Milling and turning are everyday operations in a machine shop. Both techniques use a cutting tool to remove material from a solid block to make 3D parts. Removing material is what classifies them as subtractive manufacturing processes, but there are key differences to these operations.
Turning is an operation for a lathe.
The name ‘turning’ refers to the workpiece because it rotates about a central axis. The cutting tool remains stationary and is moved in and out of the workpiece to make cuts. Turning is used to create cylindrical parts and derivatives of cylinders; think of parts shaped like baseball bats, shafts, balusters and columns, for example.
A chuck holds the workpiece centered on a rotating spindle. A base secures the cutting tool so it can move along the axis of the workpiece and in or out radially. Feeds and speeds come from the rate of rotation of the part, the radial depth of cut and the rate the tool moves along the axis of the piece.
Turning operations include OD and ID cutting and grooving, boring, treading and drilling. Since the cutting tool exerts a force on the workpiece perpendicular to its axis, it is crucial to support the piece to reduce deflection.
In milling operations, on the other hand, the cutting tool rotates while the workpiece is fastened securely to a worktable.
The cutting tool or the table can move orthogonally in the X, Y or Z direction for cutting. Milling can create more complex shapes than turning. It can even produce cylindrical shapes, but for cost effectiveness, those shapes are best left to a lathe.
In a CNC mill, a chuck holds the tool in a rotating spindle. The tool is moved relative to the workpiece to create patterns on the surface of the workpiece. Feeds and speeds are calculated based on the rate of rotation of the cutting tool, the cutting tool diameter and number of flutes, the depth of the cut and the rate the cutting tool moves across the part.
The limitation of milling pertains to whether or not a tool can access a cutting surface. Using longer and thinner tools can improve access, but these tools can deflect, causing poor machining tolerances, bad surface finishes and more wear and tear on the tool. Some advanced milling machines have articulating heads to allow for angular cuts and improved access.
Both turning and milling operations are useful for creating complex parts. The primary difference will be in the shape of the final part. For cylindrical parts, go with turning. For most other parts, milling works best.