a couple bellow making questions for y'all.
What do most of y'all use for modern bellow stiffeners? I know traditionally it was cardboard/ or stiff paper.
How much pigskin leather(most skins range around 10 sq / skin) do I need to buy if I am making an 4x5 bellows with a maximum extension of about 25 inches?
It has been about 15 years since I made a bellows, so some things may have changed regarding what materials folks use these days. However, I think most folks still use stiff card stock for the stiffeners in bellows. If you decide to use some other material, such as stiff plastic which isn't porous like cardboard, you want to be sure to use a type of glue that will stick to it and stay stuck.
I assume you are thinking of a square bellows rather than a tapered one. On such a bellows, the outer fold is typically about 6", and with 4 sides plus about 1/2" for the overlap on the bottom seam, you will need 24 1/2" minimum width. If you want to make the style with flat corners, that might influence this measurement, but someone else needs to address that, as I have never made that style bellows.
For a 25" extension, a common rule of thumb is to use material 1.4 times the maximum extension to keep from overextending the bellows to reach the maximum draw, so this would mean 35" material length. Add about 1/2" at each end for attaching the bellows and this would yield a total length of 36"
So, it looks like a skin of at least 24.5" x 36" (a bit over 6 sq. ft.) will be needed.
I cut mine from some polycarbonate sheet (Lexan). Very flexible and durable.
I just use folder stock. The buff colored stock used in filing.