Weld symbol
Plug weld symbol
A rectangle open toward the reference line calls for a plug weld: a hole made in the near member and filled with weld metal to join it to the member underneath.
Reads as: Arrow side: plug weld, 3/4 size, 1/2 depth of preparation, 45° groove angle, 4 pitch, 6 welds.
How to read it
A plug weld joins two overlapping members through a hole. The near member is drilled or punched, the assembly is clamped tight, and the hole is filled with weld metal that fuses into the far member's surface. The symbol is drawn as three sides of a rectangle with the reference line closing the fourth, and the side it sits on tells you which member is holed.
The size to the left of the symbol is the diameter of the hole at its base, not at the surface. That distinction matters when the hole is countersunk, because the angle written beyond the symbol tapers the top of the hole to help the weld metal wet the sides and to make the finished plug easier to blend flush. Reading the diameter at the top instead of the bottom oversizes every hole in the pattern.
The depth of fill is written to the left of the size. A plug does not have to be filled level with the surface, and on thick members it often is not, since only enough weld metal to develop the required shear is needed. Where the fill depth is omitted, the convention is that the hole is filled to the surface of the near member.
Number and spacing complete the callout. A count in parentheses says how many plugs the pattern contains, and a pitch to the right gives the centre-to-centre spacing along the joint. Where the pattern is more complicated than a straight line of equal spacing, the drawing normally locates the holes directly and the symbol just says what goes in them.
Where each dimension goes
- Hole diameter (left of the symbol)
- Diameter measured at the base of the hole, not at the surface, which matters when the hole is countersunk.
- Depth of fill
- How deep the hole is filled with weld metal. Omitted means filled level with the near member's surface.
- Countersink angle
- Included angle tapering the top of the hole, written beyond the symbol.
- Count (in parentheses)
- How many plug welds the callout covers.
- Pitch (right of the symbol)
- Centre-to-centre spacing between plugs along the joint.
Worked examples
Countersunk plugs in a doubler plate
“Drill six 3/4 holes countersunk at 45 degrees and fill them 1/2 deep.”
The 3/4 is the hole diameter at its base, the 45 degrees tapers the top, the 1/2 is how deep the weld metal goes, and the count in parentheses gives the number of plugs.
Plugs filled flush on a lap joint
“Fill eight 1/2 inch plug welds on four-inch centres and grind them flush.”
No fill depth appears, so the holes are filled to the surface, and the bar beyond the symbol with a G calls for the finished plugs to be ground level with the plate.
Common mistakes
- Measuring the hole diameter at the surface of a countersunk hole instead of at its base.
- Filling every plug flush when the symbol gives a shallower fill depth.
- Drilling through both members when only the near member is meant to be holed.
- Welding plugs with a gap between the members, which lets weld metal run between the plates instead of fusing them.
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.