Page:Practical Treatise on Milling and Milling Machines.djvu/14

8 The table feed has been changed from the end of the feed screw and carried up through the centre of the knee and saddle, thus allowing the table to be swiveled through a much greater arc. Power feeds have been applied to the transverse and vertical table movments, and on large machines a power fast travel for the table has taken the place of the hand quick return. The old-style elevating screw for the knee that required cutting a hole through the floor has been replaced by a telescopic screw.

Improvements have been made on the spiral head to make it more rigid and convenient to operate; differential indexing largely replaces the compound method, and refinements such as graduated index sectors, and an adjustable index crank have been added.

Such conveniences as permanent handwheels instead of cranks, adjustable dials reading to thousandths of an inch on the feed shafts, and other improvements have been added from time to time.

When the milling machine came into more general use, and its possibilities in removing metal began to be appreciated, the demand arose for the ability to make heavier cuts. These demands soon demonstrated that the method of driving the feeds through belts and cone pulleys from the spindle of the machine to the feed mechanism, was inadequate. The first improvement was to substitute chain and sprockets for the belt and pulleys and to use removable change gears to provide a variation in the rate of feed. The next step was to place all the change gears in a feed box wherein by simply shifting levers, a wide variation of feeds could be obtained.

The main spindle drive has undergone radical changes. The original machine had a four-step cone pulley mounted directly on the spindle, and many of the smaller sizes of machines today are similarly built. In order to get more power and a greater range of speeds, back gears similar to those of a lathe were added.

Following these improvements came a radical change in the whole driving mechanism of the machine. The value of feeds that were independent of the spindle speeds had become well recognized, and with the introduction of high speed steel, from which cutters could be made that would take much heavier cuts at faster speeds, and courser feeds than had ever before been the practice, there arose a demand for more powerful machines. The constant speed type of drive was therefore originated. In this type of machine any combination of table feed and spindle speed is available, because both spindle and feeding mechanisms are driven from the main shaft of