Page:Practical Treatise on Milling and Milling Machines.djvu/35

Rh but more particularly to the larger ones that are used in commercial manufacturing, where an economical production means the taking of heavy cuts at fast speeds and coarse feeds.

In driving machine tools, the power delivered to a machine depends upon the diameters of the driving pulleys, and size and velocity of the belt. A wide belt running at a high velocity on pulleys of large and equal diameters develops the maximum power, and, as its speed and width are lessened, its pulling ability decreases correspondingly. Likewise, it transmits less power, as the pulley on the machine exceeds in diameter the pulley on the driving shaft, for, when the surface contact on the driver becomes smaller, the belt has a tendency to slip.

Hence, in the factor of power is found another important difference between the cone and constant speed drive machines, with the advantage in favor of the latter.

The cone drive machine is very suitable for light and medium work, of such as the majority of milling consists, but when it comes to driving a large cutter through a heavy cut at a slow spindle speed and coarse feed, the requisite amount of power is lacking. This is due to the belt being upon the smallest step of the driving pulley, where it runs at its slowest velocity, and has a small arc and surface of contact.

On constant speed drive machines, the pulley is of the same, or almost equal diameter to that on the overhead shaft, and runs at a constant high velocity, irrespective of the spindle speed. Furthermore, a wider belt can be employed than on cone drive machines. As a result, a maximum amount of power is delivered to the machine pulley, and is transmitted through heavy gearing to the spindle, under all conditions, thus fitting this style of machine particularly well to the heavier classes of work. Another advantage of this drive is its particular adaptation to the application of a motor. The constant speed type of motor, which is more economical, both in first cost and in the amount of power consumed, than the variable speed motor, can be employed. This is also the most simple and compact form of motor drive. When applied to Brown & Sharpe Machines, the motor is mounted on a bracket at the back of the column, where it is away from dust and chips of the table (see page 176). Furthermore, by placing it in this position the floor space occupied by the machine is not increased, as it is necessary to leave room behind the machine to allow the overhanging arm to be pushed back when not in use.