Q: I have a 250 amp MIG welder and just bought a spool of .045” flux-cored, gas-shielded wire. The salesperson said to use a ¾” ESO. However, when I looked up the same information online, it said that the CTWD should be ¾” and to estimate ESO, subtract ¼” from the CTWD. I am confused. My questions are these. What is ESO? What is CTWD? In addition, how far should the contact tip be recessed in the gun nozzle?
A: Yes, this is a confusing topic. However, maintaining a constant stand off distance with your gun as you travel along the weld joint is very important for stable wire welding (cored or solid wires). As you move the gun closer or farther away from the plate, the amount of electrical resistance in the wire changes, which results in changes to arc voltage and current (i.e. “Ohms Law”). This could result in inconsistent weld quality. CTWD (contact tip to work distance) is as it says, the distance from the end of the contact tip to the plate or work piece. ESO (electrical stickout, or more technically correct, electrical extension), is the distance from the end of the contact tip to the top of the welding arc. The difference between the two is the height of the arc (which is about 1/4" (6 mm) on average). For hand held (i.e. semi-automatic) welding, you manually need to maintain a consistent CTWD. For automatic welding, it is easier to adjust the torch up or down to a preset CTWD (using a spacer or ruler) and then the automated welding devise keeps the distance constant.
For 0.045" (1.2 mm) flux-cored, gas-shielded wire, the recommended CTWD is 3/4" to 1” (19 – 25 mm). Therefore, both your earlier sources were okay. For larger diameter cored wires, the recommended CTWD is 1" – 1 1/4" (25 – 32 mm). Be careful however, not to use too long of a CTWD or you may extend the arc beyond the point of complete gas coverage. This could result in weld porosity. It may also result in an unstable arc. As a side note, solid (i.e. MIG) wires typically use a shorter CTWD, particularly with a short circuit arc mode of metal transfer, where recommended CTWD is only around 3/8” (10 mm).