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6- The Milling Machine - Changing Speeds
Milling machines have two types of drive heads: step pulley and variable. If your machine has a step pulley arrangement you should be aware of how spindle RPM changes occur. Here on the left is an animation of a step pulley system. Note that the belt has been moved to the highest place in the pulley system. If the pulley on the right of the animation was attached to an electric motor turning at a 1200 RPM then the pulley on the left would be the machine spindle and it would turn much faster. The speed change is the ratio of the pulley diameters. In this case the pulley on the right has an upper pulley diameter of 6 inches and the pulley on the left has a pulley diameter of 1.5 inches. The ratio of 6 to 1.5 is 4:1. So if the motor (driving the pulley on the right) was turning at 1200 RPM then the pulley on the left would turn at 4 x 1200 = 4800 RPM

Here is a different configuration. Note that the belt has been moved to the lowest place in the pulley system. The belt can be moved to any of four levels each with their own ratio. Using the motor speed of 1200 from the previous page then the motor (right) and the spindle (left) would be a ratio of 1:4 (just the opposite of the first animation). So now the RPM of the spindle is 1200 / 4 or 300 RPM.

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