CNC Milling vs. CNC Turning: All You Need to Know

CNC Milling vs. CNC Turning: All You Need to Know


CNC machining is a rapid manufacturing process that turns digital 3D designs into plastic or metal parts by selectively cutting away material. Many companies require CNC machining service to make parts and prototypes, and many industries use the versatile technology.






But CNC machining comes in various forms. Although all CNC machining technologies follow a similar workflow — software turns the digital design into machine instructions, which instruct the CNC machine to cut material — the hardware for cutting material can differ greatly between machines.






This article discusses the main differences between two of those machines: CNC mills and CNC turning (or lathes centers).






In the article we discuss the essential features of CNC milling parts and CNC turning while also presenting the main advantages of each technology and a selection of common parts that companies can manufacture using each process.






CNC milling is one of the most common CNC machining service, and machinists can use it to make a wide variety of CNC machined parts. Prototype companies often use CNC mills to make one-off functional prototypes.






CNC mills use computer instructions to move a rapidly rotating cutting tool along three or more axes. When the spinning cutting tool makes contact with the workpiece, it removes material in a controlled manner. The cutting tool makes a succession of passes against the surface of the workpiece until the workpiece resembles the desired part.






Most CNC mills keep the workpiece stationary, holding it down on the machine bed with a vice. However, multi-axis CNC mills may rock or rotate the workpiece to create a greater number of cutting angles. This allows the machinist to create more complex parts without having to manually reorient the workpiece.






Providers of rapid prototyping services use CNC machining because it is a one-stop, end-to-end process with short lead times.






CNC turning is a form of CNC machining that machinists use to make rounded, cylindrical, and conical parts. Although it is less versatile than CNC milling, it is one of the most popular CNC machining services and rapid prototyping services.






Machines that carry out CNC turning parts are called CNC lathes or CNC turning centers. They are different from CNC mills in that they rapidly rotate the workpiece in a chuck but do not rotate the cutting tool. The cutting tool, affixed to a turret, moves towards the spinning workpiece under computer instructions and removes material where necessary.






A CNC lathe can cut the outside of the workpiece or bore through the inside to create tubular CNC machined parts.






The turret of the machine may have multiple cutting tools that can be individually engaged as required.






Advantages of CNC Milling






CNC mills offer numerous advantages to manufacturers and prototype companies. Unlike lathes, mills are versatile machines capable of creating a range of different shapes. Furthermore, a variety of cutting tools can be used to serve different operations such as roughing and end-milling.






Although they are manufacturing machines in their own right, mills are also useful for post-machining. For example, they can be used to add details to turned, molded, or 3D printed parts.






CNC milling is also fast, repeatable, and inexpensive in low volumes — partly because it does not require tooling. It is therefore found among manufacturing services and rapid prototyping services.






Advantages of CNC Turning






The biggest advantage of CNC turning is its ability to create round profiles. It is much more difficult to achieve perfect roundness using other CNC machining parts services like CNC milling or CNC routing.






CNC turning is also highly accurate, which makes it a valuable technology for boring holes of precise dimensions with set tolerances.






CNC milling and CNC turning can be combined to reap the benefits of both processes. In most cases, CNC turning takes place first, allowing the machinist to mill further (asymmetrical) details on the part.






As a rule of thumb, CNC turning is best for parts with round, cylindrical, or conical profiles, and aluminium CNC milling parts is best for everything else. If in doubt, a machining expert can guide you to the right CNC machining service for your unique project.






That being said, CNC milling and CNC turning can be combined to good effect. If a part has a predominantly round shape but also requires asymmetrical cuts or features, CNC milling can follow CNC turning in sequence. And although it is less common, CNC turning can also follow CNC milling — if a boxy or irregular-shaped part requires a large hole bored through its center, for example.






Finally, sometimes you don’t have to choose: CNC milling-turning centers integrate both technologies into a single production device.






What is a CNC Milling Machine and how does it work? How do CNC milling machines compare to CNC Lathes? When do you need such a CNC machine tool?






Focused on milling – the process of machining using rotating tools to gradually remove material from a workpiece – CNC milling machines are a mainstay for factories around the world. These machine tools make use of a variety of cutting tools along one or more axes to remove material from a workpiece through mechanical means.






CNC milling machines are often used in a variety of manufacturing industries: from industries like aerospace, shipping, automobiles, and oil drilling / pumping and refining, to medical, FMC manufacturing, and precision engineering sectors.






Also called CNC Machining Centers, the more advanced CNC milling machines can operate along multiple-axis. These may be fitted with automatic tool changers, advanced machine coolant systems, pallet changers, and advanced software to improve the efficiency and accuracy of machining processes.






CNC Milling Machines are machine operated cutting tools that are programmed and managed by Computer Numerical Control (CNC) systems to accurately remove materials from a workpiece. The end result of the machining process is a specific part or product that is created using a Computer Aided Design (CAD) software.






These machine tools are normally equipped with a main spindle and three-linear-axes to position or move the part to be machined. More advanced versions may have a 4th or 5th rotational axis to allow for more precise shapes of varying dimensions and sizes to be machined.






CNC milling machines / machining centers normally employ a process of material cutting termed milling or machining – the milling process involves securing a piece of pre-shaped material (also known as the workpiece) to a fixture attached to a platform in the milling machine. A rapidly rotating tool (or a series of interchangeable tools) is then applied to the material to remove small chips of the material until the desired shape for the part is achieved.






Depending on the material used for the part, as well as the complexity of the machined part, varying axes, cutting head speeds, and feed rates may be applied.






Milling is normally used to machine parts that are not symmetrical from an axial perspective. These parts may have unique curvatures or surface contours, which may require a combination of drilling & tapping, grooves, slots, recesses, pockets and holes to work on them. They may also form parts of the tooling for other manufacturing processes – for example in the fabrication of 3D moulds.






In the past, milling machines were manually operated. Operators had to use a combination of machines with different tools to machine a more complex part or product. Or they had to use various settings on one machine just to complete the job. 






With the advancement of technology such a CNC controls and Automatic Tool Changers (ATCs), greater efficiency, flexibility and speed can be achieved – even for more convoluted parts. The provision of digital readouts and measuring systems has also improved the accuracy of CNC machining processes. 






To cater to manufacturers that require the flexibility of “High Mix, Low Volume” (HMLV) or “small batch” production, CNC machining centers can be fitted with Pallet Changers or other automation solutions to form part of a Flexible Manufacturing Cell (FMC) or Flexible Manufacturing System (FMS). This allows such machines to cater to a wide variety of machining demands and needs.






The general principle for a CNC milling machine or CNC machining center is that the part to be machined is clamped on top of the machine table. It could be clamped directly on the table itself, or held in place by a vice or fixture.






The spindle (moving section) including the cutting tool is then either vertically or horizontally positioned. In that configuration, the tool can reach various X-Y-Z positions on the work piece and commence cutting and shaping actions. As it does so, the work piece or part may either be fixed, mounted, or moved/positioned by the table in a linear direction to the spindle with the cutting tool. This allows material to be removed according to the desired shape needed for the machined part.






For a CNC milling machine (aka CNC machining center), the work piece is fixed or mounted in position using a vice or fixture while the cutting tool is manoeuvred on top of or around the piece. Material is then gradually removed using cutting tools or drills which rotate at high speed with varying feed rates along two or more axes. 






In the case of a CNC Lathe (aka CNC Turning Center), however, the work piece (usually cylindrical) is mounted on a rotating chuck or on the main spindle. It is then “turned” (hence the name turning Center) or rotated along a main axis while the cutting tool located in a rotational or positioning turret would move in a parallel direction to the piece. Material is then removed using stationery cutting tools.






Thus, a CNC milling machine use a spinning tool with a stationery work piece, whereas a CNC Lathe would involve spinning the material to be worked on by a stationery cutting tool.