Plug and Slot Weld Symbol
The plug and slot weld symbol is a small rectangle sitting on the reference line. It calls for weld metal to fill a round hole (plug) or an elongated hole (slot) in one of two lapped plates. Below the line means the hole is in the arrow-side member; above means it is in the other member. Size goes left, depth of filling inside, pitch right and the number of welds in parentheses.
How to read the plug and slot weld symbol
In a plug or slot weld, the top plate has a hole or slot and weld metal is laid in it to fuse to the plate underneath. The rectangle on the symbol stands for that filled opening. The same symbol is used for both; a slot weld adds its length and shows its orientation in a detail.
Side means which plate has the hole: below the line, the member on the arrow side; above the line, the other member. Plug and slot welds cannot be on both sides of one joint.
What each dimension position means
| Position | Meaning |
|---|---|
| Left (S) | Plug: diameter of the hole at its base. Slot: width of the slot. |
| Inside (D) | Depth of filling. Blank means fill the hole completely. |
| Right (P) | Plug: pitch, centre-to-centre spacing. Slot: length and pitch, written length-pitch. |
| Beyond, in parentheses (N) | Number of plug or slot welds. |
| Beyond (angle) | Included angle of countersink, when the hole is countersunk. |
Example callout, read aloud
“Make three plug welds in 3/4-inch holes in the arrow-side plate, spaced 4 inches centre to centre, each filled to a depth of 1/2 inch.”
Example: reading the callout piece by piece
- Rectangle below the line: plug welds, holes in the arrow-side member.
- 3/4 left: hole diameter 3/4 inch at the base.
- 1/2 inside: fill each hole 1/2 inch deep rather than flush.
- 4 right and (3) beyond: three welds at 4-inch pitch, so 8 inches from first to last.
Result: Make three plug welds in 3/4-inch holes in the arrow-side plate, spaced 4 inches centre to centre, each filled to a depth of 1/2 inch.
When it is used
- Joining a lapped plate where edges are not reachable or would show, such as a cover or a doubler plate.
- Adding shear strength in the middle of a large lap joint already fillet welded around its edges.
- Slot welds where more weld length is needed in the same space than round plugs give.
Common mistakes
- Thinking side means which face to weld from. Side shows which plate has the hole.
- Filling a fillet-welded hole and calling it a plug weld. A fillet around the inside of a hole is a fillet weld with all around, not a plug weld.
- Filling flush when a depth is given. A value inside the rectangle limits the depth of filling.
- Guessing slot orientation. Slot direction comes from the drawing detail.
Related weld symbols
- Spot weld: Spot welds join lapped sheet without a hole.
- Fillet weld: Fillets join the edges of lapped plates.
- Seam weld: Seam welds give a continuous lap joint in sheet.
- Weld-all-around: A fillet all around inside a hole is a different callout from a plug weld.
- Surfacing weld: Surfacing builds up a surface rather than joining plates.
Every weld symbol page: welding symbols chart
Calculators and further reading
Frequently asked questions
What does the plug weld symbol look like?
A small rectangle sitting on the reference line.
Is the slot weld symbol the same as the plug weld symbol?
Yes. A slot weld uses the same rectangle, with width on the left and length-pitch on the right, and its orientation shown in a detail.
Which plate has the hole?
The arrow-side member when the symbol is below the line; the other member when it is above.
What does the number inside the rectangle mean?
Depth of filling. With no number, fill the hole completely.
Where does the number of plug welds go?
In parentheses beyond the symbol.
Sources
- AWS A2.4 Standard Symbols for Welding, Brazing, and Nondestructive Examination, American Welding Society
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