training materials of CNC
What Is CNC?
by Mike Lynch, CNC Concepts,Inc.
CNC stands for Computer Numerical Control and has beenaround since the early 1970's. Prior to this, it was called NC, forNumerical Control. (In the early 1970's computers were introducedto these controls, hence the name change.)
While people in most walks of life have never heard of this term, CNC hastouched almost every form of manufacturing process in one way or another. Ifyou'll be working in manufacturing, it's likely that you'll be dealing with CNCon 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 oneof the simplest manufacturing processes, drilling holes, for example.
A drill press can of course be used to machine holes. (It's likelythat almost everyone has seen some form of drill press, even if you don't workin manufacturing.) A person can place a drill in the drill chuck that issecured in the spindle of the drill press. They can then (manually) select thedesired speed for rotation (commonly by switching belt pulleys), and activatethe spindle. Then they manually pull on the quill lever to drive the drill intothe workpiece being machined.
As you can easily see, there is a lot of manual intervention required to usea drill press to drill holes. A person is required to do something almost everystep along the way! While this manual intervention may be acceptable formanufacturing companies if but a small number of holes or workpieces must bemachined, as quantities grow, so does the likelihood for fatigue due to thetediousness of the operation. And do note that we've used one of the simplestmachining operations (drilling) for our example. There are more complicatedmachining operations that would require a much higher skill level (and increasethe potential for mistakes resulting in scrap workpieces) of the person runningthe conventional machine tool. (We commonly refer to the style of machine thatCNC 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 beprogrammed to perform this operation in a much more automatic fashion.Everything that the drill press operator was doing manually will now be done bythe CNC machine, including: placing the drill in the spindle, activating thespindle, positioning the workpiece under the drill, machining the hole, andturning 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 aseries of products aimed at helpingyou learn how to use CNC machines. Here we're relating how CNC works in verygeneral terms.
As you might already have guessed, everything that an operator wouldbe required to do with conventional machine tools is programmable with CNCmachines. Once the machine is setup and running, a CNC machine is quite simpleto keep running. In fact CNC operators tend to get quite bored duringlengthy production runs because there is so little to do. With some CNCmachines, even the workpiece loading process has been automated. (We don't meanto over-simplify here. CNC operators are commonly required to do other thingsrelated to the CNC operation like measuring workpieces and making adjustmentsto 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 moreprogrammable directions of motion called axes. An axis of motion can belinear (along a straight line) or rotary (along a circular path). One of thefirst specifications that implies a CNC machine's complexity is how many axesit has. Generally speaking, the more axes, the more complex the machine.
The axes of any CNC machine are required for the purpose of causing themotions 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) andmachine the hole (with the third axis). Axes are named with letters. Commonlinear 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 theworkpiece in two or more axes. Almost all CNC machines are programmable inseveral other ways. The specific CNC machine type has a lot to do with itsappropriate programmable accessories. Again, any required function will beprogrammable on full-blown CNC machine tools. Here are some examples for onemachine type.
- Automatic tool changer
- Most machining centers can hold many tools in a tool magazine. Whenrequired, the required tool can be automatically placed in the spindle formachining.
- Spindle speed and activation
- The spindle speed (in revolutions per minute) can be easily specified andthe spindle can be turned on in a forward or reverse direction. It can also, ofcourse, be turned off.
- Many machining operations require coolant for lubrication and coolingpurposes. 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 isnothing more than another kind of instruction set. It's written insentence-like format and the control will execute it in sequential order, stepby step.
A special series of CNC words are used to communicate what themachine is intended to do. CNC words begin with letter addresses (like F forfeedrate, S for spindle speed, and X, Y & Z for axis motion). When placedtogether 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 ona regular basis. So if you compare learning to write CNC programs to learning aforeign language having only 50 words, it shouldn't seem overly difficult tolearn CNC programming.
The CNC control
The CNC control will interpret a CNC program and activate the series ofcommands in sequential order. As it reads the program, the CNC control willactivate 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 otherpurposes. 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 controlallows certain important operator inputs to be specified separate from theprogram, like tool length values. In general, the CNC control allows allfunctions of the machine to be manipulated.
What is a CAM system?
For simple applications (like drilling holes), the CNC program can bedeveloped manually. That is, a programmer will sit down to write the programarmed 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 arerequired on a regular basis, writing programs manually becomes much moredifficult. To simplify the programming process, a computer aided manufacturing(CAM) system can be used. A CAM system is a software program that runs on acomputer (commonly a PC) that helps the CNC programmer with the programmingprocess. Generally speaking, a CAM system will take the tediousness anddrudgery 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. Thiseliminates the need for redefining the workpiece configuration to the CAMsystem. The CNC programmer will simply specify the machining operations to beperformed and the CAM system will create the CNC program (much like the manualprogrammer would have written) automatically.
What is a DNC system?
Once the program is developed (either manually or with a CAM system), itmust be loaded into the CNC control. Though the setup person could typethe program right into the control, this would be like using the CNC machine asa very expensive typewriter. If the CNC program is developed with the help of aCAM system, then it is already in the form of a text file . If the program iswritten manually, it can be typed into any computer using a common wordprocessor (though most companies use a special CNC text editor for thispurpose). Either way, the program is in the form of a text file that can betransferred 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 ormore CNC machines. Until only recently, rather crude serial communicationsprotocol (RS-232c) had to be used for transferring programs. Newer controlshave more current communications capabilities and can be networked in moreconventional ways (Ethernet, etc.). Regardless of methods, the CNC program mustof 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. Manymachining 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 themanufacturing processes enhanced by CNC machine usage.
In the metal removal industry:
Machining processes that have traditionally been done on conventionalmachine tools that are possible (and in some cases improved) with CNCmachining centers include all kinds of milling (face milling, contourmilling, slot milling, etc.), drilling, tapping, reaming, boring, andcounter boring.
In similar fashion, all kinds of turning operations like facing, boring,turning, grooving, knurling, and threading are done on CNC turningcenters.
There are all kinds of special "off-shoots" of these two machinetypes including CNC milling machines, CNC drill and tap centers, andCNC lathes.
Grinding operations of all kinds like outside diameter (OD) grinding andinternal diameter (ID) grinding are also being done on CNC grinders. CNChas even opened up a new technology when it comes to grinding. Contourgrinding (grinding a contour in a similar fashion to turning), which waspreviously infeasible due to technology constraints is now possible (almostcommonplace) with CNC grinders.
In the metal fabrication industry:
In manufacturing terms, fabrication commonly refers to operationsthat are performed on relatively thin plates. Think of a metal filing cabinet.All of the primary components are made of steel sheets. These sheets aresheared to size, holes are punched in appropriate places, and the sheets arebent (formed) to their final shapes. Again, operations commonly described asfabrication operations include shearing, flame or plasma cutting, punching,laser cutting, forming, and welding. Truly, CNC is heavily involved in almostevery facet of fabrication.
CNC back gages are commonly used with shearing machines to control thelength of the plate being sheared. CNC lasers and CNC plasma cutters are alsoused to bring plates to their final shapes. CNC turret punch presses can hold avariety of punch-and-die combinations and punch holes in all shapes and sizesthrough plates. CNC press brakes are used to bend the plates into their finalshapes.
In the electrical discharge machining industry:
Electrical discharge machining (EDM) is the process of removing metalthrough the use of electrical sparks which burn away the metal. CNC EDM comesin two forms, vertical EDM and Wire EDM. Vertical EDM requires the use of anelectrode (commonly machined on a CNC machining center) that is of the shape ofthe cavity to be machined into the workpiece. Picture the shape of a plasticbottle that must be machined into a mold. Wire EDM is commonly used to makepunch and die combinations for dies sets used in the fabrication industry. EDMis one of the lesser known CNC operations because it is so closely related tomaking tooling used with other manufacturing processes.
In the woodworking industry
As in the metal removal industry, CNC machines are heavily used inwoodworking shops. Operations include routing (similar to milling) anddrilling. Many woodworking machining centers are available that can holdseveral 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. Waterjetmachining uses a high pressure water jet stream to cut through plates ofmaterial. CNC is even used in the manufacturing of many electrical components.For example, there are CNC coil winders, and CNC terminal location andsoldering machines.
Job opportunities related to CNC
There is quite a shortage of skilled people to utilize CNC machines. And theshortage is growing. Everywhere I go I hear manufacturing people claiming thatthey cannot find skilled people. Unfortunately, it has also been my experiencethat pay scales have not yet reflected this shortage. Even so, you can make agood wage and develop a rewarding career working with CNC machines. Here aresome 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