I wanted some wingnuts to let me quickly clamp the bed material to the table and release it again. The XY table came from the US so it has 2BA, rather than metric threads, in it. That means I can't get them locally and would have to order them. The cost would be about $6 for 10 including postage, but I only wanted 4.
Then I remembered I have a machine than can make things so I made some knobs with captive nuts: -
Easy to design, but the hexagonal cavity is a pain to model in ArtOfIllusion. You have to start with a six sided polygon. You then convert it to a triangle mesh and then extrude it to make a hexagonal prism to subtract from the cylindrical shaft.
The three types of solid primitive: cube, cylinder and sphere all have editable dimensions but for some reason polygon primitives don't show any dimensions. To get round this you have to set the grid spacing to the dimensions you want and snap the polygon's bounding box to the grid.
I intended the nuts to be a push fit but they were too tight so I pushed them in with a hot soldering iron. The small M4 one on the left was a test to see if the design scales. I think the nut cavity needs to be a bit shorter for metric nuts.
These cost less than $0.06 each in plastic so that saved me $5.76. A good example of the economics of RepRap. Although it is no doubt cheaper to make wingnuts by traditional means in large numbers, the cheapest way for an individual to obtain them in small quantities is to RepRap them. Of course I needed some plain nuts, but they are a lot cheaper and easier to obtain.