CNC machines: a complete guide
by Duncan Geddes
What does CNC stand for?
CNC stands for Computer Numerical Control.
How does a CNC machine work?
A computer converts a design produced by computer-aided design software (CAD) into numbers. These numbers represent three-dimensional coordinates in space and direct the movement cutter or other tool.
CNC machines combine a motorised manoeuvrable tool and a motorised manoeuvrable platform, which are both controlled by a computer. In this way, the computer controls the cutting and shaping of the material. By following the coded instructions, it can machine an object without a manual operator.
Types of CNC machine
The program prescribes the movement of a spindle or item to various locations and depths. CNC mills typically have between 3 to 6 axes and can perform:
- face milling
- shoulder milling
CNC milling machine
This process cuts workpieces while they are rotated. Most of these machines will have only two axes, though some newer models have more axes for more advanced jobs.
Electric discharge machining (EDM)
Also known as spark machining, spark eroding, burning, die sinking or wire erosion, this process uses sparks to shape an object by creating a series of rapidly recurring discharges between two electrodes, separated by a dielectric fluid and subject to an electric voltage.
Also known as wire cutting EDM, wire burning EDM or travelling wire EDM, this process uses spark erosion to machine or remove material from electrically conductive materials with a travelling wire electrode. The wire is eroded during this process, so fresh wire is fed in from a spool while disposing of spent wire.
Also known as cavity type EDM or volume EDM, in this case, an electrode and workpiece are submerged in oil (or another electrical insulator that can be polarised by an applied electric field). Both are connected to a power supply which generates an electrical potential between the two parts. As the electrode gets closer to the workpiece, a plasma channel forms and a small spark jumps between the two, cutting the workpiece.
Water jet cutter
A tool capable of cutting metal, stone and other hard materials using a high-pressure jet of water – often including an abrasive substance such as sand. The process is used for operations such as cutting, shaping, carving and reaming.
This process uses a plasma torch to cut materials such as steel or other metals. It involves blowing gas (for example compressed air) at high speed out of a nozzle while an electrical arc is formed through that gas from the nozzle to the surface being cut, turning some of that gas into plasma hot enough to cut the material.
This is a type of screw machine used in mass production to cut materials into small pieces with a diverse set of tooling. This machine has a number of spindles on a drum that rotates in a horizontal manner. One rotation of the drum can complete the number of operations needed for a single part to be completed.
How is a CNC machine used?
The first step is to determine what material will be worked on, as this will dictate what type of machine is needed. For example, if you’re using wood, you may choose a CNC router; if you’re machining metal, you may choose a CNC milling machine.
The design of an item starts with CAD (computer-aided design) and follows these steps:
- Item is drawn in a CAD program
- The CAM (computer-aided manufacturing) program converts the CAD drawing into G-code
- The CNC machine reads the G-code and performs the desired action to shape the item
What are the advantages of CNC machines?
The advantages of CNC manufacturing include:
- Improved automation
- Consistency of workpieces
- Accuracy of workpieces
The introduction of CNC machining marked a major advancement from manually-operated machines, which are generally controlled by hand wheels, levers or fabricated pattern guides (often called a ‘jig’).
Manual machining is labour-intensive, requiring skilled operators to create each item during working hours. Human error can also be introduced, resulting in imperfect reproducibility and possible part defects.
Traditionally, products might require the use of a number of different tools on different machines, potentially operated by different people. Modern CNC machines combine multiple tools into a single ‘cell’ which improves efficiency by reducing the time and amount of space needed for multiple machines.
Modern CNC machines also allow for an external controller and human or robotic operators that move the component from machine to machine remotely. All of this combines to make the whole process highly automated (eradicating one-off errors) as well as producing a part that closely matches the original CAD.
The limits of CNC machining
While most parts can be made with a CNC machine, it has its limitations, for example:
- Parts with deep cavities
- Parts with thin walls
- Parts with cut-outs
- Inner vertical corners
What is CNC programming?
CNC programming is the process of using programming software to create a set of instructions – called G-code – for computers to control a machine tool.
G-code is a highly concise language and was developed in the 1960s – originally called RS-274D – and while it is intended to be a universal standard, many CNC machines have developed their own versions.
As such it is important to understand how a given machine uses G-code. The differences in G-code arose as different machines developed new processes. For example, if you use a CNC machine that can process a coordinate system rotation based on probe inputs, you will need a set of G-code commands that can enable or disable this rotation. Machines without this capability will not require that G-code.
Typically, G-code uses the following format: N## G## X## Y## Z## F## S## T## M##
Which describe: N: Line number G: Motion X: Horizontal position Y: Vertical position Z: Depth F: Feed rate S: Spindle speed T: Tool selection M: Miscellaneous functions I and J: Incremental centre of an arc R: Radius of an arc
These are used to create a program that might be: G01 X2 Y1 Z15 S1 T22 M02
Which will define the specific movements and processes for each action.
What software/programs do CNC machines use?
Creating a part on a CNC machine generally requires two software components. Firstly, CAD (Computer Aided Design) software is used to design components. Some of the most popular software packages include:
- CAD DLL Import
- Dolphin Partmaster
- SketchUp Pro
- VCarve Pro
Next, CAM (Computer Aided Manufacturing) software is used to convert CAD drawings to G-Code, which will control the machine to shape the workpiece. Popular CAM software includes:
- 17 BobCAD-CAM
- Dolphin Partmaster
- VCarve Pro
Examples of products made by CNC machines
Many items can be manufactured using CNC machines – with their versatile CNC machining parts and components, many industries rely on them. Examples of products made using CNC machines include:
- Aviation and auto parts (cam, die, connecting rod, blade, propeller, box)
- Smartphone components
- Cooking utensils
- Kitchen appliances