CNC Machining Guide
What is CNC Machining
Computer Numerical Control (CNC) Machining is a manufacturing process where a set of factory tools and machinery create parts using specific instructions from the pre-programmed computer software. CNC Machining creates custom parts by cutting pieces of material, or a workpiece, into the desired shape. Different materials can be CNC Machined, from metals such as stainless steel and brass to plastics.
CNC Machining is a form of subtractive manufacturing where the process uses a block of workpiece then removes it layer by layer to form the end part. It ‘subtracts’ the workpiece, where the resulting product has less material compared to when the process started, hence the name ‘subtractive’ manufacturing. It is distinctly different from formative manufacturing, where the material is pressed or molded to match the end product, such as Metal Injection Molding or Powdered Metal. It also differs from additive manufacturing, where the material is added layer by layer to form the product in processes like 3D printing.
The CNC Machining Process
There are three steps in the CNC Machining process. First is the design phase, where the model of the end product is created using computer-aided design (CAD) software. The second phase is to generate the CNC program with computer-aided manufacturing (CAM) software using the model. And then finally, the last phase is the execution of the program on the machine until the end product is acquired. Many parts also undergo a finishing process to achieve desired results.
In this phase, the design for the part is prepared typically using CAD software. The CAD software gives the designers the capabilities to realize a 2D or 3D model of the product and its necessary properties, such as dimension and geometries.
During the design phase, it is important to consider the strengths and weaknesses of the CNC Machining process, because its limitations can affect the shape of the end product. Tool geometry and tool access need to be considered when designing the part. As the tool has to access the workpiece from a specific angle, features that cannot be accessed this way, will be impossible to create. The tool geometry of the CNC tooling also impacts the design. As most CNC tooling are cylindrical, the parts will have curved corner sections, no matter how small the tooling is, therefore, there can be no sharp interior corners.
After the model of the part is completed, the design file is then delivered to computer-aided manufacturing (CAM) software to generate the instruction code.
Program Generation Phase
In this phase, the CAM software receives the design file created by the CAD software. Having received the design file, the CAM software subsequently generates programming code from the file. This code contains instructions for the CNC tool path, or step by step movements for the tool to create the desired machined parts. The code is written in a programming language that CNC Machines can interpret, the most common ones including G-Code and M-Code.
After the CAM software successfully generates the CNC program needed to create the product, it can be used as input for the CNC machine in the next phase.
The last one is the execution phase. Before the machine starts the fabrication process, the workpiece needs to be properly affixed to the machine. After the workpiece and all the required tooling are ready, the process is ready to be started.
In this phase, the operator runs the program code from the previous phase on the CNC Machine. It then executes the necessary machinery instructions in the CNC program to fabricate the final product.
After this phase is completed, the CNC machined parts will usually have tool marks, small imperfections on the parts that need to be removed or smoothened. To do that, various finishes can be applied to the CNC machined parts to have the desired properties.
Various finishes can be applied to the CNC machined parts. Some of the finishes are strictly for aesthetic purposes, while others also serve protective functions. These are some of the finishes that can be done to CNC machined parts.
- Smoothing and Polishing. The parts are smoothed and polished to improve the surface quality and aesthetics. Smoothing and polishing may affect the part dimension by removing a little bit of material.
- Bead blasting. The surface of the part is blasted with small glass beads to smooth the surface and protect it from corrosion. It is excellent for adding a uniform matte finish, where dimension tolerance is not a key concern.
- Anodizing adds a thin ceramic layer to the surface of the part to protect it from corrosion and wear. Only certain materials can be anodized, for example, aluminum and titanium.
- Powder coating. Powder coating adds a thin layer of polymer to protect the surface from wear. The coating is wear-resistant and can be added multiple times to improve protection. Powder coating can also be combined with bead blasting to increase corrosion resistance. This finish is excellent for CNC machined parts requiring high impact strength or for parts that cannot be anodized
Types of CNC Machining Operations
The type of CNC Machine used will vary based on how the material is removed from the workpiece. It can be done by cutting the workpiece, such as milling and turning. Or by using other mediums such as plasma, water jet, or spark. In this article, we will look at two of the most common types of CNC Machining Operations, milling and turning.
Milling is the most popular of all CNC Machining operations. Milling uses cutting tools or drills that rotate at high speed to remove the material.
In this process, first, the workpiece must be placed on the platform properly, either by using a vice or by directly mounting it on the bed. After the machine receives the CNC program of the CAD model, it can start removing the material from the workpiece. If the part needs a feature that can’t be accessed in a single run, it needs to be manipulated to a different orientation and the process needs to be repeated. After the machining process, the part needs to be deburred. Deburring is the manual process of removing small defects on the part caused by material deformation. After all these processes are complete, it is ready to be used or post-processed.
Most CNC milling systems operate on a 3-axis system, X, Y, and Z-axis, (left-right, back-forth, up-down, respectively) based on the movement of the cutting head relative to the workpiece. CNC milling systems can also have more than 3 axes. They can be 4-axis or 5-axis. These additional axes come from the ability to rotate the bed and/or the tool head.
· 3-Axis Milling
3-axis milling is the most common of the CNC milling system. This system operates using 3 axes, X, Y, and Z-axis. It is relatively easy to operate, it can create products with high accuracy, and the start-up machining cost is relatively low. But because it only has 3 axes, some areas of the part might be difficult to reach. Rotating the piece manually is still possible, but it will increase the labor cost. Therefore, 3-axis milling systems are suitable to create CNC machined parts with simple geometries.
· 4-Axis Milling
4-axis milling employs one additional axis over 3-axis milling. This additional axis, called A-axis, is a rotation on X-axis. So, in 4-axis milling, the workpiece can be rotated on the X-axis while executing the cutting process. The 4-axis milling can be cost-effective over 3-axis milling when the part needs to have features on both sides. 4-axis milling systems can run the fabrication process on a single fixture, while the 3-axis milling systems will need to run the process twice. This reduces cost and the probability of human error. 4-axis milling can machine angled features otherwise impossible on 3-axis milling.
There are two types of 4-axis milling, indexing and continuous. Index 4-axis milling rotates while the machine is not cutting the material, whereas continuous 4-axis milling can cut the material while rotating simultaneously.
· 5-Axis Milling
This type of milling adds two additional axes upon the usual X, Y, and Z-axis. This will be either the A-axis and B-axis or A-axis and C-axis. The rotational movement can be done either by the workpiece or the spindle.
There are two types of 5-axis milling, 3+2 machines, and fully continuous 5-axis machine. 3+2 machines can rotate in one axis and the other, but not simultaneously. Fully continuous 5-axis machines can rotate at both axes simultaneously. Both can create highly complex 3D shapes, but fully continuous 5-axis machines can produce parts with even higher complexity. Fully continuous 5-axis machines can even create CNC machined parts normally reserved for molding processes.
CNC Turning or CNC Lathe is a machining process where the workpiece is held on a spindle and rotated at high speed while the cutting tool traces the outer or inner perimeter of the workpiece to form the part. The tool itself does not rotate, but it may move along polar directions.
Lathes are used to create parts with cylindrical profiles, like screws or washers. They can produce parts at a much higher rate than mills with less cost.
Other than ordinary lathes, there are other turning machines with expanded capabilities, such as lathe turning with live tooling, multi-spindle turning machines, and Swiss turning machines.
Lathe Turning with Live Tooling
Tooling on a turning machine is made to carve material from the face and diameter of a spinning object held in the spindle. Live tooling means that the tooling can rotate and move using its mechanism. In other words, lathe turning with live tooling adds milling capabilities into the lathe, making the process to create complex parts to be much simpler. With an ordinary lathe, complex parts with flat features or holes need to be milled in a separate process, whereas with live tooling this can be done in a single process without the loss of the parts’ indexed position. Some of the abilities with live tooling include milling, drilling off-center, cross milling, tapping, grooving/slotting, and thread milling.
Multi-spindle Turning Machines
Multi-spindle turning machines have the advantage over ordinary or single-spindle lathes in the number of spindles that can work at any given time. While single-spindle lathes can have one spindle and one sub-spindle, multi-spindle turning machines typically have six spindles. This means that the parts can be created at a faster rate, as the work needed to fabricate the parts can be divided into six processes that can be carried out simultaneously.
Swiss Turning Machines
Swiss turning machines or sometimes called Swiss screw machines are lathes that are specially designed to create small, cylindrical parts with extremely tight tolerances. To achieve extremely tight tolerances, Swiss turning machines use a guide bushing. Its function is to minimize the wobbling of the bar caused by centrifugal force. The position of the cutting process is also different from ordinary lathes. Swiss turning machines cut the bar stock next to the guide bushing to further increase the stability.
Electrical Discharge Machining (EDM)
EDM also known as die sinking or wire burning is a process in which metal is removed from the workpiece by electrical discharges between two electrodes, separated by dielectric fluid. Imagine an electrically charged hair thin wire that will melt away tiny particles of metal. Wire EDM processing can hold incredibly tight tolerances, create complex and intricate shapes, work with extremely hard or soft materials, and make tiny parts. Advantages of EDM are:
- Can hold incredibly tight tolerances
- Create complex and intricate shapes that would otherwise be impossible with conventional machining
- Work with extremely hard or soft materials
- Very good surface finish may be obtained
- Extremely hole small holes are possible
Machined Product Applications
Due to its capabilities to manufacture both simple and complex parts with varied materials, CNC Machined parts are found in a wide range of industries. These are examples of those products.
- Aerospace: engine mounts, fuel flow components, fuel access panels.
- Automotive: gearbox, valves, axels, dashboard panels, gas gauges.
- Consumer Electronics: notebook chassis, PCBs, housings.
- Defense Industry: artillery components, fighter components, missile components.
- Healthcare: implants, surgical instruments, orthodontics.
- Oil and Gas Industry: pipeline components, valves, pins, rods.
CNC Machining Material
Material’s suitability for CNC machining depends on the manufacturing application and specifications. Most materials can be CNC machined provided they can withstand the machining process, that is having sufficient hardness, tensile strength, temperature, and chemical resistance. The most common materials in CNC machining are plastic and metal.
These are some of the metals used in CNC machining.
- Stainless steel
- Alloy steel
- Tool steel
- Mild steel
And these are some of the plastics used in CNC machining.
- POM (Delrin)
Amfas offers a wide range of CNC machining capabilities. The precision and accuracy of any machining is a function of the equipment, fixturing, cutting tools, and of course skilled machinists. Amfas’ use of the best people and equipment for the job at hand allows for tight tolerances in any production part that we fabricate.