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Basic Component of NC Machine Tools

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Basic Component of NC Machine Tools --- Aheadmold

The control system of a numerically controlled machine tool can handle many tasks com-monly done by the operator of a conventional machine. For this, the numerical control system must “know” when and in what sequence it should issue commands to change tools, at what speeds and feeds the machine tool should operate, and how to work a part to the required size. The system gains the ability to perform the control functions through the numerical input information that is the control program, also called the part program.
The work process of NC is shown in Fig. 6-1. The part programmer should study the part drawing and the process chart and then prepare the control program on a standard form in the specified format. It contains all the necessary control information. A computer-assisted NC part program for NC machining method is also available, in which the computer considerably facilitates the work of the programmer and generate a set of NC instructions. Next the part program is transferred into the control computer the wide accepted method is that the worker types the part program into the computer from the keyboard of the computer numerical control front panel. The computer converts each command into the signal that the servo-drive unit needs. The servo-drive unit drives the machine tool to manufacture the finished part.
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Fig. 6-1   The work process of NC

A typical NC machine tool has five fundamental units. (1) the input media, (2) the machine control unit, (3) the servo-drive unit, (4) the feedback transducer, and (5) the  mechanical machine tool unit. The general relationship among the five components is illustrated in Fig. 6-2.
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Fig. 6-2   Basic components of a CNC machine tool

The input media contains the program of instructions, it is the detailed step-by-step com-mands that direct the actions of the machine tool; the program of instructions is called a part program. The individual commands refer to positions of a cutting tool relative to the worktable on which the workpart is fixtured. Additional instructions are usually included, such as spindle speed, feed rate, cutting tool selection, and other functions. The program is coded on a suitable medium for submission to the machine control unit. For many years, the common medium was 1-inch wide punched tape, using a standard format that could be interpreted by the machine control unit. Today, punched tape has largely been replaced by newer storage technologies in modern machine shops. These technologies include magnetic tape, diskette, and electronic transfer of part programs from a computer.
In modern CNC technology, the machine control unit (MCU) consists of a microcomputer and related control hardware that stores the program of instructions and executes it by converting each command into mechanical actions of machine tool, one command at a time. The MCU includes system software, calculation algorithm, and transition software to covert the NC parts program into a usable format for the MCU.
The third basic component of an NC system is the servo-drive unit; the drives in machine tools are classified as spindle and feed drive mechanisms. Spindle and feed drive motors and their servo-amplifiers are the components of the servo-drive unit. The MCU processes the data and generates discrete numerical position commands for each feed drive and velocity command for the spindle drive. The numerical commands are converted into signal voltage by the MUC unit and sent to servo-amplifiers, which process and amplify them to the high voltage levels required by the drive motors.
The forth basic component of an NC system is the feedback transducer. As the drives move, sensors measure their actual position. The difference between the required position and the actual position is detected by comparison circuit and the action is taken, within the servo, to minimize this difference.
The fifth basic component of an NC system is the machine tool that performs useful work. It accomplishes the processing steps to transform the starting workpiece into a completed part. Its operations are directed by the MCU, which in turn is driven by instructions contained in the part program. In the most common example of NC, machine tool consists of the worktable and spindle.

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