tg,
How do you plan to weld them? The projections melt during welding, and the part is compressed, so the resolidified metal is evenly spaced between the nut and sheet metal. By the way, if you stick with parts that have 3 projections, you eliminate rocking since the 3 points define a plane.
Regards,
Cory
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.