope it helps