CNC stands for Computer Numerical Control and has been
around since the early 1970's. Prior to this, it was called NC, for
Numerical Control. (In the early 1970's computers were introduced
to these controls, hence the name change.)
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.
Before CNC
While there are exceptions to this statement, CNC machines typically replace
(or work in conjunction with) some existing manufacturing process/es. Take one
of the simplest manufacturing processes, drilling holes, for example.
A drill press can of course be used to machine holes. (It's likely
that almost everyone has seen some form of drill press, even if you don't work
in manufacturing.) 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.
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
There is another article included in this web site called The Basics of CNC that explains how to program,
setup, and operate CNC machines in greater detail. Additionally, we offer a
series of products aimed at helping
you learn how to use CNC machines. Here we're relating how CNC works in very
general terms.
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. (We don't mean
to over-simplify here. CNC operators are commonly required to do other things
related to the CNC operation like measuring workpieces and making adjustments
to keep the CNC machine running good workpieces.)
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 implies 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) axis 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.
Programmable accessories
A CNC machine wouldn't be very helpful if all it could only move the
workpiece in two or more axes. Almost all CNC machines are programmable in
several other ways. The specific CNC machine type has a lot to do with its
appropriate programmable accessories. Again, any required function will be
programmable on full-blown CNC machine tools. Here are some examples for one
machine type.
Machining centers
Automatic tool changer
Most machining centers can hold many tools in a tool magazine. When
required, the required tool can be automatically placed in the spindle for
machining.
Spindle speed and activation
The spindle speed (in revolutions per minute) can be easily specified and
the spindle can be turned on in a forward or reverse direction. It can also, of
course, be turned off.
Coolant
Many machining operations require coolant for lubrication and cooling
purposes. Coolant can be turned on and off from within the machine cycle.
The CNC program
Think of giving any series of step-by-step instructions. A CNC program is
nothing more than another kind of instruction set. It's written in
sentence-like format and the control will execute it in sequential order, step
by step.
A special series of CNC words are used to communicate what the
machine is intended to do. CNC words begin with letter addresses (like F for
feedrate, S for spindle speed, and X, Y & Z for axis motion). When placed
together in a logical method, a group of CNC words make up a command
that resemble a sentence.
For any given CNC machine type, there will only be about 40-50 words used on
a regular basis. So if you compare learning to write CNC programs to learning a
foreign language having only 50 words, it shouldn't seem overly difficult to
learn CNC programming.
The CNC control
The CNC control will interpret a CNC program and activate the series of
commands in sequential order. As it reads the program, the CNC control will
activate the appropriate machine functions, cause axis motion, and in general,
follow the instructions given in the program.
Along with interpreting the CNC program, the CNC control has several other
purposes. All current model CNC controls allow programs to be modified (edited)
if mistakes are found. The CNC control allows special verification functions
(like dry run) to confirm the correctness of the CNC program. The CNC control
allows certain important operator inputs to be specified separate from the
program, like tool length values. In general, the CNC control allows all
functions of the machine to be manipulated.
What is a CAM system?
For simple applications (like drilling holes), the CNC program can be
developed manually. That is, a programmer will sit down to write the program
armed only with pencil, paper, and calculator. Again, for simple applications,
this may be the very best way to develop CNC programs.
As applications get more complicated, and especially when new programs are
required on a regular basis, writing programs manually becomes much more
difficult. To simplify the programming process, a computer aided manufacturing
(CAM) system can be used. A CAM system is a software program that runs on a
computer (commonly a PC) that helps the CNC programmer with the programming
process. Generally speaking, a CAM system will take the tediousness and
drudgery out of programming.
In many companies the CAM system will work with the computer aided design
(CAD) drawing developed by the company's design engineering department. This
eliminates the need for redefining the workpiece configuration to the CAM
system. The CNC programmer will simply specify the machining operations to be
performed and the CAM system will create the CNC program (much like the manual
programmer would have written) automatically.
What is a DNC system?
Once the program is developed (either manually or with a CAM system), it
must be loaded into the CNC control. Though the setup person could type
the program right into the control, this would be like using the CNC machine as
a very expensive typewriter. If the CNC program is developed with the help of a
CAM system, then it is already in the form of a text file . If the program is
written manually, it can be typed into any computer using a common word
processor (though most companies use a special CNC text editor for this
purpose). Either way, the program is in the form of a text file that can be
transferred right into the CNC machine. A distributive numerical control (DNC)
system is used for this purpose.
A DNC system is nothing more than a computer that is networked with one or
more CNC machines. Until only recently, rather crude serial communications
protocol (RS-232c) had to be used for transferring programs. Newer controls
have more current communications capabilities and can be networked in more
conventional ways (Ethernet, etc.). Regardless of methods, the CNC program must
of course be loaded into the CNC machine before it can be run.
Types of CNC machines
As stated, CNC has touched almost every facet of manufacturing. Many
machining processes have been improved and enhanced through the use of CNC.
Let's look at some of the specific fields and place the emphasis on the
manufacturing processes enhanced by CNC machine usage.
In the metal removal industry:
Machining processes that have traditionally been done on conventional
machine tools that are possible (and in some cases improved) with CNC
machining centers include all kinds of milling (face milling, contour
milling, slot milling, etc.), drilling, tapping, reaming, boring, and
counter boring.
In similar fashion, all kinds of turning operations like facing, boring,
turning, grooving, knurling, and threading are done on CNC turning
centers.
There are all kinds of special "off-shoots" of these two machine
types including CNC milling machines, CNC drill and tap centers, and
CNC lathes.
Grinding operations of all kinds like outside diameter (OD) grinding and
internal diameter (ID) grinding are also being done on CNC grinders. CNC
has even opened up a new technology when it comes to grinding. Contour
grinding (grinding a contour in a similar fashion to turning), which was
previously infeasible due to technology constraints is now possible (almost
commonplace) with CNC grinders.
In the metal fabrication industry:
In manufacturing terms, fabrication commonly refers to operations
that are performed on relatively thin plates. Think of a metal filing cabinet.
All of the primary components are made of steel sheets. These sheets are
sheared to size, holes are punched in appropriate places, and the sheets are
bent (formed) to their final shapes. Again, operations commonly described as
fabrication operations include shearing, flame or plasma cutting, punching,
laser cutting, forming, and welding. Truly, CNC is heavily involved in almost
every facet of fabrication.
CNC back gages are commonly used with shearing machines to control the
length of the plate being sheared. CNC lasers and CNC plasma cutters are also
used to bring plates to their final shapes. CNC turret punch presses can hold a
variety of punch-and-die combinations and punch holes in all shapes and sizes
through plates. CNC press brakes are used to bend the plates into their final
shapes.
In the electrical discharge machining industry:
Electrical discharge machining (EDM) is the process of removing metal
through the use of electrical sparks which burn away the metal. CNC EDM comes
in two forms, vertical EDM and Wire EDM. Vertical EDM requires the use of an
electrode (commonly machined on a CNC machining center) that is of the shape of
the cavity to be machined into the workpiece. Picture the shape of a plastic
bottle that must be machined into a mold. Wire EDM is commonly used to make
punch and die combinations for dies sets used in the fabrication industry. EDM
is one of the lesser known CNC operations because it is so closely related to
making tooling used with other manufacturing processes.
In the woodworking industry
As in the metal removal industry, CNC machines are heavily used in
woodworking shops. Operations include routing (similar to milling) and
drilling. Many woodworking machining centers are available that can hold
several tools and perform several operations on the workpiece being machined.
Other types of CNC machines
Many forms of lettering and engraving systems use CNC technology. Waterjet
machining uses a high pressure water jet stream to cut through plates of
material. CNC is even used in the manufacturing of many electrical components.
For example, there are CNC coil winders, and CNC terminal location and
soldering machines.
Job opportunities related to CNC
There is quite a shortage of skilled people to utilize CNC machines. And the
shortage is growing. Everywhere I go I hear manufacturing people claiming that
they cannot find skilled people. Unfortunately, it has also been my experience
that pay scales have not yet reflected this shortage. Even so, you can make a
good wage and develop a rewarding career working with CNC machines. Here are
some of the job titles of people working with CNC machine tools.
