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The CNC machining process offers many advantages because it is automated and uses computer-controlled machine tools to fabricate parts from raw materials. The process can produce many different parts with varying levels of complexity. CNC machining is often used in the aerospace, automotive, and medical industries, where specific parts must meet stringent specifications and tolerances in manufacturing processes.
CNC machining has made it possible to create objects a hundred times faster than a few decades ago.
So what is CNC machining and how does it work?
CNC stands for Computer Numerical Control. It means utilizing computerized software to control how a machine moves and operates.
It works by converting a digital model of a part into a sequence of computer instructions that control the actions of the machining tools to achieve the desired output
CNC machining equipment works on a variety of parts, such as metals, metal alloys, wood, stone, and more. The size of machine tools can vary based on the specific part that requires manufacturing.
Since it is computer-controlled, CNC machining can deliver superior levels of versatility, efficiency, and precision relative to other common manufacturing methods. This makes it a popular manufacturing choice for a vast range of industries and applications.
Removing material using milling tools to shape an object has been an ongoing practice since ancient times. When it comes to the modern history of CNC machining though, the process of controlling machine tools through numerical control was invented in 1949 by John Parsons at MIT.
In 1949, MIT conducted a research project for the US air force to use motorized methods for creating helicopter blades and aircraft engines and frames. This is what gave rise to CNC technology.
Richard Kegg, in 1952, further improved the numerical control machining process, which led to CNC milling machines.
Due to the absence of computers, the automated nature of early CNC machining processes was made possible with the help of punched tape. With the advent of digital computing in the 1960s, the punched tape was replaced with computerized controls.
CNC machining relies on computer programs to create the layout of the process in which the machine tool should function. Since users cannot directly communicate with the machine tools, Computer-Aided Design (CAD) software is used.
CAD software creates the 2-dimensional and 3-dimensional models for the required CNC machined parts. With this design, the machine knows what the final part looks like.
The computerized controls do the calculation required for removing material, so the workpiece looks like the final part created in the CAD software.
Let us go through the breakdown of various processes that occur during CNC machining.
CNC machining occurs in four stages:
Before CNC machining begins, the 2D or 3D model of the final design is required. This model is created in CAD software. There are many CAD software programs available online, free and paid.
Creating CAD models is not difficult and can easily be learned. However, some complex parts might require more experience with CAD, for which expert designers can be hired.
No CNC machine understands CAD language directly. CNC machines only recognize movement based on coordinates. Therefore, the CAD model must be converted to a CNC understandable file called G code.
Many CAD software programs can write the output file directly in G code by using the particular setting before saving the file.
In other cases, converting the CAD design to G code will require dedicated software called Computer Aided Manufacturing (CAM). CAM software is a very functional tool when it comes to the automation of machine processes.
Besides using CAM software, many simple free tools can convert simple CAD designs to G code with the click of a button. However, they don’t have the vast suite of features that CAM software offers.
Before starting manufacturing processes, the CNC machine must be set up the right way.
Think of this as configuring the printer before you print something. You need to feed the printer with pages and check specific settings. CNC machines operate similarly.
Before machining begins, there are many setup processes to complete. For instance, you must ensure the workpiece is properly positioned on the machine. The dies must also be set correctly, and other position settings.
Once the configuration stage is complete, the machine operation can begin. For this, you can execute the program on the display panel of the CNC machine.
Depending on what you design, you might have to go through various program prompts to choose different types of settings and options.
Once the CNC program is executed, the machine keeps going till the end of the program. It only stops if switched off by the operator or in the case of an unexpected error or power disruption.
It is crucial to be familiar with CNC machining terminology in this field. Some common CNC-related terms include:
Computer-Aided Design is the software that creates the graphic representation of the required final part in 2D or 3D. Generally, complex parts are produced by breaking them into small parts, each having its own CAD model.
AutoCAD and FreeCAD are two of the most popular CAD applications, with the latter being completely free.
Computer Aided Manufacturing (CAM) software generates CNC programs that a machine can understand. These programs help automate different types of machines, including CNC machines.
CAM software works together with CAD programs to execute operations using a CNC machine. Most quality CNC machines come with complementary CAM software.
In a Distributed Numerical Control (DNC) setup, multiple machine tools can be connected to a central server. The central server processes the design files and sends commands to each machine tool separately.
DNC is utilized when the individual CNC machine memory is too small to process the software. It can also be used where multiple numerical control machines require synchronized working for faster operational capabilities.
Manufacturing Data Collection (MDC) is what it sounds like. Manufacturing Data Collection software collects data from machines and operators about the different manufacturing information generated in real time.
This information helps manufacturers to improvise on the existing production line. It also points out the causes of any delays and loss in production.
G-code and M-code are files that a CNC machine requires to operate. Each of these has its own importance, which we will go through one by one:
The ‘G’ in G-code refers to Geometry, making G-code a Geometric code. G-code is an alphanumeric command that tells the CNC machine how to move.
The X, Y, and Z references in G-code refer to the axes of movement, and the number pertains to the value in that particular axis. N refers to the line number.
Some particular codes refer to specific operational capabilities of the machine, such as adjusting the spindle speed or turning the spindle on and off.
M-codes are miscellaneous machine codes that perform the non-cutting actions of the CNC machine. These include starting and stopping different programs, controlling the coolant flow, or adjusting the behavior of machining tools.
A CNC machine is not one specific machine, as it is a group of different types of CNC machines working on various machining processes. Some of the most popular CNC machining operations include:
CNC milling is one of the most popular types of CNC machining processes. In fact, many professional machine shops often use a CNC machining and CNC milling process interchangeably. Face milling and peripheral milling are two of the most frequently used CNC mill applications.
In a CNC milling machine, rotating cutting tools move relative to the workpiece to remove material.
The cutting tool (also called a milling tool) is fixed on a spindle that can rotate. The rotation and movement of the spindle give CNC milling machines the ability to perform three or more axes milling operations.
The CNC drilling process is a lot simpler than using milling tools or the turning process. In CNC drilling, the workpiece is held stationary while a drill bit moves over the workpiece and creates holes.
The purpose of drilling holes might be to add screw bolts, aesthetic requirements, or any other use.
CNC grinding machines use a rotating flat abrasive wheel for removing material from rough workpiece surfaces. This machine process is usually applied to create a smooth-finished part. The grinding wheel rotates at a very high speed.
CNC routers are very similar to CNC milling machines. The main difference is that in a CNC router, the workpiece is always stationary, and the cutting tool moves in X, Y, and Z dimensions. CNC routers create faster cuts than milling machines without compromising accuracy and design complexity.
Besides the various types mentioned above, there are other CNC processes too. Some independent fabrication machines are integrated with a CNC for automatic movement. Some of these additional CNC machines are:
Broaching utilizes a toothed cutting head to create niche shapes on a workpiece. Broaching cuts are very consistent and highly accurate. These machines can be linear or rotary (with a rotating toothed cutting tool).
Sawing utilizes a toothed blade for creating straight, linear cuts. The cuts are created by the removal of material due to friction with the saw blade. When operated with a CNC, this process is usually applied for the automated cutting of materials.
Honing is similar to grinding in that it is generally used for the secondary finishing of a material. In the honing process, an abrasive stone or wheel is used for controlled grinding of the workpiece, creating the desired shape, size, or finishing.
Lapping is also similar to grinding. But, lapping uses an abrasive paste, powder, or mixture instead of a grinding wheel to create a smooth surface finish. The abrasive mixture is inserted between two materials (one of which is the workpiece) and then rubbed against each other.
CNC lathes are primary shaping tools used for machining metal or wood. In a lathe machine, the workpiece is rotated around a central axis, and the machining head moves linearly along the surface. CNC lathes can perform various functions, such as cutting, drilling, sanding, knurling, facing, and more. CNC lathes perform much better than manual lathes.
Plasma cutters are an evolved form of cutting technology, using a high-temperature plasma jet to cut material. The plasma is created by an electrical arc, so this method applies to conductive materials only.
Laser cutters use a laser beam to cut through a material. Unlike plasma cutting, laser cutting is not limited to the cutting of electrically conductive materials. Laser beams can cut through anything by adjusting the laser parameters.
Flame cutting uses an Oxy-acetylene (also known as Oxy-fuel) gaseous mixture to cut through metals. When the Oxy-fuel stream is narrowed and ignited, it creates an ultra-high temperature flame that can easily cut through metal.
The purpose of Press Brakes is to bend metal plates and sheets. The material is placed between a V-shape or a U-shape die. Then the die is pressed, resulting in the bend as required.
Electrical Discharge Machines (EDMs) are used for cutting conductive materials. In EDM, electrical pulses are emitted by a cutting head near the material, which creates an electrical arc. This arc melts and removes the material at the required position resulting in a cut.
Waterjet cutters utilize ultra high-pressure water for the cutting action. These cutters can cut through anything: metal, alloy, wood, stone, or glass. The water jet stream is controlled by CNC and moved according to the software.
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