"How does spot welding work?"

"Two copper electrodes clamp the workpieces together and pass high current (typically 5,000-20,000 amps) through the joint for a fraction of a second. Resistance at the interface generates enough heat to fuse the sheets together in a small nugget. Squeeze pressure, weld current, weld time, and hold time are the four key parameters."

"How thick can you spot weld?"

"Standard shop spot welders handle two sheets of mild steel up to about 1/8 inch each (total stack thickness of 1/4 inch). Heavy industrial units weld thicker material. Most spot welding involves sheet metal from 22-gauge to 14-gauge, which is the typical range for auto body panels, enclosures, and ductwork."

"What metals can be spot welded?"

"Mild steel and low-carbon steel are easiest. Stainless steel spot welds well but requires higher pressure and shorter weld times. Aluminum needs very high current, short duration, and frequent electrode dressing because aluminum sticks to copper electrodes. Galvanized steel welds fine but electrode wear increases due to zinc contamination."

Resistance Welding & Spot Welding Guide

Spot welding, seam welding, and projection welding explained. Electrode selection, current and time parameters, sheet metal thickness ranges, and automotive panel repair applications.

Resistance welding joins metal sheets by passing high electrical current through the workpieces while clamping them under pressure. The electrical resistance at the interface between the sheets generates heat, and that heat, combined with the clamping force, fuses the metal into a solid joint. No filler metal, no shielding gas, no flux. Every car, appliance, and HVAC unit on the market contains hundreds to thousands of resistance welds.

Types of Resistance Welding

Spot welding (RSW) is the most common form. Two opposing copper electrodes squeeze the sheets together and deliver a pulse of current. The result is a small, round weld nugget. Cycle times are typically under one second, making spot welding extremely fast for production sheet metal work.

Seam welding uses rotating wheel electrodes to create a continuous or overlapping series of spot welds along a joint. Used for leak-tight joints in fuel tanks, cans, radiators, and similar sheet metal assemblies.

Projection welding concentrates current at raised points (projections) stamped or machined into one of the parts. This allows multiple welds in a single press cycle and is standard for attaching nuts, bolts, and brackets to sheet metal panels.

Small Shop and Auto Body Applications

Portable spot welders in the $200-1,500 range handle auto body panel replacement, sheet metal enclosures, and light fabrication. Auto body work is the most common small-shop use since factory panels are assembled with spot welds. Replacing a quarter panel or rocker panel means drilling out the old spot welds and making new ones.

For proper results, clean the metal to bare steel at each weld location. Set current and time based on material thickness (most machines have a chart). Clamp tightly. Test on scrap first, then peel-test a sample weld to verify nugget formation.

Articles in This Section

Guides here cover spot welder settings by material thickness, electrode maintenance, auto body spot welding technique, and equipment reviews.