Precision Milling Processes

Milling is a process performed on milling machines using specialized cutting tools known as milling cutters. In milling operations, the cutter rotates as the primary motion, while the workpiece or cutter performs the feed motion. Milling cutters are multi-tooth tools, and different types are required based on the specific milling application, whether for metal components or molds used in injection molding plastic processes.

For planar milling, cylindrical end mills can be used for peripheral milling of workpieces, or face mills can be used for end milling, as illustrated in Figure 3-14. Compared to peripheral milling, end milling engages more cutting teeth simultaneously, resulting in smaller variations in cutting thickness. The larger contact area between the tool and the workpiece creates a more stable cutting process.

Face mills often include finishing teeth that improve the surface quality of the machined part, which is particularly important when creating molds for injection molding plastic, where surface finish directly affects the final product's appearance and functionality. Additionally, face mill arbors offer high rigidity, and their cutting sections typically use carbide inserts that allow for larger cutting parameters.

This capability often enables machining an entire work surface in a single pass, significantly increasing production efficiency. Consequently, the use of face mills on vertical milling machines for processing flat or inclined surfaces is extremely widespread in mold part manufacturing, including those used for injection molding plastic production.

The selection of appropriate tool materials is also critical. High-speed steel (HSS) tools are cost-effective for general applications, while carbide tools offer superior performance for high-speed machining and hard materials. Coatings such as titanium nitride (TiN) or titanium carbonitride (TiCN) can further enhance tool life and performance, especially in demanding applications like machining hardened steels for injection molding plastic molds.

Figure 3-14: Applications of milling (5) Face mill milling (8) Cylindrical end mill milling

In milling operations, the rotation of the milling cutter constitutes the primary motion, while the linear or curvilinear movement of the workpiece with the table represents the feed motion. Proper selection of milling parameters is crucial for achieving optimal results in terms of surface finish, dimensional accuracy, tool life, and productivity, whether machining metal parts or preparing molds for injection molding plastic processes.

The key milling parameters include cutting speed, feed rate, depth of cut, and feed velocity. Each parameter interacts with the others and must be carefully balanced based on the material being machined, the tooling used, and the desired outcome, particularly important for precision applications like injection molding plastic mold creation.

Cutting speed (vc) refers to the linear speed of the cutting edge at the maximum diameter of the milling cutter. This parameter is typically expressed in meters per minute (m/min) or surface feet per minute (sfm) and directly influences tool life and material removal rate.

Feed rates in milling can be expressed in three different ways, each serving specific purposes in various machining scenarios, including those involving injection molding plastic mold components:

1. Feed per tooth (fz): The distance the workpiece moves in the feed direction as each tooth of the cutter passes, measured in millimeters per tooth (mm/z). This is the fundamental basis for selecting feed rates and is critical for determining chip load, which affects both tool life and surface finish in applications like injection molding plastic mold machining.

2. Feed per revolution (f): The distance the workpiece moves in the feed direction during one complete revolution of the cutter, measured in millimeters per revolution (mm/r). This parameter relates directly to the rotational speed of the spindle and the feed per tooth.

3. Feed per minute (f): The distance the cutter moves relative to the workpiece in the feed direction per minute, measured in millimeters per minute (mm/min). In practical operations, this is the parameter typically used to adjust the machine feed rate, as it directly corresponds to the machine controls.

The depth of cut refers to the thickness of material removed in a single pass, which significantly impacts cutting forces and power requirements. For roughing operations, larger depths of cut are used to maximize material removal, while finishing operations employ smaller depths of cut to achieve superior surface quality, essential for injection molding plastic mold surfaces.

Figure 3-15 illustrates common milling surfaces, showing how different combinations of parameters and tool paths can create various geometric features. Proper parameter selection is especially critical when transitioning from roughing to finishing operations, ensuring that the final dimensions and surface quality meet the required specifications for applications like injection molding plastic production.

Modern CNC milling machines often include software that recommends optimal parameters based on material and tooling, but experienced machinists can make adjustments to optimize performance for specific applications, including the precise requirements of injection molding plastic mold manufacturing.