Page:Practical Treatise on Milling and Milling Machines.djvu/34

28 and constitutes a particular point wherein this type of drive differs from that known as the cone drive.

The cone drive machine is admirably adapted to all classes of work where it is not necessary to use combinations of extreme speeds and feeds. In these cases, however, it cannot fulfill the requirements. For instance, it is impossible to obtain a course enough feed for a cutter of very large diameter, because the feeding mechanism is invariably driven from the end of the spindle, and is subject to the speed variations of this part. Consequently, when a large cutter is being used, the spindle is usually driven at its slowest speed, and the fastest feed that is then available is not coarse enough. Likewise, a correct combination of speed and feed cannot be had for a small mill, as this should run at the fastest spindle speed, and, when it does, the finest feed obtainable is much too coarse. The majority of work, however, does not require such combinations, and when medium-sized mills are used and work of ordinary classes is done, the cone drive machine is very satisfactory.

Owing to the dependence of the feeds upon the spindle speeds in the cone drive machines, it is necessary to rate them as so much per revolution of the spindle. This requires that the feed being used be multiplied by the spindle speed, in order to obtain the rate of production in inches per minute—the most generally accepted standard.

With the constant speed type of drive any combination of spindle speed and table feed within the ranges of the machine can be obtained, and thus the large, medium, or small sizes of cutters can all be run at the most practical speeds and feeds. This is due to the fact that the spindle and feeding mechanisms are driven independently of each other from the same main shaft, which revolves at a constant velocity at all times. Feeds obtained in this manner can be rated directly in inches per minute, a point that in itself constitutes an important advantage.

On practically all of the Brown & Sharpe constant speed drive machines, sixteen changes of spindle speed, and at least sixteen different feeds are available, while some sizes have as many as twenty feeds. Their range varies slightly in the different sizes of machines, but is such in every case that the correct combination can be had for any cutter that is used.

Power. A milling machine must have ample power, or its use is exceedingly limited. This applies to all styles and sizes of machines,