ABSTRACT:CNC stands for Computerized Numerical Control and has been
around since the early 1970s. Prior to this,  it was called NC, for numerical control.
While people in most walks of life have never heard of this term, CNC has touched
almost every form of manufacturing process in one way or another. If you'll be
working in manufacturing, it's likely that you'll be dealing with CNC on a regular
basis'.6381
KEY WORDS: CNC, Computer integrated manufacturing, CAM system
CNC machines typically replace (or work in conjunction with) some existing
manufacturing processes. Take one of the simplest manufacturing processes, drilling
holes, for example.   
A drill press can of course be used to machine holes. A person can place a drill
in the drill chuck that is secured in the spindle of the drill press. They can then
(manually) select the desired speed for rotation (commonly by switching belt pulleys),
and activate the spindle. Then they manually pull on the quill lever to drive the drill
into the workpiece being machined.
  As you can easily see, there is a lot of manual intervention required to use a
drill press to drill holes. A person is required to do something almost every step along
the way! While this manual intervention may be acceptable for manufacturing
companies if but a small number of holes or workpieces must be machined, as
quantities grow, so does the likelihood for fatigue due to the tediousness of the
operation. And do note that we've used one of the simplest machining operations
(drilling) for our example. There are more complicated machining operations that
would require a much higher skill level (and increase the potential for mistakes
resulting in scrap workpieces) of the person running the conventional machine tool.
(We commonly refer to the style of machine that CNC is replacing as the conventional
machine.)
By comparison, the CNC equivalent for a drill press (possibly a CNC
machining center or CNC drilling & tapping center) can be programmed to perform
this operation in a much more automatic fashion. See fig. 1. Everything that the drill
press operator was doing manually will now be done by the CNC machine, including:
placing the drill in the spindle, activating the spindle, positioning the workpiece under
the drill, machining the hole, and turning off the spindle.
 How CNC works
As you might already have guessed, everything that an operator would be
required to do with conventional machine tools is programmable with CNC machines.
Once the machine is setup and running, a CNC machine is quite simple to keep
running. In fact CNC operators tend to get quite bored during lengthy production runs
because there is so little to do. With some CNC machines, even the workpiece loading
process has been automated. Let's look at some of the specific programmable
functions.
Motion control
All CNC machine types share this commonality: They all have two or more
programmable directions of motion called axes. An axis of motion can be linear
(along a straight line) or rotary (along a circular path). One of the first specifications
that imply a CNC machine's complexity is how many axes it has. Generally speaking,
the more axes, the more complex the machine. The axes of any CNC machine are required for the purpose of causing the
motions needed for the manufacturing process. In the drilling example, these (3) axes
would position the tool over the hole to be machined (in two axes) and machine the hole (with the third axis). Axes are named with letters. Common linear axis names are
X, Y, and Z. Common rotary axis names are A, B, and C. These are related to  the
coordinate system. See fig.2.
Programmable accessories
A CNC machine wouldn't be very helpful if all it could only move the 计算机数字控制机床英文文献和中文翻译:http://www.751com.cn/fanyi/lunwen_3907.html