Page:Practical Treatise on Milling and Milling Machines.djvu/15

 the machine, which revolves at a constant high velocity at all times. The table feeds are therefore entirely independent of the spindle speeds. A powerful drive is also transmitted to the spindle from the driving pulley of large diameter and wide face on the main shaft of the machine through a train of heavy spur gearing in which are certain change gears that can be manipulated to give a wide range of spindle speeds.

At the same time that the constant speed type of drive was evolved, the machine was redesigned and made stronger throughout in order to better fit it for the heavy cuts that had become the practice.

Later improvements have been the extension of the flat bearing surface on the front of the column to the top, the application of a friction clutch in the driving pulley with levers at the sides of the machine for operating it, the power fast travel for quick movement of the table, and other improvements of lesser importance.

It is not to be assumed that the constant speed type of drive has been developed to the exclusion of the cone type, for there are many pieces of work that can be done to good advantage on this machine. The modern cone type of machine embodies all of the previously mentioned improvements, except those relating particularly to the constant speed drive, and there is still, and probably always will be, a steady demand for this machine.

Two other types of machines known as Plain and Vertical Spindle Milling Machines have kept pace with the development of the universal milling machine.

Milling Machines of the Planer and Manufacturing types have also come into extensive use, the former producing a wide range of work that is of too large dimensions for the previously mentioned machines, and the latter manufacturing in large quantities, small duplicate parts of machinery, tools, etc.

With the improvements that have been made on the machines and their equipment, milling has become indispensable in the modern shop. Interchangeable pieces can be easily made, and work is produced at a low cost because of the continuous operation and inexpensiveness of cutters for a given amount of production. We, therefore, recommend the milling machine to manufacturers desirous of obtaining the best results at the lowest cost on all classes of work to which the machine is adapted. And we trust that a careful reading of the following chapters will be of material assistance in understanding the process of milling and how to use the machines.