there is a good load of math if you really want to think about it huh - I ended up with an ODE at one stage and good ol' root2 then decided ah crap just draw it out because my knife isnít going to be cutting to that level of detail, and how exactly do you cut some thing to the length of a irrational number ?
if you were mass producing them it would be a good idea... you could make a calc program in your PC to tell you the measurements for any front and rear size and start (or end) rib width- even go so far as to plot them out on a sheet for you with optimum use of cuts space on the sheet ...
however - I havenít seen reference to it but I used a little trick which although didnít use the card efficiently it made placing them a breeze (resource = money, but so does time!) :
I first made a quick 3 sides only bellows out of paper to make sure I got my measurements correct (turned out that bellows get larger when folded by ~one rib width) - one I had that I spray glued one of the sections directly onto the card, and then placed another piece of card under that and cut the slightly offset ribs deeply through it all,
but heres the trick: I left them all in place and ran a long piece of electrical tape down the centre of them - once that was stuck on each (i.e. one piece on both) I cut off the outer bit much like the leftover bit around a sticker so I was left with the ribs only hanging off the tape... it looks like one of those wind chime things that spin around in the wind if you give it a small twist .. the reason i did this was so that I could simply lay this out on the material and no more placement would be necessary, it was in place already (;
one issue tho: donít stretch the tape as you apply it, or the ribs will all contract once they are free of the rest of the cardboard and your bellows will be smaller *^%%$ you say (yes this happened to me) - you could use a non-stretch tape, I just found electrical tape had the best tack (not too sticky)
you now have the top and bottom (or left and right) done - you could glue another set on with a left over template from your earlier paper prototype but I simply offset the original one by one large rib and poked through the paper at the corner points to another two sheets of card below (making them 1mm longer to account for the offset and fudging the one missing rib) - I did all this before the tape thing btw...
I hope this makes sense and it helps some people, if it doesnít make sense now it prob will when you are making one, sorry I didnít have a digital cam on hand when I was doin it -
in any case, there are many ways to skin a cat and all the time I spent thinking about the best/fastest way to do it accounted about for the time spent saved in doing that way so I can up even! thinking probably with a superior product than if I hadn't tho... pix in the tech gallery soon