Automation is the key to increasing the productivity and performance of a system, and a Computer Numerical Control (CNC) machine is a great way to automate a manufacturing process.
CNC systems are now integrated with different machines, resulting in the development of a CNC for almost every manufacturing application.
But what exactly is a CNC machine? How does it work? What are the axes in a CNC machine? And is it worth upgrading to CNC machining?
In this article, I will dive into the topic of CNC machines, discuss various types of CNC machines available in the market, and shed some light on the advantages and limitations of using CNC machines.
What is a CNC Machine?
A Computer Numerical Control (CNC) machine is an automated machine that uses a computer system to control the motion of the cutting tool and make the desired cut. CNC machines can produce precise cuts with minimum cycle time and a high repeatability index, making them ideal for large-scale production.
A numerical control machine consists of basic elements such as a computer system loaded with CNC software, a CNC controller, a CNC shield, stepper motor drivers, and a drive system generally operated by stepper motors.
These elements function in coordination to produce the desired motion in the cutting tool.
CAD software is used to prepare the design and export the design file.
The design file is then imported into CAM software that converts the design into G-codes and sends them to the controller.
These electrical signals control the movement of the stepper motor, thereby controlling the motion of the cutting tool.
Open and Closed Loop CNC Machines
Open-loop CNC systems use a one-way communication channel where the computer system controls the movement of the cutting tool.
Most entry-level and hobbyist CNC machines fall under this category, where you feed the input design to execute the cut.
In an open-loop system, the controller translates the G-codes and energizes the motors to execute the cut without monitoring the process.
This means that the system does not identify the irregularities in the process and continues with the next task until stopped manually or by triggering of limit switches.
This system is generally applicable for CNC machines using stepper motors.
Whereas in a closed-loop system, generally servo motors are used, and there is two-way communication between the tool and the controller.
In this system, the motor sends a feedback signal to the controller after executing every task.
This continuous feedback generated from the tool helps identify any irregularity in the process and rectify it automatically.
Furthermore, it can even stop the process if the detected error is beyond rectification.
Generally, a closed-loop system is used in high-end industrial CNC machines involved in precision work where there is no scope for error.
Fully Automatic CNC Machines
Most CNC machines are semi-automatic and require human assistance for various tasks such as tool change, work holding, loading, unloading, etc.
However, new generation CNC machines are capable of performing fully automatic processes with features such as automatic tool change, part detection for automatic work holding, and robotic mechanism for loading and unloading.
These machines require minimal human supervision during the process and, therefore, generally consist of a closed-loop system.
Types of CNC Machines
Numerical Control machines were introduced as semi-automatic tools that took the input in the form of a numerical punch tape.
However, the advancement in technology and the development of computer systems enhanced the flexibility of numerical control systems.
This enabled the numerical control system to be integrated with different machines to produce semi-automatic or fully-automatic CNC machines.
These machines are used in CNC manufacturing process and are available in various types.
A CNC lathe is a versatile machine capable of turning, face milling, tapping, drilling, etc. These machines generally consist of a 2-axis system, with the plunging of the cutting tool into the surface of the workpiece as the first axis and the cutting tool moving along the length of the workpiece as the second.
A CNC lathe is ideal for machining axially symmetric parts, such as a baseball bat, tumbler, a log of wood, etc.
In this machine, the workpiece is mounted on a chuck that spins rapidly at high RPM, and the cutting tool moves along the surface of the workpiece to produce the desired cut.
The computer system controls the RPM of the spinning workpiece and the movement of the cutting tool to execute the desired process with high precision.
Apart from CNC turning, these machines can be used for performing various operations like facing, knurling, taping, threading, boring, etc.
Depending on their speed, torque, and tool holding parameters, lathes can be classified as wood lathes and metal lathes.
Generally, machining metal requires low speed and high torque configuration, which is opposite to that of wood lathes, but a wood lathe can be used for machining metal with satisfactory results.
CNC Milling Machine
A CNC mill consists of a workbed, where the workpiece is placed and held securely. These machines generally consist of a 3-axis system with a rotating tool (spindle) that plunges into the workpiece (Z-axis) while the workbed moves along the X and Y axes to produce the desired cut.
Milling machines are generally used for making gears and cutting grooves or slots in flat workpieces.
However, as the number of axis of the CNC milling machine increases, its ability to perform complex and intricate cuts also increases.
A 4-axis milling machine has the ability to rotate the workpiece along X-axis, while a 5-axis CNC mill is more sophisticated with rotating X and Y-axis.
A CNC router is similar to a CNC mill, with the only difference being that, in a router, the tool or spindle moves along the X, Y and Z-axis while the workpiece remains stationary on the worktable. Whereas in a CNC mill, the workpiece moves along the X and Y axes.
CNC routers provide a larger work area when compared to CNC milling machines which is ideal for working on large-size applications such as wood carving.
However, CNC mills provide more rigidity and higher spinning torque than routers, making them ideal for machining harder materials like steel, cast iron, and titanium.
Most CNC routers provide an optional rotary kit that transforms your machine into a 4-axis CNC router.
CNC Plasma Cutter
A CNC plasma cutter is a metal cutting machine that uses a jet of hot plasma to cut through a conductive material. A computer system controls the movement of the cutting head and maintains a constant gap between the workpiece and the cutting torch to produce the desired cut.
Plasma cutting involves forcing a jet of high-velocity gas (oxygen, nitrogen, argon, etc.) through a nozzle and initiating an electric arc between the workpiece and the electrode.
This electric arc increases the temperature of the high-velocity gas, thereby transforming it into hot plasma.
CNC plasma cutting is ideal for sheet metal applications as it can cut through almost any metal with high precision.
CNC plasma cutters usually consist of a 2-axis system that supports the movement of the plasma torch along the X and Y axes.
CNC Laser Cutter
CNC laser cutters work by using a high-energy laser beam to melt and vaporize the material, producing the desired cut. A jet of auxiliary gas blows the molten material out of the cut, thereby enhancing the ability of the laser to perform a clean cut with quick cycle time.
These machines can be used for laser cutting and engraving various metals and non-metals like paper, acrylic, leather, plastic, polypropylene, fabric, foam, plywood, glass, vinyl, aluminum, brass, etc.
The computer system guides the movement of the laser head to trace the path of the required design and execute the cut.
With constant development, these CNC machines are available in various sizes ranging from large and powerful industrial laser cutters to low-powered desktop laser cutters ideal for small-scale businesses and hobbyists.
Laser cutters are one of the most precise CNC machining techniques that provide fast processing speed, ideal for mass production.
However, these machines are generally suitable for cutting very thin workpieces, and their performance degrades with the increase in the thickness of the material.
For example, when comparing laser cutting with plasma cutting, laser cutting provides a faster cutting speed for thin metal sheets.
But as the thickness of the metal increases, plasma cutting outperforms laser cutting by providing better cutting speed and smoother surface finish.
CNC Water jet Cutter
CNC waterjet cutter uses a high-pressure (above 50000 psi) jet of water to erode the material from the surface of the workpiece and create the desired cut. Generally, for cutting hard materials, tiny abrasive particles are added to the water, which enhances the material removal ability of the waterjet.
It is a highly precise cutting technique that can produce intricate cuts with tight tolerances at the cost of cutting speed.
This means that, when comparing water jet cutting with laser and plasma cutting, waterjet can produce better quality cuts but at a comparatively slower cutting speed.
Moreover, the use of a high-pressure compressor and abrasive particles results in a comparatively higher overall cost of waterjet machining than laser cutting.
Industrial waterjet cutters, such as Omax waterjets, can produce tapered cuts by tilting the cutting head at an angle.
This enhances the functionality of the waterjet and enables to produce 3D cuts with complex geometry.
CNC Electric Discharge Machining (EDM)
Electric Discharge Machining (EDM), also known as spark erosion or spark machining, is a high-precision machining technique that uses electrical discharge (sparks) to cut through a conducting material. These sparks raise the temperature of the material to around 8000 - 12000 ℃ and produce the required cut by vaporizing it.
There are various types of EDM processes, such as Die-sinking EDM, Wire EDM, and hole drilling EDM.
Wire EDM is one of the most popularly used EDM technique that uses a thin wire as the cutting tool and produces cuts with almost zero tolerance.
This means the cuts produced by Wire EDM have an extremely narrow kerf width, which makes it an ideal machining process for performing intricate cuts on metal surfaces.
Industrial Wire-EDM machines, such as UON-01 by Makino, can produce extremely precise cuts with a kerf width of around 1.52μm (~0.00006") and showcase a high positional accuracy of +/-0.5μm (~0.00002").
3D printing is an additive manufacturing process. Unlike other subtractive CNC machining methods that involve the removal of material, 3D-printer prints a product by adding layers of material on top of the other, while a computer system controls the movement of the nozzle in the XYZ plane to print the required shape.
A 3D printer prints layers of building material such as plastic, cement, composites, or even metal in liquid or powdered form, which then fuses together to form a solid object.
These machines are generally available in small-size desktop variants that are highly popular among hobbyists and DIY enthusiasts.
However, rapid development in the field of 3D printing has introduced large-size printers capable of printing an entire house.
How to Use a CNC Machine
Although the process varies depending on the type of CNC machine, the fundamental process is similar for most CNC machines.
Preparing the Design for CNC Machining
Preparing a design is one of the most important and fundamental steps in almost every CNC machining process.
It involves using CAD software to create a 2D or 3D design that replicates the final cut in a graphical form.
When preparing the design, a good designer should always be mindful of the characteristics of the material to be machined.
For example, materials like aluminum and copper are ductile in nature and tend to undergo slight deformation when milled on a CNC machine.
Therefore, a good design will provide tolerance in the dimensions to account for this deformation.
Generally, it is recommended that the thickness of each design element should be greater than the thickness of the material being cut.
This ensures the structural integrity of the workpiece after carving out the design.
CAD software prepares the design and exports the design file in formats such as DXF, SVG, PNG, BMP, JPG, etc.
The design file is then sent to CAM software, which converts it into G-codes.
Most CAM software also act as control software that provides the interface to control the various machining parameters such as speeds and feeds.
After preparing the design and setting the process parameters, you can use a simulation software to simulate the cut virtually and verify the process for any abnormal behavior.
Furthermore, if you are a beginner with poor designing skills, you can look for CNC machines that include easy-to-use dedicated CNC software loaded with free design templates and pre-set machining parameters for various materials.
Selecting the Tool for CNC Machining
After preparing the design, the next step is to select the appropriate cutting tool for the application.
The selection of the CNC machine tool depends on various factors such as the nature of the cutting process, type of material to be cut, intricacy of the design, etc.
Depending Upon the Nature of the Cutting Process
The nature of work plays an important role in tool selection because different types of cutting processes require different tools.
While some tools remove material faster, others provide a higher surface finish.
For example, an end mill and a router bit perform a similar function of removing the material by spinning around their axis.
However, an end mill is generally used for profiling, contouring, reaming, slotting, and counter-boring operations.
Whereas a router bit is ideal for woodworking operations like joint making, shaping wood edges, and engraving various designs by making depressions.
Depending upon the Type of Material to be Cut
The type of material to be cut also plays an important role in tool selection for a CNC process.
A tool meant for cutting soft materials like wood and plastic will not be effective on hard materials like stainless steel and titanium.
Generally, carbide end mills are recommended for machining hard metals because they deliver high cutting force and retain tool sharpness even after prolonged use.
Moreover, when delivering high cutting force, a thick end mill with a small shank is comparatively less prone to snapping than a thin end mill with a long shank.
Similarly, different types of lasers are used for cutting different materials.
A fiber laser is recommended for cutting and engraving metals, whereas a CO2 laser is ideal for processing non-metals, and low-powered diode lasers perform well for laser engraving non-metals.
Depending Upon the Intricacy of the Design
The intricacy of the design determines the size of the tool to be used for the cutting process.
A design consisting of an intricate pattern generally requires a tool with a small diameter.
When selecting the tool, ensure that the selected tool is smaller or equal to the smallest width of the design element that is to be cut by that tool.
For example, it is physically impossible for a 1/4" end mill to cut a 1/8" slot.
Performing Test Runs to Set the Optimal Machining Parameters
After preparing the design and selecting the appropriate cutting tool, perform test runs on a scrap piece of the material.
These test runs enable the user to determine the behavior of the material when machined under different process parameters.
Although the optimal settings for almost every material are readily available on the internet, it is always recommended to perform test runs before executing the actual cut.
This is because the optimal settings for a process vary from one setup to another, and executing the actual cut without performing the test run risks ruining your workpiece.
Generally, when performing test runs, it is advised to start with the lowest recommended settings and work your way up until you find the optimal results.
Placing the Workpiece on the Worktable of CNC / Workholding
After performing the test runs and finding the optimal parameters, prepare the workpiece for the cut.
Placing the workpiece securely on the worktable plays a vital role in determining the quality of the cut.
Workholding of the workpiece depends on the type of CNC machining to be performed.
Non-contact machining processes like laser cutting, EDM, plasma cutting, etc., do not exert strong cutting force on the material and therefore do not require strong clamping.
Whereas, when performing machining processes like CNC milling and waterjet machining that deliver strong cutting force to the workpiece, it is required to hold the workpiece securely in place by applying a strong clamping force.
Clamping of soft materials like aluminum and plastic can result in the deformation of the workpiece, and therefore, it is recommended to use an appropriate clamping system to hold them securely.
Executing and Monitoring the Machining Process
After placing the workpiece on the table, it's time to execute the cut.
CNC machines are automatic machines that can execute a process just by clicking a button.
However, it is always recommended to monitor the machining process for abnormal behavior and to avoid the risk of accidents.
Especially when working with CNC lasers, any abnormal behavior of the system can lead to fire and laser hazards.
Significance of The Number of Axes on a CNC Machine
The number of axes on a CNC machine determines its degree of freedom.
Most CNC machines are capable of producing 2D, 2.5D, or 3D cuts.
A 2D cut involves the movement of the cutting tool along the X and Y axes. This type of cut is used for simple applications like slot cutting and facing.
In 2.5D cutting, the position of the cutting tool along the z-axis can be altered.
However, when executing the cut, it can only translate in one plane (XY-plane) and is generally used for cutting 3D contours with no overhanging elements.
Whereas in 3D cutting, the tool can be moved along the X, Y, and Z-axis simultaneously.
Therefore, a CNC machine with a higher degree of freedom can produce complex shapes with a quick cycle time.
Generally, the higher the number of axes on a CNC machine, the higher its functionality and cost will be.
3-axis CNC Machines
A 3-axis CNC system is one in which the tool moves along the X, Y, and Z axes to perform the desired cut.
The X-axis runs along the length of the work table, Y-axis involves the movement of the tool towards the front and back of the work table, and Z-axis represents the plunging of the tool in and out of the workpiece.
Generally, a 3-axis CNC consists of a stationary workpiece placed on the worktable while the tool moves along the 3-axis to trace the required pattern.
Whereas certain CNC machines, like milling machines, consist of a stationary tool that moves up/down along the z-axis while the workpiece moves in the XY plane to perform the desired cut.
3-axis CNC machines are ideal for simple cuts that are relatively shallow and do not involve intricate patterns.
A 3-axis system can also perform complex cuts similar to a 4 or 5-axis system, but at the expense of quality and time.
This reduces productivity and makes 3-axis system inefficient for producing complex geometries.
Nevertheless, it is one of the most popular systems in CNC machines that are used for applications such as boring, tapping, milling, drilling, laser cutting/engraving, etc.
4-axis CNC Machines
4-axis CNC machines support the movement of the cutting tool in the X, Y, and Z planes along with a rotary axis.
The tool or the workpiece rotates either along the horizontal axis (A-axis) or along the vertical axis (B-axis).
This improves the functionality of the CNC machines and allows to make cuts or drill holes in the side of the workpiece.
One of the most common applications of 4-axis CNC machines is the ability to cut or engrave cylindrical objects.
Most CNC manufacturers provide an optional rotary kit that transforms your 3-axis CNC machine into a 4-axis system.
5-axis CNC Machines
5-axis CNC machines are one of the most complex CNC machines that can make cuts along the XYZ plane and support two rotating axes.
These machines allow the machinist to work on the workpiece from 5 sides without the need of re-orienting the workpiece.
A 5-axis CNC machine can produce complex geometries with a quick cycle time, ideal for processes such as micro-machining of medical and aerospace products.
Furthermore, having a high degree of freedom eliminates the need for a long tool, thereby reducing the chances of tool failure and increasing the ability to deliver strong cutting force.
However, the CAD/CAM programming involved with a 5-axis system can be extremely complicated and requires an experienced machinist with good designing skills.
Moreover, the initial cost and maintenance costs of these machines can be extremely high.
How Much do CNC Machines Cost?
CNC machines are ideal for automating a process to produce intricate designs, but how much do CNC machines cost?
With constant development, these machines are available in various sizes, suitable for large-scale industries to small-size Hobbyist workshops.
Based on a CNC machine's size and machining capabilities, it can cost as low as $150 to as high as $300k or even higher.
For beginners who want to explore and tinker with CNC machines, a 3D printer or a laser cutter/engraver under $500 is the ideal choice.
Whereas for an experienced user or a user looking to work with wood projects, a CNC router under $5000 should be a preferable tool.
Advantages of Using a CNC Machine
CNC machines provide various advantages over traditional cutting tools, which leads to their increasing popularity in the manufacturing industry.
Automation and Flexibility
One of the basic advantages of a CNC machine is the ability to automate a process.
Automation reduces the risk of human injury and enables to perform machining on applications that could be significantly dangerous for a human operator.
It also eliminates the factor of human error during machining and produces high-quality outputs.
Furthermore, the ability to perform multiple operations on a single CNC machine enhances its flexibility.
For example, a CNC laser cutter can cut and engrave a material just by changing its machining parameters without changing the tool.
Precision and Speed
CNC machines can produce cuts with high precision and minimum cycle time.
This enhances the productivity of the process, thereby increasing the production rate.
Furthermore, with good designing skills and a capable CNC machine at hand, you can produce intricate patterns with utmost precision in almost any material.
Quality and Repeatability
The cuts made by a CNC machine produce smooth edges with a good surface finish and often do not require any secondary finishing process.
Furthermore, after preparing a design and generating the G-codes, a CNC machine can execute the cut repeatedly with identical results every single time.
This makes them ideal for mass production, where a similar product is produced in large volumes.
Limitations of CNC Machines
Despite the advantages offered by CNC machines over traditional cutting tools, various limitations discourage users from upgrading to CNC machines.
High Initial Cost
Although CNC machines are available in cheap price ranges, a machine suitable for industrial applications can be quite costly.
Furthermore, the operating cost of a CNC machine is generally higher than their traditional counterparts.
This prevents small-scale businesses from upgrading to CNC machines.
A cheaper alternative is to look for used CNC machines.
Used CNC machines are readily available as large-scale industries are always looking to upgrade their tools with best-in-class CNC technology.
However, when purchasing a used CNC machine, it is recommended to have an experienced machinist by your side to evaluate the condition of the machine.
Steep Learning Curve
Operating a CNC machine can be quite complicated for an amateur machinist or hobbyist.
It requires familiarity with CAD software and good drafting skills to prepare a good design.
Furthermore, perfecting the machining parameters require adequate working experience with the machine.
However, community forums and tutorial videos from the manufacturer will help minimize the learning curve, and once you get the hang of it, CNC machines are relatively easy to operate.
CNC machines are capable of machining workpieces of various thicknesses.
However, as the thickness of the material increases, the machining speed decreases.
Furthermore, machines such as laser cutters and entry-level CNC routers are generally not capable of cutting thick materials.
This makes CNC machines preferable for cutting relatively thin workpieces over thick workpieces.
Frequently Asked Questions (FAQ)
What products can be made on a CNC machine?
A CNC machine can produce various types of products ranging from paper cutouts to metal artwork. CNC machines can be used for making DIY projects from various materials like wood, acrylic, leather, plastic, metal, etc. Apart from that, you can also make various profitable CNC projects that you can sell.
Should I buy a CNC router or a 3D printer?
The answer to the debate between CNC routers and 3D printers lies in your requirement. A CNC router can produce smoother parts with a high surface finish, whereas a 3D printer is more suitable for prototyping.
Furthermore, a CNC router is a preferable choice for producing high-precision mating parts.
Can you perform CNC machining on 3D-printed parts?
Yes, you can perform CNC machining on 3D-printed parts. The parts produced by a 3D printer fuse into a solid object that can undergo various CNC machining processes such as laser engraving, CNC milling, turning, etc.