Weld symbol
J-groove weld symbol
Half of a U: one member is machined to a curved, near-vertical face with a radiused root while the other stays square. A broken arrow shows which member gets the preparation.
Reads asArrow side: J-groove weld, 1/2 size, 20° groove angle, 1/8 root opening.
How to read it
The J-groove symbol is drawn with a straight perpendicular leg on the left and a curved face on the right, which is exactly half of the U symbol. It says one member is prepared with a radiused groove and the other is left square, the same single-sided idea as a bevel groove but with a curved profile instead of a straight cut.
As with any single-member preparation, the symbol cannot say which physical member is cut, so the leader is normally drawn with a break that swings toward the prepared piece. On a tee joint that is usually the attached member, on a butt joint between unequal thicknesses it is usually the thicker one, but read the break rather than guessing from the geometry.
The angle beyond the curved face is that face's angle off the plate surface, and on a J it is small for the same reason it is small on a U: the root radius, not the angle, gives the electrode room at the bottom. The root opening sits inside the symbol, the depth of preparation sits to the left, and the root radius normally appears on the joint detail rather than on the symbol.
J preparations earn their machining cost on thick single-sided joints where a straight bevel would open into a very wide groove. They also produce a gentler transition at the root than a sharp bevel, which matters where the joint sees fatigue loading and where a sharp root corner would concentrate stress.
Where each dimension goes
- Groove angle (beyond the curved face)
- Angle of the single prepared face off the plate surface, small because the radius provides root access.
- Root radius
- Curve machined at the bottom of the preparation. Given on the joint detail rather than in the symbol.
- Root opening (inside the symbol)
- Gap between the prepared face and the square member at the root.
- Depth of preparation (left of the symbol)
- How far the groove is cut into the prepared member and filled with weld metal.
- Broken arrow
- The kink in the leader identifies which member carries the J preparation.
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: J-groove on a heavy tee
TitleJ-groove on a heavy teeRefJ-groove weldEx1 / 2Reads as
Machine a J into the member the broken arrow shows, 3/4 deep at 20 degrees.
Which mark says so
- J-groove symbolBelow the reference lineA J-groove weld on the arrow side of the joint
- 3/4Left 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
- Broken leaderOn the arrow itselfThe arrow points at the member to be prepared
The break in the leader picks the prepared member, the 20 degrees is that single face's angle, and the 3/4 to the left is the depth of the groove filled with weld metal.
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: J-groove with a back weld
TitleJ-groove with a back weldRefJ-groove weldEx2 / 2Reads as
Fill the J from the arrow side, then gouge and back weld the far side.
Which mark says so
- J-groove symbolBelow the reference lineA J-groove weld on the arrow side of the joint
- 5/8Left of the arrow-side symbolGroove weld size - how deep the weld itself goes
- 20°Beyond the outer face of the arrow-side symbolIncluded angle the joint is prepared to
- 1/16Inside the arrow-side symbolRoot opening - the gap left between the members before welding
- back symbolAbove the reference lineA back weld on the far side of the joint
- Broken leaderOn the arrow itselfThe arrow points at the member to be prepared
The J below the line is the main preparation and the semicircle above it calls for a back weld deposited after the root is cleaned out from the opposite face.
And what it does not say
- No length is given, so each weld runs the full length of the joint.
- There is no circle at the elbow, so the weld does not continue around the joint.
Common mistakes
- Cutting a straight bevel because the small J angle looks like a bevel angle on a poor print.
- Ignoring the broken arrow and machining the wrong member.
- Leaving a sharp corner at the root instead of the specified radius, which defeats the reason for choosing a J.
- Confusing the root opening inside the symbol with the root radius on the detail.