Weld symbol
U-groove weld symbol
A U-shaped symbol calls for a groove with curved, near-vertical prepared faces and a radiused root. It uses far less weld metal than a vee on thick material but has to be machined or gouged.
Reads asArrow side: U-groove weld, 3/4 size, 20° groove angle, 1/8 root opening.
How to read it
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.
Where each dimension goes
- Included angle (beyond the U)
- Total angle between the two prepared faces, small on a U because the root radius provides access instead.
- Root radius
- The curve at the bottom of the groove. Normally dimensioned on the joint detail rather than on the symbol itself.
- Root opening (inside the symbol)
- Gap between the two root faces before welding.
- Groove weld size (left of the symbol)
- Depth of the joint filled with weld metal.
Worked examples
Each one is a drawing, the sentence it comes out as, and then the mark-by-mark account of why. Read the sheet first and say the sentence yourself before you look down.
Example 1: Deep U-groove from one side
TitleDeep U-groove from one sideRefU-groove weldEx1 / 2Reads as
Machine a 20 degree U 1-1/4 deep with a 1/8 root gap and fill it from the arrow side.
Which mark says so
- U-groove symbolBelow the reference lineA U-groove weld on the arrow side of the joint
- 1-1/4Left of the symbolGroove weld size - how deep the weld itself goes
- 20°Beyond the outer face of the symbolIncluded angle the joint is prepared to
- 1/8Inside the symbolRoot opening - the gap left between the members before welding
The 20 degrees is the small included angle typical of a U, the 1/8 inside the symbol is the root gap, and the 1-1/4 to the left is how deep the groove is filled.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- Nothing sits above the reference line, so the far face of the joint stays bare.
- There is no circle at the elbow, so the weld does not continue around the joint.
Example 2: U-groove finished flush with the plate
TitleU-groove finished flush with the plateRefU-groove weldEx2 / 2Reads as
Fill the U-groove 1 inch deep and grind the face flush with the plate surface.
Which mark says so
- U-groove symbolBelow the reference lineA U-groove weld on the arrow side of the joint
- 1Left of the symbolGroove weld size - how deep the weld itself goes
- 15°Beyond the outer face of the symbolIncluded angle the joint is prepared to
- 1/16Inside the symbolRoot opening - the gap left between the members before welding
- flush contourBeyond the outer face of the symbolThe completed face is to be flush
- GBeyond the contour on the symbolThe method used to reach that contour, not the finish itself
The straight bar across the outer face of the symbol calls for a flush contour, and the G beside it says grinding is the method used to get there.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- Nothing sits above the reference line, so the far face of the joint stays bare.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Cutting the small U angle as though it were a vee angle, which produces a groove too narrow to weld.
- Ignoring the root radius on the joint detail and gouging a flat-bottomed groove instead.
- Choosing a U preparation where a vee is cheaper overall because the material is not thick enough to repay the machining.
- Reading the root opening inside the symbol as the root radius. They are different dimensions.