The Milling Machine - 6: Changing Speeds
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WARNING: Machine tools present a safety hazard. Improper operation can result in severe injury. These topics are for non-laboratory study only and are not to be used in conjunction with the operation of any tool or machine described herein. Never use a machine tool without the supervision of a qualified instructor.
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.

This video is a procedure for making step pulley RPM changes.

  • Turn off main power
  • Remove the safety cover
  • Loosen the clamp that holds the motor housing in place.
  • Pull the motor forward to decrease the center line distance from motor pulley to spindle pulley.
  • Move the belt by hand to the desired location. (If the belt does not move easily from one step to the next, try rotating the spindle by hand and let the belt "climb" to different levels)
  • Push the motor housing back to get maximum tension on the belt and tighten the motor housing clamp.
  • Replace the safety cover

If your machine has a variable drive system you should be aware of how spindle RPM changes occur: This animation shows how varying ratios can be obtained by squeezing two halves a special pulley together at the motor. By squeezing them together they force the belt to rise higher on the diameter of the pulley being squeezed. By relaxing the squeeze the belt can slip lower. The spring loaded pulley halves on the other shaft compensate accordingly, thus producing variable ratios and variable RPM.
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