A Slip-On Flange is bored so that it does indeed slip onto the pipe so that the end of the pipe is just inside the flange a fraction of an inch from the flange face. The pipe end has been cut square. The flange bolt holes are aligned and the flange face squared to the pipe. Then a fillet weld is run around that end of the pipe to fuse it to the inside of the bore of the flange. Another fillet weld is run around the back end of the flange hub fusing it to the outside of the pipe. This is the most common welding flange. Before the final full fillet welds are made, temporary tack welds are made until alignment is assured. These tacks are easy to cut out so a correction can be made.