Seam Welding Machine

Overview

A seam welding machine creates a continuous weld along the overlap of two sheet metal parts using rotating wheel electrodes. Unlike spot welding, which creates discrete nuggets separated by unwelded gaps, a seam weld is a continuous line of overlapping nuggets. The lower Wheel Electrode rotates against the workpiece, advancing at a set speed (0.3–10 m/min), while the upper wheel presses down with controlled force (500–5,000 N). Current pulses are timed to the wheel rotation, typically 1–3 pulses per revolution, so each revolution creates one or more overlapping weld nuggets that fuse into a continuous seam. Seam welding is used for tanks, boxes, fuel tanks, and any application requiring a fluid-tight or air-tight joint without added sealant.

The Seam Weld Transformer supplies low-voltage high-current power. The Seam Drive & Synchronization rotates the lower Wheel Electrode and times the Seam Weld Control Unit current pulses to the wheel position. The Electrode Force System maintains constant Load Cylinder load on the upper wheel. The Wheel Cooling System continuously circulates water through hollow Upper Wheel Spindle and Lower Wheel Spindle passages to keep the wheels below 80 °C.

Seam geometry and nugget overlap

A continuous seam is formed by overlapping spot-like nuggets. If the wheel rotates once per second and fires 2 pulses per rotation, the machine creates 2 weld nuggets per second. Each nugget grows to 4–6 mm diameter; if nuggets are spaced 2 mm apart axially (along the seam), they overlap by 2–4 mm and fuse into a continuous nugget-chain. The overlap is the key: insufficient overlap leaves gaps and the seam leaks; excessive overlap creates a wider, weaker seam and wastes energy.

The seam strength comes from two sources: the nugget cross-sectional area and the number of nuggets per unit length. A 5 mm diameter nugget in 0.8 mm mild steel has roughly 2–3 mm² fusion area; with 3–4 overlapping nuggets per centimeter, the total fused area per cm is 6–12 mm², comparable to a mechanical rivet.

Wheel materials and design

The Wheel Electrode is typically 50–150 mm diameter and 10–20 mm thick. Smaller wheels are used on tight corners (radius 10–20 mm); larger wheels are used on flat seams where cooling is easier and mechanical stiffness is desirable.

Wheel material is hardened copper alloy (chromium-copper, beryllium-copper) at 55–70 HRC, or tungsten-carbide overlay. The wheel surface wears over time: after 5,000–20,000 meters of seam, the wheel face flattens and contact area grows, reducing current density and weld quality. At that point, the wheel is re-faced: the seam is run against a grinding stone to restore roundness and contact. A worn wheel can reduce nugget size by 20–30 %.

Optional Knurling Insert inserts impart a crosshatch knurl pattern to the weld nuggets, mechanically interlocking the sheets and increasing shear strength. This is common on fuel tank seams.

Synchronization and pulse control

The Drive Motor continuously rotates the lower wheel. The Wheel Encoder on the wheel spindle outputs a pulse once per revolution (or multiple pulses per revolution). The Seam Weld Control Unit PLC counts these pulses and triggers the SCR Phase Controller current phase controller to fire a current pulse each time the encoder pulse arrives.

For example: wheel speed 30 rpm (0.5 revolutions per second); 2 pulses per revolution → 1 weld pulse per second → 60 weld nuggets per minute. Wheel diameter 80 mm, circumference 250 mm → linear advance 15 m/min (rough estimate). The actual seam speed depends on wheel slip against the workpiece (typically 5–10 % slip).

The timing jitter must be < 10 ms to keep nuggets aligned; any wobble or slip causes misalignment and overlapping welds become staggered, weakening the seam. This is why the wheel speed is kept constant (no load disturbance) and the encoder trigger is monitored.

Electrode load and contact stress

The Load Cylinder applies 500–5,000 N to the upper wheel, distributed over the contact patch (typically 5–10 mm²). Contact pressure is 50–500 MPa depending on application. Too light a load and contact resistance is high, wasting current heating the wheels; too heavy and the wheels slip or skid across the workpiece, misaligning the seam.

The Electrode Force System uses a Pressure Regulator to hold load constant despite variations in sheet springback. Some machines use a load cell feedback (Pressure Sensor) that modulates the cylinder pressure to maintain setpoint load.

Water cooling during continuous operation

Seam welding generates continuous heat: unlike spot welding, which has idle time between welds, seam welding fires a weld pulse every 0.5–1 second. The Wheel Cooling System must circulate 15–50 L/min of chilled water through Water Cooling Sleeve passages in both wheel spindles to keep them below 80 °C. Without cooling, electrodes overheat and soften within minutes, destroying the weld quality.

The cooling water enters through the upper Upper Wheel Spindle via rotating seals, flows through hollow passages, and exits through a separate seal. The lower wheel is similarly cooled. The Cooling Sump must be large (100–300 L) to absorb heat; some machines add a chiller or fan-cooled radiator for high-duty applications.

Material-specific seaming

Mild steel seams at standard settings. High-strength steel (HSLA 350–500 MPa) requires higher current or longer pulse time. Stainless steel (304, 316) demands even higher current (20–30 % more) and tungsten-carbide wheels (copper wheels gall on stainless). Aluminum has high conductivity so current must increase 2–3 fold compared to steel; many aluminum seam machines run at 50–100 Hz with 1–2 pulses per revolution for slow-speed precise control.

Thin sheet (< 0.5 mm) is prone to burn-through; the current is reduced and pulse time shortened to 1–2 cycles. Thick sheet (> 1.5 mm) requires extended weld time (5–10 cycles) to grow nuggets large enough to span both layers.

Seam tightness and leak testing

Seam-welded tanks must be fluid-tight. The overlap distance between nuggets directly affects leak rate: 2–3 mm overlap is typical, but critical applications (pressure vessels, fuel tanks) require 4–5 mm overlap and 100 % ultrasonic or radiographic testing. The Seam Weld Control Unit can be programmed to vary pulse frequency along the seam, faster pulses (more nuggets per cm) at seams prone to leaking.

Post-weld inspection often includes air-pressure testing (fill the tank with compressed air and submerge to detect bubbles) or helium mass-spectrometry leak detection for aerospace tanks.

Changeover and maintenance

Changing from one part to another requires adjusting wheel load, current, and pulse frequency in the HMI Control Panel, typically a 5–10 minute job. Wheel re-facing is done every 1–2 weeks on high-production machines, using a mounted grinding stone that skims the wheel surface while it rotates. Complete wheel replacement occurs every 3–6 months depending on throughput.

The Water Cooling Sleeve and cooling passages must be flushed quarterly to prevent scale buildup; hard water areas may require water softening or de-ionization cartridges.