Resistance spot welding squeezes two sheets between copper electrodes and fires thousands of amps through them — a nugget forms in fractions of a second. The fastest joining process in the shop.
Spot welding — resistance spot welding, or RSW — is a resistance process. Two overlapping sheets are clamped between copper electrodes, and a short pulse of high current flows through them. The joint itself is the resistance: it heats, melts, and freezes into a round nugget that fuses both sheets.
No filler metal, no shielding gas, no flame. The entire weld happens in a few cycles of mains power — which is why car plants, sheet-metal shops, and appliance lines run on it. The cost per weld is pennies, and robots place thousands a shift.
Mild steel sheet, RSW — the daily bread of every body shop.
| Sheet | Tip dia | Force | Current | Time |
|---|---|---|---|---|
| 0.6 mm | 3/16 in | 400 lb | 7–9 kA | 5–8 cyc |
| 1.0 mm | 3/16 in | 600 lb | 9–11 kA | 7–10 cyc |
| 1.6 mm | 1/4 in | 800 lb | 10–12 kA | 8–12 cyc |
| 2.0 mm | 1/4 in | 1000 lb | 11–13 kA | 10–15 cyc |
| 3.0 mm | 5/16 in | 1400 lb | 13–16 kA | 12–18 cyc |
| Property | Spot / RSW | MIG Tacking |
|---|---|---|
| Time per weld | 0.1–0.5 s | 2–5 s |
| Consumables | Copper tips only | Wire + gas |
| Filler metal | None | Wire |
| Heat & distortion | Localized, low | Higher, more |
| Access | Both sides | One side |
| Appearance | Dimple marks | Beads to grind |
Transformer or inverter supply — delivers the kA pulse with a programmable schedule.
Browse power sources →C-type or X-type guns — the arms hold the electrodes on both sides of the joint.
Browse automation →RWMA Class 2 copper tips — the wearable that defines the weld shape.
Browse consumables →Timers and monitors set cycles, current, and force — and log every spot.
Air or servo cylinders hold electrode force steady through the pulse.
Manual or automatic cutter that restores tip face shape between runs.
Water-cooled cables and electrode barrels — resistance welding runs on cooling.
Spot welding doesn't fight gravity the way arc welding does — the electrodes clamp the joint, so the gun defines the position.
A pedestal gun at waist height — the classic shop floorwork setup, pedal-operated.
Manual guns reach into assemblies — the spot gun comes to the joint.
Servo guns on robots place thousands of spots per shift, logged per weld.
Rotating wheel electrodes make seams; projection dies weld formed features.
The machine runs the schedule — the craft is writing it, maintaining it, and reading the results.
The recipe of force, current, and time per material and thickness — start from published data.
Two or more short pulses with cool periods — tames aluminum and coated steels.
Automatic dressing every few hundred welds keeps nuggets identical all shift long.
Rotating wheel electrodes roll a continuous overlap of nuggets — tanks, drums, ducts.
Formed bumps concentrate current at exact spots — brackets and nuts on sheet metal.
Hold time after current stops lets the nugget quench under pressure — skip it and it cracks.
Most spot weld defects are invisible from the outside — the nugget hides inside the stack.
Appearance
Molten metal sprayed out of the joint — a cratered dimple.
Causes
Current too high, force too low, or worn tips.
Prevention
Balance the schedule and dress the tips on time.
Appearance
A good-looking dimple with a small, weak nugget inside.
Causes
Low current, short time, or shunting from close spacing.
Prevention
Verify with peel tests; keep pitch per the schedule.
Appearance
Fine cracks in or around the nugget — often found at the edge.
Causes
Hold time too short or force released while the nugget is hot.
Prevention
Extend hold time — keep force on until the weld cools.
Appearance
Darkened, rough, or discolored marks at the electrode contact.
Causes
Poor tip contact, dirty metal, or overheating the face.
Prevention
Dress tips, clean sheets, check cooling water.
Appearance
The tip welds itself to the sheet — tearing metal on release.
Causes
Excess heat at the electrode face or contaminated tips.
Prevention
Cool the face, dress tips, lower the current a notch.
Appearance
The peel test pulls the nugget apart through its middle — flat fracture faces.
Causes
Undersized or brittle nugget — the classic hidden failure.
Prevention
Nugget must pull a button; schedule or material is wrong.
Appearance
The nugget breaks through the sheet edge — melted corner.
Causes
Spots placed too close to the edge of the sheet.
Prevention
Keep edge distance — generally 1.5× the tip diameter.
Appearance
Copper-colored deposits on the weld or brittle spots in the nugget.
Causes
Tip face overheated — copper transferred into the weld.
Prevention
Cool, dress, and keep the schedule inside the window.
The documents behind spot weld quality, schedules, and acceptance.
| Standard | Covers |
|---|---|
| AWS C1.1 | Recommended practices for resistance welding of steel |
| AWS C1.5 | Resistance welding of aluminum |
| AWS D8.9 | Spot weld testing of automotive materials |
| RWMA Bulletin 16 | Resistance welding equipment standards and electrodes |
| ISO 14373 | Spot welding of uncoated and coated low-carbon steel |
| ISO 14324 | Destructive testing of resistance spot welds |
| ISO 17652 | Spot welding of coated sheet — quality requirements |
| ISO 16432 | Spot welding of aluminum and its alloys |
All-in-one machine settings helper for a fast spot schedule starting point.
Current × time × resistance — the number that makes the nugget.
Cooling water flow for electrodes and cables — RSW runs on water.
RSW uses no filler — this covers the repairs and reinforcements around it.
What current and force fit foil vs heavy sheet and plate.
See tip dressing, peel tests, and robotic cells on real sheet metal.
Watch the schedule sections — squeeze, current firing, and peel testing — on real metal.
Watch VideosA shop-floor cheat sheet with the schedule table, nugget size rule, and maintenance intervals.
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