Work from the joint. If the tube is thin-walled, the internal bead is critical and there is clearance to close a chamber round it, an enclosed head gives the better weld with less setup. If wall thickness needs filler, the diameter exceeds the enclosed range, or access is restricted on one side, an open head with wire feed is the only option.

Enough for the body to close fully around the tube, which is considerably more than the tube diameter itself and is the constraint most often missed at design stage. Manufacturers publish a clearance envelope per head. On congested skids and pipe racks it is worth checking the envelope against the drawing before the joint is designed, not after.

They adapt one head to the several diameters within its range, gripping the tube concentrically so the joint runs true under the electrode. A set is specific to both the head and the tube outside diameter. Worn or wrong-size inserts let the tube sit off centre, which shows up as arc length variation and inconsistent penetration around the circumference.

Open heads can, through a cold wire feeder that meters wire into the leading edge of the pool, and this is standard for multi-pass work. Enclosed heads generally cannot; they are designed for autogenous fusion of prepared square-cut ends. A few enclosed designs accept wire for specific applications, but adding filler is normally the reason to move to an open head.

Smaller heads run air-cooled on the duty cycles typical of thin-wall tube. Higher currents, larger diameters and continuous production need liquid cooling, with a recirculator supplying coolant through the head cable. Running a liquid-cooled head dry, or with degraded coolant, damages the head quickly, so flow and coolant condition belong on the pre-weld check.

Rotor and gear cleanliness above all, since weld spatter and grinding dust in the drive are the usual cause of uneven rotation. Beyond that: electrode condition and stickout set to the program, clamp inserts checked for wear, gas seals and diffusers on enclosed heads inspected for leaks, and cable assemblies checked for coolant weeps and broken conductors.

Welding tubes into the tubesheet of a heat exchanger, boiler or condenser, where hundreds of identical joints sit close together on one face. The head locates in the tube bore, expands to centre itself, and rotates the electrode around the tube-to-sheet junction. Access between adjacent tubes and the tube pitch determine which head fits.