Wednesday, February 17, 2010
I've been reading the new book about forming up sheet metal and not surprisingly, it looks like it could be a little more work than what I had originally thought. The book shows several different methods of achieving the part shape, including using a draw ring. In theory, using a draw ring is quite simple. You have a ram or top punch made to the shape of the part, a pressure plate to hold the aluminum sheet flat while forming and the draw ring which is basically just a heavy plate with a hole cut in it to match the outline of the ram plus clearance for the metal. The ram comes down and pushes the sheet through the draw ring with the pressure plate holding the sheet down to prevent wrinkling. No bottom mold or cavity required. Pretty slick, actually. The only problem I see, regardless of the type of operation chosen, is the problem of getting it right the first time. If one goes to the trouble of building some type of press and tooling it all up, then putting some expensive aluminum sheet in it only to have it come out as wrinkled as Grandma's arse, you're not getting much of a return on your investment. I need to have a confab with Surly and see what he thinks about the whole idea. We spoke just briefly about it but in the meantime, I'm working out the shape I want and maybe I'll start on a model of some type.
The tank on the BSA Spitfire above or one like they put on the 441 Victor look good. Maybe make it just a little narrower in the rear. The straight edge along the bottom would make installation of the inner part real easy. The Greeves Scottish is basically the type of bike I'm trying to build. Named for and designed to compete in the Scottish Six Day Trials, it's all business and has a lovely little aluminum tank, especially nice with the Monza style filler cap. That's the tank I'd like to be able to make but everything is curved. That's why I've been considering something other than hand forming. Eventually it will come to the point of either putting this project in mothballs or just getting after it. I'm determined to see this one through to the end, however. I'm not going to start any new projects, I'm going to finish this one. I've got too many that I've started and put on hold already. I want to get after the 900 one of these days, as well. I'd be real nice if they were both done by the time school's out.
Posted by Shop Teacher Bob at 6:00 AM