What is a CNC process?
CNC stands for “Computer Numerical Control” and means that the machine is controlled by a set of commands issued by a controller. The command codes issued by the controller are usually in the form of a list of coordinates, called G-codes. Any machine controlled by such codes can be called a CNC machine, including milling machines, lathes and even plasma cutters. In this article, we will focus on the different types of CNC mills and lathes and their combinations.The movement of a CNC machine can be defined by their axes, which include the X-axis, Y-axis, and Z-axis, with more advanced machines also including the A-axis, B-axis, and C-axis.The X-axis, Y-axis, and Z-axis represent the major Cartesian vectors, while the A-axis, B-axis, and C-axis represent the rotation of the axes.A CNC machine will typically use up to 5 axes. Typical CNC machines are listed below.
CNC Lathe - This type of lathe works by rotating material in the lathe's chuck. The tool then moves on 2 axes and cuts out the cylindrical part. CNC lathes are able to create curved surfaces, whereas manual lathes make curved surfaces difficult, if not impossible. The tool is usually non-rotating, but can be moved if it is a power tool.
CNC Milling Machines - CNC milling machines are usually used to create flat parts, but the more complex machines have more degrees of freedom and are capable of producing complex shapes. The material is stationary and the spindle rotates with the tool, which moves in 3 axes to cut the material. In some cases, the spindle is stationary and the material moves.
CNC Drilling - This machine is similar to a CNC milling machine, but it is specifically designed to cut in only one axis, i.e., it only drills down into the material along the Z-axis and never cuts in the X- and Y-axis.
CNC Grinder - This type of machine allows grinding wheels to come into contact with the material to produce a high quality surface. It is designed to remove small amounts of material from hard metals; therefore, it is used as a surface preparation operation.
Reduced Material Manufacturing
CNC machining produces parts through subtractive manufacturing. This type of machining basically removes material from a solid billet, resulting in the desired shape. It can be accomplished by any of the methods we mentioned above, such as milling, turning, grinding, or drilling. Additive manufacturing is the opposite process, where material is added from scratch to form the final part, such as 3D printing.
Tooling
Tooling performs all cutting. The tools are usually mounted in a tool holder or loaded onto a spindle as needed. Many different tools are used in the manufacture of complete parts, and there is no one-size-fits-all manufacturing method. The following is a list of tools commonly used in typical machining.
Milling Tools
End Mills- An end mill is a common tooling tool that is usually capable of cutting in 3 directions. It comes in different styles such as flat, fillet radius, ball and taper shanks; with different number of flutes, helix angles, base materials and coating materials.
Face Milling Cutter- A face milling cutter cuts over a large surface area, also known as positive plane milling. The cutting edge is usually on the edge of the tool and the teeth are usually carbide inserts.
Thread Mills - Thread milling cutters create threads and work by rotating in a threaded fashion around an axle to cut a threaded shape.
Notch Mills - T-slots can be formed along the length of a part with these cutters. Due to the geometry of this tool, it must enter and exit through the open end of the material.
Lathe Tools
OD Turning - As the name implies, this tooling is designed to cut on the outside diameter of a part. It may be a solid tooling to machine the part to the desired shape, or it may be a carbide insert.
I.D. Grooving and Threading - These tools are usually thin enough to get inside the part after drilling to groove the I.D. or form threads on the inside.
Cutting - Cutting tools are used to cut the part as a final operation after all other operations have been completed.
Drilling - Used to drill holes in the longitudinal direction of the part, which must be reamed or drilled out to final tolerances.
Tooling Material
Tool types can be further subdivided by the material of the tool itself. The following is a list of commonly used tooling materials:
High Carbon Steel - It is the least expensive machining tool and does not last long. It loses its hardness at a temperature of about 200 degrees Celsius.
High Speed Steel (HSS) - It is more commonly used than carbon steel tools because it lasts longer and loses its hardness at 600°C, allowing it to cut faster.
Carbide - Carbide tools are harder than HSS, but less rigid and can break if not handled properly. It can withstand temperatures up to 900°C.
Ceramic - This type of cutting tool is extremely hard and is usually only used to cut hard materials at very high temperatures. It is available in two common materials, namely aluminum nitride and silicon nitride.
Cubic Boron Nitride - These tools are ideal for hardened steels and high temperature alloys and have excellent friction resistance and thermal resistance.
Advantages and Disadvantages of CNC Machining
CNC machining has gradually become a staple in the manufacturing industry because it is more efficient than using manually operated machines. Listed below are some of the advantages and disadvantages of CNC machines.
| Vantage | Drawbacks |
| Faster than manual operation | Expensive machine costs |
| Manual operation is not comparable to CNC machines in terms of speed and accuracy, and in a high-volume production environment, the use of manual machines can lead to economic losses | CNC machines are very advanced machines with very high tolerances and rigidity. It allows users to make millions of parts and guarantee high quality, but high quality also means high cost; and the more advanced the machine, the higher the cost. |
| Lower production costs | Highly skilled operators needed |
| If material loading and unloading is further automated, the CNC machine will be able to run uninterrupted without the need for personnel. Additionally, one operator can operate multiple machines, thereby offsetting the labor costs of turnaround. | Although the number of operators required is small, CNC machines require highly skilled operators, thus increasing labor costs. |
| Increased efficiency | Increased maintenance costs |
| CNC machines can be switched from one operation to the next in less than a second, and tool changes can be done very quickly because some machines have many tools pre-installed on the turret or have a tool magazine that allows new tools to be loaded onto the spindle when needed. | Due to the complexity of CNC machines, their maintenance costs are much higher than manual machines |
Types of CNC Milling and Turning Machines
CCNC Milling Machines
Vertical Machining Center (VMC) - The spindle of a vertical machining center remains in the same position and the lathe moves under it. In some cases, the lathe moves upward to contact the spindle, or the spindle can move up and down along the Z-axis. This type of machine is very rigid and therefore capable of producing high precision components. The disadvantage is that the working area is relatively small. a VMC may have 3 axes (X, Y, Z), 4 axes (X, Y, Z, A) or even 5 axes (X, Y, Z, A, B).
Horizontal Machining Centers (HMC) - HMC machines have a horizontal spindle rather than a vertical one. This type of machine is ideal for long production runs because it can machine up to three times as many parts as a VMC given enough workload. HMCs are much more expensive than VMCs. The HMC is much more expensive than the VMC. A piece of material can be fixed on the lathe of the machine while another part is being manufactured. This allows for continuous production, and the spindle can be easily moved to the next piece of material that is ready to be changed quickly.
CNC Lathe
CNC lathes are capable of machining with only one chuck and two spindles.CNC lathes are categorized into the following types:
General Lathe- It is basically a standard lathe and is relatively common. Its name includes the word “Engine” as this type of lathe used to be driven by pulleys of an engine mounted outside the machine. An ordinary lathe is a lathe with an electric motor on the lathe.
Turret Lathe - A turret lathe can significantly speed up production because all the tools needed are loaded onto the turret before they are made. When a new tool is needed, it is simply rotated to the appropriate position.
Toolroom Lathes - Toolroom lathes are used for high precision, low volume work. As the name suggests, these lathes are used to make tools and molds. Tool room lathes are also very versatile.
High Speed Lathe- This type of lathe is mainly used for light work and has a very simple structure consisting of a spindle box, tailstock and tool holder.
CNC Turning Centers- These lathes are very advanced and offer a range of features including milling and turret tool holders and even a second spindle. Turning centers are also classified as vertical or horizontal. Horizontal lathes allow the chips that fall from the part to go into a chip conveyor, while vertical lathes allow gravity to help remove the chips when the part is chucked into the chuck. Vertical lathes are easier to automate. The question of which type of lathe is more suitable depends on the specific application.
Materials
CNC machines are capable of handling a wide range of materials, from aluminum to high-temperature alloys such as Inconel. Each material has its own set of challenges and requires specific tooling, speeds and feeds.
Aluminum
Since aluminum is a very soft metal, there is a risk of sticking to the cutting tool. Given its low melting point, tempering aluminum properly to increase its hardness can improve its machinability.
Carbon Steel
Since there are many grades of steel, many factors affect the overall machinability of the material, such as cold work, chemical composition, microstructure and other factors. In general, elements such as lead and tin increase cutting speed due to lubrication, and sulfur reduces strain hardening of chips.
Titanium
Titanium is available in a number of alloy types, each with its own set of challenges. Ideally, the tool must be in constant interaction with the material, as staying in one area will result in friction, heat buildup, work hardening and tool wear. Pure titanium, which has properties similar to aluminum, also sticks to cutting tools, but its alloys are typically harder, which can lead to heat buildup and tool wear. Low rotational speeds and high chip loads can extend tool life by allowing temperatures to be reduced.
High Temperature Alloys
High-temperature alloys are very strong at high temperatures, making them difficult to machine. More powerful machines are required to machine these materials. High-temperature alloys work harden quickly, making subsequent machining more difficult. It is generally recommended to keep cutting speeds low.
Copper
Copper is known to be a very difficult material to machine as it has spreading properties and often coils around the tool and cannot be cut. It is mainly used in electrical components and heat exchanger assemblies where high electrical conductivity and high heat transfer coefficients are required. For pure copper, high speed feeds can usually be used. Copper alloys are much easier to machine than pure copper.
Plastics
Plastics are categorized into thousands of types, ranging from thermosets to common thermoplastics. The hardness and mechanical properties of plastics also vary widely. Only rigid plastics can be machined well enough to stay within tolerances, while soft plastics often deform when passed through cutting tools, resulting in parts that are out of specification in terms of form factor. Since plastic is an insulator, heat often builds up at the cutting edge and if care is not taken, the plastic will melt.
What can go wrong?
While CNC machines fulfill a wide range of uses and functions, there are some risks involved. Listed below are some of the errors that often occur in CNC machining.
CNC System Crash - A CNC machine does not think on its own; it only follows human instructions. If not programmed correctly, the machine can have a cutting tool cut into itself in a millisecond's time. The machine will usually detect a system crash and stop running, but by this time the damage may have already been done. There are a variety of software tools available to help minimize such risks. Before uploading the code to the machine, the tool's path of operation can be simulated. It is difficult to simulate more complex 5-axis machines using standard Computer Aided Manufacturing (CAM) software, and additional software is required between the time the CAM code is written and the time it is uploaded to the machine.
Improper speeds and feeds - Speed and feeds are critical to producing high quality machined components. If the wrong settings are used, they will accelerate tool wear and result in substandard surface finishes and tolerances. The proper setting of speed and feed is a complex subject, as each material and its alloy requires different settings to achieve the desired cut. It usually takes several attempts to achieve the right settings.
Lack of Maintenance- As with any complex machine, lack of maintenance can cause a CNC machine to break down quickly. Machines must be kept clean and follow a strict OEM maintenance program.
Major Industries Using CNC Technology
Any industry involved in the production of components is directly or indirectly affected by CNC machining. Below is a list of some of the major industries that utilize CNC machining.
Aerospace - Aerospace requires highly accurate and repeatable components, including turbine blades in engines, tooling to make other components, and even combustion chambers used in rocket engines.
Automotive and Machine Building - The automotive industry needs to make high precision molds for casting components such as engine seats or machining high tolerance parts such as pistons. Gantry machines can cast clay modules that are used in the design phase of an automobile.
Military - The military industry uses high precision components with tight tolerances, including missile components, gun barrels, etc. All machined parts in the military industry are molded to tight tolerances. All machined parts in the military industry benefit from the precision and speed of CNC machines.
Medical - Medical implantable devices are often designed to fit the shape of a human organ and must be manufactured using high grade alloys. CNC machines are a necessity as there are no manual machines capable of producing such shapes.
Energy- The energy industry encompasses all areas of engineering, from steam turbines to cutting-edge technologies such as nuclear fusion. Steam turbines require high-precision turbine blades to maintain equilibrium in the turbine, and the R&D plasma containment cavities in nuclear fusion have complex shapes made from advanced materials that require CNC machine support.
Current Trends in CNC Technology
As the pace of technological development has accelerated in recent years, we have a feeling that additive manufacturing will become the mainstream of CNC machining, but what is more likely to happen is that more and more emerging manufacturing centers are combining a variety of technologies into a single machine, thus taking full advantage of the strengths of subtractive and additive manufacturing machines, and developing machines with capabilities that are stronger than the sum of the two. Early applications of such machines are already emerging.
In addition, the great advances in automation made through the Fourth Industrial Revolution will lead to the development of even more automated systems that are capable of self-diagnosis and self-optimization with little need for human intervention. In the future, it is expected that products will be manufactured to the individual requirements of the consumer, and CNC machines, with their remarkable flexibility, can make this vision a reality.