U-groove weld symbol

The U-groove symbol is drawn as a U sitting on the reference line: straight sides that curve into a rounded bottom. That shape is a description of the joint cross section. Both members are prepared, each with a small included angle and a generous root radius, so the groove opens only slightly as it rises rather than fanning out the way a vee does.

The number beyond the open end of the U is the included angle between the two prepared faces. On a U it is small, often in the region of fifteen to twenty degrees, because the root radius rather than the angle is what gives the electrode access to the bottom of the joint. The root opening inside the symbol is the gap between the two root faces, and the size to the left is the depth filled.

The economics are the whole point. On very thick material a vee's cross-sectional area grows with the square of the depth, while a U's grows close to linearly once past the radius. Halving the weld metal on a four-inch joint saves hours of arc time, kilograms of consumable, and a great deal of shrinkage. The cost is preparation: a U has to be machined, gouged, or cut with a shaped torch rather than a straight bevel.

Everything else reads as it does on any groove. A U above the line as well as below means both faces are prepared, a semicircle above a U means a back weld follows, and a length to the right limits the run. Contour and finish symbols beyond the outer face describe how the completed weld face is to be left.