Body shop: spot welding and robotic welding lines

Why it matters

A car body-in-white (BIW) is held together by roughly 3,000–5,000 resistance spot welds, laid down by a few hundred robots in a line that must keep pace with the assembly takt. Weld size, current, time and electrode condition decide crash strength and fatigue life, while the number of robots per station decides whether the line meets its output. Spot-weld heat calculations appear in welding questions in GATE, and line-sizing logic is everyday work for a body-shop engineer.

Key ideas

Resistance spot welding (RSW). Two or three overlapping sheets are squeezed between two water-cooled copper-alloy electrodes and a large current (typically 6–15 kA) is passed for a short time (a few tenths of a second). Heat is generated by Joule heating, I²R, mainly where resistance is highest — at the faying (sheet-to-sheet) interface. A lens-shaped molten nugget forms there, then solidifies under pressure. No filler or shielding gas is used.

Where the resistance is. The total resistance in the current path is the sum of: electrode–sheet contact resistances (top and bottom), the bulk resistance of each sheet, and the sheet–sheet contact resistance. Electrodes are copper alloys (Cu–Cr, Cu–Cr–Zr) with high electrical and thermal conductivity, so the electrode faces stay cool and the nugget forms at the interface rather than at the surface. Contact resistance falls as electrode force rises, so force and current must be set together.

Weld schedule. One spot follows a timed sequence:

1. Squeeze — electrodes close and build force before current flows.
2. Weld — current flows; on 50 Hz AC time is often counted in cycles (1 cycle = 20 ms).
3. Hold — force maintained while the nugget solidifies, avoiding porosity and cracks.
4. Off — electrodes open and the gun moves to the next spot.

Weld lobe and defects. For a given sheet stack there is a window of current and time (the weld lobe) that gives a nugget above the minimum size without expulsion (molten metal squirting out, seen as spatter). Too little heat gives small or no nugget ("stick weld"); too much gives expulsion, deep indentation, electrode sticking and voids. Galvanised (zinc-coated) steels need higher current and wear electrodes faster, so tips are dressed regularly and current is stepped up as tips mushroom.

Weld quality checks. Destructive chisel and peel tests and periodic teardown (nugget diameter measured from the button pulled out), non-destructive ultrasonic testing, and in-process monitoring of current, voltage and dynamic resistance by the weld controller.

Robotic welding lines. The BIW is built up in stages: underbody (floor and rails), side-frame sub-assemblies, then the framing (geometry) station, where the underbody, side frames and roof are clamped in a precise jig and tack-welded to fix the body geometry. Re-spot stations then add the remaining welds without fixtures. Robots carry servo-electric guns — X-type (scissor) guns for wide flanges and C-type guns for deep reach — and modern lines use medium-frequency DC controllers that give steadier current. Other joining methods also appear: MIG brazing, laser welding of the roof, adhesive bonding, self-piercing rivets and roller hemming of closures.

Line sizing. The body shop must deliver one body per takt time. Each robot can make only as many welds as fit into the takt minus the time for transfer and clamping, so the welds required at a station set the number of robots. Station layouts are limited by reach and by robots interfering with each other.

Formulas

Q = I² · R · t

• Q = heat generated (J); I = welding current (A, rms); R = total resistance of the current path (Ω); t = weld time (s). Note this is energy, not power; average power is P = I² · R (W).

Q_m = V_n · u_m with V_n = (π/4) · d_n² · h_n

• Q_m = heat needed to melt the nugget (J); V_n = nugget volume (mm³); d_n = nugget diameter (mm); h_n = nugget thickness (mm); u_m = energy to melt unit volume of the metal (J/mm³, from a data table).

η = Q_m / Q

• Thermal efficiency of the weld (–); the rest of the heat is conducted into the sheets and electrodes.

d_min ≈ 4·√t to 5·√t

• Empirical minimum nugget diameter (mm) for sheet thickness t (mm); the exact factor comes from the OEM or welding standard in use.

T_takt = available working time / required output

• Takt time (s per body).

n_robots = ⌈ N_w · t_w / (T_takt − t_h) ⌉

• N_w = welds at the station; t_w = time per weld including gun travel (s); t_h = transfer, clamp and unclamp time per cycle (s); ⌈ ⌉ means round up.

Worked examples

Example 1 (standard) — heat in one spot. Given: I = 10 kA, total resistance R = 100 μΩ, weld time 0.25 s.

1. Q = I²·R·t = (10,000)² × 100 × 10⁻⁶ × 0.25
2. = 10⁸ × 10⁻⁴ × 0.25 = 2,500 J
3. Average power P = I²·R = 10⁸ × 10⁻⁴ = 10,000 W; 0.25 s is 12.5 cycles at 50 Hz.

Answer: Q = 2.5 kJ (average power 10 kW).

Example 2 (GATE level) — efficiency of a spot weld. Given: two 1 mm steel sheets; I = 5,000 A, R = 200 μΩ, t = 0.1 s; nugget 5 mm diameter and 1.5 mm thick; energy to melt steel u_m = 10 J/mm³.

1. Q = I²·R·t = (5,000)² × 200 × 10⁻⁶ × 0.1 = 25 × 10⁶ × 2 × 10⁻⁴ × 0.1 = 500 J
2. V_n = (π/4)·d_n²·h_n = 0.7854 × 25 × 1.5 = 29.45 mm³
3. Q_m = V_n·u_m = 29.45 × 10 = 294.5 J
4. η = Q_m / Q = 294.5 / 500 = 0.589

Answer: about 58.9 % of the heat goes into melting the nugget.

Example 3 (line sizing). Given: 900 bodies per day over two shifts of 7.5 h net each; a re-spot station needs 60 welds; each weld including gun travel takes 3 s; transfer and clamping take 12 s per cycle.

1. T_takt = (2 × 7.5 × 3600) / 900 = 54,000 / 900 = 60 s
2. Time available for welding per robot = 60 − 12 = 48 s → 48 / 3 = 16 welds per robot.
3. n = ⌈60 × 3 / 48⌉ = ⌈3.75⌉ = 4

Answer: 4 robots at the station.

Common mistakes

• Calling I²Rt "power". It is heat energy in joules; power is I²R in watts.
• Forgetting to convert μΩ to Ω or kA to A; the current is squared, so a slip of 10³ becomes 10⁶.
• Assuming all the heat goes into the nugget; a large part is conducted away, which is why efficiency is well below 100 %.
• Rounding robot count down — 3.75 robots means 4, not 3.
• Thinking higher current is always better: beyond the lobe it causes expulsion and electrode sticking.
• Ignoring electrode wear: a mushroomed tip lowers current density and gives undersized nuggets at the same settings.

For GATE ME

Welding questions ask for heat generated in resistance welding, heat needed to melt the nugget and the resulting efficiency, often with mixed units (kA, μΩ, cycles of 50 Hz). Conceptual questions test why copper electrodes are used, where maximum heat is generated, and the stages of a weld schedule. Practise unit conversion and reading weld time given in cycles.

Quick check

1. Heat generated with I = 8 kA, R = 150 μΩ, t = 0.2 s?
2. At which interface is the most heat generated in a good spot weld?
3. What is expulsion and what usually causes it?
4. A weld time of 15 cycles on 50 Hz mains equals how many seconds?
5. Why is the framing station critical in a body shop?

Answers: 1. 64 × 10⁶ × 150 × 10⁻⁶ × 0.2 = 1,920 J. 2. The sheet-to-sheet (faying) interface. 3. Molten metal ejected from the nugget, from too much current or too little electrode force. 4. 0.3 s. 5. It fixes the overall geometry of the body-in-white.