What Is CNC

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What Is CNC? by Mike Lynch, CNC Concepts, Inc. 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
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  What Is CNC? by Mike Lynch, CNC Concepts, Inc. CNC stands for  C omputer  N umerical C ontrol and has been around since theearly 1970's. Prior to this, it was called NC, for  N umerical C ontrol. (In theearly 1970's computers were introduced to these controls, hence the namechange.)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.If you'll be working in manufacturing, it's likely that you'll be dealing with CNCon a regular basis. Before CNC While there are exceptions to this statement, CNC machines typicallyreplace (or work in conjunction with) some existing manufacturingprocess/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 almosteveryone has seen some form of drill press, even if you don't work inmanufacturing.) A person can place a drill in the drill chuck that is secured inthe spindle of the drill press. They can then (manually) select the desiredspeed for rotation (commonly by switching belt pulleys), and activate thespindle. Then they manually pull on the quill lever to drive the drill into theworkpiece being machined.As you can easily see, there is a lot of manual intervention required to use adrill press to drill holes. A person is required to do something almost everystep along the way! While this manual intervention may be acceptable for manufacturing companies if but a small number of holes or workpieces mustbe machined, as quantities grow, so does the likelihood for fatigue due to thetediousness of the operation. And do note that we've used one of thesimplest machining operations (drilling) for our example. There are morecomplicated machining operations that would require a much higher skilllevel (and increase the potential for mistakes resulting in scrap workpieces)of the person running the conventional machine tool. (We commonly refer tothe style of machine that CNC is replacing as the conventional  machine.)  By comparison, the CNC equivalent for a drill press (possibly a CNCmachining center or CNC drilling & tapping center) can be  programmed  toperform this operation in a much more automatic fashion. Everything that thedrill press operator was doing manually will now be done by the CNCmachine, 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 ingreater detail. Additionally, we offer a series of productsaimed at helpingyou learn how to use CNC machines. Here we're relating how CNC works invery general terms.As you might already have guessed, everything  that an operator would berequired to do with conventional machine tools is programmable with CNCmachines. Once the machine is setup and running, a CNC machine is quitesimple to 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. (Wedon't mean to over-simplify here. CNC operators are commonly required todo other things related to the CNC operation like measuring workpieces andmaking 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 moreprogrammable directions of motion called axes . An axis of motion can belinear (along a straight line) or rotary (along a circular path). One of the firstspecifications that implies a CNC machine's complexity is how many axes ithas. 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)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 areA, B, and C. Programmable accessories 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 programmablein several 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 examplesfor one machine type. Machining centersAutomatic tool changer  Most machining centers can hold many tools in a tool magazine. When required, the required tool can beautomatically 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 forwardor 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 programis nothing more than another kind of instruction set. It's written insentence-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 themachine is intended to do. CNC words begin with letter addresses (likeF for feedrate, S for spindle speed, and X, Y & Z for axis motion).When placed together in a logical method, a group of CNC wordsmake up a command  that resemble a sentence.For any given CNC machine type, there will only be about 40-50 wordsused on a regular basis. So if you compare learning to write CNC  programs to learning a foreign language having only 50 words, itshouldn't seem overly difficult to learn CNC programming. The CNC control  The CNC control will interpret a CNC program and activate the seriesof commands in sequential order. As it reads the program, the CNCcontrol will activate the appropriate machine functions, cause axismotion, and in general, follow the instructions given in the program.Along with interpreting the CNC program, the CNC control has severalother purposes. All current model CNC controls allow programs to bemodified (edited) if mistakes are found. The CNC control allowsspecial 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 lengthvalues. In general, the CNC control allows all functions of the machineto 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 theprogram armed only with pencil, paper, and calculator. Again, for simple applications, this may be the very best way to develop CNCprograms.As applications get more complicated, and especially when newprograms are required on a regular basis, writing programs manuallybecomes much more difficult. To simplify the programming process, acomputer aided manufacturing (CAM) system can be used. A CAMsystem is a software program that runs on a computer (commonly aPC) that helps the CNC programmer with the programming process.Generally speaking, a CAM system will take the tediousness anddrudgery out of programming.In many companies the CAM system will work with the computer aideddesign (CAD) drawing developed by the company's designengineering department. This eliminates the need for redefining theworkpiece configuration to the CAM system. The CNC programmer willsimply specify the machining operations to be performed and the CAMsystem 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 CAMsystem), it must be loaded into the CNC control. Though the setup
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