I want to know which type of screw will be good to join two piece of aluminum structure, one with greater pitch or less pitch.which one will provide more strength? any help would be appreciated.thanks

 

Hey, you probably are not interested in an answer now, but here is a quick answer. Strength-wise: fine pitch is better. But try to use coarse pitch unless it's really needed. On a side note, since aluminum is a soft material in comparision to the screw/bolt, use a washer.

 

My main concern would be the thread strength in the aluminum part. I would use a coarse pitch fastener as they are less likely to strip out. With the exception of thin wall materials where you can not get at least a couple full threads without using a fine pitch fastener but I would try to avoid this case.

Edit: I assumed you were threading into the aluminum ...not sure if that is actually the case. Fine pitch fasteners have a higher tensile strength so the strongest fastener depends on your design.