In today’s world of industrial automation, it is inevitable to meet the stringent demand for quality to produce consistent products with competitive prices.
One of the most remarkable and influential steps in the industrial automation sector was the development of CNC machines (Computer Numerical Control).
Until the 1980’s CNC machinery was used only in the areas which required a high level of precision, but now, the advancement in technology has allowed the use of this machinery in all industrial and production areas.
Using advanced software programs for design, coupled with the high precision of CNC machinery brings out a drastic reduction in production cost with superior quality products.
What is a CNC machine?
CNC stands for computer numerical control, which is a manufacturing process in which a programmed computer software directs the movement of factory tools and machinery.
The CNC process can automate a range of manufacturing techniques such as water jet cutting, milling, laser cutting materials etc. The process can control a range of complex industrial machineries such as mills, lathes, grinders, and routers.
By definition, the CNC machine stands for a programmable machine that is capable of automatically performing the functions of CNC machining.
The machine is fully equipped for three-dimensional cutting tasks as per the instructions that are fed through a CAD file.
CNC manufacturing is a subtractive manufacturing process in which layers of materials are removed from the blank or workpiece.
The subtractive process is the method by which layers of materials are removed from the workpiece to produce shapes and designs. On the other hand, the additive process is a formative process in which layers of materials are assembled up to produce the desired shape.
This process is apt for a wide range of materials such as wood, foam, plastic, glass, composites and even metals. The CNC process enables the production of products with extremely high precision and accuracy from simple parts.
Types of CNC machines
Several different types of CNC machines are available, each having its own unique function.
1. CNC Milling machine
These are one of the most common types of CNC machines. CNC milling machine has built-in tools for cutting and drilling.
The CNC milling machines are available in three- to six-axis configurations.
The material to be drilled will be placed inside the machine, and the built-in computer will guide the tools precisely as per the instructions in the program to perform a drill or cut.
2. CNC plasma cutting machine
The plasma cutting machine is used to cut the materials similar to that of a CNC Milling machine. The main difference here is that a plasma torch is used.
A high powered plasma torch is generated, which can cut through even the rigid materials such as metals. The temperature of the plasma torch can go as high up to 50,000 degrees Fahrenheit.
3. CNC Lathe Machine
These machines can rotate materials while it is in operation. When compared to the CNC milling machine, the lathe machines have lesser number of axis, making it more compact.
These machines have a lathe in the centre which helps to move the material as per the desired way.
4. CNC Electric Discharge Machine
Quite popularly known as the spark CNC machine, this device uses electric sparks to change the materials to reach the desired shape.
One unique feature of electric discharge is that it can cause an erosive effect. This means that it can deform the materials and manipulate them to reach the desired shape.
By producing controlled sparks inside the CNC electric discharge Machine, the desired shape can be obtained.
The material will be placed between the top and bottom electrode, after which the computer will generate precisely the required amount of electric spark required for the operation.
5. CNC Laser-Cutting Machine
This CNC laser cutting machine is somewhat similar to that of a CNC plasma cutting machine. The main difference here is that instead of plasma, a high beam of the laser is used to perform the cutting.
The main advantage of using a laser is that you can achieve a high level of accuracy, which is not possible in the case of CNC plasma machines.
6. CNC water jet cutters
The CNC water jet cutter uses high-pressure water to cut various metals.
For cutting softer materials such as wood or rubber water alone can be used, and for cutting hard materials, water is mixed with an abrasive substance such as an aluminium oxide or garnet.
The water pressure is usually between 20,000 to 55,000 PSI. Water is induced into the CNC jet unit using a high-pressure pump and a filter. Here jewel nozzle is selected for corrosion resistance.
Types of CNC manufacturing processes
The great thing about the CNC manufacturing process is that it can be used for a wide variety of industries such as aeroplane engines, aerospace, construction, automotive and even in agriculture.
This technique has the potential to produce a range of products such as simple garden tools to highly complex aerospace equipment.
The CNC manufacturing process is a unique blend of controlled machining operations such as thermal, electrical and mechanical processes.
Let us examine some of the most common mechanical CNC machining operations.
This is one of the simplest processes for removing large amounts of materials in one go. Turning involves clamping a workpiece firmly on to a rotating plate or mandrel.
While the workpiece spins, a cutting tool will be firmly held against it on a moving slide or in a fixture.
By moving the slide up and down, it is possible to remove a large amount of materials quickly and effectively.
The process of turning can produce smooth and uniform surface finishes.
Precise holes can be drilled down the centreline of the workpiece with a drill bit mounted on the tailstock.
Milling is used to make squares/flat faces, notches, chamfers, channels, profiles, keyways and other features that require precisely cut angles.
The majority of CNC operations are done by milling and turning.
The fundamental difference between turning and milling is that unlike the former here the workpiece is held stationary and the cutting tool will rotate on a spindle.
The spindle holds a variety of cutting tools and can move in the X, Y and Z-axis. The workpiece will usually be mounted on a table which can move in the X, Y and Z-axis.
Milling excels at removing stock from highly complex and asymmetrical parts. A cooling fluid is used to cool down the workpiece, lubrication and to flush away the metal parts.
For many applications, it is necessary to have a smooth and flat surface on the metal piece. Grinding is a method which is best suited for creating a flat surface on the metal piece.
The workpiece is mounted on a table and moved back and forth laterally beneath the spinning disk to which an abrasive grit of a specific coarseness is attached.
The abrasive chosen will be depending on the type of material that needs to be lubricated.
Sometimes a different approach is taken where the abrasive disk is held firm, and the workpiece is moved back and forth.
The whole process of grinding will create a large amount of heat due to friction which needs to be regulated by carefully controlling the number of spins and temperature.
4. Optical Grinding
This technique is used for making optics in glass or plastic. As we all know, optics require a very fine surface with extremely close tolerance.
For this specialised grinding equipment is used that spins a grinding wheel against a surface while rotating in multiple axes. The grinding equipment is specially designed to minimise deviating from the normal, which is a common occurrence while grinding glass.
Grinding paste is also used as a lubricant to create a polished surface.
5. Wire EDM
This uses a copper wire as a conductor for high voltage electric charge. The new wire has to be constantly fed from the spool as the conductor will be steadily eroded during the process of cutting.
6. Solid sink EDM
In this method, an electric current is used to shape the material. The workpiece will be immersed in the dielectric fluid, and conductive electrodes will be placed close to it.
The electrodes don’t physically touch the workpiece but need to be replaced constantly.
As the electrode discharges, the metal will be forced off the workpiece to create the desired shape.
EDM is used for making textured surfaces or logos and lettering. It is also used for plastic injection and pressure die casting.
How does a CNC machine work?
The CNC manufacturing process involves computerised controls to fully automate and operate the cutting tools to make sure that exact details of the dimensions fed into the software program are carved out from the workpiece.
In a typical CNC manufacturing process, the fundamental principles remain the same. This includes the following stages:
- Designing the CAD model
- Converting the CAD file to a CNC program
- Preparing the CNC machine
- Executing the work
1. CAD model design
The machining process starts with designing the parts in CAD software. The designers can build digital tool libraries that interface with the actual tools. Unlike the previous CNC machines, modern equipment is fully automated. The digital files having instructions about the tools and cutting trajectories can be fed into tho the machine.
The machine is capable of mimicking the effect of several different tools at the same time.
The 3D model created from the CAD software will determine the properties and necessary dimensions for the final part.
2. Converting the CAD file
Once the CAD design has been finalised, it is run through a CAM (computer-aided manufacturing) software to extract the geometrical features and convert them into digital programming code.
The programming code will later control the CNC machine tools to carve out the exact dimensions as per the program.
Several programming languages are used for the CNC machine. This includes M-code and G-code.
- G-code(Geometric code)- Controls how and where the machine tools move.For example: how fast to travel to a particular location, when to turn on and off etc.
- M-code( Miscellaneous function code)- Contains auxiliary functions of the machine. For example: automating the removal and replacement of the machine cover at the start and end of production, use of coolant, tool change etc.
Once the CNC program is generated, it is loaded into the CNC machine.
3. Preparing the machine
Before the technician runs the CNC program, it is necessary to prepare the CNC machine for the desired operation.
- Attaching the toolings to the machine components such as end mills, toolings etc.
- Affixing the workpiece directly into the holding devices in the machine.
4. Operating the machine
The technician will run the CNC program once the machine is fully set up. The program will dictate the commands to the integrated computer within the machine. All tooling actions will and movements of the machinery will be done precisely as per the instructions in the program.
Once the operation is started, the program will continually guide the tools to perform all the operations, as mentioned in the program.
What engineering materials are suitable for CNC machining?
The CNC machines can handle a variety of engineering materials such as
The material selection for the CNC manufacturing process varies greatly on the applications and specifications required.
The physical properties of the workpiece material are first analysed to determine the optimal cutting speed, feed rate and depth of cut.
The cutting is performed in two different phases. In which during the first phase the material is cut to custom-designed dimensions. The second phase is the finishing phase in which the material is polished to the exact dimensions with a lower cut-rate.
The CNC machine can handle almost all types of materials, provided it has enough tensile strength, sufficient hardness, chemical and temperature resistance.
The CNC machining is a great boon for today’s manufacturing sector. The components of several devices that we use on a day to day basis are manufactured through this process.
This includes computer parts, automobile equipment and even aerospace components.
In this article, we have covered some of the basics of the CNC machining process, various CNC machining operations, their required equipment, and the factors that have to be considered before deciding if CNC manufacturing process is the best choice.
As this technology continues to evolve, we will be able to produce equipment which is highly precise and complex with less cost.