A Computer Numerical Control (CNC) is a computer controlled machine used to cut or route material, usually plastic, wood or aluminium. It can work to very fine tolerances (+/- 0.01mm) and the final product can be reproduced. It is usually programmed via Computer Aided Design (CAD) drawings. They are usually mills, drills or lathes.
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How does a CNC work?
To answer this question we will consider a complex mill. These can effectively cut a 3-D object out of a block of material. To do this the cutting head can be controlled up and down, backwards and forwards and left to right. The cutting head can also be rotated vertically and horizontally. As the CAD drawing is a 3-D representation of the work in progress, by varying the signals to each of the motors we can accurately position the cutting tool, the angle at which the tool approaches the work, the rotational cutting speed and the translational speed of the tool. The cutting tool can also be changed, thereby increasing the versatility of the machine.
CNC machines use motors in order to be able to operate, using servo motors and stepper motors. Stepper motors are usually used to control the positioning of the tool while the servo motors are used to control the rotation of the cutting head.
Servo motors are high torque, high speed devices with built in feedback devices capable of fine control and calibration. As the design has magnets rotating in a magnetic field there is no requirement for brushes or commutators to maintain electrical connection, which wear with use, thus the only moving part subject to wear and tear is the bearings. Larger 3 phase models are capable of delivering more torque than a stepper motor and are ideal for applications requiring continuous rotation.
A stepper motor moves the shaft by a predetermined angle in response to an electronic pulse. This is typically about 1.8 degrees. They are also capable of being micro stepped allowing for very fine adjustment (0.007 degrees per step). While the speed of rotation can be controlled by the frequency of pulses sent, this is means the motor does not rotate smoothly and at low speeds this may be visible to the naked eye. Due to fact the rotor moves by a known amount for each impulse received a feedback device is not required. As no feedback device is required they can be physically smaller than servo motors.
Why use a CNC?
While most tasks a CNC performs can be done manually, a CNC is quicker and more reliable. They reduce the rejection rate and increase productivity. The accuracy is often limited by the cutting tool - every time it is used it wears slightly, therefore requiring recalibrating or replacement.
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