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).
Figures show CTWD, ESO or Electrode Extension and Arc Length.
ESO = CTWD – Arc Length
Pictures are from the “GMAW Welding Guide (C4.200) p. 17
Regarding positioning of the end of the contact tip to the end of the gas nozzle, for flux-cored, gas-shielded wires (or solid wires in a spray arc mode of metal transfer), the contact tip should be recessed about 1/4" (6 mm). For solid wires in a short circuit mode, the contact tip should be even or flush with the nozzle. If you have an adjustable gas nozzle, slide it to the appropriate position. If you have fixed gas nozzles (i.e. insulator is built into them), then use an “R” series for a recessed tip or an “F” series for a flush tip. You may even occasionally need to use an “S” series for an extended tip, which gives you better access into tight joints. Note that with a recessed contact tip, we introduce another variable called "VSO" (visible stickout). With your flux-cored, gas-shielded wire and a 1/4" (6 mm) recessed tip, the CTWD is still 3/4" (19 mm), but the length of wire visible past the nozzle is only 1/2" (13 mm). Therefore, that is the length of wire you see with your eye and want to keep consistent.