Abstract: A carriage is adapted to travel along a guide-band mounted about a pipe section. The carriage has a frame that includes a platform for carrying the welding apparatus. Connected to the frame are swing arms whose angular positions relative to the frame are adjustable to allow the carriage to accommodate the curvature of the pipe section. Indexing assemblies that include indexing discs and locking pins are co-operable with each swing arm and elements of the frame to allow each swing arm to be fixed in one predetermined angular position selected from a set of discrete angular positions relative to the frame. The carriage also has an arrangement of drive and idler wheel assemblies which engage opposite edges of the guide-band to effect travel of the carriage in a constrained motion about the guide-band. The wheel assemblies are arranged generally symmetrically about the carriage and are evenly distributed about the carriage.