CNC stands for "Computer Numerical Control," meaning a machine is controlled by a set of commands issued by a controller. The command codes issued by the controller are typically in the form of a list of coordinates called G-codes. Any machine controlled by this type of code is considered a CNC machine, including milling machines, lathes, and even plasma cutters. In this article, we'll focus on different types of CNC milling and lathes, as well as combinations thereof. The movement of a CNC machine can be defined by its axes, including the X, Y, and Z axes, and more advanced machines also include the A, B, and C axes. The X, Y, and Z axes represent the primary Cartesian vectors, while the A, B, and C axes represent the rotations of the axes. CNC machines typically use up to five axes . Typical CNC machines are listed below.
· CNC lathes – These lathes work by spinning the material in the lathe's chuck. The tool then moves along two axes , cutting out cylindrical parts. CNC lathes can create curved surfaces that would be difficult or impossible with a manual lathe. The tool typically does not rotate, but can move if it is a powered tool.
· CNC milling machines – CNC milling machines are typically used to create flat parts, but more complex machines have more degrees of freedom and can produce intricate shapes. The material is stationary while the spindle rotates along the tool, which moves along three axes to cut the material. In some cases, the spindle is stationary while the material moves.
· CNC Drill – This machine is similar to a CNC mill, but it is specifically designed to cut along one axis only. This means it only drills down into the material along the Z axis and never cuts along the X and Y axes.
· CNC Grinding Machine – This machine uses an abrasive wheel to engage the material, producing 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.
CNC machining produces parts through subtractive manufacturing . This process essentially removes material from a solid blank to create the desired shape. This can be accomplished using any of the methods mentioned above, such as milling, turning, grinding, or drilling. Additive manufacturing , like 3D printing, works in reverse, adding material from scratch to create the final part.
The tooling performs all the cutting work. Tools are typically mounted on toolholders or loaded onto spindles as needed. Many different tools are used to create a complete part; there's no one-size-fits-all approach. The following list includes tools commonly used in typical machining operations.
End mills – End mills are a common tooling tool that typically cuts in three directions . They come in different styles, such as flat nose, corner radius, ball nose, and tapered shank . They also have varying numbers of flutes , helix angles, base materials, and coatings.
Face milling cutters – Face milling cuts are used to cut over a large surface area, also known as flat face milling. The cutting edge is usually on the tool edge, and the milling teeth are usually carbide inserts.
Thread Mills – Thread mills create threads and work by rotating in a threading pattern around the axle, cutting the thread form.
Slot Mills – Use this type of mill to create T-slots along the length of a part. Due to the geometry of this tool, it must enter and exit from the open end of the material.
OD Turning – As the name implies, this tooling is designed to cut on the outside diameter of a component. It may be a solid tooling that machines the component to the desired shape, or it may be a carbide-inserted Taiwan precision grinder.
ID Grooving and Threading – These tools are typically thin and are designed to reach into a component after drilling to create a groove on the inside diameter or to create internal threads.
Cut-off – Cut-off tools are used to cut off parts after all other operations are completed, with CNC surface grinding as the final operation.
Drilling – Used to drill holes in the longitudinal direction of a component. The holes must be reamed or drilled out to achieve final tolerances.
Tool types can be further subdivided according to the material of the tool itself. The following is a list of commonly used tooling materials:
· High Carbon Steel – This is the lowest cost steel for machining tools, but does not last very long. It loses hardness at approximately 200 °C .
· High Speed Steel (HSS) – This is more commonly used than carbon steel tools because it lasts longer and does not lose hardness until it reaches 600°C, allowing it to cut at faster speeds.
· Carbide – Carbide tools are harder than HSS but less rigid and can break if not handled properly. They can withstand temperatures up to 900 °C .
· Ceramics – These cutting tools are extremely hard and are generally only used to cut hard materials at very high temperatures. Two common materials are aluminum nitride and silicon nitride.
· Cubic Boron Nitride – These tools are well-suited for hardened steels and high-temperature alloys, offering excellent friction and thermal resistance.
Advantages and disadvantages of CNC machining
CNC machining has gradually become mainstream 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.
CCNC milling machines
· Vertical Machining Center (VMC) – In a vertical machining center, the spindle remains in the same position, while the lathe moves underneath it. In some cases, the lathe moves upward to contact the spindle, or the spindle can move up and down along the Z axis. These machines are highly rigid, enabling the production of high-precision components. Their disadvantage is their relatively small work envelope. VMCs may have three axes (X, Y, Z), four axes (X, Y, Z, A), or even five axes (X, Y, Z, A, B).
· Horizontal Machining Center (HMC) – An HMC machine has a horizontal rather than vertical spindle. These machines are ideal for long production runs because, given sufficient workload, they can produce up to three times as many parts as a VMC . HMCs are significantly more expensive than VMCs. A piece of material can be secured to the machine's lathe while another part is being manufactured. This allows for continuous production, allowing the spindle to easily move to the next piece of material as it becomes available, allowing for quick changes.
CNC lathes
CNC lathes are capable of machining with only one chuck and two axes. CNC lathes are classified into the following types:
· A standard lathe – This is essentially a standard lathe that's relatively versatile. The word "Engine" is in its name because these lathes were traditionally driven by a pulley from an engine mounted outside the machine. A standard lathe is one that has an internal motor.
· Turret Lathes – Turret lathes significantly speed up production because all the required tools are loaded onto the turret before they are manufactured. When a new tool is needed, it is simply rotated into position.
· Toolroom Lathes – Toolroom lathes are used for high-precision, low-volume work. As the name suggests, these lathes are used to create tools and dies. Toolroom lathes are also very versatile.
· High-speed lathes – These lathes are mainly used for light work and have a very simple structure consisting of a headstock, tailstock and tool holder.
· CNC turning centers – These lathes are highly advanced and offer a range of capabilities, including milling, turret tooling, and even a second spindle. Turning centers are also categorized as either vertical or horizontal. Horizontal lathes allow chips falling from the part to enter a chip conveyor, while vertical lathes use gravity to remove chips as the part is engaged in the chuck. Vertical lathes are also more easily automated. The most appropriate type of lathe depends on the specific application.
Material
CNC machines are capable of processing a wide range of materials, from aluminum to high-temperature alloys like Inconel . Each material presents its own set of challenges, requiring specific tooling, speeds, and feeds.
aluminum
As aluminium is a very soft metal, there is a risk of it sticking to cutting tools. Given its low melting point, proper tempering of the aluminium to increase its hardness can improve its machinability.
carbon steel
Since steel comes in many grades, many factors affect the overall machinability of the material, such as cold work, chemical composition, microstructure, etc. Generally speaking, elements such as lead and tin can increase cutting speed due to lubrication, and sulfur can reduce strain hardening of the chip.
titanium
Titanium comes in many alloy types, each presenting its own challenges. Ideally, the tool must be in constant contact with the material, as resting in one area will cause friction, heat buildup, work hardening, and tool wear. Pure titanium behaves similarly to aluminum and will also stick to cutting tools, but its alloys are generally harder, potentially leading to heat buildup and tool wear. Slow rotational speeds and high chip loads extend tool life by keeping temperatures down.
High temperature alloy
High-temperature alloys are difficult to machine due to their high strength at high temperatures. More powerful machines are required to process these materials. They work harden quickly, making subsequent machining more difficult. It is generally recommended to keep cutting speeds low.
copper
Copper is notoriously difficult to machine due to its ductility, often curling around tools and preventing cutting. It is primarily used in electrical components and heat exchangers, where high electrical conductivity and heat transfer coefficients are required. High feed rates are often possible with pure copper. Copper alloys are significantly easier to machine than pure copper.
plastic
There are thousands of different types of plastics, ranging from thermosets to common thermoplastics. Their hardness and mechanical properties also vary widely. Only rigid plastics can be machined well within tolerances, while soft plastics often deform when passing through cutting tools, resulting in parts that are not dimensionally specified. Because plastic is an insulator, heat often accumulates at the cutting edge, melting the plastic if care is not taken.
While CNC machines offer a wide range of uses and capabilities, they also come with some risks. Listed below are some common mistakes that can occur during CNC machining.
CNC system crashes – CNC machines don't think for themselves; they simply follow instructions. If programmed incorrectly, a machine could cause a cutting tool to cut into itself in a millisecond. The machine typically detects a system crash and stops, but by then damage may have already been done. Several software tools can help mitigate this risk. Tool paths can be simulated before uploading the code to the machine. Simulating complex 5-axis machines is difficult using standard computer-aided manufacturing (CAM) software, requiring additional software between writing the CAM code and uploading it to the machine.
Improper Speeds and Feeds – Speeds and feeds are critical to producing high-quality machined components. Using the wrong settings will accelerate tool wear, resulting in substandard surface finishes and tolerances. Properly setting speeds and feeds is a complex subject, as each material and its alloy requires different settings to achieve the desired cutting results. It often takes several attempts to achieve the right settings.
Lack of Maintenance – As with any complex machine, a CNC machine can quickly break down due to lack of maintenance. The machine must be kept clean and the OEM maintenance schedule must be strictly adhered to.
Any industry involved in component production will be directly or indirectly impacted by CNC machining. Listed below are some of the major industries that use CNC machining.
Aerospace – Aerospace requires components with high precision and repeatability, including turbine blades in engines, tooling for making other components, and even combustion chambers used in rocket engines.
Automotive and machine building – The automotive industry requires the creation of high-precision molds for casting components, such as engine blocks, or machining high-tolerance parts, such as pistons. Gantry machines cast clay modules and are used during the design phase of a car.
Military – The military industry uses high-precision components with tight tolerances, including missile components and gun barrels. All machined parts in the military industry can benefit from the accuracy and speed of CNC machines.
Medical – Medical implants are often designed to fit the shape of human organs and must be manufactured from high-grade alloys. Since no manual machine can produce these shapes, CNC machines are a necessity.
Energy – The energy industry encompasses all areas of engineering, from steam turbines to cutting-edge technologies like nuclear fusion. Steam turbines require highly precise blades to maintain balance within the turbine. Nuclear fusion R&D involves complex shapes of plasma suppression cavities, manufactured using advanced materials, requiring the support of CNC machines.
With the rapid pace of technological development in recent years, we feel that additive manufacturing will become the mainstream of CNC machining, but it is more likely that more and more emerging manufacturing centers will combine multiple technologies into a single machine, thereby leveraging the advantages of subtractive and additive manufacturing machines to develop machines that are more powerful than the sum of their parts. Early applications of such machines are already emerging.
Furthermore, the Fourth Industrial Revolution has significantly advanced automation, leading to the development of increasingly automated systems capable of self-diagnosis and self-optimization, requiring minimal human intervention. In the future, products are expected to be manufactured to individual consumer specifications, and the exceptional flexibility of CNC machines makes this vision a reality.
