Look around and identify and name all the products around you, including cell phones, tablets, laptops, watches, vehicles, and tables. Most of them are the final products of CNC machining. The computerized nature of CNC machining, combined with high precision, accuracy, and wide material compatibility, makes it one of the most popular manufacturing processes today. For example, tech giant Apple had to abandon traditional manufacturing and rely on CNC machining to produce one-piece molded casings.
Since the invention of the world’s first numerical control machine tool by the Massachusetts Institute of Technology (MIT) in 1952, CNC machines have been widely used in the manufacturing industry, especially in the automotive, aerospace, and military industries. CNC technology has seen rapid development in both hardware and software aspects. Do you want to learn about the working principles and CNC machining processes of CNC machine ? You’ve come to the right place!
What is CNC and CNC machining?
CNC, represents Computer Numerical Control, it is a method of automating machines. This control system can logically process computer instructions and utilize CAM (Computer-Aided Manufacturing) softwares to control, automate, and monitor the movements of the machine tool to create the required parts.
CNC machining is a subtractive manufacturing process that removing part of a workpiece with instructions from a CAD model. The CNC machine operations include turning, drilling, and milling, according to codes (g-code and m-code) obtained from the CAD model and containing instructions on machining parameters.
CNC Machining Process Steps
We can divide the CNC machining process into five different steps:
1. Creating 2D and 3D Designs
The first step in the CNC machining process is to use CAD&CAM software, such as SolidWorks and Autodesk Inventor, to create 2D engineering drawings and 3D engineering model of the parts. These engineering drawings must specify all critical dimensions, features with tolerances as GD&T symbol, and surfaces with specific surface finish requirements. The goal is to accurately represent the part (or product) you intend to create.
2. Using the Best 3D CAD File Formats
Two commonly used CNC-compatible file formats in CNC machining are STEP and IGES file formats. STEP format (sometimes referred to as STP format) is widely considered the best file format for sharing 3D models. This is because the format is neutral and not tied to any specific 3D CAD software vendor. Therefore, regardless of which 3D CAD software you use to create the design, your product development team (or machine shop) can access (or modify) your design without any issues.
3. Generating Tool Paths
Tool paths are a series of coordinate positions (or spatial paths) that CNC milling tools will follow during the machining process to produce the desired geometric shape of the workpiece. To better understand “tool paths” in CNC machining, let’s consider the example of a block with features on the XY plane. To create the desired part (a block with two rectangular cavities), you would first position the cutting tool to cut along the Z-axis and then move it along the XY plane. CAM software typically allows machinists to define their machining strategies, including the type of cutting tool to use, feed rates, and cutting tool speeds.
4. Converting Parameters to CNC-Compatible Files
The final step for machinists using CAM software is to convert their machining strategies into what’s known as G code. G code contains information such as when and where the machine motors should move and how fast they should run. These motors are guided by the instructions in the G code to remove material from the appropriate sections of the workpiece. Once the G code is generated, machinists export it to the CNC machine.
5. Executing the Machining Operations
This is the stage where machinists insert the workpiece into the CNC machine and set up all the necessary tools for the machining operations. After completing these steps, the machinist initiates the machining program to autonomously create the part. Following all the instructions, the result will be a completed part that matches the original design made on the CAD software. The part can then be used for its intended purpose, further improvement, or duplicated as needed.
Types Of CNC Machining
The common types of CNC machining processes include the most prevalent CNC milling and CNC turning, as well as grinding, electrical discharge machining, and more. During milling, rotating cutting tools are used on the surface of the workpiece, moving along 3, 4, or 5 axes.
Three-Axis CNC Machining
Three-axis CNC milling remains one of the most popular and widely used machining processes. In 3-axis machining, the workpiece remains fixed, while the rotating tool cuts along the x, y, and z axes. It is a relatively simple form of CNC machining and is suitable for manufacturing products with simple structures. However, it may not be ideal for machining complex geometries or products with intricate components. Since it can only cut along three axes, the machining speed might be slower compared to four-axis or five-axis CNC, as the workpiece may need manual repositioning to achieve the desired shape.
Four-Axis CNC Machining
In four-axis CNC milling, the fourth axis is added to the motion of the cutting tool, allowing rotation around the x-axis. Now there are four axes – x, y, z, and a-axis (rotating around the x-axis). Most four-axis CNC machines also allow the workpiece to rotate, known as the b-axis, which enables the machine to function both as a mill and a lathe. Four-axis CNC machining is an excellent choice when you need to drill holes on the side of a part or on curved surfaces of a cylinder. It significantly speeds up the machining process and offers high machining precision.
Five-Axis CNC Machining
Five-axis CNC milling adds an additional rotational axis compared to four-axis CNC. The fifth axis is typically around the y-axis or the b-axis. Some machines may have the workpiece rotating as well, sometimes referred to as the b-axis or c-axis. Five-axis CNC machining is highly versatile and used to manufacture complex and precise components, such as medical parts for artificial limbs or skeletons, aerospace components, titanium parts, oil and gas machinery components, military products, and more.
CNC Machining Methods
Depending on the material and requirements of the workpiece, different machining methods can be chosen. Understanding common machining methods and their applicable ranges can help us find the most suitable way to manufacture parts.
CNC turning machininig refers to the method of machining parts using a lathe. The characteristic of CNC lathes is their ability to rotate (turn) the material during the machining process. The lathe cutting tool moves linearly along the rotating workpiece, removing material around the circumference until the desired diameter and features are achieved. CNC turning can be used for threading, facing, and machining eccentric shafts, among others.
Applicable Range: Center drilling, drilling, reaming, tapping, external turning, boring, facing, grooving, contouring, taper turning, knurling, thread cutting.
CNC Milling machining is a machining process that uses a rotating multi-edge tool (milling cutter) on a milling machine to process the workpiece. The main cutting motion is the rotation of the tool. Milling can be divided into climb milling and conventional milling, depending on whether the direction of the main motion speed is the same or opposite to the feed direction of the workpiece.
Applicable Range: Milling flat surfaces, milling steps, milling grooves, contour milling, helical milling, gear milling, cut-off.
Planing is a method of machining that uses a planer to perform reciprocating linear motion relative to the workpiece, removing excess material. Planing machining accuracy can reach IT8-IT7, and the surface roughness is Ra6.3-1.6μm. It has advantages in processing large castings.
Grinding is a machining method that uses high-hardness artificial grinding wheels (abrasive wheels) as tools to cut the surface of the workpiece on a grinder. Grinding machining accuracy can reach IT6-IT4, and the surface roughness can reach Ra1.25-0.01μm, or even 0.1-0.008μm. Grinding can also process quenched metal materials and is often used as the final processing operation.
Applicable Range: External grinding, internal grinding, surface grinding, contour grinding, thread grinding, gear grinding.
The process of machining various internal holes on a drilling machine is called drilling. Drilling machining accuracy is relatively low, generally IT12, IT11, and the surface roughness is Ra5.06.3μm. After drilling, semi-finishing and finishing processes such as reaming and tapping are often used. Reaming machining accuracy is generally IT9-IT6, and the surface roughness is Ra1.6-0.4μm.
Applicable Range: Drilling, reaming, tapping, thread cutting, counterboring, spot facing.
Boring uses a boring machine to enlarge existing holes and improve quality. Boring machining accuracy is relatively high, generally IT9-IT7, and the surface roughness is Ra6.3-0.8mm, but the production efficiency is low.
Applicable Range: Machining high-precision holes, multiple-hole precision machining.
CNC Electrical Discharge Machining (EDM) uses highly controlled electric sparks to machine materials into the desired shapes. It is also known as spark erosion, spark machining, or wire burning. The component is placed under the electrode wire, and the machine is programmed to emit discharges from the wire, generating intense heat (up to 11700 ℃). The material is melted or washed away to create the desired shape or feature. EDM is commonly used to create precise micro-holes, slots, tapers, angles, and various other complex features.
- Machining hard, brittle, tough, soft, and high-melting-point conductive materials.
- Machining semiconductor materials and non-conductive materials.
- Machining various shaped holes, curved holes, and tiny holes.
- Machining various three-dimensional curved surface cavities, such as forging dies, die-casting dies, and plastic mold cavities.
CNC Laser Cutting
CNC laser machines have a highly focused laser beam that is used to precisely cut, slice, or engrave materials. The laser heats the material and either melts or vaporizes it, forming the cut in the material. This process can generate more versatile designs compared to traditional cutting equipment (lathes, machining centers, milling machines) and often produces cuts and/or edges that do not require additional finishing processes. CNC laser machines typically consist of a laser, power supply, optical system, and mechanical system.
Applicable Range: Diamond wire drawing dies, watch jewel bearings, porous skins of diverging air-cooled blades, engine nozzles, aviation engine blades, and various metal and non-metal materials cutting.
Common materials used in CNC machining
Advantages of CNC Machining
Precision: CNC machines are highly accurate, even more precise than skilled human engineers. Most CNC machines have a precision level of around 0.02 millimeters, allowing for the manufacturing of complex parts with tight tolerances.
Reproducibility: CNC machines can repeatedly execute the same instructions without any changes. As long as the machine is well-maintained, it can produce identical parts over and over again, which is challenging to achieve with manual techniques.
Scalability: Reproducibility also benefits production scalability. Since CNC machining can be repeated multiple times (and on multiple machines if needed), companies can quickly scale up from a single prototype to producing hundreds of thousands of identical parts.
Labor Cost Savings: Using CNC machines can reduce production costs compared to traditional manual machining. Once the G-code is created and sent to the machine, it can run without human supervision, leading to potential labor cost savings.
Versatility: CNC machines can work with various materials and create a wide range of complex shapes, making them suitable for a diverse range of applications, from automotive parts to food containers.
Continuity: Most CNC machines can work continuously, with relatively simple maintenance. With necessary upkeep and tool changes when required, CNC machines can achieve impressive continuous production capabilities.
Disadvantages of CNC Machining:
CNC machines are significantly more expensive than manual machines, and this cost is reflected in the pricing of CNC-machined parts. However, as CNC technology becomes more widespread, the price difference is gradually decreasing.
Cost: The cost-effectiveness of CNC machining largely depends on the quantity of parts. For one-off parts, 3D printing may be a more cost-effective alternative to CNC machining. On the other hand, for large quantities of parts (in the hundreds of thousands), mass manufacturing processes like injection molding may be more cost-effective.
It’s important to consider both the advantages and disadvantages of CNC machining when deciding on the most suitable manufacturing process for a specific project.
Main industries with CNC machining
CNC machining, as a continuously evolving technology, is constantly advancing and improving. With the progress of artificial intelligence and machine learning, CNC machining is moving towards a more intelligent and automated direction. Intelligent CNC machines will be able to adapt to different workpieces, automatically adjust processing parameters, and select tools to enhance processing efficiency and quality. This will result in higher production efficiency, more precise machining results, and reduced labor costs and time savings.