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Tech Talk... Bad CNC machine and your power bill

Nov 2, 2015

A very sophisticated and capable spindle design will not be acceptable on a low-cost machine tool. Consequently, an advanced machine tool design can justify the higher cost of a more capable and complex spindle package. The column talks on how a bad machine increases the spindle load and your power bill

Machines of two different makers may look the same, and we wonder why one costs so less than the other. The reason may be that the cheaper machine is badly designed and made with cheaper components. The poor build quality and design increases power consumption in spindle motor, axes motors, coolant motor, chip conveyor, etc. The below points explain on how a poor spindle drive train can result in a higher spindle load and raises power bills.

  • A poorly designed spindle that is heavier or has a larger diameter than necessary or having more moment of inertia, consumes more power to rotate it

  • Poor bearings that result in increased friction as the spindle rotates, and need more energy to rotate the spindle.

  • Poor belt design and belt quality, that increases power consumption because of underbelting (belt with not enough load capacity) or overbelting (belt with too much capacity). Underbelting causes excessive stretch and increased slip, hence there is energy loss. Overbelting involves use of a thicker or wider belt than necessary that has more resistance to flexing as it moves around a pulley and more energy loss to bend the belt.

Because of energy losses in the belt drive and in bearings, and the power required to rotate the spindle against its moment of inertia, the power available at the chuck in a CNC lathe or machining centre is always less than what is available at the motor. The losses cannot be eliminated fully. They can only be reduced. On a reasonably well made machine, the losses will be 15 to 20%, and on a low cost machine, they can be as high as 40%. This means that on the latter machine only 60% of the spindle motor power is available at the chuck, and the power bill with the latter machine is 33% more that with the former machine.

Action point

Instead of deciding on a machine purely based on its low initial capital cost (a cheap machine), you can consider its running costs too, like the power consumption.

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