In welding, butt joints are used to join the edges of two plates or surfaces located in approximately the same plane. Plane square butt joints in light sections are shown in figure 6-17. Grooved butt joints for heavy sections with several types of edge preparation are shown in figure 6-18. These edges can be prepared by flame cutting, shearing, flame grooving, machining, chipping, or carbon arc air cutting or gouging. The edge surfaces in each case must be free of oxides, scales, dirt, grease, or other foreign matter.

Welding butt joints in light sections.

Welding butt joints in heavy sections.
 The square butt joints shown in figure 6-16 are used for butt welding light sheet metal. Plate thicknesses 3/8 to 1/2 in. (0.95 to 1.27 cm) can be welded using the single V or single U joints as shown in views A and C, figure 6-18. The edges of heavier sections (1/2 to 2 in. (1.27 to 5.08 cm)) are prepared as shown in view B, figure 6-18. Thicknesses of 3/4 in. (1.91 cm) and up are prepared as shown in view D, figure 6-18. The edges of heavier sections should be prepared as shown in views B and D, figure 6-18.

The single U groove (view C, fig. 6-18) is more satisfactory and requires less filler metal than the single V groove when welding heavy sections and when welding in deep grooves. The double V groove joint requires approximately one-half the amount of filler metal used to produce the single V groove joint for the same plate thickness. In general, butt joints prepared from both sides permit easier welding, produce less distortion, and insure better weld metal qualities in heavy sections than joints prepared from one side only.