Guide written by:
Michael, Professional and passionate welder, Sheffield
Without necessarily realising it, you come face to face with spot welding every day when you get into your car. On average, 4000 welding points are needed to assemble a car. Portable vs. workshop welding machines: spot the difference!
How does spot welding work?
Electric welding relies on the Joule Effect. This is the thermalresult of the electrical resistance, occurring when an electric current passes through a conductive metal - in this case metal sheets for assembly. If that last sentence went over your head, here's how it works: to weld two or more sheets together without adding a filler metal, they are tightly compressed between two heat-resistant electrodes (i.e. non-melting), generally made of copper, and a high-intensity current is applied to melt the plates together at that point. The result is a small merging of metal which constitutes a welding point. The welding time is very short, between one and two seconds, and the shape of the resulting welding spot depends on your choice of electrodes.
What phases does the process consist of?
Spot welding is a form of precision welding consisting of four separate phases:
- Docking: metal sheets are placed between the electrodes;
- Welding: current is applied across the plates;
- Forging: sheets are held under pressure;
- Release: electrodes are pushed apart to release the plates.
Rest assured, all four steps are coordinated by means of settings controlled by the operator.
What does a spot welding machine look like?
As is often the case with machine tools, there are two types: portable (for ease of use but with limited performance); and stationary (better suited to intensive work and thicker metal sheet).
- Portable spot welders are compact, lightweight and handy for difficult-to-access joints. As such, they are most commonly used in coachbuilding. They have an average welding capacity of2 + 2mm and are relatively inefficient due to the fact that the electrodes aren't cooled between cycles. Automatic adjustment makes it easier to adjust the spot parameters according to the thickness of sheet and welding head used.
- Stationary spot welders are mounted on a column with a jack and mobile arms. The frame is heavy and bulky and the machine has a welding capacity of 0.5–10mm. The body of the machine often equipped with a 380V generator and two arms with a mechanical, pneumatic or hydraulic jack to adjust operating height, as well as heat-resistant electrodes made of copper or copper-chromium alloy. No electric arc is produced, and the arms are water-cooled in closed circuit. The arms and electrodes are interchangeable to vary spot size and form across different types of weld.
What type of sheets can be welded?
Rust-free, non-painted sheetsof the same ordifferent metals can be welded provided they are compatible alloys with a very similar melting point.
Metals such as stainless steel, aluminium, steel alloys and galvanized steels can be spot welded, subject to operating adjustments (current, welding time, intensity of compression).
Note that the coating on galvanized metals tends to clog the electrodes - which must be cleaned regularly!
How to choose your generator?
Your generator must be suited to the type of work you intend to carry out. What type and thickness of sheet are you working with? There are three points to consider:
- Mains voltage (i.e. 230V) will allow you to run a portable machine; for a fixed welder, you'll need 400V and probably a compressed air supply for the actuator;
- Power has a direct influence on a machine's ability to weld thicker sheet metal;
- Operating factor is expressed as a percentage, for a given amperage (eg 70% at 100A): for 10 minutes of use, this corresponds to 7 mins of welding and 3 mins of cooling. This factor is inversely proportional to the operating current (so a higher-current device offers better operating efficiency).
And for a bit of clarification on some other spot welder operating characteristics:
- Spot current is of the order of 3–6000A for portable machines, and up to 15000A for stationary machines, its value dictating the quality of the weld;
- Secondary vacuum voltage is the voltage measured at the terminals of the welding machine before the welding current is applied - a key indicator of priming quality;
- An isolator fuse provides protection against voltage surges.
You can also get various welding accessories:
- Interchangeable arms to suit different formats of metal parts;
- A wide range of electrodes will let you produce a range of different spot welds.
Why choose spot welding over other techniques?
For one thing, spot welding considerably reduces deformation of the welding surfaces due to very brief fusion phase. Take a lampshade frame assembled by spot welding, for instance: what would be the result with arc welding, flame welding, etc?... Enough said!
Are there other variants of spot welding machines?
Yes - two!
In the same category you also find generators, always operating on direct current, intended for car bodywork includingrepairing dented panels. The machine consists of a holder which lets you position your welding gear precisely in front of the deformed zone, and uses mechanical or hydraulic force to restore it to its original shape. All you have to do is crack out your hammer to fine-tune the finish.
The "gougeonneuse", as it's known in France, can be used to mend tow rings and other specific parts. The huge advantage of being able to return a panel or component to its original shape without needing to disassemble the car body makes this machine a must-have tool for bodywork!
Can you summarize the applications of a spot welder?
Spot welding machines represent a significant long-term investment that can decline in value. Their high performance has led to widespread use in automotive manufacturing; their use can be automated by combining them with extremely fast, precise robots.
Radiators can be welded by the same process except using rollers to obtain a continuous, watertight weld. Industrially made furniture and other functional metal objects such as lampshade frames are also spot-welded. The process gives an impressive quality of weld without deforming the components it is used on.
A word about safety
Personal protection is less important than with arc or flame welding, however gloves and clear goggles are still essential. Always keep your equipment clean and dry!
Learn more about metalworking tools...
To find out more about welding equipment, follow our editors' advice and check out their other guides:
- How to choose your torch, cutter, pressure regulator and flame welding fittings?
- How to choose your electrode holder and earthing clamp for arc welding?
- How to choose your welding mask?
Guide written by:
Michael, Professional and passionate welder, Sheffield, 71 guides
I was trained as a pipe worker and a pipe-welder and after having traveled for 35 years working around the UK, I became the head a metal shop and then a designer and in the end the head engineer. I have designed and built a workshop where I make metal sculptures: I managed to find a piece of paradise where I can to let my imagination run wild. Auctions and garage sales are no secret to me. I find unusual objects and old tools there that I collect or transform into works of art. I also like decoration, painting on canvas, and gardening. I am developing new technologies concerning tools. To share my passion and humbly advise you in your choice of materials is a real pleasure.