CNC | Principle of Operation of a Numerical Controlled Machine

The CNC machine differs from a conventional manual machine in several respects. The principle of operation of a numerical controlled machine can be explained with the help of Fig. 12.1. The figure shows a vertical milling machine. For carrying out an operation like end milling the spindle head is to be positioned in Z- axis and the table in X and Y coordinate axes. The feed movement is to be realized by the individual or simultaneous movement of X and Y axes. Thus the milling machine requires three slide movements, which are usually referred as axes feed drives. A special feature of a CNC machine is that a separate motor called a servomotor individually drives each axis. AC servomotors are the preferred choice for this purpose today. DC servomotors were widely used earlier. The slides are driven by the servomotors through recirculating ball screw and nut assemblies. The use of re-circulating ball screw reduces friction, backlash and wear. The low friction reduces the torque required at the motor and the lost motion through torsional deflection of the screw. The use of ball screws also improves the dynamic response of the system. In some modern designs, particularly in the case of high-speed machines, linear motors are used in the place of servomotor ball screw combination.

Fig. 12.1 Sketch of a Numerically Controlled Machine Tool

In order to carry out the milling operation on the workpiece the coordinate information (X, and Y coordinates) of the starting point and the ending point has to be coded in the NC program. Similarly, the information regarding the direction of rotation and speed of the spindle, use of coolant, and the feed rate is also coded suitably. The CNC controller decodes the positioning information coded in the NC program and the slide is moved to the programmed position at the required feed rate. Each slide is fitted with a feedback transducer, which continuously monitors the slide position and compares with the programmed position as well as the feedrate. The feedback transducer is mounted either on the slide or on the servomotor and measures the displacement or position of the slide.
For example, let us assume that a hole is to be drilled at the location X = 100.0 and Y = 150.0. The corresponding block of the program is read by the control system and the necessary inputs are sent to the X and Y-axis servomotors. These motors drive the respective slides to the commanded position. When the distance information from the feedback devices equals the programmed values, the slide movement stops. The input is then given to the Z-axis servomotor to perform the drilling operation. The system switches on the motor driving the spindle carrying the drill and commands the Z-axis servomotor to move at the programmed feedrate into the workpiece till the programmed depth is reached. The Zaxis feedback transducer ensures the correct depth and feedrate. The drill is then withdrawn to the desired height and the machine starts the positioning movements to the next location of the drill. This procedure thus ensures production of accurate workpieces.
The feedback transducer used may be analog (synchro resolver) or digital (encoders). Another classification of feedback transducer is based on their nature of measurement absolute or incremental. They are also classified as linear or rotary depending upon their construction. Linear feedback devices include optical scales, inductosyn etc. Rotary feedback devices are mounted either on the ball screw or on the motor shaft and measure the slide position indirectly. Rotary encoders, optical scales, and synchro resolvers are the commonly used rotary feed back devices.
The measured and the targeted positions are compared and the servo system ensures that the correct positioning is achieved to make this error (difference between targeted and measured positions) zero. The resolution of axis feed drive is usually one micrometre. Since positioning is done electronically, it is possible to achieve accuracy and repeatability of the order of 5-10 micrometres even under heavy-duty cutting conditions. Two servo loops are incorporated in feed drive – one for the position and the other for the feed. In addition to this, the selection of spindle speed is also under servo control.