Numerical control/ What are NC Machines/ What is CNC
Numerical control (also computer numerical control, and commonly called CNC) is the automated control of machining tools (drills, boring tools, lathes) and 3D printers by means of a computer. A CNC machine processes a piece of material (metal, plastic, wood, ceramic, or composite) to meet specifications by following a coded programmed instruction and without a manual operator.
Numerical control, popularly known as the NC is very commonly used in the machine tools. Numerical control is defined as the form of programmable automation, in which the process is controlled by the number, letters, and symbols. In case of the machine tools this programmable automation is used for the operation of the machines. The digital control of a physical machine that consists of a series of integrated actuators, power electronics, sensors, and dedicated computer running under a real-time operating system.
Computer Numerical Control (CNC) Machining is the process through which computers control machine-based processes in manufacturing. The kinds of machines controlled include lathes, mills, routers and grinders – all used for manufacturing of metal and plastic products.
A CNC machine is a motorized maneuverable tool and often a motorized maneuverable platform, which are both controlled by a computer, according to specific input instructions. Instructions are delivered to a CNC machine in the form of a sequential program of machine control instructions such as G-code and then executed. The program can be written by a person or, far more often this century, generated by graphical computer-aided design (CAD) software. In the case of 3D Printers, the part to be printed is "sliced", before the instructions (or the program) is generated. 3D printers also use G-Code.
CNC is a vast improvement over non-computerized machining that must be manually controlled (e.g., using devices such as hand wheels or levers) or mechanically controlled by pre-fabricated pattern guides (cams). In modern CNC systems, the design of a mechanical part and its manufacturing program is highly automated. The part's mechanical dimensions are defined using CAD software, and then translated into manufacturing directives by computer-aided manufacturing (CAM) software. The resulting directives are transformed (by "post processor" software) into the specific commands necessary for a particular machine to produce the component, and then are loaded into the CNC machine.
The first NC machines were built in the 1940s and 1950s, based on existing tools that were modified with motors that moved the tool or part to follow points fed into the system on punched tape. These early servomechanisms were rapidly augmented with analog and digital computers, creating the modern CNC machine tools that have revolutionized machining processes.
Within the numerical systems of CNC programming it is possible for the code generator to assume that the controlled mechanism is always perfectly accurate, or that precision tolerances are identical for all cutting or movement directions. This is not always a true condition of CNC tools. CNC tools with a large amount of mechanical backlash can still be highly precise if the drive or cutting mechanism is only driven so as to apply cutting force from one direction, and all driving systems are pressed tightly together in that one cutting direction. However a CNC device with high backlash and a dull cutting tool can lead to cutter chatter and possible workpiece gouging. Backlash also affects precision of some operations involving axis movement reversals during cutting, such as the milling of a circle, where axis motion is sinusoidal. However, this can be compensated for if the amount of backlash is precisely known by linear encoders or manual measurement.
The high backlash mechanism itself is not necessarily relied on to be repeatedly precise for the cutting process, but some other reference object or precision surface may be used to zero the mechanism, by tightly applying pressure against the reference and setting that as the zero reference for all following CNC-encoded motions. This is similar to the manual machine tool method of clamping a micrometer onto a reference beam and adjusting the Vernier dial to zero using that object as the reference.
M-codes are miscellaneous machine commands that do not command axis motion. The format for an M-code is the letter M followed by two to three digits. G-codes are used to command specific movements of the machine, such as machine moves or drilling functions. The format for a G-code is the letter G followed by two to three digits; for example G01. G-codes differ slightly between a mill and lathe application.
Motion Control Systems
Point-to-Point systems (Also called position systems)
System moves to a location and performs an operation at that location (e.g., drilling)
Also applicable in robotics
Continuous path systems (Also called contouring systems in machining)
System performs an operation during movement (e.g., milling and turning)
NC Application Characteristics (Machining)
Batch and High Volume production
Repeat and/or Repetitive orders
Complex part geometries
Many separate operations on one part
N refers to the block number.
G refers to the G code (Preparatory function).
X refers to the absolute/incremental distance travelled by the slide tool in the X axis direction.
Y refers to the absolute/incremental distance travelled by the slide tool in the Y axis direction.
Z refers to the absolute/incremental distance travelled by the slide tool in the Z axis direction.
F refers to the feed rate.
M refers to the M code (Miscellaneous function).
S refers to the spindle speed.
T refers to the tooling management.
*Each block, or program line, contains addresses which appear in this order : N , G , X , Y , Z , F , M , S , T ; This order should be maintained throughout every block in the program, although individual blocks may not necessarily contain all these addresses.
Fitzpatrick, Michael (2019), "Machining and CNC Technology".