What is CNC Machining? | Definition Process, and Equipments
CNC machining is a manufacturing process that shapes intricate metal, plastic, and resin parts into products. CNC machining process is an extension of subtractive manufacturing process which means it removes material from the starting part known as the blank or workpiece.
If you’ve ever wondered what is CNC machining and how tiny components are able to fit into an enclosure and how metal parts like bolts and nuts pair, the answer probably is CNC machines. But there’s much more to know in the CNC machining process and in this guide, I will share with you everything from the process to the tools.
What is CNC Machining?
CNC is an acronym for Computer Numerical Control, CNC Machining is a subtractive manufacturing process. CNC works by taking instructions in the form of code or more specifically a g-code or an m-code. The software interpreting the code and the computer controlling the cutting tools are all embedded into modern CNC systems requiring little to no manual control and high precision.
CNC machines rely on Computer-Aided Design(CAD) and Computer-Aided Manufacturing (CAM) software as a set of instructions to carry out machining work on the blank. The CAD software is used to model the intended design with measurements and tolerances, while the CAM software turns it into a series of instructions that translates to tool speed, tool movement, translating axes, and motion. The instructions are repeated in precise movement to ensure identical parts are made.
Overview of CNC Machining Process
In CNC machining, a CAD (Computer-Aided Design) model of the desired part is created, which serves as a blueprint for the machining process. The CAD model is then converted into a CNC program that contains instructions for the machine tool to follow.
Designing the CAD model
CNC machine is a programmable machine, but before programming, the part geometry, structure, and design are made in CAD software. You can choose to build your own 3D CAD model or insert a 2D image that can be turned into a 3D model.
Converting the CAD file to a CNC program
A CNC machine is compatible with computer-aided manufacturing software that can turn the 3D part into a CNC machine-specific code. This code tells the machines how to move the cutting tool, adjust speeds during, rotate, move machining parts, angles and depths of cut, etc.
Here's an overview of the process:
Importing the CAD File:
In the CAM software, import the CAD file that contains the 3D model of the part. The software should support common CAD file formats such as STEP, IGES, or STL.
Defining Machining Operations:
In the CAM software, specify the machining operations based on the part’s requirements. This includes selecting cutting tools, defining tool parameters, and specifying the machining strategy.
Generating Toolpaths:
The CAM software calculates optimal toolpaths based on selected operations, considering factors like tool geometry, cutting conditions, material, and machine capabilities. It offers customization options to optimize toolpaths for efficiency and quality.
Setting Machining Parameters:
Specify machining parameters like cutting depths, stepovers, clearances, and tool change positions to ensure safe and efficient tool movements, preventing collisions and excessive material removal.
Post-Processing:
The CAM software performs post-processing to convert the toolpaths into a CNC program for your specific CNC machine. The post-processor generates G-code instructions to control machine movements, tool changes, and other machine-specific commands.
Preparing the CNC machine
Before a CNC machine can remove material, there are many checks and practices needed to ensure optimum performance and correct tooling. Often, there are more than one ways to machine the part and the CNC machinist begins by selecting the machining operation.
This is followed by, the processing sequence which breaks down the steps and tools to be used. Finally, he selects the machine which can accommodate the CNC machine tools and the manufacturing process.
Executing the machining operation
The blank or workpiece is placed inside the chuck or jaws and the machining process code is delivered to the CNC machine. The machine starts removing material in an automatic repetitive manner and the entire process is automated to speed up production.
There are several different ways to clamp workpieces in CNC machining, depending on the size, shape, and requirements of the part. Here are some common methods:
Vise Clamping:
Vises are commonly used in CNC machining to securely clamp workpieces of different sizes and shapes. They provide a versatile and adjustable grip, allowing for manual or pneumatic tightening as needed.
Fixture Clamping:
Fixtures are custom-made devices specifically designed to hold and position workpieces during machining operations. They provide precise and repeatable positioning, especially for complex or irregularly shaped parts.
Modular Clamping Systems:
The modular clamping system features adjustable and interchangeable clamps, brackets and adapters for flexible configuration of different machining Settings.
Collet Chucks:
Collet chucks are commonly used in CNC turning operations to securely hold cylindrical workpieces. Collet chucks provide high concentricity and gripping force, ensuring stability during rotational machining processes.
The Different Types of CNC Machines
CNC-machined parts are often made from multiple machining operations. Most CNC machines are specialized to perform single CNC machining operation like milling or turning. CNC machine terminology is based on the number of axes a CNC machine can control. Different types of CNC machines are categorized according to the number of axis freedom they have.
Below is a breakdown of the different types of CNC machines
CNC Mills
CNC mills are CNC machines designed to remove material using a rotating tool spindle from a workpiece. CNC mill has a base where the workpiece is attached. In most CNC mills, the tool is able to move in 3 axes, alternatively, the base can also move in 3 axes.
3-Axis CNC machines
3-axis CNC machines can move in 3 directions. Unlike 2-axis CNC machines, they can move in the x, y, and z axis unlocking many cutting operations. The most common 3-axis CNC machine is the milling machine which can either have its table moving in two directions and relies on the tool’s movement for the third direction or have two directional movements provided by the tool and the last or additional coming from the table.
4-Axis CNC machines
Going up from 3-axis CNC machines does not bring a new dimension to the tooling, but adds a rotational axis which makes the CNC more sophisticated and precise. A 4-axis CNC machine has movements in 3 axes and a rotational movement. This brings a new planar movement to the standard 3 planes. Typical operations of 4-axis CNC machines are cutting at angles, cutting along an arc, carving and circular engravings.
5-Axis CNC machines
5-axis CNC machine has the standard three dimensional movement along x, y and z axes. Along with a rotational movement on the x-axis like the 4-axis CNC machine. However, it goes further to add another rotational movement giving it a new depth and ability to cut exact designs. These new rotational axes are called the A and B axes.
5-axis CNC machines are the most sophisticated types of CNC machines and approach a workpiece from any side or angle. The biggest advantage over standard 4 and 3 axis CNC machines is you don’t have to adjust or change the orientation of the part manually.
Lathes
CNC lathes are able to shape cylindrical blanks by rotating the part at high speeds and driving it on the intended axes. The cutting tool remains stationary and the chuck rotates. Another type of CNC lathe is the Swiss-type lathe. Swiss CNC machine is a relatively modern addition to the host of CNC machines that uses a sliding headstock and a guide bushing to support long cylindrical parts. In addition to this, Swiss-type lathes also have milling capability.
CNC turning machines are specifically used to provide some of the standard turning operations done by stationary cutting tools like reaming, drilling, spherical balls, pins, grooves, or tappings and tapers. Keep in mind, they are strictly for cylindrical pieces.
Electric Discharge Machines
Electrical Discharge machines remove metal by continuously sparking and producing heat on the metal. It works by applying a DC electric current to the electrode and the metal part below, developing an electric field. The tiny metal parts suspended in the electrode form at the center of this field and because metals conduct electricity, these tiny metal impurities heat up and spark removing metal where the spark contacts the metal piece.
Wire EDM:
Wire EDM or wire cutting is an electrical discharge material removal process that uses a metal wire to cut a metal piece. The wire is passed through the material using a hole and a pre-programmed path is determined by the EDM machine for the desired shape. High voltage passes through the wire, instantly removing material around the wire.
The entire part and wire are submerged in deionized water to make sure, there’s no unwanted conductivity and the sparked metal is removed from the cut. This process is also called a 0 tolerance process.
Sinker EDM:
Sinker EDM uses the same electrical discharge principle but utilizes different tooling to remove material. Unlike wire EDM, sinker EDM makes use of an electrode that is machined to make a positive copy of the final design that is intended for the workpiece.
Sinker EDM is primarily used to machine intricate cavities. The electrode tool is lowered into dielectric oil, where the workpiece sits and features are burned into the part using sparks. The oil helps flush away any metal pieces and waste.
Types of CNC Machining Operations
There are many ways to classify CNC machines. The number of axes, cutting tools, and speed are ways to differentiate between them, but another way to categorize them is by using the type of CNC machining operation they can perform.
There are many computer numerical control machining operations, other than the standard cutting process, like water jet cutting and presses that are not used in typical industries.
Common CNC machining operations are listed below in detail.
CNC Drilling
CNC drilling involves a cutting tool (drill bit) mounted perpendicular to the surface of the workpiece that rotates to cut cylindrical holes inside the workpiece. Usually, these are made for bolts and screws. Drilling bits are of standardized sizes, meaning most holes are of a pre-determined standard like Metric.
However, holes with any diameter can be made if the multi-point drill bit can be designed. Drill holes are not limited to a vertical orientation and can be drilled from any angle. CNC drills can also perform other drilling operations including broaching, reaming, and tapping.
CNC Milling
CNC milling is another numerical controlled machining operation that utilizes a multi-point rotating cutting tool to remove material from a workpiece. The part is fixed on the CNC mill‘s table and the rotating spindle feeds into the workpiece.
There are many CNC milling machining operations depending on the axes of rotation of the spindle. CNC milling is the most common CNC machining operation because of its high versatility and accuracy. CNC mills can produce many types of shapes and pats using the same CNC mill setup.
They can be used to cut slots, form shapes, and simply remove a layer of material from the workpiece. CNC milling machines are able to perform many machining CNC machining operations like form milling, angular milling, face, plain milling, side and gang milling.
An important distinction to draw here is between CNC milling machines and CNC routers. Both are capable of performing the same function, use milling tools, and are operated by numerical control. The difference between a CNC router and a CNC mill is that a CNC router is used for softer materials like wood and is not as precise as a professional CNC milling machine.
CNC routers have higher RPMs and spindle speeds but don’t draw enough power to cut industrial metals like basic mills. A CNC router has a quicker completion time because it cuts softer materials and is great for contouring wooden signs and cardboard.
CNC Turning
Turning is a machining process that cuts material using a stationary cutting tool and a rotating workpiece. Although manual lathes are still common, most turning operations are done using lathes.
The CNC lathe holds the workpiece in a rotating chuck, and computer numerical control programs perform all the movement of the single-point cutting tool. CNC machined parts produced by CNC lathes are much more accurate and precise.
Types of CNC Machining Support Software
CNC machining requires pre-programmed computer software to perform the actual machining. Since the workpiece or machine tool is fed automatically, most common CNC machines rely on one of these software to interpret the desired design.
CAD Software
CAD software is the entry point to part design. CAD software allows you to make 2D drawings and drafts of final CNC machined parts and it can produce 3D renders or allow you to work in 3 dimensions on the part itself. CAD is primarily design software and cannot be used directly with a CNC machine. Common CAD software includes Solidworks, AutoCAD, Fusion, Creo Element, and CATIA.
CAM Software
CAM is the next step in the CNC system. CAM software can turn the CAD model into a g code, m code, or other numerical control code for a CNC machine. CAM programs create the instructions that automate the CNC machine and remove the need for a CNC operator. Typical CAM file types are 3DM, DWG, DXF, and IGES. CNC machinists usually know the ins and outs of CNC programming and some machines require CNC machinists to write or fix any errors.
CAE Software
CAE software can be considered a plug-in to CAD software with a host of analysis tools before the manufacturing processes start. CAE is strictly used at the design stage, to mimic real life scenarios through simulations, dynamic tests, diagnosis, and reports. The most common use of CAE is pre-production testing for dynamic loads, structural analysis, thermal expansion and finite element analysis on moving assemblies.
Materials Compatible With CNC Machining
Most common CNC machines can work with a range of materials as long as the workpiece fits the workable dimensions. CNC machining process is identical for various materials, but the cutting tools need to be changed depending on the type of material being CNC machined. CNC-machined parts are made from one of these materials.
Metals
Plastics
Wood
Foam
Composite
One reason CNC machining process is so common in the manufacturing industry is that it is precise, repeatable, and efficient. Choosing the correct material is important as the tool doesn’t wear out, the CNC machine doesn’t produce excess heat and the CNC machinist doesn’t have to spend time replacing the machine tool and coolant.
To select the material, the designer and the CNC machinist need to know the conditions the part will be used in, strength requirements, weight, and compatibility or conductivity. These factors determine the machinability of the workpiece and become the selection criteria for CNC machining process.
Another important factor in CNC machining is dimensional tolerance, which I will talk about in detail in the next section.
CNC Size Considerations
CNC machining is mostly reserved for small to medium-sized parts. In many cases, CNC machined parts are made and assembled into larger parts to give the impression of a large CNC machined part. Large sizes pose a great challenge to CNC machining, as most CNC machining operations require rotating the part or cutting tool, which can be difficult to do with heavy workpieces.
Most common CNC machines have the following tolerances.
# 1. CNC Machining Tolerances
Linear Tolerances in CNC Machining
Angle Tolerances in CNC Machining
# 2. Challenges of Machining by Part Size
CNC machining can cater to a wide variety of materials, and shapes and is not limited to a single CNC machining process at any one time. However, there are many machining parameters that need to fit on the workpiece before CNC machining can commence. Other than the achievable tolerances above, part size can limit CNC machining in many ways.
For a very large part
Accuracy errors are magnified as the size increases
More heat is produced
4 and 5-axis CNC might not cover the entire surface of the part
Requires complex machinery
Large parts are prone to vibrations
New CNC program for large parts
High pressure coolant delivery that can add cost
Prone to stress concentrations
For a bulky and heavy workpiece
Hard to rotate and reposition
Bench or CNC table might not support the extra weight
Produces more stress on the CNC
Alternative Manufacturing Processes To CNC Machining
CNC machining may sound like a do it all machining process, but there are many machining processes that are more useful than CNC machining despite all its advantages. One of these processes is 3D printing.
3D printing produces quick and easy plastic parts which require almost 0 input from the user. Metal 3D printing has become popular, producing 3D-printed metal parts using high-intensity lasers and metal powder.
Injection molding is another mass-production process that can produce intricate geometries like the 5-axis CNC machines but with a much simple and cheaper process. Injection molding requires no g-code and is ideal for plastic parts, but it can produce metal products using similar dies. Other CNC machining alternatives include laser cutting, water jets, EDM cutters, and chemical machining.
Author
Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.