Machine Photogravure

Still another method of grinding that has been developed in recent years involves the use of broad faced wheels, up to approxi mately 12 in. in width, and there is no reason to believe that this is the limit. In cylindrical work, under the name of the plunge cut grinder, the wheel is fed straight into the work instead of being traversed along the length of the piece as by the older method. A new development in fine grinding is known as super finishing, developed by D. A. Wallace of the Chrysler Corp. It resembles honing but must have at least three motions between the abrasive blocks and the work. It uses abrasives up to Soo fine. It removes minute high spots on bearing surfaces that might puncture the oil film and cause wear. It produces surfaces with scratches not exceeding 6-millionths of an inch deep.

Even more interesting than the development is the growth of the centreless grinding. Beginning with the grinding of small round pins, with more or less errors as to roundness and size, centreless grinding is now being used to secure extreme ac curacy. Starting with a grinding wheel and two stationary rests, which supported the work as it revolved, one rest was abandoned in favour of a second abrasive wheel. This second wheel runs at a slower speed than the main, or grinding wheel, and is known as the feed wheel. Its main object is to insure that the work shall revolve at all times, which overcomes the occasional stoppage and the consequent flattening of the work which sometimes occurred in the older machines. One of the economical features of the cen treless grinding machine is the continuous feed that can be secured by setting the feed wheel, or even a stationary rest, at a slight angle to the grinding wheel. All that is necessary is to place the work in an inclined trough leading to the opening between the wheels, and the work automatically feeds into and through the machine. In some cases two machines are placed in tandem, the first acting as a rougher while the second finishes the work. While centreless grinding is primarily for grinding straight work that can pass between the wheels continuously, the method is also used on work of different diameters or with shoulders and tapers. For this work the wheels are set parallel and the work fed between the wheels at right angles to the axis of the wheel spindles. While not as fast as the straight through feed, it is usually more rapid than work ground between centres. Centreless grinding now in cludes internal work with automatic sizing for both types of machines by mechanical, electrical, and pneumatic control. The

latter uses an air jet that builds up back pressure and stops the feed when the correct size is reached.

Broaching.

Broaching, which is the forcing of a special cutter through an opening or beside a piece of metal in order to enlarge a hole or shape the exterior, continues to play an important part in machine work. In some cases the finishing of surfaces in this way is applied to large sizes. A broach may be likened to a special chisel, with teeth spaced at frequent intervals and of such shape as to produce the desired surface in the work. Broaching is con fined to internal work in most cases, being originally used for holes that were not round. Present day practice, however, uses broaching very largely as a method of producing round holes accurately and economically. Broaches are made in both the "pull" and "push" types. The former uses a long broach and frequently finishes a hole with a single pull of the broach. Push broaches on the other hand are usually rather short and several are required to cut the hole to the desired shape or size. The number of broaches depends largely upon the length of the piece to be broached, the metal used and the accuracy required. The length is determined by the amount of metal that can be cut successfully by each tooth of the broach, as on this depends the number of teeth required. Where great accuracy is required the amount cut per tooth is very small : as the broach reaches the maximum size, it is frequently neces sary to use two or more of the long pull broaches. Outside broach ing, which is the finishing of the outside surfaces of machine parts by forcing a broach past them while the work is rigidly held and the broach firmly guided, is used in some cases. Gear teeth have been cut in this way and similar work is also handled. The applica tion of outside broaching is however, comparatively limited. Broaching machines are made in both horizontal and vertical types, the former usually for the pull broach and the vertical for the shorter push broach. Broaching is now replacing milling to some extent in the machining of outside surfaces such as automobile cyl inder blocks, for straddle milling, and other surfaces where the quantity warrants the first cost of machines and broaches.

Illustrations relating to machine tool practice are shown in Plates I. to IV. (F. H. C.) See TOOL, and also AUTOMATIC MACHINES, TOOL STEEL, GRINDING-MACHINES, PRESSES AND PRESSWORK, LATHE, MILLING MACHINES, PLANING-MACHINES, PUNCHING AND SHEARING MACHINES, SAWING MACHINES, STEAM-HAMMER, WOODWORKING MACHINERY, etc.