All About CNC Machining: Process, Materials, Applications, and Cost
Learn more about this technology and how it can be used in manufacturing.
CNC (Computer Numerical Control) machining is the process of using a machine to automatically remove material from raw stock based on a set of computer-generated instructions. It can be performed on almost any material from expanded polyurethane foam to nickel superalloys. Typical applications for CNC machining include fabricating, ornamental furniture, and turbine blades. The cost of CNC machining depends heavily on the required precision, surface quality, quantity, material used, and overall part complexity. This article will detail how CNC machining works, describing the overall process as well as commonly used materials and applications.
What is CNC Machining?
CNC machining is a subtractive manufacturing technology that automatically removes material from raw stock based on a set of computer-generated instructions. Generally, CNC machines are divided into two types of machines that keep the raw stock stationary and rotate the cutting tool, and machines that rotate the raw stock while moving it against a stationary tool. There are, of course, many variations on these basic approaches, and some machines employ both techniques to produce parts with complex features.
How Does CNC Machining Work?
There are many different CNC machine configurations. However, they all have the same fundamental mode of operation: specialized software converts a 3D model into a set of instructions for the CNC machine to follow to create the final part. These instructions are written in a computer language called G-codes. Once the machine begins operating, the process does not require any human intervention apart from loading material, unloading the final part, and potentially re-orienting or re-fixturing the workpiece to allow the machine to reach previously unreachable areas.
What is the CNC Machining Process?
The overall CNC machining process consists of four main steps, as listed below:
1. Designing the CAD Model
A CAD (Computer Aided Design) 3D model is developed by an engineer or designer. This part is designed according to specific CNC design principles, which are included in the overall DFM (Design for Manufacturing) philosophy. DFM ensures that the part can be efficiently manufactured on a CNC machine. For CNC parts, it is not strictly necessary to create 2D drawings of the part, since the machine instructions are created using CAM software which generates machine instructions. However, 2D drawings are often generated to indicate key dimensions for quality control purposes.
2. Converting the CAD File to a CNC Program
The CAD file is exported to a CAM (Computer Aided Manufacturing) software package that contains a range of tools to convert the 3D model into a set of instructions that can be understood by the CNC machine. CAM software is normally used by CNC machine operators to generate G-code. At the end of the process, a G-code file is saved to the machine. This file contains all the relevant instructions required to manufacture the part.
3. Preparing the CNC Machine
Preparing the machine to fabricate a new part number starts with setting up the required fixturing to hold the raw stock material in place while the cutting tool removes material. Alternatively, the raw stock is fixed in a spindle and stationary cutting tools are presented to the rotating stock piece to remove material. The required tooling must be checked for wear or chips and placed into the machine. More advanced CNC machines have automatic tool changers and tool libraries that allow the machine to automatically select the relevant tool needed to machine a specific feature without having to wait for the operator to swap out tools.
4. Executing the Machining Operation
When the material is securely clamped into the machine, the correct tooling is selected and the G-code is loaded, the actual machining can commence. Once the cycle-start button is pressed the machine will select the appropriate tool, move to the workpiece and begin removing material as per the G-code instructions and will continue until the part is complete. For operations that require machining with more than one type of tool, or with the part in more than one orientation, some CNC machines may require the operator to reposition the in-process work material between operations.
What are the Materials Used in CNC Machining?
CNC machines are used on a vast array of materials. Each material has its own optimal set of machining parameters (speeds and feeds) to successfully machine the material. The most common materials are:
Meal is easily the most commonly used material in CNC machining. CNC machines can cut almost any type of metal from free-machining brass to nickel superalloys like Inconel. Metal machining covers a range of applications from injection molds to shafts and gears.
While the vast majority of plastic parts are manufactured using injection molding, CNC machining may be used to produce certain plastic components. Typical materials can include ABS (acrylonitrile butadiene styrene), nylon, and polycarbonate. Plastic machining applications can include valve bodies, bushings, and injection molding prototypes to check the overall function of the part before investing in expensive molding tools.
CNC routers are most often used to cut wood and are generally cheaper than standard metal cutting CNC machines. CNC machining of wood is most commonly done for decorative purposes. Common applications include furniture, window frames, and ornamental panels.
Polyurethane foam (either closed or open-cell) is commonly used in CNC machining applications. Foam blocks can be cut into snug packaging for high-value products using a CNC router. One example is the foam used in toolboxes to keep the tools secure during transport.
CNC machines are regularly used when processing composite parts. Composites can include anything from aramid to fiberglass to carbon fiber. These materials are extremely abrasive to cutting tools. Aerospace and marine composite components are machined to add fastener holes and for general trimming after the molding process.
What Are the Applications of CNC Machining?
The list of potential CNC machining applications is almost endless. However, listed below are some examples of what is possible with CNC machining:
1. Woodworking Industries
Wooden furniture is often CNC machined to save time and reduce cost. Manually carving wooden parts is extremely time-consuming and costly. CNC machines can be used to create furniture with complex geometries that would have otherwise required a master woodworker to achieve.
2. Lettering and Engraving Systems
Some applications don’t require the heavy-duty removal of material, but rather make use of an engraving tool to mark patterns or text onto the surface of a part. This application can be done for artistic applications or serialized parts.
3. Electrical Industry
Demand for CNC-machined parts for the electronics industry covers a wide spectrum of applications. Machining component mounting holes into printed circuit boards and machining aluminum or copper heat sink for heat-generating electrical components are some examples of the use of CNC machining in the electronics industry.
4. Pharmaceutical Industry
CNC machines are used to manufacture precise equipment for use in the production of pharmaceutical products. Applications can include extremely precise metering pumps, ingredient-dispensing nozzles, and packaging.
5. Food and Beverage Industry
The fast-moving consumer goods industry makes use of a wide range of custom material-handling machinery that makes use of CNC machined parts. Food packaging is also made from injection molded plastics whose molds are CNC machined.
What Are the Benefits of CNC Machining?
CNC machines have become popular due to some key benefits as described below:
- Produce parts with extremely high levels of precision, consistently meet quality requirements, and eliminate the risks of human error.
- Produce parts 24/7 provided they are regularly supplied with raw material and fresh cutting tools. Robotic arms can be used to load raw material and unload finished parts with zero human intervention.
What are the Limitations of CNC Machining?
Despite the popularity of CNC machining, there are still a few limitations to the technology:
The operation and programming of CNC machines is a complex task and as such, requires specialized skills. These skills do not come cheaply.
- Due to the high levels of machine stiffness and highly accurate components required, CNC machines are very costly. Investing in one must only be done after carefully considering the current workload and potential payback period.
How Much Does CNC Machining Cost?
The cost of CNC machining a part depends on a range of factors, some of which are listed below:
- Material: Some materials are cheaper and easier to process with CNC machines than others. For example, machining Inconel will be more expensive than machining aluminum due to increased tool wear and slower cutting speeds.
- Complexity: The more complex features a part has, the more expensive it will be to machine. Complex surface shapes will be significantly more expensive than flat surfaces, for example.
- Tolerances: The tighter the dimensional tolerance requirements, the higher the cost. Needlessly specifying tight tolerances on non-critical features will increase the cost of CNC machining.
- Surface Finish: Mirror finishes on parts require specialized tooling and machining strategies. This additional requirement increases the machining time, and therefore increases the cost.
- Quantity: Low-volume production is inherently more expensive per part than large-volume production. This higher fee is because initial setup and programming costs are spread over more parts.
What Are the Different Types of CNC Machines?
There are many different types of CNC machines. They often borrow attributes and features from each other. However, at the most fundamental level, there are two main technology types, as described below:
- CNC Lathes: A CNC lathe consists of a spindle and a tool post. The spindle is able to secure (typically) cylindrical raw stock in a chuck. The chuck then spins at a high rotational speed and the tool post moves a cutting tool perpendicularly and parallels the axis of the rotating raw stock. CNC lathes can only produce parts with a consistent profile around their long axis. Other more complex machines like swiss or turret lathes are able to machine non-cylindrical features.
- CNC Mills: A CNC mill consists of a tool spindle and a table. A vise is typically mounted onto the table, and the raw stock is secured in the vise. In more automated machines the vise is pneumatic and a robot arm places the stock into an open vise which then clamps down automatically. The appropriate cutting tool is mounted on the spindle, which rotates at a high rate of speed and moves to the correct position on the raw stock to make the desired cuts. Some mills only have axes of motion, meaning that the table can move along the x (left and right) and y (back and forth) axes, while the cutting tool moves up and down along the z-axis. More complex machines have five or even six axes of motion. They can both translate and rotate the cutting tool to allow machining on five of the six possible faces of the workpiece.
For more information, see our guide on the 12 Types of CNC Machines.
What Are the CNC Machining Services Available?
Common CNC machining services include CNC turning, milling, cutting, engraving, and grinding. Learn more about our CNC Machining Services.
This article presented CNC machining, explained what it is, and discussed how this technology can be used in manufacturing. To learn more about CNC machining, contact a Xometry representative.
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