Weld symbol
Double J-groove weld symbol
One member machined to a J profile from both faces while the other stays square. Two half-U symbols sit on the reference line, and the broken arrow still shows which member is cut.
Reads as: Arrow side: J-groove weld, 1/2 size, 20° groove angle, 1/8 root opening. Other side: J-groove weld, 1/2 size, 20° groove angle.
How to read it
A double J is the single J done twice on the same member, once from each face, with the second member passing through square between them. The symbol shows the half-U shape above the reference line and again below it, and the broken arrow still resolves which of the two members carries both preparations.
Each face is dimensioned independently. The small angle beyond each curved face is that face's groove angle, and the depth to the left of each symbol is how far that preparation is cut into the member. The root opening between the prepared member and the square member is written once, and the land left between the two preparations comes from the joint detail.
The reason for the arrangement is the same as any double preparation on thick material: half the volume of a single deep groove, balanced shrinkage across the joint, and the ability to alternate passes side to side. Adding the J profile on top of that keeps the groove narrow all the way down, which matters most where the member is several inches thick.
Expect this preparation on heavy machined weldments and on nozzle attachments where both faces can be reached and where the joint is going to be inspected volumetrically. Its cost is real — two machined profiles and a part that has to be turned — so its appearance on a drawing usually signals a critical joint rather than a routine one.
Where each dimension goes
- Groove angle on each face
- Angle of that face's prepared surface off the plate, small on a J because the radius gives root access.
- Depth on each face
- How far that preparation is machined into the prepared member and filled.
- Root opening
- Gap between the prepared member and the square member, written once for the joint.
- Root face or land
- Unmachined thickness left between the two J profiles, given on the joint detail.
- Broken arrow
- Identifies which member is machined from both faces.
Worked examples
Symmetric double J
“Machine J profiles 1 inch deep into both faces of the prepared member at 20 degrees.”
Matching half-U symbols above and below the reference line describe the same member cut twice, so the preparation is symmetric about its mid-thickness and shrinkage balances.
Double J finished flush on the accessible face
“Fill both J grooves and machine the arrow-side face flush with the plate.”
The bar across the outer face of the lower symbol calls for a flush contour, and the M beside it says machining rather than grinding produces that surface.
Common mistakes
- Machining a J into each member instead of both faces of one member.
- Losing track of the broken arrow when both symbols look alike above and below the line.
- Assuming the two depths match when the symbol dimensions them separately.
- Welding the second side without gouging back to sound metal at the land.
Typical joints
Related symbols
Practise reading welds, not just symbols
The drawing tells you what the weld should be. The gauge trainer is where you find out whether you can tell what it actually is.