The first development away from the engine lathe was to replace the tail-stock, or back centre, with a turret for holding several tools which could be presented to work held in a chuck, one after the other. This only affected chucking work and not work held between centres. The multiple tooling permitted several operations to be performed in quick succession, at one setting of the work. These have been developed in many forms and in the semi-automatic or full automatic form ; they now produce intricate pieces with great accuracy and speed. Hand-operated turret lathes have standardized the tools used so that they can now be used on a large variety of work and only require a minimum of time to set them up for a particular job. Semi-automatic machines, in which the tools are fed to the work automatically after the piece has been put in place, are in common use. The automatic feeds are secured by cams, gears and screws, or, in some cases, by hydraulic pressure. Semi-automatic machines of the lathe family are used in nearly all metal manufacturing. Another and later development retains the tail stock and is used for "centre work," the tool carriage being provided with multiple tools and the feeding movements, both longitudinally and as it affects diameter, automatically controlled.
Modifications of turret lathes have also been made in which several work spindles are indexed from one position to another so as to present different tools to the work in the proper succession. An excellent ex ample of the station type machine is the Bullard Mult-au-matic, which is in reality a multi-spindle, multi-tooled vertical lathe, or a number of lathes. This machine has six vertical work spindles, each carrying a table, or horizontal face plate as in the vertical boring mill. Above and in vertical alinement with these spindles are five tool slides each carrying tool heads. The sixth place is vacant so that the work can be loaded and unloaded at this point. When the empty spindle has been loaded and the index ing takes place, all six spindles move one-sixth of the circle. Each spindle stops during the indexing but resumes speed as soon as it is in the new position. Its speed varies with the operation being performed at the particular station, being slow for large diam eters or broad cuts and fast where the work and tools permit.
Although the use of planers and shapers have not increased as with some other types of machine tools, they have been greatly improved. Planer beds have been made heavier, stiffer and longer ; the planer table no longer over runs the bed, but is fully supported at all parts of even the longest stroke. Driving gears are wider and heavier, and are of steel instead of cast iron, and in some cases have helical or herringbone teeth. Shafts and shaft bearings are also much heavier and forced lubrication supplies all the driving mechanism with oil. Reversing electric motors that eliminate the use of shifting belts are now in common use. Another innovation is the application of individual motors to various functions such as raising and lowering the cross rail and side heads, and of moving the tool heads or saddles on the cross rail. A more common method however is to secure power from the main drive but to control each movement from either end of the cross rail, by individual control handles. They are so arranged that all movements can be made by power and at high speed if desired. The open side type of planer handles a wide variety of work that cannot be done on the regular type planer.
The old knee type machine has, to a great extent, given way to other designs. In a type known as the
manufacturing design, a heavy bed supports a long deep table while on both sides is a massive column each with a heavy spindle driven by hardened and ground steel gearing. The spindle sleeve or quill, moves in or out and supports the cutters with a minimum of overhang. In addition, the heavy overarms can be used to provide additional bearings for the cutter arbours and so counter act the tendency to spring under a heavy cut. The planer type machine, which resembles the planer in general appearance, has the cross-rail carrying milling cutters instead of a head for single point tools. Side milling heads are also used, the total number of milling heads frequently reaching ten or more on a single machine. As each milling head represents a surface that can be machined at a single pass of the work, the capacity of such machines can easily be estimated. A particularly interesting development of the milling machine is the drum, or rotary type. This is a continuous operation machine, the drum carrying the work past a number of milling cutters.
Another and special form of milling machine has been built to handle the increased use of dies for forging, stamping, etc. Until about 1921, machines for cutting dies required the use of models made of such materials as bronze, cast iron or electrotypes. These were of two distinct types : reducing machines in which the model is larger than the die required, and duplicating machines in which the model is the same size as the die. Both these types are still in use, but they have their limitations. To meet the demand for a machine on which models made of softer materials could be used, the full electric controlled machine was devised in 1921. This machine is, in its essentials, a milling machine. The movement of the cutting tool is positive, and under control of a tracer or finger which fol lows the surface of the model by means of an electrical mechanism. The main bed carries a column sliding thereon, and operated by a lead screw driven through suitable gearing by a pair of oppo sitely rotating magnetic clutches. On the column is the saddle, operated vertically by a vertical lead screw, which is driven by a similar mechanism as the main column. Mounted on this saddle is the cutter head designed to slide in a transverse direction and operated by a lead screw, again in the same manner as the other two motions. The cutter head carries the cutter spindle and driving gears, also the motor supplying power to the cutting tool. On the front part is a long bracket extending vertically for the support of the tracer. The tracer operates the three slides of the machine in such a manner as to cause the cutting tool to follow the same path over the die as the tracer point follows over the model—thereby reproducing the exact shape of the model in the cast iron or steel die block. The model is usually made of wood, plaster, cement or other convenient material. The pressure of the tracer point on the model is only a few ounces, but the cutter by means of the lead screws and power driven magnetic clutches, is forced to cut the material of the die. This machine has played an important part in the mass production of automobiles and other articles manufactured on a large scale by greatly reducing the time necessary to produce a set of tools for body-work on forging parts, etc., and also cutting materially the cost of the same. Since the machine will follow the outline of a male or female thin sheet metal template with unusual fidelity, it is also a very efficient profiler. It may also be conveniently used for jig boring operations.