Pulished on Jan. 19, 2024
Mill-turn machines encompass a range of configurations, starting from the basic 3-axis lathes (X, Z & C). In these machines, the spindle serves as a distinct controllable angular axis for milling operations. More advanced versions include 6-axis machines, featuring an additional linear Y-axis, W-axis, and a secondary programmable sub-spindle or counter-spindle. In cases with a secondary spindle, the W-axis is employed to position the secondary spindle for machining operations.
In the image above, it is evident that the Y-axis is implemented through a narrowly angled "wedge" design. However, even on machines with this configuration, the Y-axis consistently moves in a perpendicular motion to the X-axis.
A common puzzle in understanding mill-turn technology revolves around determining the necessity of a Y-axis and when a specific feature can be machined using a simpler 3-axis machine with only the C-axis. Essentially, a Y-axis is required only when the tool needs to deviate from the centerline of the part. Typically, this is primarily necessary for features machined into the circumference of the part. In many cases, features on the face of the part can be accomplished solely with the use of the C-axis.
The components shown below serve as excellent examples of features that can be produced with the addition of only the C-axis. Even in the presence of holes illustrated in the blueprint that are not aligned with the centerline of the part, the C-axis can adjust its position, enabling the X-axis to move into the appropriate position for machining the feature.
The decision on which type of CNC machining to employ—mill, lathe, or mill-turn—can be daunting. All three methods fall under subtractive manufacturing, involving the removal of material rather than adding it, as seen in additive processes like 3D printing. The key distinction among them lies in how they remove material and which component rotates: the cutter or the workpiece.
Milling is the most commonly employed process among the three. In milling, a rapidly rotating multi-bladed cutting tool removes material, while the workpiece remains stationary or moves along a predetermined path. Milling centers vary, ranging from 2-axis milling machines that cut holes and slots along the X- and Z-axes (horizontally and vertically) to multiaxis milling machines that handle more intricate geometries using both rotational and linear axes.
Vertical milling centers position the spindle and cutter above the table, holding the material to be cut. There are two primary types of vertical centers: turret and bed. Turret vertical mills keep the spindle stationary while the table moves along linear axes for cutting direction. Bed vertical mills enable the spindle to move along the Y-axis, while the table moves along the X-axis. In contrast, horizontal milling centers have the spindle oriented horizontally instead of above the table.
A lathe machine is engineered to rotate its workpiece at high speeds, while a stationary, single-bladed cutting tool removes material. Turning is particularly effective for crafting cylindrical or radially symmetrical parts, and most lathes can also perform drilling, boring, grooving, and threading operations. Their design is well-suited for handling oversized workpieces.
Mill-turn techniques seamlessly integrate the capabilities of both a mill and a lathe. A mill-turn center is essentially a hybrid machine that leverages the tool rotation characteristic of milling and the workpiece rotation feature of turning. This approach enables users to efficiently machine complex parts without the need to switch between separate machining centers, significantly reducing processing time. On average, a mill-turn center can accomplish four operations for every one performed by a standalone mill or lathe. This method is particularly well-suited for the production of intricate parts that demand both milling and turning processes.