216 Modern Welding Copyright Goodheart-Willcox Co., Inc. Contact tips are available with a variety of inside diameters (ID) and lengths. The contact tip must be designed for the diameter of welding wire being used. Common sizes include .030″ (0.8 mm), .035″ (0.9 mm), and .045″ (1.2 mm). The inside diam- eter of the contact tip is critical because the contact tip must maintain good electrical contact with the welding wire as the wire slides through it. Each time the wire diameter is changed, the contact tip must be changed so the ID matches the diameter of the wire. Most manufacturers of contact tips make them in different lengths. Different lengths help obtain dif- ferent transfer methods. The longest contact tips for a gun are usually used for short circuiting transfer. With short circuiting transfer, the contact tip should be fl ush with the end of the nozzle or should stick out about 1/16″ (1.6 mm) beyond the end of the nozzle. Resistance heating of the electrode takes place after the welding wire exits from the contact tip. The elec- trode extension distance is the distance over which the electrode is heated. The longer this distance, the more heating takes place. A long contact tip minimizes the electrode extension and reduces the resistance heating of the welding wire. See Figure 8-40. A shorter-length contact tip is used for spray trans- fer. The end of a shorter-length contact tip is usually inside the end of the nozzle. This provides a longer electrode extension and allows the welding current to preheat the wire more. A B C D Welpers Goodheart-Willcox Publisher Figure 8-39. Assembling the front end of a GMAW gun. A—Install the adapter. B—Install the contact tip. C—Secure the contact tip. D—Install the nozzle. Goodheart-Willcox Publisher Goodheart-Willcox Publisher Goodheart-Willcox Publisher