Rebar Cage Welding Machine

Overview

A Rebar Cage Welding Machine builds the cylindrical steel reinforcement cages that sit inside bored piles, drilled shafts and large columns. The cage is a set of straight longitudinal main bars arranged on a circle, wrapped by a continuous helical stirrup wire that is welded to every bar it crosses. Done by hand, that is slow and the pitch wanders. The machine fixes the main bars in a rotating ring, spins them, and lays the spiral at a set pitch while a welding head fuses each crossing, so a cage that took a crew most of a shift is finished in a few minutes.

The machine has the form of a long lathe. A driven Rotating Head grips the bars at one end, an idle Tailstock supports the far end, and a Welding Carriage travels the length of the Bed and Rail Frame between them. As the cage turns, the carriage moves along at a matched rate so the welded spiral advances at the chosen pitch. Diameter ranges from about 300 mm for small piles to 2500 mm for large drilled shafts, and cage length runs from 3 m up to 18 m on a long bed.

Holding and turning the cage

The Rotating Head is the heart of the machine. Its Head Face Ring is a slotted faceplate that locates the longitudinal bars on a pitch circle set to the cage diameter. Each bar is clamped by a Main Bar Collet, a radial jaw pair closed by a hydraulic cylinder and held by a serrated jaw that bites into the bar so it cannot slip under welding or rotation loads. A cage carries 6 to 30 main bars from 12 to 40 mm diameter, so the head is built for high clamp force and torque.

The whole faceplate turns on a Head Slewing Bearing, a large four-point-contact ring driven through a Head Ring Gear and pinion. At the other end the Tailstock carries a free-running support ring on its own slewing bearing, so the far end of the cage spins with the head. The tailstock sits on a sliding carriage positioned by a ball screw along the bed, which is how cage length is set. Centring rollers keep the cage concentric in the ring as it turns.

Feeding the spiral

The helical stirrup is drawn from a coil and shaped on the fly by the Spiral Wire Feed. Wire of 6 to 16 mm comes off a powered Wire Payoff Reel that can hold up to about 2 t, with a brake holding back-tension so the coil does not overrun. Coiled wire keeps its curl, so it passes through a Wire Straightener, a cassette of grooved straightening rollers in two planes that flex the curl out before it reaches the cage.

A servo Wire Drive Unit then meters the wire through knurled pinch rolls. Metering the wire to track cage rotation is what fixes the spiral pitch: feed faster relative to rotation and the helix opens out, feed slower and it tightens. Pitch is programmable from about 50 to 350 mm. A dancer arm takes up slack between payoff and drive, and a guide arm lays the wire onto the rotating bars exactly at the weld point.

Welding the intersections

Each place the spiral crosses a main bar is fused by the Welding Head on the carriage. Most cage machines use resistance welding: two copper electrodes clamp the wire and bar together under force from a force cylinder, then a heavy low-voltage current passes through the joint and the resistance at the contact melts and fuses the two. Electrode force is watched by a pressure sensor so each weld lands with repeatable pressure, and the holders and electrodes are water-cooled because the duty is continuous.

Current comes from the Welding Power Source. A primary weld transformer steps mains down to a few volts at thousands of amps, an SCR contactor switches and phase-controls the current, and a weld timer sets squeeze, weld and hold times for the bar and wire sizes in use. A closed-loop cooling unit circulates water through the electrodes and transformer. Running flat out the head can place up to about 60 welds per minute.

Drives, control and motion

Three motion axes must stay in step: cage rotation, carriage travel and wire feed. Each runs on a servo: the Rotation Servo Motor through a planetary Rotation Gearbox into the head ring gear, the Carriage Servo Motor through the Carriage Gearbox and a pinion on the bed rack, and the Wire Feed Servo Motor on the pinch rolls. The servo drives take their commands from a motion controller so the three move as a locked group; the operator enters diameter and pitch at the HMI Terminal and the controller derives the matched speeds. The PLC Panel handles weld sequencing, clamping and interlocks, with an Operator Pendant for jogging during setup. A Hydraulic Power Unit drives the collets and tailstock clamps.

Materials and use

The load-bearing structure is welded structural steel: the bed, head and tailstock frames in S355, with the collet jaws and carriage pinion in hardened alloy steel. Electrodes and holders are CuCrZr copper alloy for conductivity and wear life. The machine is a fixed installation in precast yards and pile contractors, grouted level on its levelling feet. Output feeds bored-pile and drilled-shaft foundations, bridge piers, and precast columns, where a consistent spiral pitch and a sound weld at every crossing are what let the finished cage be lifted and lowered into a borehole without distorting.