CNC machining is a popular method for shaping plastic parts because it ensures precise and accurate dimensions, which are critical for functional plastic parts. Tolerances can be tightly controlled, resulting in consistent part quality. Milling and turning are two commonly used methods for machining plastic. While both fall under the CNC machining umbrella, there are differences in what they do and how they do it. Understanding the differences between milling vs. turning will help you decide which option is best for your projects.

Understanding CNC Machining

While CNC has become common vernacular, it is an acronym for computerized numerical control. The machines produce highly accurate parts directly from a CAD or CAM file. G-code and M-code are part of the CNC programming that tells the machine to perform an action (e.g., move to a position, adjust spindle speed) and manages the machine functions (e.g., starting, stopping, changing tools), respectively. These codes are essentially sequences of numbers that control the CNC machine’s behavior, hence the name numerical control.

The ability to CNC machine plastics has been highly beneficial for many industries. Plastics are increasingly replacing metals in a wide variety of applications because they are lightweight, and many offer comparable mechanical properties and chemical resistance. While the same machining methods are used to shape plastic and metal, plastic’s properties, such as thermal expansion, softening temperatures, and elasticity, make parts more challenging to produce. Part manufacturers must consider polymer selection, tooling, feed rates, stabilizing methods, and more to achieve high-quality results.

Milling vs Turning: Choosing the Right Method

When deciding between milling vs. turning for plastic parts, consider the following factors:

  • Design Requirements: Does your design call for flat surfaces, intricate shapes, or cylindrical or symmetrical features?
  • Material Properties: Different plastics behave uniquely—some may warp or crack if mishandled.
  • Precision: What level of precision is required for your project?

Remember, each project is unique, and the choice between milling and turning depends on your specific requirements. Understanding these methods will empower you to work with your machining partner to make the right decision for your plastic parts. Because plastics behave differently from metals when machined, always partner with a supplier with experience in plastic machining. Below, we discuss the differences between milling and turning plastics.

What is Milling?

CNC milling involves high-speed rotation of a cylindrical cutting tool to remove plastic chips from a stationary workpiece. When milling, the workpiece usually remains fixed. The cutting tool performs the work in a linear fashion but operates on several axes, giving it flexibility for complex parts. The machine is called a CNC mill.

There are two different processes for milling plastics: down milling (also called climb milling) and up milling (also called conventional milling). The difference between them is the direction the cutter is rotating and the direction of the feed. With down milling, the tool rotation and feed direction are in the same direction. The cutting edge initially engages the workpiece at its thinnest point, gradually increasing the material thickness as it progresses. Chips move downward in the same direction as the cutter’s rotation. Down milling is generally preferred for milling plastics. With up milling, the cutter rotation is against the feed direction. As the cutting edge contacts the material and shears material away from the workpiece, chips move upward against the feed direction. Opposite of down milling, up milling produces thicker initial chips, and as cutting progresses, the chips become thinner and less substantial. A challenge with up milling is thick chips and higher temperatures. The heat generated can melt the plastic, causing it to stick to the tool and potentially reducing the cutting quality.

Three common techniques for milling used with either up or down milling are end, peripheral, and stepped milling. End milling involves cutting with a tool with teeth on its end and sides to create slots, pockets, or contours in the workpiece. Peripheral milling uses a cutter with teeth on its circumference to remove material from the surface of the workpiece, commonly used for creating flat surfaces or precise shapes. Stepped milling involves making successive stepped cuts to achieve specific profiles or features on the workpiece, often used for creating shoulders, grooves, or steps.

CNC milling excels at creating flat or intricate shapes on plastic parts. However, the process generates heat, negatively affecting plastic parts if not managed.

Understanding Turning

CNC turning is performed on a lathe. One way it differs from milling is the plastic workpiece rotates on its axis, and the cutting tool is stationary as it shapes the part. It is critical that plastic part manufacturers use appropriate cutting tools, cutting speeds, and stabilization and cooling methods for optimal results.

Turning excels at creating parts with cylindrical or symmetrical features. It’s ideal for crafting rods, tubes, bushings, and other components with a central axis. Unlike milling’s multi-axis tool movement, turning relies primarily on the rotation of the workpiece and the linear movement of the cutting tool. This simplicity translates to efficient production of these rotationally symmetrical parts.

Like milling, there are factors to consider for optimal turning of plastics. Cutting tool selection is crucial. Sharp, high-quality carbide inserts are preferred for their clean cuts and ability to minimize friction, which can generate unwanted heat. Maintaining proper speed and feed rates is also essential. Certain plastics often need slower speeds to prevent melting or burning at the cutting point.

When comparing milling vs turning, turning offers a distinct advantage – the generation of continuous chips. This eliminates chip re-cutting, a potential issue in milling that can lead to poor surface finishes. Additionally, turning generally produces a smoother surface finish on the final part due to the continuous contact between the cutting tool and the rotating workpiece.

However, turning also has limitations. It’s unsuitable for creating complex geometries with intricate features on multiple planes. For these situations, milling’s multi-axis capabilities are better suited.

Choosing an Experienced Partner

Plastic requires different machining techniques than metal, making it critical to work with an experienced plastic machining partner who understands the nuances and is able to machine your parts with the correct speeds, feeds, and tooling to avoid warping, melting, or cracking the material. There are many types of plastics, each with its own properties. A partner experienced in plastics can help you choose a suitable material for your application, considering factors like strength, flexibility, heat resistance, and chemical resistance.

Ensinger is an industry leader in plastic manufacturing, specializing in CNC machining, injection molding, plastic fabrication, and advanced manufacturing. Our plastics design and engineering services can help you decide what materials and processes are best for your project. Contact us to experience Ensinger’s excellence in plastic part manufacturing.