You probably know what CNC turning is and how it works, but let's remember a few basics to fully understand its operation and how to best use this machine. Want to start your adventure with CNC? Familiarize yourself with basic knowledge about the construction of this type of machines. Read our guide, which discusses the most important elements. Welcome!
How does a CNC lathe work?
On a CNC lathe, the geometry of the workpiece is shaped by removing material. During turning, the workpiece material rotates around a hypothetical axis mounted in the holder. This axis is the Z-axis and contains the main movement, the rotational movement. The cutting tool performs the secondary movement, the feed. When the tool comes into contact with the surface of the workpiece, the material is removed, this is how cutting on a lathe takes place. In practice, it looks like this: the cutting tool is on one side of the diameter of the workpiece on the X-axis, and the diameter of the machined surface is created around the Z-axis. The cutting tool can move longitudinally or transversely. It is worth noting that objects can have different geometries. We note that the geometry of the workpiece depends on the combination of two axes (rotation and feed) and the cutting tool used. In turn, the cutting tool to be used depends on the type of surface produced and the type of turning operation.
What are 2-axis and 3-axis lathes?
While reading basic materials about the construction of CNC machines, you will certainly find information about 2-axis and 3-axis lathes. The difference between them lies in the number of axes - as the name suggests.
- In a 2-axis lathe, machining takes place on 2 axes.
- In a 3-axis lathe, machining takes place on 3 axes.
It is said that three-spindle lathes offer more options. But let's explain this a bit better. What we've said so far applies to 2-axis lathes: X and Z; both axes are programmed. Then the tool moves linearly while the part rotates around its own axis. The linear movement of the tool can run along or perpendicular to the Z axis of the part. Three-axis lathes include an additional axis that can be rotated. This is called the C axis. From a 3-axis lathe, you can certainly expect more. Thanks to the 3-axis lathe, some milling operations can be handled and programmed. To be more precise, you can change the position of the tool around the rotation axis of the part and perform slots, holes, and several peripheral elements according to the drawing requirements.
Why are CNC lathes so popular?
The lathe is one of the oldest machines ever invented. However, modern lathes have undergone significant modernization to incorporate several technological achievements. Lathes have evolved in parallel with other types of machine tools and are now widely used in the industry. But why has the lathe gained the trust of the industry and such worldwide popularity? What are the advantages of modern lathes and turning centers? The answer to this question is speed, accuracy, and production automation. Thanks to a wide range of lathes and special-purpose products currently available on the market, many industrial applications can be serviced with high productivity. Additionally, thanks to advances in hard coating technology and tools, a typical lathe can process:
- ferrous and non-ferrous metals,
- polyamides,
- thermoplastic materials,
- non-metallic engineering materials,
- wood and many others.
All this makes CNC lathes and turning centers the most efficient machines. It is estimated that 40% of all metal cutting operations are performed on CNC lathes, where most of the production involves rotating parts. Therefore, it should not surprise anyone that this equipment is so popular!
Milling vs. turning in CNC machines
If you understand how turning works, you will easily understand how milling works, as it is the reverse process. During turning, the tool moves linearly and the part rotates, but during milling, the tool rotates and the part moves linearly along the x and y axes of the table. In milling, the Z axis is the vertical axis of the tool. This is their fundamental difference. Moreover, in CNC turning, single-point tools are often used, so turning is a "continuous cutting" process. On the other hand, mills (e.g., end mills, finger mills, etc.) are multi-point cutting tools, in other words, they have multiple tips. Therefore, milling is theoretically an "interrupted" process. Attention should also be paid to primary and secondary movements.
- Secondary motion is a linear motion or feed.
- Primary motion is always a rotary motion.
This means that during CNC turning, the main motion takes place in the part, and the secondary motion in the tool. During CNC milling, you make large movements on the tool and minor movements on the part.
What element can be turned using a CNC machine?
Thanks to the above information, you can easily understand the application of turning and milling, considering the geometry and features of your part. When it comes to rotary parts, choose turning for the machine. For prismatic parts with minor milling properties, you can choose the Milling option. However, if your workshop is equipped with additional materials, such as an indexer or a faceplate for milling or a third axis (C axis) for CNC turning, you can perform milling operations on a lathe on a CNC milling machine using dynamic tools and turning operations. Additional equipment, however, involves a significant financial cost, which must be justified by the cost of producing the product. In CNC machining, productivity and success depend not only on the proper selection of the machine tool, but also on other factors such as:
- the proper tool,
- conditions for clamping and cutting.
It's not enough to know which part will be better for what kind of processing. It is always necessary to determine any additional conditions that must be met to ensure the best results.
CNC lathe construction
Have you seen a typical CNC lathe? Its system consists of several elements, each of which improves efficiency in its own way. However, six of them are the most important:
- bed frame,
- spindle,
- feed drive gear,
- "Norton" gear,
- lathe carriage,
- taper,
- lathe chuck.
Let's examine the above elements in order to understand the structure of a CNC lathe. The bed frame is the main body of the machine. All other major elements are connected and attached to it. Due to its high compressive strength and excellent lubrication properties, it is most often made of cast iron. The bed is made from castings and attached to the base. The guide rails placed on the bed of the machine ensure smooth and precise linear movement of the carriage and tailstock on the base.
Spindle
The spindle contains a drive unit for rotating the spindle, which in turn rotates the workpiece. The spindle is located on the left side of the table. The spindle contains a gearbox and a main shaft. Outside the spindle, there is a regulator and related modules
Feed gear wheel or "Norton"
The feed gearbox contains a system that regulates and maintains the linear speed of the tool in relation to the spindle of the workpiece. It consists of a feed controller and a lead screw, which rotates at the appropriate speed to achieve the desired feed rate.
Lathe carriage
The carriage is powered by a lead screw, so it follows the feed speed controlled by the Norton gearbox. The sliding table moves in a direction perpendicular to the carriage movement. By moving the carriage, the tool can be moved parallel to the working axis, which facilitates turning. By moving the slider, the tool can be radially inserted into the workpiece for planning, profiling, or cutting operations. Cutting tools are held in tool holders attached to cross slides mounted on the carriage. The carriage slides on the lathe rails, feeding the tool parallel to the axis of rotation and parallel to the spindle axis with high precision. The rails are built into the lathe bed and provide a rigid frame for the lathe.
Center
The center is located on the other side of the spindle, where the support center is mounted for the free end of the workpiece to balance it. In addition to centering, which is the main role of the center, drilling is performed using centering drills and conventional ones installed in the center assembly.
Lathe Chuck
The holder is used to hold the workpiece. It is mounted on the spindle of the rotary holder and the workpiece. A typical lathe holder may have three, four, or five jaws, depending on the machining requirements, standard part geometry, and the desired clamping force.
What are CNC lathe tools?
There are many different engineering materials available on the market, so it makes sense for cutting tools to be made from various materials. When you need to choose a material for your tool, never forget that during machining the tool is exposed to:
- high temperatures,
- high contact stresses
- strong shearing.
The material of the tool should be chosen appropriately for the product being processed. Your main goal should be to maintain the hardness and durability of the tool, especially at high temperatures. This will most likely result in the machining of parts with high-quality surfaces. However, other factors such as cutting conditions and the material being processed, which are crucial for success, also influence the final result. Cutting conditions are very important for the entire process. To maintain the stability of machining and tool wear at the lowest possible level, it is recommended to set the cutting conditions according to technical manuals and the tool manufacturer's recommended specifications. Proper setting of cutting conditions will also extend the tool's lifespan, reducing tool replacement costs. Other important factors to consider when choosing the material of a cutting tool are its wear resistance and chemical stability. Of course, the higher the wear resistance, the longer the tool's lifespan. As for the chemical stability of the tool, the rule is that the higher the better, as it prevents harmful reactions that can accelerate tool wear. The most commonly used tool materials for CNC turning are:
- carbide tools,
- ceramic tools,
- coated tools.
Using a CNC machine – benefits
No matter what part you want to make using CNC turning, below we present the advantages that will certainly convince you to this type of solution:
- Performance improvement and high precision - CNC turning involves simultaneous rotation of the part and linear movement of the tool. Thanks to this synergy of movements, the lathe is a very efficient machine tool. Moreover, due to the very high stability of CNC turning centers, a very high precision can be achieved.
- Generating less waste - As with any CNC machining process, the operator is responsible for setting up the task and ensuring that the CNC program is error-free and efficient. Operators do not participate or interfere with the production process. This, in turn, eliminates human errors, and thus significantly reduces the amount of waste.
- Constant working time - Free time includes tool exchange, part alignment and orientation, and rapid movements. Thanks to the automation built into CNC technology, downtime is significantly reduced.
- Safety - Due to the fully automated production environment in CNC turning, operators are kept away from the working area and are not physically involved in the production process.
We hope that the above guide has helped you understand the basic issues related to the use, operation, and construction of CNC machines. As you can see, CNC machines are a great tool that can help automate and improve the entire production process on a large scale.