Spot Welding: Simple Rules For Strong Sheet-Metal Joints

Spot welding fuses sheet metal by squeezing two copper electrodes on each side of an overlap and pushing high electric current through the stack. The metal softens, a “nugget” of molten steel grows, and pressure forges the spot solid again—all within 200 ms. Because a robot can repeat that cycle ten times a second, spot welding remains the backbone of car bodies, appliances, and battery packs. This guide shows, in plain language, how to choose settings, design parts, and check weld quality for dependable production.

目次

• Why spot welding dominates thin-steel joining
• Understand current, pressure, and time
• Pick metals that weld cleanly
• Select electrode shape and material
• Calculate setup values with a quick chart
• Step-by-step fixturing and cooling
• Check nugget size and shear strength
• Prevent common spot-welding defects
• Robotic and portable-gun options
• Energy use and cycle-time basics
• Need integrated metal services?
• Fast checklist before production starts

Why Spot Welding Dominates Thin-Steel Joining

In the 0.5–3 mm thickness range, no other method beats spot welding’s mix of speed, low filler cost, and clean surfaces. Rivets add weight; MIG leaves spatter; adhesives need hours of cure. A spot weld forms in milliseconds, needs no filler metal, and demands only a two-side electrode squeeze—perfect for overlapping panels that will be painted later.

Understand Current, Pressure, And Time

Spot welding relies on three variables. Adjust one and you must balance the others:

Most setups adjust current in steps of 0.5 kA, force in 0.2 kN, and time in 2 ms. Push too much current without force and hot metal spurts (expulsion). Squeeze too hard with low current and the spot remains weak.

Pick Metals That Weld Cleanly

Resistance welding likes low electrical resistance (for current flow) and high contact resistance (to localize heat). These two needs fight each other, so alloy choice matters. The table ranks popular sheet metals by ease of spot welding.

*Five stars means easiest, one star means special process or electrode alloy required.

Select Electrode Shape And Material

Electrodes carry heat out and set the contact area. Choose shape first, then alloy.

• Class 2 CuCrZr domed tips. Standard for mild steel ≤1.5 mm per sheet.
• Class 3 Cu-cobalt flat tips. High-strength steel; more wear life.
• Dome radius. 25–40 mm spreads pressure, cuts indentation.
• Tip face. 4–6 mm Ø for thin sheet; 6–8 mm Ø for 2+ mm sheet.

Calculate Setup Values With A Quick Chart

Start with this rule-of-thumb chart, then tune by peel testing.

Sheet stack (mm) |  Current kA  |  Force kN  | Weld time ms
------------------------------------------------------------
0.6 + 0.6        |   8–9        |   1.5      |   10–12
1.0 + 1.0        |  11–12       |   2.0      |   12–14
1.2 + 1.2        |  13–15       |   2.5      |   14–18
1.6 + 1.6        |  16–18       |   3.0      |   18–22

Cross-wire or three-sheet stacks need 20–30 % extra current. For galvanized steel, add 10 % current and 4 ms to burn off zinc.

Step-By-Step Fixturing And Cooling

1. Clean mating surfaces with solvent wipe; no oil films thicker than 2 µm.
2. Clamp parts with locating pins; leave 12 mm clearance for electrodes.
3. Set hold timing: squeeze 50 ms before current, hold 50 ms after current.
4. Use internal water at 2 l/min per tip, 20 °C inlet. Tip face must stay under 45 °C.
5. Dress tips every 1000 welds with diamond dresser; restore 4 mm tip face.

Check Nugget Size And Shear Strength

An ideal nugget diameter equals 5 × √sheet-thickness (in mm). For a 1 mm + 1 mm stack aim for ~7 mm.

Sheet   | Target nugget Ø mm  | Min peel force N
0.8+0.8 | 6.3                 |  600
1.0+1.0 | 7.1                 |  850
1.2+1.2 | 7.7                 | 1100

Peel force measured by clamping one coupon and peeling the other at 180°. Failure should tear sheet metal, not the weld interface.

Prevent Common Spot-Welding Defects

Robotic And Portable-Gun Options

For mass production, servo-gun spot cells deliver repeatable squeeze and positional accuracy ±0.2 mm. C-frames handle mid-size brackets; X-guns reach deep pockets in car bodies. Portable rocker-arm machines still pay off in job shops that weld small batches or repair runs. Pick transformer size so 50 % of max current covers your thickest part—this leaves headroom and lowers conductor heat.

Energy Use And Cycle-Time Basics

Spot welding uses short bursts of high power. The bar plot compares kWh per thousand welds:

Mild steel 1 mm      █ 0.4 kWh
Galvanized 1 mm      ██ 0.6 kWh
AHSS 1.2 mm          ███ 0.9 kWh
Aluminum 1 mm        █████████ 3.2 kWh

Copper electrodes conduct away 80 % of heat; only 20 % melts the nugget. Water cooling costs less than compressed-air chillers by a wide margin—install closed-loop chillers to reuse heat.

Need Integrated Metal Services?

Alongside CNC and sheet-metal cutting, our shop runs servo spot-weld cells for prototype or mass builds. Send your STEP file and get one quote for cutting, forming, and welding.

Fast Checklist Before Production Starts

1. Verify sheet chemistry and coating weight—process drift here kills welds.
2. Use water-cooled Class 2 electrodes and dress tips before first run.
3. Start with current-force-time from the quick chart; adjust until peel tears base metal.
4. Log electrode life—expect 10 k welds on steel, 3 k on galvanized, 500 on aluminum.
5. Space welds ≥10 mm or 5× sheet thickness to avoid shunting.

Follow these steps and your spot-welded assemblies will survive crash tests, leak tests, and the hands of the final customer.