Shorter and thicker billets of steel can more easily be bored. When the holes have been enlarged by pushing larger and larger-nosed rams through them in a hydraulic press, they can be rolled down to size over a mandrel, just as lap-welded pipe is rolled, only in this case they are put through several times and considerably reduced in diameter. In this way some of the seamless tubes are made. The important starting point in all processes for seamless steel tubes is the piercing operation and it is mostly in the method of getting the first hole through the billet that they differ, since the hot-rolling and the cold-drawing processes by which they are finished have long been known. One of the most important modern processes for seamless tubes, the Mannesmann, is based upon the principle that if a white-hot round steel bar is rapidly rotated between “cross rolls,” a longitudinal rupture which is almost a hole forms along its center. We may liken the motion of the bar in the rolls to the whirling of a lead pencil between the palms of our hands, except, of course, that the bar is kept rotating in one direction only. Though something like this tendency of a steel bar to open along the center through pressure applied at two opposite points on the outside, seems to have been known to forgemen, the Brothers Mannesmann came upon the similar tendency under action of the rolls, by accident. They were German tool steel manufacturers. A critical customer wanted perfectly round and surface-polished bars of steel. They attempted to give his bars this perfect shape and smoothness by finishing them between cross rolls which spun the bars rapidly around while they were slowly passing along through the machine. The pieces were perfect outwardly, but, much to the steel makers’ chagrin, the customer reported that the quality of the steel was not as satisfactory as that which he previously had been receiving. Upon investigation it was found that this cross-rolling under pressure tended to form a small hole along the center of the bar with slight cracks in the metal all around it. Upon this happenstance discovery is based the Mannesmann process for piercing the bar, which consists in pushing over a piercing head such a center-weakened piece as it comes through the cross rolls. One or two modifications of the Mannesmann piercing method are also in use. The material generally used for seamless steel tubes is medium soft open-hearth steel of .15% to .25% carbon. It is received as billets which are rolled down and cut at the mill or they are purchased as 3″ to 6″ “rounds” and cut into such lengths as give proper amounts of steel for the tubes which are to be formed. Usually the bars cut for tubes are from three to five feet in length. They are heated in a furnace and after the end has been dented at the center, they go into the rolls. [Illustration: PIERCING A SOLID BILLET BY THE MANNESMANN PROCESS ] [Illustration: ROLLING DOWN THE PIERCED TUBE ]