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latter type, and the back gears referred to are enclosed at the front of the column, where they are rigidly mounted closely together to overcome torsion and cutter chatter. The feeding mechanism is driven from the rear end of the spindle by a chain and sprockets, and is subject ot the speed variations of the spindle.
When the cone method of drive is employed for vertical spindle milling machines, the belt usually leads from the cone pulley on the countershaft to one on a shaft at the back of the machine. Power is transmitted thence to the spindle on the lighter machines, by means of a quarter-turn belt. An application of this method of drive is shown in the illustration on page 36. The heavier machines are fitted with bevel gears, and a vertical shaft from which the spindle is driven by a chain and sprockets.
Constant Speed Drive. The invention of the gear type of drive, or, as it is better known, the "constant speed drive," is, without doubt, the most valuable improvement in design brought out in many years. It is the result of a demand for a machine in which the feeds would be entirely independent of the spindle speeds, and all speeds and feeds would be self-contained, thus doing away with complicated overhead works, or permitting the machine to be driven by a constant speed motor. More power and greater convenience in changing speeds and feeds were also important factors leading to the development of this type of drive.
The introduction of high speed steel marked a new era in cutter manufacturing, and brought about conditions that necessitated machines of higher efficiency. This added impetus to the already growing interest in a machine offering possibilities such as those of the constant speed drive, and, early in 1904, the Brown & Sharpe Mfg. Company placed the first constant speed drive machine upon the market. From the beginning, it was conceded an important improvement, especially for the larger sizes of heavy service machines, where an abundance of power is required, and this has led to its becoming almost universally adopted by milling machine manufacturers. Several examples of constant speed drive machines are shown in this treatise, notably those illustrated on pages 16, 19, and 44.
The general features of this drive are as follows: the belt delivers power to the driving pulley that runs loose on a sleeve on the main shaft of the machine. By means of a friction clutch on the main shaft, operated by levers at each side of the column, power is transmitted from the driving pulley to a train of hardened gears
