ELEMENT OF CNC
A CNC system consists of three basic components (see Figure 13.6 in the text):
- Computer for storing the part programs.
- Machine Control Unit (MCU) for processing programs.
- Machine Tool with Motors for physically executing the program.
Computer Part Program
The part program is a detailed set of commands to be followed by the machine tool. Each command specifies a position in the Cartesian coordinate system (X,Y,Z) or motion (workpiece travel or cutting tool travel), machining parameters and on/off function. Part programmers should be well versed with machine tools, machining processes, effects of process variables, and limitations of CNC controls. The part program is written for manually or p using computer-assisted language such as APT (Automated Programming Tool).
Machine Control Unit
The machine control unit (MCU) is a microcomputer that stores the program and executes the commands into actions by the machine tool. The MCU consists of two main units: data processing unit (DPU) and the control loops unit (CLU). The DPU software includes control system software, calculation algorithms, translation software that converts the part program into a usable format for the MCU, interpolation algorithm to achieve smooth motion oh the cutter, and editing of part program (in case of errors and changes). The CLU consists of the circuits for position and velocity control loops, deceleration and backlash take up, and function controls such as spindle on/ off.
The DPU processes the data from the part program and provides it to the CLU, which operates the drives, attached to the machine leadscrews and receives feedback signals on the actual position and velocity of each one of the axes. A driver (DC motor) and a feedback device are attached to the leadscrew.
Machine Tool
The machine tool could be one of the following: lathe, milling machine, laser, plasma coordinate measuring machine, etc. A right hand coordinate system is used to describe the motions of a machine tool.There are three linear axes (X,Y,Z) and three rotational axes (U,V,W)
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