You probably know what is CNC turning and how it works; but let’s remember again some of its basic operations so that you fully understand its fundamentals and make the best use of the overall article.
How a CNC Lathe Machine works
With CNC lathes, the geometry of machined components is formed by removing the material. During turning, the workpiece rotates about a hypothetical axis, which is held in the lathe chuck. This axis is the Z axis and holds the major motion which is the rotary motion. The cutting tool has the secondary motion that is the feed.
When the cutting tool comes in contact with the workpiece surface, the material is removed; this is how cutting is done with lathes. In more detail, the cutting tool is on one side of the workpiece diameter on X-axis and the machined surface is produced diametrically, around the Z axis. The cutting tool can move either longitudinally or transversely (i.e. in a straight line parallel to the workpiece’s Z-axis, or transverse to the axis of the piece) or even as a combination of longitudinal and transversal motions.
The workpiece can take several geometries. Which geometry the workpiece will take depends on the combination between the two axes (rotation and feed) as well as the cutting tools to be used. In turn, the cutting tools to be used depend on both the kind of the desired surface to be produced as well as the type of turning operation (for instance, either internal or external).
But it is not only about the typical CNC turning. In modern CNC lathes, a variety of other machining operations can be performed. These include:
• Contour turning – the tool follows a contour that is other than straight, thus creating a contoured form in the turned part.
• Taper turning – the tool is fed at an angle, thus creating a tapered cylinder or conical shape.
• Chamfering – the cutting edge of the tool is used to cut an angle on the corner of the cylinder, forming a “chamfer.”
• Facing – the tool is fed radially into the rotating workpiece starting from the outside or the inside diameter on one end, to create a flat surface on the face of the workpiece.
• Form turning – the tool has a shape that is imparted to the work by plunging the tool radially into the work).
• Boring – a single-point tool is fed linearly, parallel to the axis of rotation, on the inside diameter of an existing hole in the part.
• Drilling – it can be performed on a lathe by feeding the drill into the rotating work along its axis. Reaming can be performed in a similar way.
• Grooving/Cut-off – the tool is fed radially into the rotating work at a location along its length to either groove or to cut off the end of the part. Cut-off operation is sometimes referred to as parting.
• Threading – a pointed tool is fed linearly across the outside surface of the rotating work part in a direction parallel to the axis of rotation at a large effective feed rate, thus creating threads in the cylinder.
• Knurling – this is not a machining operation because it does not involve cutting of material. Instead, it is a metal forming operation used to produce a regular cross-hatched pattern in the work surface.
2-axis and 3-axis lathes
You surely have heard about 2-axis and 3-axis lathes. Their difference is what their names imply. With 2-axis lathes the work is done on 2 axes, whereas with 3-axis lathes the work is done on 3 axes. Apparently, 3-axis lathes offer more possibilities.
But let’s shed some more light on this.
What we have said so far refers to 2-axis lathes: X and Z; and you program two axes. Then, the tool moves linearly as the part is rotated around its axis. The linear travel of the tool can be longitudinally or vertically to the imaginary axis (Z-axis) of the part. This is the simple kinematical principle for a lathe which is shown in the figure below.
3-axis lathes include an additional axis, which is rotary. It is the so-called C-axis. With 3-axis lathes, you can surely expect to do more.
With 3-axis lathes, you can handle and program some milling operations. More specifically, you can change the orientation of the tool with respect to the part’s rotational axis and perform slots, holes, and several peripheral features, according to your drawing’s requirements.
By using a “live” tool (C rotary axis), you practically add a rotational motion to your tool. In order for the “live” tools to be functional, they should have their own servomotor or an identical motion-transmission system.
To better understand the kinematics of a CNC turning center, we will show you a video later. It is the video that shows how the final part of the picture below is machined.
But first, let's examine some features of the final part. As you see, it is a rotational part, yet with a milled surface in its outer diameter. On the surface, four holes have been drilled; two of them have been tapped as well. The flange of the part has peripheral holes. It is a complete part with turning and milling features.
The video below shows how this part is made in a 3-axis CNC turning center. You will see 2-axis, as well as 3-axis machining operations, all done in the same setup. Let's watch it :)
For a better understanding, let's describe below in more detail the steps of the machining operations shown in the video. The pictures below are screenshots of the video.
First, the part is clamped to the lathe chuck and a face cutting is performed by vertically moving the tool in X-axis while the part is rotated.
What follows next, is the roughing operation for the part’s diameter. This is done by moving the tool longitudinally in Z-axis. In addition, you can see that a smaller diameter is also machined in the part.
After changing the tool to select a thread cutting tool, a threading operation is performed in the small diameter of the part. This is done by moving the tool longitudinally in Z-axis in a sequential manner (as shown in the video).
So far you have seen indicative examples for 2-axis CNC turning, right? Now let's see the rest operations which are all based on the capabilities that the 3rd axis offers.
Next is a milling operation performed using a flat end-mill which has a rotational motion as a “live” tool. The tool machines a flat surface on the cylindrical contour of the part.
Having machined the flat surface on the cylindrical contour of the part, drilling is conducted, to produce the holes. You will notice that the drill has its own rotational motion with respect to the part. The part has stopped its rotation!
Next, you can see that a tapping tool is selected to perform tapping operations in the holes. This is also a cutting tool with its own rotation with respect to the part.
In the last picture, a drill is now ready to machine a set of peripheral holes to the part’s flange. In this case, the lathe chuck operates as an indexer, to properly position the part at the correct angle.
Why are CNC lathes so popular?
The lathe is one of the oldest machines invented; however, modern lathes have been significantly upgraded by incorporating several technological achievements. The lathes have evolved in parallel with other types of machine tools and as a result, they are widely used by industry today.
But why the lathes have gained the trust from the industry? What are the benefits of modern lathes and turning centers?
In a few words, the answer is production speed, accuracy and automation.
But it’s not only that. Due to the variety of specialized turning machine tools and lathes which are available in the market today, numerous industrial applications can be handled with high level of productivity. Also, with the progress of technology in hard coatings and cutting tools, a typical lathe can process ferrous and non-ferrous metals, non-metallic engineering materials, polyamides, thermoplastics, wood, and so on.
All the above have made CNC lathes and turning centers the most productive machines. Actually, it is said that 40% of metal cutting operations are executed in CNC lathes; and most of the production with CNC lathes is about rotational parts. This is the nature of a lathe!
Differences between CNC Turning & Milling
If you understand how turning works you can easily understand how milling works; because it’s exactly the opposite. In turning the tool moves linearly and the part rotates; but in milling, the tool rotates and the part moves linearly in X and Y axes of the working table. In the case of milling, Z-axis is the vertical axis of the tool.
That’s their fundamental difference. But it is not their only difference.
In CNC turning, you normally use single-point cutting tools; this is the reason why turning is a “continuous cutting” process. On the other hand, milling tools – such as cutters, end-mills and others – are multi-point cutting tools; in other words they multiple cutting edges. Therefore, milling machining is theoretically an “interrupted cutting” process.
You should also be aware of the major and secondary motions. The major motion is always the rotational motion, whereas the secondary motion is the linear motion, i.e. the feed rate. This suggests that in CNC turning, you have your major motion in the part and your secondary motion in the tool. In CNC milling, you have the major motion on the tool and the secondary motion on the part!
Is my part a good fit for CNC turning?
Based on the above information you might easily understand the applicability of turning and milling given the geometry and features of your part. When it comes to rotational parts, you select turning to machine. If it is about prismatic parts with several milling features, you select milling.
Nevertheless, if your machine shop is equipped with additional apparatus like indexers or trunnion tables for milling, or a third axis (C-axis) for CNC turning, you can execute milling operations in turning machines (with “live” tools) and turning operations in CNC milling machines. However, the additional equipment comes with its cost and this should be justified by the cost of products you manufacture
In CNC machining, productivity and success not only depend on the proper selection of the machine tool, but also on other factors such as suitable tools, work-holding and cutting conditions. It is not enough to know what part is more proper for which type of machining. You should always be sure about some additional conditions that need to be satisfied on order to secure the best possible outcome.
Let’s dive into this a bit more.
In overall, no matter what type of machinery you have or what type of machining you have to execute, there are some general tips and guidelines you should keep in mind. For example, you should always check the coolant type and your cutting conditions; also, you should be aware of the work-piece material and its properties in order to ensure the best possible result and accuracy of your machining work; that is because different materials with different properties behave differently in machining.
In addition, keep in mind that the stability and power of your machine are key factors affecting the accuracy and productivity of your manufacturing process, no matter whether you work in milling or turning. It is critical to know the possibilities of your machines in terms of stability and power – as these are defined in the manufacturer’s technical specs – so that you can judge how demanding work they can execute, basically in terms of high-speed machining, different-to-cut materials, etc.
An important issue will always be the configuration of your equipment, given any of its technological constraints. Generally speaking, the simpler your machinery, the more limited work it can execute.
CNC Turning Centers
The most critical components of a lathe
Have you ever had the chance to observe a typical CNC lathe? Several components comprise its system and each one contributes to operational efficiency with its own way.
However, six of them are the most critical:
- Machine bed
- Feed gearbox or “Norton” gearbox
Let’s examine them one by one.
The machine bed is the main body of the machine. All other main components are attached and fastened on it. It is usually made with cast iron due to the high compressive strength and high lubrication quality of this alloy. The machine bed is made by casting operations and it is secured on floor space. The guideways – which are positioned on the machine bed – ensure the smooth and accurate linear motions of carriage and tailstock on the bed.
The headstock contains the drive unit to rotate the spindle, which in turn rotates the workpiece. The headstock is on the left upper side. The headstock contains the gearbox and the spindle. The outer part of the headstock contains the speed controller and its associated modules.
Photo source: https://www.quora.com/What-is-a-lathe-headstock
Feed gearbox or “Norton” gearbox
The feed gearbox contains the system which is responsible for adjusting and maintaining the linear speed of the cutting tool in relation to the spindle of the workpiece. It consists of the feed controller and the lead screw that rotates at the proper speed to obtain the desired feed rate.
Photo source: https://www.pinterest.com/pin/598626975464894518/
The carriage is driven by the lead screw and thus it follows the feed rate controlled by the Norton gearbox. The cross-slide feeds in a direction perpendicular to the carriage movement. By moving the carriage, the tool can be fed parallel to the work axis to perform straight turning. By moving the cross-slide, the tool can be fed radially into the work to perform facing, form turning, or cut-off operations. The cutting tool is held in a tool post fastened to the cross-slide, which is assembled to the carriage. The carriage slides along the slideways of the lathe to feed the tool parallel to the axis of rotation with great precision for parallelism, relative to the spindle axis. The slideways are built into the bed of the lathe, providing a rigid frame for the lathe.
Photo source: http://www.lathes.co.uk/wabeco/page8.html
The tailstock is placed at the opposite side of the headstock, in which the center is mounted to support the free end of the workpiece to counterbalance it. Apart from centering, which is the major role of the tailstock, drilling is also performed by using center drills and common drills which are attached in the tailstock unit.
The chuck is used to clamping and hold the workpiece. It is fastened on the spindle that rotates the chuck and workpiece. Typical lathe chucks can comprise three four or five jaws depending on machining requirements, standard part geometries, and clamping forces required.
Types of CNC turning centers
There is a wide range of CNC Turning Centers in the market. Usually, the available CNC Turning Centers are categorized based on the number of their working axes and their spindle orientation.
Types of CNC Turning Centers based on the number of working axes
With reference to the number of working axes, there are the following types:
- 2-axis CNC turning centers. They are the simplest CNC turning machine tools and can execute typical machining operations, such as internal / external diameter machining, facing, drilling and tapping.
- 3-axis CNC turning centers. They are equipped with X axis, Z axis, and one rotary axis, the so-called “C” axis.
- 4-axis CNC turning centers. In addition to the X axis, Z axis and the “C” axis, they are equipped with the “Y”-axis, which allows them to execute “off-center” machining operations, required for complex components.
- 5-axis CNC turning centers. In addition to the 4 axis mentioned above, they are equipped with an additional turret that allows two cutting tools to operate simultaneously.
Types of CNC Turning Centers based on the orientation of the spindle
With reference to the orientation of the lathe spindle, there are two types of CNC Turning Centers in the market: the vertical and the horizontal ones.
- Vertical CNC turning machines. The spindle is perpendicular to the machine table. The machine table is rotational and has a large diameter to accommodate large parts.
- Horizontal CNC turning machines. This type is the most common type of CNC turning machining centers. The part is normally clamped to chuck which is rotated, while the tool executes the linear motion resulting to feed per revolution of the part.
Cutting Tools for CNC Turning
There is a wide variety of engineering materials in the market, so it is very reasonable that there is also a wide range of materials which cutting tools are made from.
When you need to select the material for your cutting tool, never forget that during machining, the cutting tool is subjected to high temperatures, high contact stresses and high shearing forces. The material of the cutting tool should be selected based on the material that the cutting tool will machine.
Your major goal should be to maintain the hardness and strength of the tool, especially at elevated temperatures. This will most likely lead to machined parts with high-quality surfaces. However, other factors affect the final outcome, such as the cutting conditions and the material to be machined, which are critical success factors.
Cutting conditions are very critical. In order to maintain stability of machining and keep the tool wear rate as low as possible, it is highly recommended that the cutting conditions are set according to technical handbooks and recommended specs of the tool manufacturer. Setting the cutting conditions properly will also result in a longer tool life and consequently, reduce the tool replacing costs.
Other important factors you should take into consideration when selecting the cutting tool material, are its wear resistance and chemical stability. Obviously, the stronger is the wear resistance, the longer the tool life. Regarding the chemical stability of the tool, the higher it is, the better prevents adverse reactions that may accelerate tool wear.
The most often-used cutting tool materials for CNC turning are carbides, ceramics and coated tools. These cutting tools are commercially available in the form of cutting inserts with standard geometry, as you can see in the following picture.
The geometry you should select depends on the application.
Another factor playing a critical role is the tool holders. Specific geometries of cutting inserts come with their dedicated tool holders. In order to maintain cutting efficiency and stability, you should select the proper tool holder depending on the geometry of the insert. Having selected the right tool holder will also extend the tool life.
In the figure below, you can see an assembly between the tool holder and its corresponding cutting insert. On the left side of the figure, you can see the special geometry of the tool holder, where the insert should be mounted. In the center of the figure, you can see the complete tool holder assembly. On the right side, you can see the type of cutting insert geometry corresponding to the tool holder.
CNC turning services
As expected, there are quite many CNC machine shops providing turning services in the market. Not all of them are equally good or experienced at their job.
Let’s see below what are the main qualities you should be looking for when selecting your CNC turning partner.
Experience matters! Only CNC machine shops with long experience can offer a variety of solutions by taking advantage of the technologies presented above. Moreover, technologies evolve rapidly; thus, CNC machine shops should be prepared to keep up with the technological advances and implement them at their work when needed.
Besides delivering complete and high-quality machined parts, your selected CNC machine shop should additionally be able to provide supplementary supportive services such as polishing, sandblasting, anodizing, sterilization, laser marking, assembling, fast “on-time” product delivery, etc.
The cutting tools play a critical role in CNC turning. In other words, keep in mind that your selected CNC turning provider must have the right tools to provide high-quality turning work. It’s always beneficial to discuss with your partner the materials and methods they are going to implement so that you have a better understanding of their overall knowledge, experiences, and resources to complete your work at the expected standards.
Many CNC machine shops are equipped with ERP systems (Enterprise Resource Planning), online monitoring, and full production and batch traceability. These resources are evidence of good management of their internal operations, securing that the right processes are always followed and minimizing the risks associated with the quality of your CNC parts.
Uses and applications
CNC turning is used when rotational parts are to be machined. This includes parts with milling features provided that your CNC turning machine supports the third axis.
A typical list with parts you can produce by implementing CNC turning is the following:
- Shafts in different lengths and diameters
- Threaded rods
- Gun Barrels
Regardless of the part that you want to produce with CNC turning, there are standard benefits that you can expect to gain. Let’s see them one by one.
Productivity increase and high precision
CNC turning engages the simultaneous rotational motion of the part and the linear motion of the tool. Owing to this synergy of motions, the lathe is a very productive machine tool. In addition, owing to the very high stability of the CNC turning centers, very high precision is achieved.
Reduction of scrap owing to automation
As it occurs in any CNC machining process, the operator is responsible to set the job and make sure that the CNC program is error-free and productive. Operators do not involve or interfere during the manufacturing process. This, in turn, eliminates human errors, and thus, scrap material is greatly reduced.
Reduction of idle time
Idle time involves tool changes, part setup and orientation and rapid traverse motions. Owing to the automation that CNC technology embeds, idle time is dramatically reduced.
Due to the fully automated production environment in CNC turning, the operator keeps distance from the working area and is not involved physically in the production process.