A spot welder joins overlapping metal sheets by squeezing them between copper electrodes and sending a large electric current through a small contact area. The electrical resistance of the metal turns electrical energy into heat, so the sheets melt locally and form a small fused region called a weld nugget. Spot welding matters because it is fast, repeatable, and widely used in cars, appliances, battery tabs, and sheet metal fabrication.
It is a practical example of electricity, heat transfer, force, and materials science working together in one machine.
The basic cycle is squeeze, current, hold, and release. First, the electrodes press the sheets together to reduce gaps and keep the joint aligned. Then a short pulse of high current creates heat mainly at the interface where the two sheets touch, and the hold time allows the molten nugget to solidify under pressure.
Good welds require the right balance of current, time, electrode force, sheet thickness, material conductivity, and clean contact surfaces.
Understanding Tools & Workshop Machines: Spot Welder
Inside a spot welder, a transformer changes the electrical supply into a form suited to welding. The transformer has many turns of wire on its input side and very few on its output side. This greatly lowers the voltage while allowing a huge current to flow.
Thick copper arms and cables are needed because ordinary wire would heat up too much. The current follows a closed path from one electrode, through the sheets, then back through the other electrode.
The shape and size of each electrode tip control how concentrated that path becomes. A smaller tip can concentrate heating, but it wears faster and can leave deeper marks.
The hottest region is not always the metal with the highest bulk resistance. Real sheet surfaces are rough at a microscopic scale. They touch first at tiny high points called asperities.
These small contact points restrict current flow and create extra heating. Surface oil, rust, paint, or oxide layers can make this effect unpredictable. During the weld pulse, the contact points soften and change shape.
Resistance therefore changes from moment to moment. This is one reason a setting that works on clean steel may fail on dirty steel.
Aluminium is especially challenging because it conducts electricity and heat very well. It usually needs much higher current, careful surface preparation, and electrodes designed for the job.
The molten region must grow to the right size before it freezes. If too little energy enters the joint, the sheets may appear joined but separate easily in a peel test. This is called an undersized nugget.
If too much energy enters, liquid metal can be forced out from between the sheets. This expulsion creates sparks, weakens the joint, and damages the electrode faces. A dark surface mark does not prove that a weld is strong.
Manufacturers often test sample welds by pulling or peeling the sheets apart. They inspect the size and shape of the torn metal left behind. They may also measure electrical current, electrode movement, and voltage during each weld to detect faults early.
Students can see the same ideas in car body panels, metal filing cabinets, food cans, and rechargeable battery packs. A car factory uses robots because a single body can need thousands of welds placed in nearly the same positions every time. Battery manufacturing needs extra care because a hot weld near a cell can damage sensitive parts.
When studying this process, separate the roles of current, resistance, time, force, and cooling. They affect one another rather than working independently. Clean surfaces and consistent electrode tips matter as much as machine settings.
Spot welding is not a safe classroom experiment without trained supervision. The equipment can deliver dangerous current, create hot metal, eject sparks, and pinch fingers between moving electrodes.
Key Facts
- Joule heating in the joint is given by Q = I^2Rt, where Q is heat energy, I is current, R is electrical resistance, and t is time.
- Spot welding uses low voltage and very high current, often thousands of amperes, to heat a small region safely and quickly.
- Copper electrodes conduct current well and carry heat away, which helps focus heating inside the overlapped sheets.
- The welding cycle is squeeze time, weld time, hold time, and off time.
- Electrode force affects contact resistance: too little force can cause arcing, while too much force can reduce heating and make a weak weld.
- A strong spot weld forms a weld nugget large enough to hold the sheets without excessive surface burning or deep electrode marks.
Vocabulary
- Resistance spot welding
- A welding process that joins overlapping metal sheets by passing high current through them while they are clamped between electrodes.
- Weld nugget
- The small fused region of metal that forms between the sheets during a spot weld.
- Electrode
- A conductive part, usually made of copper alloy, that presses on the workpieces and delivers welding current.
- Contact resistance
- The electrical resistance at the surfaces where materials touch, which helps determine where heat is produced.
- Hold time
- The time after current stops when the electrodes continue pressing the joint so the molten metal solidifies properly.
Common Mistakes to Avoid
- Using dirty or painted metal surfaces, which is wrong because coatings and dirt change contact resistance, create fumes, and can prevent a reliable weld nugget from forming.
- Increasing current without adjusting time or force, which is wrong because excessive heat can burn through the sheet, cause spatter, or damage the electrode tips.
- Assuming higher electrode force always makes a stronger weld, which is wrong because too much force lowers contact resistance and can reduce the heat needed to fuse the joint.
- Touching electrodes or workpieces immediately after welding, which is wrong because the metal can remain hot enough to cause burns even after the glowing spot disappears.
Practice Questions
- 1 A spot welder sends 6000 A through a joint with a resistance of 0.00008 ohm for 0.20 s. Using Q = I^2Rt, how much heat energy is produced?
- 2 A welding transformer delivers 2.5 V at 8000 A during a weld pulse lasting 0.10 s. Using P = IV and E = Pt, find the power during the pulse and the energy delivered.
- 3 Two welds use the same metal sheets and current, but one has dirty, oxidized surfaces and the other has clean surfaces. Explain how the surface condition can change heating, weld quality, and safety.