Page:Practical Treatise on Milling and Milling Machines.djvu/26

 leading to the spindle, and in which there are certain change gears operated by levers at the right-hand side of the column. The belt and main driving pulley run at a constant high velocity regardless of the spindle speed, which is entirely dependent upon the ratio of gearing that may be in mesh. The power at the spindle is therefore constant, regardless of its speed.

The mechanism of constant speed drive vertical spindle machines is essentially like that outlined above, except that a pair of bevel gears and vertical shaft are introduced to transmit power to the spindle head, from whence it is communicated to the spindle itself by spur gearing.

The feed changing mechanism is driven from the main shaft by means of a chain and sprockets in all constant speed drive machines. Hence it is completely separated from the spindle drive, in so far as its speeds are concerned, permitting the full range of feeds to be available for every spindle speed. Such an arrangement also permits the table feeds to be rated directly in inches per minute, which is an advantage in that it enables the production of a machine to be ascertained at a glance.