● Plug weld
Plug weld · Soldadura en plano · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
On the drawing, “● Ø10 full fill in plate” states a plug weld requirement.
What controls
Weld deposited inside a hole made in the top plate of a lap joint, fusing it to the plate underneath. The symbol is accompanied by the hole diameter, the depth of filling when it is not complete, and the number and pitch if there are several. It joins through the area of the hole, not its perimeter: it is an alternative to a rivet or resistance spot weld when there is access from one side only. Do not confuse it with welding around the edge of the hole and leaving it hollow, which is something else entirely.
How to verify
The hole diameter is checked with calipers before welding and the fill afterward: visually and, if the drawing gives a flush or fill depth dimension, with a straightedge or depth gauge on the surface. The number of plugs and their pitch are counted and measured with a tape measure. Actual fusion to the bottom plate cannot be seen from above; it is demonstrated with process coupons (destructive peel test) or by a qualified procedure.
Common mistakes when interpreting it
- Welding only around the edge of the hole instead of filling it: that is not a plug weld and the joint is much weaker.
- Drilling the hole to whatever diameter is at hand: the diameter is a functional dimension of the joint, not a clearance hole.
- Forgetting the pitch when there are several plugs: the strength of the joint depends on how many there are and how they are distributed.
How to get it in the workshop / if it comes out of tolerance
The hole is made beforehand in the top plate and its diameter is set by the drawing; drilling it larger than specified changes the strength of the joint. Clean the faying surfaces between the plates: paint, oil or galvanizing trapped between them causes porosity and spatter. If the drawing calls for complete fill, the weld must finish flush, not sunken.
If the plug breaks loose under load, the problem is fusion at the root: dwell the arc longer at the center before starting to fill, and make sure the plates are in firm contact with no gap between them. Porosity in the fill usually comes from coatings between the plates: clean them or use a filler metal tolerant of that coating. Excess fill that stands proud has to be ground back, and it thins the plate if you overdo it.
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