Posted March 18, 2021

Difference Between CNC Milling and Turning: Learning About Manufacturing Processes

CNC machine performing a milling application.

CNC milling and CNC turning are subtractive manufacturing processes that generally fall under the umbrella of “CNC machining.” Both processes use computer numerical control (CNC) to sequentially remove material from a raw block to get the specified dimensions of a final product. Despite those basic similarities, the CNC milling and turning process have significant differences. The decision to use one, or the other, or both, depends on a part’s features and design. Understanding the distinction between these terms is useful because they’re standard in the manufacturing industry. This blog will unpack the differences between each method to understand CNC machining processes comprehensively.

Understanding CNC Milling and Turning

An engineer at a machine shop will determine whether a part needs milling or turning. Typically they select turning for cylindrical parts and milling for more complex parts because it offers more dimensional flexibility. Some parts may also involve a combination of both methods. 

CNC Milling

The CNC milling process allows the workpiece to stay in one place while the machine tool rotates and moves around it, sequentially removing material using various tools. A CNC machine uses a multi-axis cutting tool to mill a wide variety of materials. A spindle holds the cutting tool, which spins at pre-set RPMs. 

A traditional 3-axis machine cuts on X, Y, and Z axes. A machine with four or more axes generally includes rotation of the tool, the work table, or both. CNC milling machines with 5-axis, 5-axis-indexed, and 3-axis machining capabilities can provide standard tolerances of +/- 0.005” (+/- 0.13 mm) for metals and +/-0.008” (+/- 0.2mm) for plastics, and holes with diameters as small as 0.020” (0.51mm). Depending upon the part geometry, some machines can produce tighter tolerance of +/- 0.001” (+/- 0.03mm). Tight tolerances and high-quality finishes are possible because the initial block of material is larger than the finished part. 

CNC Turning

CNC turning machines or “lathes” rotates the workpiece against a stationary machine tool to obtain desired geometry—typically axially symmetrical parts. In this process, a clamping system such as a chuck or arbor grips the workpiece, and a “spindle” spins both as it moves against a tool. A single-point cutting tool attached in a “turret” cuts away the spinning material. A part’s geometric specifications and material determine the spindle, tool, and speed options.

A standard CNC turning machine can produce a tolerance of +/- 0.005” (+/- 0.13mm) for metals and +/- 0.008” (+/-0.2mm) for plastics. Some CNC turning involves “live tooling.” Live tooling brings a mill into a CNC lathe. Typically, the live tool is simply an end-mill within the lathe that adds flat surfaces and holes to a part without removing it and additional turning steps. Live tools can include a coupler that allows the turret to drive the live tool spindle or an air-driven spindle. A machine shop can help determine the most efficient method. 

Difference between CNC Milling and Turning

There are distinctions between CNC milling and turning processes in terms of basic operational features and their use in fabricating parts with different complexities.

Features

CNC Milling

CNC Turning

Function

The milling process removes material by rotating the tool around the workpiece.

In the CNC turning process, the tool works upon a rotating workpiece.

Machine used

CNC mills or milling machines.

CNC lathes or turning machines.

Parts formed

CNC milling is suitable for creating geometrically complex parts. Although, it can also create flat or cylindrical parts.

 

CNC turning mainly fabricates round, conical, or cylindrical parts like tubes, ball bearings, or washers. Live tooling can augment this operation.

Cutting process

The tool intermittently disengages from the workpiece.

The tool is continuously in contact with the workpiece.

Tool features

CNC mills use multi-point cutting tools such as end mills, reamers, or milling cutters. The tool has two or more cutting edges that engage with the workpiece for the fabrication purpose.

 

CNC turning processes use single-point cutting tools such as fly cutters for material removal. The single point means only one point of the tool will engage with the workpiece for fabrication purposes.

Ejected material

As the cutting process is intermittent, ejected metal chips are discontinuous.

 

The continuous cutting process produces fragmented, discontinuous, or continuous ejected metal chips.

 

CNC Milling and Turning for Your Manufacturing Operation

Learning industry-standard terminology is valuable for anyone planning on using CNC services. Ultimately, one can safely assume that a shop will use a lathe for cylindrical parts and a mill for more complex shapes with features. An experienced machine shop will know which method or combination of methods is best for machining a part. They base the selection mainly on the required design specifications and features. CNC milling and turning operations can fabricate quality custom parts at low or high volumes, offering precision and tight tolerances.  

Plethora is an ISO 9001-certified machine shop specializing in precision parts using advanced CNC milling and turning technologies. For questions about our CNC machining solutions, give us a call at 415-726-2256, or you can upload your design files to Quote My Part and get your project started today.

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The Plethora Team

The Plethora team is your go-to CNC manufacturer for hardware done right the first time. We have the tools and experience needed to create high quality custom parts quickly and with precision, whether you need a prototype or production run.

Topics: materials, Design, Manufacturing, finishing, CNC machining, Quality, Prototyping, Milling vs. turning

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