Weld symbol
Seam weld symbol
A circle with a bar through it calls for a seam weld — a continuous or closely overlapping resistance weld made by rolling wheel electrodes along the joint instead of stopping at each spot.
Reads as: Arrow side: seam weld, 1/4 size, 6 long, 1 pitch.
How to read it
The seam weld symbol is the spot weld circle with a straight bar drawn through it along the direction of travel. That bar is the whole distinction: it says the weld runs as a line rather than existing as isolated points. In practice the weld is made by wheel electrodes that roll along the overlapping sheets while current pulses through them.
The number to the left of the circle is the weld's width, meaning how wide the fused band is across the seam. A length to the right says how far the seam runs, and where the seam is not continuous a pitch after the length gives centre-to-centre spacing of the individual weld runs, which is how a roll spot pattern is called out.
Continuous seams are chosen when the joint has to hold pressure or keep liquid in, because a line of separate spots leaves paths between them. Intermittent roll spot seams are chosen when only strength is needed, since running the wheel continuously puts far more heat into thin sheet than the joint requires.
Like a spot weld, the seam forms between the sheets rather than on a visible face, so the symbol may sit centred on the reference line when the process determines access. Where it is drawn clearly above or below, read the side in the normal way. In every case the drawing rather than the symbol tells you which sheet the wheels run on.
Where each dimension goes
- Width (left of the symbol)
- How wide the fused band is across the seam, measured at the faying surface.
- Length (right of the symbol)
- How far the seam runs along the joint.
- Pitch (after the length)
- Centre-to-centre spacing where the seam is made as a series of runs rather than one continuous line.
- Bar through the circle
- The mark that separates a seam from a spot: it says the weld is a line, not a point.
Worked examples
Continuous seam on a tank lap
“Run a continuous 3/8 wide seam weld the full six inches of the lap.”
The 3/8 to the left is the fused band's width and the 6 to the right is the run length. With no pitch, the seam is continuous rather than a series of separate runs.
Roll spot seam
“Make 1/4 wide seam runs one inch long on three-inch centres.”
Adding a pitch after the length turns the continuous seam into a series of short runs, which puts far less heat into thin sheet while still developing the required strength.
Common mistakes
- Missing the bar through the circle and reading a seam callout as a spot weld.
- Treating the width to the left as a nugget diameter rather than the width of a fused band.
- Making a continuous seam where a pitch says the weld should be intermittent, overheating thin sheet.
- Assuming a seam weld is automatically pressure-tight when the callout gives an intermittent pattern.
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.