What is CNC?
The CNC process, computer numerical control, is a method of manufacturing where programmed software directs the operation of factory tools and machinery. It is designed to manage a wide range of complex machines from grinders and lathes to mills and routers. The purpose of CNC is to overcome the limitations associated with manual operation of machinery where buttons, levers, controls, and wheels have to be hand activated. In concise terms, CNC is three dimensional cutting completed by a single set of programmed instructions.
CNC Machines operate using a numerical control system where a software program controls production. The language for the process is written in G code and controls the multiple operations of a machine. CNC allows an operator to pre-program the performance of a machine to complete repetitive, predictable functions. The accuracy and efficiency of CNC has made it a popular manufacturing method primarily in the metal fabricating and plastic production industries.
The programming of CNC begins with a CAD rendering that is translated into a computer language for the CNC system. To determine the effectiveness of the programming, the machine is put through a trial run to detect any flaws or bugs. Once any errors have been eliminated, the program is fed into the machine and it begins production.
Open loop and closed loop machine systems
The manipulation of a material on a CNC Machine is determined by either an open looped or closed loop system. An open loop system runs in one direction until the completion of the process while a closed loop system has the ability to accept feedback and make corrections of any possible errors, which could be speed of the feed or positioning of the material.
An open loop control system is simple whose accuracy depends on how it is calibrated. Material is fed into the system and a completed part is returned. There aren't any in-process corrections, but the calibration has to be meticulously and carefully completed to avoid human errors. A good example of an open loop system is a washing machine where the clothes are put in the machine and removed when fully washed. The machine does not stop to correct itself or determine if the clothes are sufficiently clean.
A closed loop system is dependent on the controller and is more complex and intricate than an open loop system. An open loop system has a controller process while a closed loop system contains an amplifier, controller, and receives feedback during the process. This can be seen in a heating system in a house where the heater activates when a set temperature is not present and deactivates or adjusts when there is enough heat. Since there is constant monitoring of the process, a closed loop system operates slower and carefully but is easy to calibrate since the system takes most of the responsibility.
CNC equipment operates by referencing multiple axis points to remove material from the workpiece. The number of axis can vary and includes 3, 4, or 5 axes. With the basic three axes points, the material remains in the same position while being worked by the machine, which moves along an XYZ plane. The four axes points operate exactly like the three axes method with the addition of a fourth operation that may include cutting a hole or making a special cut. The five axes points method is more complicated than either of the other two. It manipulates five sides of the material and is used on highly technical complex components.
The CNC manufacturing process
The CNC process is different from the traditional additive processes since it is a subtractive process where layers of material are removed to create custom shapes. Additive processes add layers. CNC is fully automated manufacturing with high reliability and precision machining.
The key to the success of CNC manufacturing is the initial programming. The software must be coded with the proper instructions keeping the machine within the limitations of its parameters. The thinking of CNC equipment is derived from the person who creates its instructions in CAD, computer aided design, a basic engineering software. It is important that care be given to the development of the programmed instructions to avoid errors and loss of production time.
An approved CAD design is sent to a CNC compatible file format such as STEP or IGES. A STEP file, standard for the exchange of product data, contains three dimensional data capable of being read by multiple programs. It is designed in the way that PDF files are created to save and share documents from different formats. IGES, initial graphics exchange specifications, is similar to STEP and used to share CAD files in the form of circuit diagrams, wireframes, or solid modeling. Either form is acceptable to CNC processing.
CNC uses many different programming languages, which include G-code and M-Code. The preferred language for CNC is general or geometric code, G-code, which controls machine movements such as speed, on and off switches, and paths. M-code, miscellaneous code, controls functions at the start and end of the CNC process.
Before the machine can be activated to read the CNC program, it has to be set up and prepared. This includes inserting the workpiece into the devices that will hold it in place during the completion of the operation and adding the necessary tools such as lathes, plasma cutters, or water jet cutters. Each of the tools has to be carefully installed and accurately aligned.
Much like verbal instructions given to a person, a CNC program gives direction to the machine through a set of commands that determine tool actions and movements. Once the program is initiated, the production process begins, guided step by step by the programming. At the completion of the process, a custom designed part or product is produced.
Types of CNC machines and CNC operations
The CNC process uses several types of machining processes from mechanical and electrical to chemical and thermal. Each type is designed to remove material from the workpiece. Below are common machine operations.
In CNC, lathes are used to produce complex designs that are impossible to produce manually. The limitation of a lathe is that it operates on a X and Y axis.
Plasma cutters cut with a plasma torch. To produce the speed and heat necessary to cut metal, plasma is generated through a combination of compressed-air gas and electrical arcs.
Electric Discharge Machines
Electric-discharge machining (EDM), die sinking or spark machining, molds workpieces into shapes using electrical sparks. The EDM discharges current between two electrodes, which removes sections of the workpiece. As the space between electrodes gets small, the electric field becomes more intense and stronger removing portions of the workpiece by electrodes.
Water Jet Cutters
Water jets cut hard materials, like granite and metal, using high-pressure water. In some cases, the water is mixed with sand or other abrasives. Factory machine parts are shaped by this process. Water jets are a cooler alternative for materials unable to bear heat-intensive processes. They are also useful for applications requiring intricate cuts since the lack of heat prevents changes to the materials properties.
Drilling employs multi-point drill bits to create cylindrical holes. The CNC machine feeds the drill bit perpendicularly into the plane of the workpiece producing vertically-aligned holes. Angular drilling operations are performed using specialized machine configurations using work holding devices. Capabilities of the drilling process include counterboring, countersinking, reaming, and tapping.
Milling uses rotating multi-point cutting tools. The CNC machine feeds the workpiece to the cutting tool in the same direction as the cutting tool’s rotation. The milling process includes face milling, cutting of shallow, flat surfaces and flat-bottomed cavities into the workpiece while peripheral milling cuts deep cavities, such as slots and threads.
Turning is a single-point cutting tool. The cutting tool is fed in a linear motion along the surface of the rotating workpiece, removing material around the circumference until the desired diameter is achieved producing cylindrical parts with external and internal features. The turning process is used for boring, facing, grooving, and thread cutting.
Types of CNC software
Though CAD is one of the central formats for programming CNC equipment, it is not the only type of software available. As any engineer will explain, there are a variety of software programs that can render three dimensional images to be fed into CNC equipment. The choice of software takes several directions and depends on the preference of the engineer.
Computer-aided design (CAD) software drafts 2D vector or 3D solid parts and surface renderings with technical documentation and specifications for the parts. The designs and models for CAD are used by CAM to create the program to produce the part in a CNC machine. CAD software can be used to determine and define part properties, evaluate and verify part designs, simulate products without the need for a prototype, and provide design data.
Computer-aided manufacturing (CAM) software extracts the technical information from CAD and generates the machine program necessary to run the CNC machine to manipulate the tooling. CAM software enables the CNC machine to complete its function without operator assistance and helps automate finished product evaluation.
Computer-aided engineering (CAE) software is used by engineers during the pre-processing, analysis, and post-processing phases of process development. CAE software is a supportive tool for engineering analysis applications, such as design, simulation, planning, manufacturing, diagnosis, and repair, to help to evaluate and modify product design. Types of CAE software include finite element analysis (FEA), computational fluid dynamics (CFD), and multibody dynamics (MDB) software.
The CAD/CAM/CAE software combines all three software platforms into one easy to access format. This combined form of integrated platforms is a single software capable of managing the fabrication process from design to analysis to production.
What is a CNC machine?
A CNC machine is a numerically controlled automated machine that uses a set of tools to create a custom precision part by the process of removing access materials and shaping. A three dimensional image programmed by CAD software is downloaded into the computer of the CNC machine. A workpiece made of metal, plastic, wood, ceramic, or a composite is placed in the CNC machine to be processed using coded programmed instructions. Aside from the initial set up of the device, a manual operator is not required.
The program for a CNC machine is created by a person using CAD software, which develops a set of input instructions, which are delivered using either general or miscellaneous coding. The process of 3D printing has become a popular method for creating a rendering for CNC machines. Unlike CAD, a 3D printer part has to be sliced before the instructions can be input and uses G-Code. 3D printers are very useful with soft materials, but parts have to be finished once they are removed from the CNC machine unlike CAD parts, which are already finished.
CNC machines are a totally automated process that does not require any manual handling of materials during production. The dimensions and specifications for a part are predetermined by CAD software, translated in directives by CAM software, and sent to a STEP or IGES file so that it is compatible for the CNC machine.
The majority of CNC machines require several tools to make the appropriate cuts. Normally, the machine moves along the X and Y axes. This type of machine is a single cell since the workpiece does not change direction and is not turned. When several tools are required, they are combined, and the part moves along the X and Y axes as well as the Z axis. In this case, the machine is multiple cells able to change the direction of the workpiece and turn it. Multi axes machines can flip and turn a part automatically. The fully automated cuts are more precise and accurate. More complex parts may require extensive programming in order to include all of the necessary elements.
CNC machining is essential for the production of computer parts and fasteners as well as auto parts and aerospace components. The advanced technology of CNC processing has enhanced the production process and enabled it to produce sophisticated household and manufacturing products. The complex method of developing the code for a CNC machine is challenging and requires intelligent manipulation of multiple variables.
The beauty of CNC machines is the flexibility to design, program, and produce parts that exactly fit the specific requirements of a job down to the smallest detail. In many ways, there are few limitations to the capabilities of a CNC machine, though it does have restrictions. Complicated cuts at multiple angles can be performed in minutes. All that is required is planning and proper programming. Once a CNC machine has processed the information and machined the part, a finished product is produced.
The final word about CNC
The CNC process has been a perfect match for manufacturing in the 21st Century. It has combined the components of proven machine tools and the elements of the computer age to produce a device that can create flawless products. In the last fifty years, CNC machines have taken giant leaps forward and become an essential part of several industries. It is very likely this technology will continue to advance as it integrates AGV material handling processes.
There are many advantages to CNC Machining. It is more precise and repeats each operation in the same manner. Processes that were considered to be impossible and beyond the capability of machines are easily being completed by CNC machines. It can take any project and turn it into a finished precise part ready for the part’s application.
CNC Machining is used for jobs requiring a high level of precision as in the creation of prototypes and experimental parts. Precision, without variation, is a reason that it has become the foundation of modern production. CNC Machining is a step and process that has taken us into the future of production and manufacturing.