What Is CNC?
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.
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 theconventional 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 CNCthat 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.
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.
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.
- 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.
- 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 commandthat 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.
Working for manufacturing companies:
CNC tool setters
CNC setup people
CAM system programmers
CNC maintenance personnel
Working for companies that sell CNC machines
CNC service technicians
CNC applications engineers
Working for schools
The Different Types of CNC Machines
When CNC was first invented it was a technology that was adapted to fit existing machines. Today CNC technology is still being retrofitted to various machine tools but there are also many machines which are created for the sole purpose of being CNC machines.
Machines that are Retrofitted
Milling machines are often retrofitted with CNC technology. This process involves removing all the mechanisms built into the machine to make it easy for a human to operate, such as: hand wheels and DRO (Digital Read Out) electronics.
The machine will usually have its old lead screws replaced with new very high accuracy ball screws and various new mounts built for mounting the actuators to the machine.
Just like the milling machine, lathes are also commonly retrofitted with CNC technology in the exact same way.
Machines that are Custom Built for CNC Operation
CNC Routers are a very common piece of machinery you will see a lot when learning about CNC. These are machines built exclusively to be operated by CNC technology and have no human interface other than through the computer.
Routers are generally for producing larger work and more commonly built with the idea of cutting wood, plastics and sheet metal in mind. Router also are most commonly found in a 3 axis setup (X, Y and Z). This set up will allow cutting of basic profiles and 3 dimensional relief machining. There are also CNC router which are 4, 5 or even 6 axis, these machines are more suited towards cutting more complex shapes or prototype models.
There are many milling machines today which were built specifically for CNC as opposed to being retrofitted at a later stage. Some of these machines can be absolutely massive and have built in tool changers, auto-feed mechanisms for loading in material and various electrical sensors for safe monitored cutting.
CNC Plasma Cutter
CNC plasma cutters are very similar to CNC routers in size and setup, however plasma cutters don’t require as much of a powerful set up because as opposed to dragging around a spinning tool in material they fly above the table with a plasma torch.
Plasma cutters are made for cutting 2 dimensional profile shapes into sheet metal.
CNC Laser Cutter
CNC laser cutters follow the same principle as the plasma cutter. However laser cutters use a much less destructive force than a plasma torch – A laser. Laser cutters are often good for cutting wood, plastic and metal. Each will need a different strength of laser suited for the material.
A 3D printer or 3 Dimensional printer uses a similar set up as a CNC router or laser cutter, except it uses a plastic extruder. This plastic extruder pushes out hot plastic through a tiny hole and slowly, layer by layer deposits enough plastic to build up a completed part.
Pick and Place Machine
A pick and place machine again uses a similar set up as a CNC router or laser cutter. This time there a multiple small nozzles, that pick up electrical components and then move into a desired location and place that electrical component down. Hence the name pick and place!
Pick and place machines move very quickly and are used to place the many hundreds or thousands of electrical components that make up devices such as computer motherboards, phones / tablets, and pretty much everything else that has a printed circuit board.