Table 8-2. - Approximate Cup Size for GTA Welding

Table 8-3. - Approximate Current Ranges for Tungsten Electrodes

the proper insulation for the maximum current ranges to ensure 'operational safety. Current is transmitted-from the weld-rig machine through the power cable to a collet holding the tungsten electrode. A variety of collet sizes are available, depending on the diameter of the electrode. Figure 8-8 shows the various parts of a typical GTA torch.

Gas is fed to the welding zone through the torch nozzle that consists of a ceramic cup. Nozzles also are made of steel (chrome plated), plastic, and glass (Pyrex) materials. These nozzles (gas cups) vary in size, depending upon the type and size of torch and the diameter of the electrode. See table 8-2 for sizes.

Electrodes

The electrode should extend beyond the end of the gas cup a distance equal to its diameter for butt welding and slightly further for fillet welding. Selecting the right size electrode for each job is important to prevent electrode damage and poor welds caused by too high or too low a current. Excessive current causes tungsten particles to transfer to the weld, while not enough current allows the arc to wander emetically over the end of the electrode.

The diameter of the electrode selected for GTA welding is governed by the amount of welding current used. Remember: DCRP requires larger electrodes than DCSP. Recommended electrode sizes for various ranges of welding current are shown in table 8-3. These current ranges are broad. You should refer to the manufacturer's recommendations for specific current ranges and electrode sizes based on the type of material you are welding.

Basic diameters of nonconsumable electrodes are .040, 1/16, 3/32, and 1/8 of an inch. They are either pure tungsten or alloyed tungsten. The alloyed electrodes are of three types: 1% thorium alloy, 2% thorium alley, and zirconium alloy. Pure tungsten is routinely used with ac welding and is sufficient for most GTA welding operations. The thoriated types are normally used for DCSP welding. These electrodes give slightly better penetration and arc-starting characteristics over a wider range of current settings. The zirconium alloy is excellent for ac welding and has high resistance to contamination. The electrodes alloyed with thorium and zirconium are primarily used for critical weldments in the aircraft and missile industries.