Abstract: An electric resistance welding apparatus is shown for attaching insulation to duct work comprising a supporting anvil with a surface for supporting a sheet metal duct work with a covering sheet of insulating material and a movable hammer member spaced from and movable relative to the anvil. The hammer member has one or more magnets for supporting a resistance welding pin and is movable to drive the pin through the insulating material to engage the sheet metal duct work. The hammer member and anvil are connected in an electric welding circuit to supply current from the hammer through the pin to the anvil to fuse the end of the pin to the sheet metal with the head of the pin holding a washer tightly against the insulating material. The hammer member and anvil are both copper with recesses having cylindrical copper electrode members removably positioned therein. In one embodiment, the hammer member recess is deeper than the copper electrode member, and the magnet fits the recess behind the electrode member.