Anyone BIW expert who can answer me this,
I want to know the minimum distance of FU than we can maintain while designing BIW panels,
detail explanation would be great. we generally keep 6mm.
That depends on what you are using to fasten them together, the thickness of the material, the materal. Guessing you are talking 0.8-1.0mm steel, and spotwelds, it will depend on what your machine is set to, and how accurately you can place the electrodes and how accurately the clinch joint is jigged.
I'm not a body guy, these are just the obvious points. The obvious thing to do is to cut the body up and do strength tests and cut sections through welds.
It looks like a bonded joint. In this case it is configured as a salt, dirt, and moisture trap. The longer overlap it is without sealant, the faster it will eventually blow out from rust/corrosion.
Usually, this sort of joint is on door-skin outers to inners, or similar situations (the cosmetic outer is hemmed around the edge of the structural inner), so no spot-welding. Yes, you need to make sure the adhesive/sealant squeezes completely out both ends of the joint to avoid moisture trapping points. Beyond that, it's going to depend upon (a) how strong the joint needs to be, (b) how good (accurate/consistent) your stampings are, (c) what your hemming tooling is capable of. 6mm sounds about right, though I've seen more.
