Computer Numerical Control: Difference between revisions

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Revision as of 14:39, 27 January 2009

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:'''''CNC''' redirects here. For other uses, see [[CNC (disambiguation)]].''

[[you are gay!are completed, the machine is given another command to cancel the cycle, and resumes operation.

[[Image:Small CNC Turning Center.jpg|thumb|A CNC Turning Center]]

[[Image:CNC panel Sinumerik.jpg|thumb|CNC panel [[Siemens]] Sinumerik]]

[[Image:CNC panel.jpg|thumb|upright|Siemens CNC panel]]

<!-- Unsourced image removed: [[Image:Nichols-ducati-part.jpg‎|thumb|CNC product|{{Typical CNC tool enclosure|1=Thursday, 14 February 2008}}]] -->

'''Computer numerical control''' ('''CNC''') is a computer "controller" that reads [[G-code]] and M-code commands and drives a [[machine tool]], a powered mechanical device typically used to fabricate components by the selective removal of material. CNC numerically interpolates the points along a cutting tool's toolpath and directs the [[servomechanism]]s that translate the data into movement. The operating parameters of the CNC can be altered via the Master Control Unit (MCU) of the machine.

==Historical overview==

CNC was preceded by [[numerical control|numerically controlled]] (NC) machines, which were hard-wired and their operating parameters could not be changed. NC was developed in the late 1940s and early 1950s by John T. Parsons in collaboration with the [[MIT]] Servomechanisms Laboratory. The first CNC systems used NC style hardware, and the computer was used for the tool compensation calculations and sometimes for editing.

Punched tape continued to be used as a medium for transferring G-codes into the controller for many decades after 1950, until it was eventually superseded by [[RS232|RS232 cables]], floppy disks, and now is commonly tied directly into [[Computer network|plant networks]]. The files containing the G-codes to be interpreted by the controller are usually saved under the .NC extension. Most shops have their own saving format that matches their [[International Organization for Standardization|ISO]] certification requirements.

The introduction of CNC machines radically changed the manufacturing industry. Curves are as easy to cut as straight lines, complex 3-D structures are relatively easy to produce, and the number of machining steps that required human action have been dramatically reduced.

With the increased automation of manufacturing processes with CNC machining, considerable improvements in consistency and quality have been achieved with no strain on the operator. CNC automation reduced the frequency of errors and provided CNC operators with time to perform additional tasks. CNC automation also allows for more flexibility in the way parts are held in the manufacturing process and the time required to change the machine to produce different components.

== Production environment ==

A series of CNC machines may be combined into one station, commonly called a "cell", to progressively machine a part requiring several operations. CNC machines today are controlled directly from files created by [[Computer-aided manufacturing|CAM]] software packages, so that a part or assembly can go directly from design to manufacturing without the need of producing a drafted paper drawing of the manufactured component. In a sense, the CNC machines represent a special segment of industrial robot systems, as they are programmable to perform many kinds of machining operations (within their designed physical limits, like other robotic systems). CNC machines can run over night and over weekends without operator intervention. Error detection features have been developed, giving CNC machines the ability to call the operator's mobile phone if it detects that a tool has broken. While the machine is awaiting replacement on the tool, it would run other parts it is already loaded with up to that tool and wait for the operator. The ever changing intelligence of CNC controllers has dramatically increased job shop cell production. Some machines might even make 1000 parts on a weekend with no operator, checking each part with lasers and sensors.

== Types of instruction ==

[[G-code]]s are known as preparatory and movement codes. Depending on the number directly following the G, they can perform an extremely wide range of commands. The G00 code for example, stands for Rapid Traverse, which moves the machine to a given point at top speed. In the industry it is known as the WFO command. The G01 command is the most common command, in which it tells the machine to move in linear interpolation to a given point. The G02/G03 (clockwise/counter clockwise) command is circular interpolation allowing the machine to make near perfect circles with one move.

The M-codes are the Misc. functions of the machine. They do things like turning the coolant on and off, activating drill cycles and starting chip conveyors. They complement the G-codes, for example the M03/M04 commands turns the [[Spindle (tool)|spindle]] on/off. The command M30 marks the end of a program.

===Movements===

Lately, some controllers have implemented the ability to follow an arbitrary curve [[Nonuniform rational B-spline|(NURBS)]], but these efforts have been met with skepticism since, unlike circular arcs, their definitions are not natural and are too complicated to set up by hand, and CAM software can already generate any motion using many short linear segments.

=== Drilling ===

A tool can be used to drill holes by pecking to let the [[swarf]] out. Using an internal [[screw thread|thread]] cutting tool and the ability to control the exact rotational position of the tool with the depth of cut, it can be used to cut screw threads.

A drilling cycle is used to repeat drilling or tapping operations on a workpiece. The drilling cycle accepts a list of parameters about the operation, such as depth and feed rate. To begin drilling any number of holes to the specifications configured in the cycle, the only input required is a set of coordinates for hole ___location. The cycle takes care of depth, feed rate, retraction, and other parameters that appear in more complex cycles. After the holes are completed, the machine is given another command to cancel the cycle, and resumes operation.

=== Parametric programming ===