ON TILGHMAN'S PATENT SAND BLAST, For Cutting, Grinding, Engraving, and Ornamenting, Glass, Stone, Wood, Iron, and other hard substances. COMMUNICATED BY W. E. NEWTON, C.E.. The cutting, grinding, engraving, and ornamenting of glass, stone, wood, iron, and other hard substances, are operations reguiring a considerable expenditure of time and labor, and some of them a large amount of skill. The object and result of Mr. Tilghman's ingenious and simple invention is to vastly economise time and labor in these operations, and to very considerably reduce the amount of skilled labor required to produce ornamental patterns and architectural devices in stone and other hard substances. The invention is based upon the fact that if grains of sharp sand are driven with a certain velocity against a harder surface, such as glass, stone, wood, or iron, such surface will be gradually cut away. As a matter of fact the action of the sand on the hard surface of the glass or stone is very rapid, as will be seen hereafter; and, therefore, if a sheet of plain polished glass be subjected to the sand blast, it will be quickly depolished or ground. If now, before the polished sheet be subjected to the action of the sand blast, a portion of its surface be protected by covering it with soft or elastic substance, such as india rubber, paper, or other suitable material, cut to any particular pattern or device, all those parts so covered will remain intact, while the exposed surfaces will be ground or cut away by the impact of the sand. In this process a stream of sand is fed into a jet or current of steam or air, so as to acquire a high velocity, and is then directed upon the surface of the stone, glass, wood, or metal, which it grinds or wears away with a degree of rapidity dependent upon the velocity of the sand and the nature of the material operated on. For work, such as cutting stone, where a considerable quantity of material is to be removed, a steam jet, of from 60 to 120 lbs. pressure on the square inch, has generally been used as the propelling agent. The machine employed to direct the sand on to the object to be operated upon, resembles a Gifford's injector. The central tube is to be supplied with a jet of steam, or a stream of air, under considerable pressure, and sand is used instead of water, the grains of sand being drawn by suction into the steam or air jet, and then projected forward with a velocity proportioned to its pressure, in the same way as the water in the Gifford's injector is propelled or carried along by the steam. The general nature of the process, and the construction of the apparatus, will be understood by referring to Fig. 1 of Plate x, which represents a machine for grinding or ornamenting glass. In the stone-cutting machine now in operation at the International Exhibition,* the sand is introduced by a central iron tube, such as that shewn in the drawing at Fig. 2. This tube is about inch bore, and the steam issues through an annular passage (7-16ths of an inch external and 5-16ths internal diameter), surrounding the sand tube. A tube of chilled cast iron, 6 inches long and 7-16ths of an inch bore, is fixed as a prolongation of the steam passage, and serves as the gun or tube in which the steam mixes with the sand, and imparts velocity to the latter. The central sand tube is connected, by means of a flexible tube * 1873. |