For light materials the square-edge butt joint is the easiest to prepare and can be welded without filler rod. It consists of “butting” two pieces of metal up against one another (no overlapping) and then welding along the seam between them. If the weld is to be made without filler rod, extreme care must be taken to avoid burning through the metal.
The single-V butt joint is preferable on material ranging in thickness from 3/8” to 1/2” in order to secure complete penetration. It is prepared like a regular butt joint except that the top edge of each piece is chamfered in order to reduced the area of contact between the two. The included angle of the V formed by the chamfering should be approximately 60° with a depth of about 1/8” to 1/4”.
The double-V butt joint is needed when the metal exceeds 1/2” thickness and the design is such that the weld can be made on both sides. This is like a single-V joint except that both the top and the bottom edges of the pieces are chamfered, and welding is performed on both sides. With a double V there is greater assurance that penetration will be complete.