There may be a money opportunity here for a programmer. In Visio (at least through 2010), you need to build weld symbols manually. Someone may want to look at building an Add-on (?) to sell to MS that takes user input and builds the weld symbol automatically. See atch photo for an example of how a CAD program does it: input window and weld symbol output. I would stick with just the weld symbols in Visio. If you want to see all welding symbols, the American Welding Society site has them.
Probably a low demand for this in Visio, so may not want to put too much effort into it. Another money possibility would be to port it to an Android or Apple app.
