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 asArrow 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
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: Countersunk plugs in a doubler plate
TitleCountersunk plugs in a doubler plateRefPlug weldEx1 / 2Reads as
Drill six 3/4 holes countersunk at 45 degrees and fill them 1/2 deep.
Which mark says so
- plug symbolBelow the reference lineA plug weld on the arrow side of the joint
- 1/2Left of the symbol, before the sizeDepth of preparation - how far the joint is cut back
- 3/4Left of the symbolDiameter of the hole, or width of the slot
- 45°Beyond the outer face of the symbolIncluded angle the joint is prepared to
- (6)In parentheses beside the symbolHow many welds are called for
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.
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: Plugs filled flush on a lap joint
TitlePlugs filled flush on a lap jointRefPlug weldEx2 / 2Reads as
Fill eight 1/2 inch plug welds on four-inch centres and grind them flush.
Which mark says so
- plug symbolBelow the reference lineA plug weld on the arrow side of the joint
- 1/2Left of the symbolDiameter of the hole, or width of the slot
- -4Right of the length on the symbolPitch - centre to centre spacing of an intermittent weld
- (8)In parentheses beside the symbolHow many welds are called for
- flush contourBeyond the outer face of the symbolThe completed face is to be flush
- GBeyond the contour on the symbolThe method used to reach that contour, not the finish itself
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.
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
- 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.