Chapter 21 Introduction to CNC Machining 401 Copyright Goodheart-Willcox Co., Inc. GREEN MACHINING Efficiency in CNC Machining CNC machine shops can ensure energy- and cost-effi ciency by streamlining shop operations and maintenance. High-effi ciency practices include the following: • Perform large work volumes on a complicated setup, rather than repeatedly resetting for smaller batches. • Minimize maintenance costs through regular preven- tion and troubleshooting. • Provide easy access to machines to reduce setup and breakdown time. Special equipment, such as an induction motor with dual winding, can also be used to increase spindle effi ciency. When a spindle is not turning in operation, shops can turn off CNC motors, fans, pumps, and con- veyors to save energy. In addition, simple shop upkeep, such as adjusting the layout of milling equipment and maintaining a clean shop, can improve a shop’s overall effi ciency. 21.7.1 Stepper Motors Stepper motors are often used in less expensive CNC machines, especially machines that are used for woodwork- ing purposes, Figure 21-21. They do not require encoders to keep track of the motor’s revolutions relative to the posi- tion of the axis they are driving. An encoder is a transducer that measures the position of moving axes and provides elec- tronic feedback to the control. Stepper motors are typically used in open-loop systems. An open-loop system has no means to provide feedback to the control relative to the position of the axis. This requires the machine to be reset to its zero position, often called machine homing, more frequently than a machine using servomotors. Another major drawback of stepper motors is that they lose torque at higher speeds, which means their machine cycle times must be reduced when cutting forces require higher torque. 21.7.2 Servomotors Servomotors do not lose as much torque at high speeds as stepper motors, so machines equipped with servomotors can remove more material faster, Figure 21-22. However, because of their design, they require encoders to provide feedback to the control relative to the position of the axis they are driv- ing. This is known as a closed-loop system. The use of encod- ers is critical when precision machining is needed. 21.7.3 Lead Screws and Lead Nuts Both stepper and servomotors drive the axes of a machine using lead screws. Lead screws are designed to translate the rotating motion of an electric motor into straight-line, or linear, motion. As lead screws rotate, they cause a lead nut to move along the threads of the screw, Figure 21-23. Lead nuts are attached to the moving table of a milling machine or to the cross-slide of a lathe. Goodheart-Willcox Publisher Figure 21-21. This stepper motor controls the Z axis movement on a small benchtop turning center. Stepper motors are commonly used in open-loop systems. 180° X axis (polar axis) 0° CCW (+) CW (–) Y axis 90° r = radius a = polar angle Polar coordinates: a and r Cartesian coordinates: x and y 270° a x r y Goodheart-Willcox Publisher Figure 21-20. Polar coordinates describe a new location from the present location (usually the center of a circle) by showing the value of the radius (r) and the polar angle (a). Counterclockwise rotation is positive, clockwise rotation is negative.