Weld symbol
Convex contour symbol
An arc bowing away from the reference line calls for a weld face that stands proud of a straight line between its toes, giving extra metal in the middle of the bead.
Reads asArrow side: fillet weld, 5/16 size, convex contour.
How to read it
The convex contour symbol is an arc drawn beyond the weld symbol's outer face, curving away from the reference line. It says the finished weld face should bulge outward rather than sit flat or dish inward. On a fillet, that means the face stands above the straight line joining the two toes; on a groove weld it means reinforcement above the plate surface.
A modest convexity is what most welds naturally form, so the symbol usually appears when the shape has to be guaranteed rather than merely allowed. That happens where extra metal is wanted for wear, where the joint sees mainly static load and reinforcement adds margin, or where the face will be machined later and needs stock left on it.
Convexity has limits, though the symbol itself does not state them. Piling weld metal too high creates a sharp angle where the face meets the base metal, and that abrupt change in section concentrates stress right at the toe. Under fatigue loading a heavily convex fillet can perform worse than a flat one with the same throat, which is exactly why fatigue-sensitive joints usually call for flush or concave instead.
The important measurement point is the throat, not the face. A convex fillet's extra crown does not add to the effective throat that carries load, since the throat is measured from the root to the straight line joining the toes. Adding convexity therefore costs consumable and heat without adding capacity, which is worth remembering before over-welding a joint that only asked for a shape.
Where each dimension goes
- Arc bowing away from the reference line
- Calls for a weld face that stands proud of a straight line joining the weld toes.
- Weld size
- Still read to the left of the weld symbol. The contour describes shape, not size.
- Finish letter
- Names how the convex face is produced or dressed where the shape has to be worked to rather than welded to.
- Effective throat
- Unchanged by convexity, since the throat is measured to the straight line between the toes rather than to the crown.
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: Convex fillets on both faces of a tee
TitleConvex fillets on both faces of a teeRefConvex contourEx1 / 2Reads as
Weld 3/8 fillets both sides and leave the faces crowned rather than flat.
Which mark says so
- fillet symbolBelow the reference lineA fillet weld on the arrow side of the joint
- 3/8Left of the arrow-side symbolLeg size of the fillet
- convex contourBeyond the outer face of the arrow-side symbolThe completed face is to be convex
- fillet symbolAbove the reference lineA fillet weld on the far side of the joint
- 3/8Left of the other-side symbolLeg size of the fillet
- convex contourBeyond the outer face of the other-side symbolThe completed face is to be convex
An arc bowing away from the reference line appears on each side, so both fillet faces are to stand proud of the straight line joining their toes.
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.
Example 2: Convex groove weld left for machining
TitleConvex groove weld left for machiningRefConvex contourEx2 / 2Reads as
Fill the groove with a crowned face and machine it back afterwards.
Which mark says so
- V-groove symbolBelow the reference lineA V-groove weld on the arrow side of the joint
- 1/2Left of the symbolGroove weld size - how deep the weld itself goes
- 60°Beyond the outer face of the symbolIncluded angle the joint is prepared to
- convex contourBeyond the outer face of the symbolThe completed face is to be convex
- MBeyond the contour on the symbolThe method used to reach that contour, not the finish itself
The convex arc leaves reinforcement above the plate, and the finish letter tells the shop that machining will bring the face down to its final level later.
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
- Treating extra convexity as extra strength. The effective throat is measured to the line between the toes, not to the crown.
- Piling weld metal so high that the toe angle becomes sharp, which concentrates stress under fatigue loading.
- Applying convexity to a joint where the drawing actually calls for a flush or concave face.
- Reading the arc as a size dimension rather than as a shape instruction.