13-04-2013, 02:24 PM
Computer Numerical Control (CNC)
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Definition
Computer Numerical Control (CNC) is one in which the functions and motions of a machine
tool are controlled by means of a prepared program containing coded alphanumeric data.
CNC can control the motions of the workpiece or tool, the input parameters such as feed,
depth of cut, speed, and the functions such as turning spindle on/off, turning coolant on/off.
[b]Applications[/b]
The applications of CNC include both for machine tool as well as non-machine tool areas.
In the machine tool category, CNC is widely used for lathe, drill press, milling machine,
grinding unit, laser, sheet-metal press working machine, tube bending machine etc. Highly
automated machine tools such as turning center and machining center which change the
cutting tools automatically under CNC control have been developed. In the non-machine tool
category, CNC applications include welding machines (arc and resistance), coordinate
measuring machine, electronic assembly, tape laying and filament winding machines for
composites etc.
Part Program
1 . Part program
2 . Machine Control Unit (MCU)
3 . Machine tool (lathe, drill press, milling machine etc)
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 manually or by using computerassisted
language such as APT (Automated Programming Tool).
Machine Control Unit
MACHINE TOOL
Figure 2. A typical numerical control system for a milling machine
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: the 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 of the
cutter, editing of part program (in case of errors and changes). 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.
The CLU consists of the circuits for position and velocity control loops, deceleration and
backlash take up, function controls such as spindle on/off.
Continuous Path Systems (Straight cut and Contouring systems)
These systems provide continuous path such that the tool can perform while the axes are
moving, enabling the system to generate angular surfaces, two-dimensional curves, or threedimensional
contours . Example is a milling machine where such tasks are accomplished
(Figure 5). Each axis might move continuously at a different velocity. Velocity error is
significant in affecting the positions of the cutter (Figure 5). It is much more important in
circular contour cutting where one axis follows sine function while the other follows cosine
function. Figure 6 illustrates point-to-point and continuous path for various machines.
Open Loop Control Systems
The open-loop control means that there is no feedback and uses stepping motors for driving
the leadscrew . A stepping motor is a device whose output shaft rotates through a fixed angle
in response to an input pulse (Figure 9) . The accuracy of the system depends on the motor's
ability to step through the exact number. The frequency of the stepping motor depends on the
load torque. The higher the load torque, lower would be the frequency . Excessive load torque
may occur in motors due to the cutting forces in machine tools. Hence this system is more
suitable for cases where the tool force does not exist (Example: laser cutting) .