What is a plug weld? – Going into the issue

A spot weld is fast and convenient, and you can get quality steady joints within a couple of minutes. But there are situations when the shape of metal sheets doesn’t let you do a spot weld.

The welding method called plug welding (another name for this welding type is a rosette weld) can be extremely useful in situations when a welder doesn’t have access enough for a spot weld.

The joint received by plug welding tends to be steadier than that of spot welding and has tons of other benefits. Now that we’ve introduced the topic, let’s get into it and see, whether it’s worthwhile (and why!).

What is a plug weld (rosette weld)?

Long story short, a plug weld slightly resembles some kind of putting rivets. This type of welding is applied to the welding of two metal plates, one of which has holes drilled up preliminarily.

The welding process goes through these holes, stuffing the space within them with filler material.

This approach leads to the robust conjunction, due to its mechanical component. The rivet-like geometrical shape contributes to the overall sturdiness and endurance of this conjunction.

Blueprint plug weld symbols

Construction projects that need plug welding are denoted with welding symbols: rectangles. The exact position is indicated with an arrow so that the rectangle is beneath the shelf. This denotes clarifying numbers to the left and to the right of the rectangle, as well as inside and underneath it.

The sign of a diameter standing before the plug weld symbol and a number next to it denotes the diameter of a circular hole for the plug welding.

Some constructive peculiarities imply not the entire hole volume to be stuffed with too much filler material. In such cases, the filling depth is designated with a number inside the rectangle.

Figures to the right of the rectangle denote a pitch of welding holes, which means nothing short of a distance welding holes are separated from each other.

The last mark you can notice on the plug weld symbol is the number of welds to apply. It’s designated with a number in parentheses just under the rectangle, in a case when more than one weld is needed.

The markings inside and underneath the plug weld symbols are less common than diameter and pitch designations, but anyway, a welder has to be aware of them.

Plug welding procedure

The plug welding process comprises several operations, and surprisingly, preps must be done before plug welding begins.

Metal preps

Any surface taking a weld must be cleaned with sandpaper or a grinder, and the grease and dirt must be removed with solvents or detergents. Then make marks for holes as per the drawing. If you don’t have one, don’t place them too far away from each other: for a thin sheet, it must be less than a couple of inches.

You can drill the plug weld holes or just use a simple hand punch if the metal sheet isn’t too thick.

The rule of thumb is that the thicker a sheet of metal is, the wider must be a hole. An average diameter hole for a 1/25 inch steel sheet is about 1/4 inch.

The next step is fixing the plates of metal together: you can do it either with a plug weld clamp or fix it with rivets which will be removed aftermath. The good idea is to make a joggle on both plates using a tipping wheel or a bead roller.

Welding process

Finally, after all these preps, you can put a weld into holes. If you use TIG weld, put it around a circle starting from an edge of a hole and moving spirally to its center, touching all the holes consequently that way. For MIG welding, it’s more reasonable to start from the center of a hole.

Possible plug welding process problems

One must tighten plates of metal together as strongly as it’s possible since even the smallest gap between them will worsen the conjunction quality. If the joint surfaces are warped somehow, maybe clamps won’t be enough to keep them close; hence temporary rivets may be a good solution.

Don’t use more filler than needed, – that’s point is inherited from the previous clause. Extra filler in a joint worsens its strength and endurance characteristics.

Plug welding scope

This kind of welding is especially widespread in the automotive manufacturing industry.

It overplays its competitors, for instance, due to being applicable for welding curved or hard-to-reach spots and the high sturdiness of a junction.

Plug weld is also widely utilized in the aerospace industry since, again, it provides the best results and quality in conjunction to work with cylindrical or even more complex shapes of metal parts. It’s also applicable for repairing perforated curved metal plates just by putting patches.

Advantages of plug welding

Thus, having described the plug weld scopes, we can distinguish its strong points and weaknesses.

First, a plug weld is relatively fast to accomplish, especially when the surface is preliminarily prepared. A second major advantage of a plug weld is its flexibility: a welder can adjust equipment to plug welding works in a tight space or to weld complex shape surfaces.

The joint received with a plug weld is pretty strong and endurable due to the mechanical preparation of the material. Besides, this approach applies to most rolled metal plates like aluminum alloys, cast iron, and stainless steel.

A plug weld doesn’t need much finishing to obtain a flush surface. Metal splatters are few, – you’d just grind a bit the knobs above the holes and the surrounding area.

Hence, plug weld is relatively safe for a welder as well, since there’s not so much excessive heat, slag, and splatters. And hence, it’s a good choice for welders, who aren’t sufficiently experienced.

And the last thing worth mentioning is that a plug weld closely resembles a slot weld in terms of material and energy consumption, whatever welding – MIG or TIG – is utilized.

Fly in the ointment

The downsides of a plug weld are rare, and most of them are more or less regarded to the welder and his proficiency. Although the mechanical part of work, like drilling or punching holes, someone may consider it rather as a drawback than an advantage.

Reversed wheels and a plug weld

Automotive history highlights some curious moments about plug weld scope. In the mid-60s, enthusiastic drivers started the trend of upgrading their vehicles by reversing wheels so that the inner steel ribs turned out to be the exterior.

To do so, they had to drill out wheel hubs originally fixed with a spot weld, and having reversed them secured them in a new position by putting a plug weld.

Such upgrades not only changed the look of cars – chromium-plated, they were cool – but made their suspensions more reliable and enduring than ever before.

A plug weld and changing a car’s floor

One more detail we would like to mention, a plug weld is replacing a vehicle’s rusty floor. This type of welding is the best choice for accomplishing that since it gives the most endure and sturdy conjunction of the floor. As the safety issue is at stake here, the widespread of a plug weld for that purpose is quite meaningful.

What’s the difference between a plug weld and its rivals?

The two methods are truly great ones, and they both share the same merit: they’re really fast.

Nevertheless, it’s worth mentioning some crucial differences between them.

And as we’ve already mentioned as well, plug welding can be put in the most hard-to-reach spaces, unlike spot weld, which requires a relatively large machine. Plug welds are more sturdy than those spot welds. Although, a spot weld requires less expendable materials than a plug weld.

The types of plug weld conjunctions

There are mainly five types of conjunctions produced with a plug welding process. Here they are:

Edge or butt conjunctions

That’s a humble joint when two metal plates are joined with an overlap with their adjacent edges. The additional mechanical fixation is provided by making joggles or grooves. Edge joints’ subtypes are U-, V-, J-groove welds, and also bevel, square, and corner flange welds.

Corner conjunctions

A corner conjunction, as it becomes clear from its title, is the L-shaped joint of two sheets of metal.

There is a set of corner conjunction subtypes intersecting with the edge joint subtypes: U-groove, V-groove, and J-groove welds, a bevel groove as well, but also a spot weld, square grove weld, fillet welded holes, corner flange weld, and a flare V-groove weld.

What’s the maximum thickness of the steel sheet for a plug weld?

Thick metal plates apply additional tension upon plug weld conjunctions, therefore, the depth of an upper sheet shouldn’t be more than 1/8 inch (3 mm). This value covers most sheet-rolled metals.

Conclusion

A plug weld is one of the best ways to join sheet metal plates of the most complex shapes. It doesn’t require a large machine like a spot weld. This type of weld provides high-endurance joints.

Hence, it’s so widespread in the automotive industry, as well as in the aerospace industry, and both of them have pretty high safety demands. Besides, putting plug welding is extremely fast, in comparison with other weld types (except for a spot weld, but it overlaps rapidly with lots of other welding defects).

A plug weld is the best choice for an under-experienced welder, especially when using a TIG weld. An amateur welder can easily utilize a plug weld in a household to join any kind of thin metal plates, using just a welder machine and a hand punch.

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