Automatic Welding Process

          Automation, like in other fields, in welding too, increases production, improves quality and lowers cost. Automatic welding employs various automatic power devices (variable speed drive motors), gas controls, coolant fluid, controls, sequence controls, instruments, gauges, timers, limit switches, etc.

Some of the advantages and limitations of automating arc-welding are listed below.

Advantages:

           High weld rate; uniform weld height, width, fusion and penetration ; low electrode stub loss due to the continuous feed from a reel of welding wire; use of high currents; automatic care of arc length, speed and other variables to produce quality weld.

Limitations:

           Correctly fitting weldment components have to be used to obtain all the advantages, no compensation (in form of speed control) for poor fits and alignments is possible, once the automatic controls have been adjusted.

The various components involved in automatic welding are described briefly below :

Adjustable speed drive motors:

          These are used to move the torch/electrode along the work at a controlled rate, and to feed the welding wire from coils into the weld area. In arc welding the adjustment of arc length is important to obtain quality welds. Change in arc length due to poor fits, etc. is sensed by the voltage changes in the welding transformer and a signal is sent to wire drive motor to speed up if voltage rises and vice versa. The best motor for such cases is the low voltage series wound universal motor whose speed varies proportionally to the voltage. These motors are then electrically connected to the arc circuit so that as arc length increases, voltage in the arc circuit increases which automatically increases the feed rate to adjust the arc length. Manual'override controls to feed wire are also provided.

Gas Controls:

           Gas is used in gas welding and in inert gas arc welding. These consist of pressure regulators, flow meters, and solenoid valves for gas flow control. The solenoid valve may bq energizing by pressing a push button, or automatically as follows. When arc is struck, current flows in the secondary winding which can be used to energizing a relay, which in turn energizing a solenoid. Since gas flow is required for a short time on breaking of arc in order to protect the electrode from contamination during cooling, a bimetal strip is utilized to de-energisc the solenoid valve on gas line with time delay. As long as gas arc is on, current through secondary windings also keeps solenoid energizing. When arc is broken, current in secondary winding falls to zero, but bimetal strip takes time in cooling and changing its position. Thus solenoid valve remains open for some extra time on breaking of arc.

Water controls:

           Some coolant (usually water) is required for cooling transformers, electrodes (in resistance welding), and electrode holders (in GTAW and GMAW welding). The pressure and flow oil cooling medium must be properly controlled to reduce wear and increase efficiency of welding. Interlocks to stop welding machine in case of inadequate coolant supply are provided. The solenoid valve in coolant supply line is energizing  only when welding is “on” by energizing it through the current in the secondary circuit.

         Three- way solenoid valves are used to direct the hydraulic pressure to be communicated to either operate a cylinder to perform certain operation or to reservoir of oil.

        The operation phases of a welding cycle arc controlled by timers which may be mechanical type (cam or clock) or electronic type. Sometimes exact timings have to be controlled as in the case of spot welding of aluminium or stainless steel. This can be done by thyratr on controls in conjunction with ignition controls which permit up to 1500 interruptions per minute. It is possible to control current flow in welding operation to produce regulated amount of heat by phase shifting the AC cycle. Sometimes sequence controls are utilized to stagger starting operation of various welding machines in a shop to reduce the instantaneous heat load on AC supply lines. Sufficient instrumentation is provided in automatic welding operation to monitor working of various electrical, electronic, cooling, pneumatic and hydraulic circuits. Parameters like weld pressure, forge pressure weld current, weld time and forge time are monitored and recorded on a recorder for permanent records. It has been possible to fully automate practically all welding processes, viz. gas welding, arc welding, resistance welding, brazing, cutting, metal surfacing etc.

      It may be understood that automatic welding calls for more safety precautions than manual welding. It should be thoroughly ensured before starting automatic operation that various movements are without any obstruction and hindrances and that various devices during their operation do not move in hazardous area.