rollers 58 may indent walls of the conduit to form grooves and ridges in the conduit ridges formed by rollers 58 may then contact rollers 58 u2032 rollers 58 u2032 may push the ridges towards an axis of central passage 60 i e towards the longitudinal axis of the central passage grooves between ridges may also move towards the axis of central passage 60 fig1 depicts an embodiment of roller 58 of conduit modifier 54 having rollers 58 and rollers 58 u2032 an outer surface of roller 58 may have a convex shape fig1 depicts an embodiment of roller 58 u2032 an outer surface of roller 58 u2032 may have a concave shape the concave shape may conform better to a shape of a ridge formed by roller 58 in an embodiment of conduit modifier used to form corrugated conduit from 5 inch nominal diameter pipe rollers may be about 3 inches in diameter widths of the rollers may be chosen to form grooves and ridges of desired sizes in an embodiment a width of each roller is about 0 75 inches in some embodiments rollers may not need to be formed of very hard materials to indent conduits rollers may be made of material that is harder than the conduits being corrugated a significant portion of force between the rollers and a conduit being corrugated may be borne by bearings supporting the rollers supporting a significant portion.