What is “electrode extension,” what is “contact tip to work distance,” and how far should the contact tip be recessed in the gun nozzle?
Q: I have a 250-amp MIG welder and just bought a spool of 0.045-in. flux-cored, gas-shielded wire. The salesperson said to use a 0.75-in. ESO. However, when I looked up the same information online, it said that the CTWD should be 0.75-in. and to estimate ESO, subtract 0.25-in. 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: 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.
Contact tip to work distance (CTWD) is, as the name indicates, the distance from the end of the contact tip to the plate or work piece. Electrical stickout (ESO), or more technically, 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-in. (6-mm) on average. For hand-held (i.e. semi-automatic) welding, you need to maintain a consistent CTWD manually. 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 will keep the distance constant.
For 0.045-in. (1.2-mm) flux-cored, gas-shielded wire, the recommended CTWD is 0.75 to 1 in. (19 to 25 mm). Therefore, both your earlier sources were okay. For larger-diameter cored wires, the recommended CTWD is 1 to 1.25 in. (25 to 32 mm). Be careful, however, not to use a CTWD that is too long or you may extend the arc beyond the point of complete gas coverage. This could result in weld porosity. It also may result in an unstable arc. As an aside, 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 0.375 in. (10 mm).
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 0.25-in. (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 occasionally need to use an “S” series for an extended tip, which will give 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 0.25-in. (6-mm) recessed tip, the CTWD is still 0.75 in. (19 mm), but the length of wire visible past the nozzle is only 0.50 in. (13 mm). Therefore, that is the length of wire you’ll see with your eye and want to keep consistent.
Tom Myers is a Senior Application Engineer for The Lincoln Electric Company. As a member of the Application Engineering Department, he specializes in flux-cored and stick welding processes. He has over 22 years of welding experience with Lincoln Electric, including 10 years as a Technical Sales Representative, eight years in the Technical Training Department, with concurrent positions as the Corporate Sales Training Manager and Educational Services Manager. In these roles, he was responsible for training of Lincoln Electric’s technical sales force, as well as many customer and distributor training programs. In addition, he was the coordinator for Lincoln’s educational programs and services available to public and private welding schools.
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