This article throws light upon the top three types of MIG or GMA equipments used in welding. The types are: 1. Torch or Welding Gun 2. Power Source 3. Wire Feeder.
MIG or GMA Equipment: Type # 1. Torch or Welding Gun:
The Welding Torch or the Gun is normally 3.4 meter long.
There are different categories of Torches, such as:
(i) Gas-cooled Goose-neck type (200-450 amps)
(ii) Water-cooled Goose-neck type (up to 600 amps)
(iii) Water-cooled/Gas-cooled Pistol type (up to 600 amps).
The Torch must feed the wire and current smoothly into an arc that is protected by a gas shield. The wire must be fed into the arc at the correct rate and the exactly required current and voltage must be supplied. An advantage of MIG/GMA welding is that the equipment provides a self-regulating arc; this is called “semi-automatic” welding.
This arc regulation allows the welder to move the Torch to the work and the arc will maintain a constant length (Fig. 27.2).
MIG or GMA Equipment: Type # 2. Power Source:
In Advanced process, the power source consists of a Transformer/Rectifier of constant potential. The current is related to the arc voltage by the electrical characteristics of the power source. This type of power source supplies current that increases rapidly as the potential falls .This is called “constant potential”.
The ‘constant potential’ source does not give constant current unless all the other factors controlling burn-off are held constant. If the Torch is moved towards the ‘work’ the wire extension is shortened and the resistance of the system is reduced, causing a rapid increase in the current. This is called ‘self-regulating constant potential system’.
It can also be explained by showing that the choke (inductance) is provided basically to limit the short circuit currents being drawn by the power source and also to control spatter and bead shape. In the ‘Dip Transfer’ process the choke plays an important role in the control of the frequency of the short circuit.
MIG or GMA Equipment: Type # 3. Wire Feeder:
In the most popular system, the wire is pushed through the guide tube to the gun. This is particularly suitable for hard wires such as steel, stainless steel, and heat-resistant steel. In the second system, the motor drives the gun.
The wire is pulled from the reel through the guide tube to the gun. In the push type, the wire diameters are 0.8, 1.2, 1.6, 2.4 mm hard wire and 1.2, 1.6, 2.4 mm soft-wire, and 3.2 mm flux cored wires are supplied on reels of 15, 25, and 30 kg etc.
These wire reels are mainly for steel, low alloy-steel, stainless steel, hard facings, aluminium, bronze and copper. All these wires pass between motor-driven feed and pressure rollers at a speed range of the wire feeder of (approximately) 2.5 and 15-18 per minute.
The wire feeder consists of a motor with gear box, wire drive rolls, pressure counter rolls and a wire spool holding arrangement with brake to prevent overrun of wire. Earlier wire feeders were of the A.C. motor type controlled by a governor. Later, the D.C. motor was introduced. Recent developments have introduced SCR controls for speed variation and control of the motor.
The average motor capacity of a wire feeder for solid wires up to 2.4 mm dia. is 120 watts. Normally, this motor actuates a two-roll drive through a gear box. For tubular or flux cord wires, specially for wire sizes over 1.6 mm dia, a higher capacity D.C. motor is used in 180 watts; with this motor and gear box a 4-roll drive is incorporated.
This gives a better grip on the wire and has a tendency of straightening the wire before entering the torch.
There are some basic factors to be noted in relation to the wire feeder:
(1) Tension on the spool-holder;
(2) Selection of the guide tube;
(3) Drive roller;
(4) Tension on the electrode wire;
(5) Quality of the electrode wire.