Weld symbol
Projection weld symbol
A resistance weld located by an embossed dimple or raised boss rather than by the electrode. The circle sits on the side of the reference line carrying the projections.
Reads asArrow side: spot weld, 1/4 size, 2 pitch, 6 welds.
How to read it
Projection welding is resistance welding where the location of each weld is decided by the part rather than by the electrode. One member carries a formed dimple, an extruded boss, or a machined ridge, and when current passes through the stack that raised feature concentrates it, so the nugget forms exactly there. A flat electrode can therefore make several welds at once.
The symbol borrows the resistance-weld circle. What tells you it is a projection weld rather than a plain spot is which side of the reference line the circle sits on: the side carrying the projections. That is a meaningful placement here in a way it is not for ordinary spot welding, because the projections physically belong to one of the two members.
The dimensions read as they do on a spot weld. A number to the left gives the required nugget diameter or the strength each weld must develop, a count in parentheses says how many, and a pitch gives spacing where the welds fall in an evenly spaced line. The projections themselves - their height, diameter, and shape - belong on the part drawing, not on the weld symbol.
Projection welding is chosen for production volume and for joining parts of unequal thickness or dissimilar geometry, such as a nut or stud onto a panel. Because the projection collapses as the weld forms, controlling force and current is what makes the process repeatable, and the drawing will usually reference a procedure for that rather than encoding it in the symbol.
Where each dimension goes
- Nugget size or strength (left of the circle)
- What each weld must achieve - a fused diameter at the interface or a load capacity per weld.
- Count (in parentheses)
- How many projection welds the callout covers, often made simultaneously in one electrode stroke.
- Pitch (right of the circle)
- Centre-to-centre spacing where the welds fall in an evenly spaced row.
- Side of the reference line
- Identifies which member carries the formed projections, which is the placement's real job here.
- Projection geometry
- Height, diameter, and shape of the raised feature, given on the part drawing rather than on the weld symbol.
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: Four projections in a row
TitleFour projections in a rowRefProjection weldEx1 / 2Reads as
Make four projection welds with 3/16 nuggets on one-and-a-half inch centres.
Which mark says so
- spot symbolBelow the reference lineA spot weld on the arrow side of the joint
- 3/16Left of the symbolSize of each weld - diameter for a spot, width for a seam
- -1-1/2Right of the length on the symbolPitch - centre to centre spacing of an intermittent weld
- (4)In parentheses beside the symbolHow many welds are called for
The circle sits on the side of the line whose member carries the dimples, the 3/16 is the required fused diameter, and the count and pitch describe the row of welds.
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: Projection welds specified by strength
TitleProjection welds specified by strengthRefProjection weldEx2 / 2Reads as
Attach the nut with three projection welds each holding 600 pounds.
Which mark says so
- spot symbolBelow the reference lineA spot weld on the arrow side of the joint
- 600 lbLeft of the symbolSize of each weld - diameter for a spot, width for a seam
- (3)In parentheses beside the symbolHow many welds are called for
Replacing the diameter with a load value controls the result rather than the nugget geometry, which suits fastener attachment where pull-off strength is the requirement.
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
- Reading the circle as an ordinary spot weld and forming the projections on the wrong member.
- Looking for projection height and diameter on the weld symbol instead of on the part drawing.
- Assuming the count means sequential welds when projection welding usually makes them all at once.
- Ignoring that projection collapse makes electrode force as important as current for a sound weld.