Spot Welding Machine Product

Overview

A spot welding machine joins two or more sheets of steel or aluminum by locally melting a nugget at the interface. Two water-cooled copper electrodes squeeze the sheets together and pass a massive current (1,000–5,000 A) through them for a fraction of a second. Resistive heating at the faying surface creates a weld nugget 4–15 mm in diameter. The process is fast (one weld in 100–500 milliseconds), repeatable (±10 % nugget size variation), and requires no filler or flux — the base metal itself is the weld. Spot welders are ubiquitous in automotive assembly, appliance manufacturing, and sheet metal fabrication.

The Step-Down Transformer steps the 400 V three-phase mains down to 2–10 V on the secondary side while current increases to 1,000–5,000 A. The Electrode Arm Assembly pairs (upper and lower) hold Electrode Tip electrodes that touch the workpiece. The Electrode Force Cylinder applies squeeze force (200–2,000 N), and the Weld Control Unit initiates a timed current pulse. The Cooling Water System circulates chilled water through the Cooling Water Hose in each arm to keep electrode temperature below 80 °C, preventing softening.

Weld nugget formation

When current flows from the upper Electrode Tip through the top sheet, across the faying surface, through the bottom sheet, and back to the lower electrode, the highest resistance is at the faying surface interface. Ohmic heating (I²R) raises this surface to melting point (1,600 °C for steel). The nugget grows from both sides, inward, forming a lens-shaped fusion zone. Surrounding this is the heat-affected zone where grain growth softens the metal but there is no melting.

The nugget grows with time: at 1 cycle (20 ms) it is tiny (2–3 mm); at 5–10 cycles (100–200 ms) it is 6–10 mm — the typical range. After current stops, the nugget cools under the clamped Electrode Force Cylinder force, solidifying and gaining strength as it cools. A "hold time" (1–10 cycles post-weld) keeps the electrodes pressed together during cooling to prevent cracking.

Electrode materials and wear

Spot weld electrodes are class II or class III copper alloys (typically chromium-copper or beryllium-copper) hardened by precipitation and cold-working to 60–80 HRC. Pure copper would be too soft and mushroom immediately. The Electrode Tip is either welded or press-fit into a Electrode Holder. Electrode life is finite: after 1,000–5,000 welds, the tip flattens and erodes, reducing contact area. At that point, the tip is reversed (flipped and remachined) or replaced entirely.

The Cooling Water Hose supplying chilled water through each electrode is essential — without cooling, the electrode softens and loses hardness, accelerating wear. A small blockage in the water passage (scale, algae) can allow an electrode to overheat and fail in hours instead of months.

Transformation and current control

The Step-Down Transformer is a large, heavy device because it must conduct 1,000–5,000 A on the secondary side. Copper losses and eddy current losses in the core are significant, so the Cooling Fins provide convective surface area, or the transformer is oil-cooled on very large machines.

The Current Control Module modulates the primary current using a silicon-controlled rectifier (SCR) or IGBT circuit. By phase-shifting the triggering angle relative to the AC zero-crossing, the RMS current delivered to the secondary is controlled. For full current, the SCR fires at zero degrees (full sine wave); for half current, it fires at 90 degrees (truncated half-sine wave). Modern machines use IGBT switching at high frequency (1–10 kHz) to reduce transformer size and improve current regulation.

Squeeze force and pressure feedback

The Electrode Force Cylinder is supplied by compressed air (typically 6–8 bar shop air). A Proportional Solenoid Valve proportional solenoid reduces this to a lower pressure signal (1–5 bar) that adjusts the electrode squeeze force. The Pressure Sensor on the cylinder rod measures actual force and feeds back to the Weld Control Unit PLC, allowing closed-loop force control. If workpiece thickness varies, the force can be held constant despite mechanical variation.

Correct force is critical. Too little force and contact resistance is high, wasting current on heating the electrodes instead of the workpiece. Too much force can splash liquid metal out of the nugget (spatter) or crush the sheets. The setpoint is typically 300–1,000 N for thin steel sheets.

Sequence timing

The typical weld sequence is:

1. Squeeze time (1–5 cycles): Electrodes contact and clamp the workpiece, but no current flows. This allows the parts to settle and ensures good contact.

2. Weld time (3–15 cycles): Current flows, heating the faying surface. The Current Control Module ramps current in steps or holds it constant.

3. Hold time (1–10 cycles): Current stops, but electrodes stay clamped while the nugget cools and solidifies.

4. Off time: Electrodes release and the workpiece is removed or advanced to the next spot.

The Time Adjustment Dial or digital panel on the Weld Control Unit sets each phase independently. Typical programs: a soft ramp (current increase over 3–5 cycles) reduces spatter; a hard step (full current immediately) maximizes nugget growth but often generates spatter.

Water cooling requirements

The Cooling Water System supplies 10–30 L/min of chilled water through the Cooling Water Hose in both electrode arms. On low-duty machines (1–5 welds per minute), tap water at 15–20 °C suffices. On high-duty machines (20–50 welds per minute), the electrodes overheat and the Chiller Unit becomes necessary; it uses a thermoelectric cooler or a small refrigeration compressor to maintain 10–15 °C.

Mineral scale and algae in untreated water block passages and cause electrodes to fail thermally. Many machines include water treatment cartridges (de-ionization or softening resin) to extend electrode life.

Material and thickness considerations

Mild steel (< 250 MPa) is easiest to weld and sets the baseline current and time. High-strength steel (HSLA 400–500 MPa) requires higher current or longer time because its electrical resistivity is slightly higher. Stainless steel (304, 316) has higher resistivity and work-hardening; typical current is 10–20 % higher. Aluminum is highly conductive (low resistivity), so current must be 3–5 times higher and weld time is shorter — aluminum spot welders are much larger machines.

Sheet thickness is the other major variable. For two 1 mm mild steel sheets, a 5–10 cycle weld at 2,000 A is sufficient. For two 3 mm sheets, the time must extend to 20–30 cycles to grow a nugget deep enough to span both sheets. Thin gauge (< 0.7 mm) requires lower current to avoid burn-through; thick gauge (> 2 mm) requires higher current.

Changeover between jobs

Moving from one workpiece to another typically requires changing the Electrode Tip size and adjusting the Weld Control Unit sequence timers and force setpoint. The work table height (on the Work Table) may also be adjusted. On manual machines, this is a 5–10 minute manual task. On automated welding lines, job changeovers are done offline at a tool station, and multiple machines are swapped in sequence.