What is milling?

What is milling?

milling

Milling equipment

In the milling of mold parts, the main types of milling machines include horizontal milling machines, vertical milling machines, gantry milling machines, and universal milling machines. The most widely used are vertical milling machines and universal milling machines, which can achieve machining accuracy of IT8 or higher and surface roughness Ra of 1.6μm. If high-speed, low-volume milling is used, the workpiece accuracy can reach IT7, and the surface roughness Ra can reach 0.8μm. During milling, a 0.05mm finishing allowance is left, and finishing by a fitter is sufficient. When high precision is required, milling is only an intermediate process, and further finishing processes are needed after milling.

Selection of milling tools

Milling is a machining method performed on a milling machine using a milling cutter. During milling, the milling cutter performs the main rotary motion, while the workpiece or the milling cutter performs the feed motion. A milling cutter is a multi-tooth cutting tool, and different types of milling cutters are required depending on the object being milled.

For planar milling, cylindrical end mills can be used for peripheral milling, or face mills can be used for end milling, as shown in Figure 3-14. Compared to peripheral milling, end milling involves more teeth working simultaneously, results in less variation in cutting thickness, a larger contact area between the cutter and the workpiece, a smoother cutting process, and the finishing teeth on face mills can polish the machined surface, resulting in better machining quality. Furthermore, face mills have high rigidity, and the cutting part mostly uses carbide inserts, allowing for larger cutting parameters. Often, the entire working surface can be machined in a single pass, resulting in high production efficiency. Therefore, the use of face mills on vertical milling machines for machining planar or inclined surfaces is widely used in the machining of mold parts.

Workpiece clamping and positioning methods

The main clamping methods for parts on milling machines are: clamping with a flat-jaw vise; clamping with a universal indexing head; directly clamping the workpiece on the milling machine worktable with a clamping plate and bolts; and using special fixtures for clamping in batch production, such as the following fixtures and mechanisms that are compatible with milling machines:

① Positioning and support mechanisms used to directly position and clamp workpieces onto the worktable; lever-type clamping mechanisms consisting of pressure plates and bolts or eccentric components.

② General-purpose precision flat-jaw vises for positioning, clamping, and fixing on the worktable.

③ Universal indexing heads mounted on the worktable.

④ Spindle-type, horizontal-spindle-type, and universal rotary indexing worktables mounted on the milling machine worktable.

⑤ Commonly used manual and motorized rotary worktables.

Workpieces and machined surfaces that can be milled are shown in Figure 3-15. Figure 3-15(a) shows plane milling, Figures 3-15(b) to 3-15(d) show three-dimensional concave cavity milling, and Figure 3-15(e) shows forming punch milling. During machining, since the workpiece blank is mostly hexahedral, its bottom plane is often used as the primary machining datum, positioned on the milling machine table, restricting the workpiece's three degrees of freedom: z (rotation), y (rotation), and z (translation). Simultaneously, the two sides of the workpiece are used as positioning datum planes in the X and Y directions to restrict z (rotation), x (translation), and y (translation). After clamping the workpiece, all six degrees of freedom are completely restricted.

A special fixture can also be designed and manufactured based on the workpiece's six positioning datums. When the workpiece is clamped in the fixture for milling, the following basic requirements should be ensured:

① Ensure that the machining dimensional errors of each machined surface are within the allowable range.

② Ensure the shape accuracy of each machined surface, i.e., ensure that the shape tolerances of the machined workpiece meet the design requirements.

③ Ensure the positional accuracy requirements between the machined surfaces.

Milling parameters

In milling, the rotation of the milling cutter is the primary motion, while the linear or curvilinear movement of the workpiece along the worktable is the feed motion. Key milling parameters include milling speed, feed rate, depth of cut, and feed rate.

Milling speed (vₙ) refers to the linear velocity of the cutting edge at the maximum diameter of the milling cutter.

Feed rates in milling are expressed in three ways:

① Feed per tooth f_z: The distance the workpiece moves along the feed direction per tooth the milling cutter passes, measured in mm/z. f_z is the basis for selecting the feed rate.

② Feed per revolution fᵣ: The distance the workpiece moves along the feed direction per revolution of the milling cutter, measured in mm/r.

③ Feed per minute fₘ: The distance the milling cutter moves relative to the workpiece in the feed direction per minute, measured in mm/min. In practice, fₘ is generally used to adjust the machine tool feed rate.

The following relationships exist between f_z, fᵣ, and fₘ:

In the formula, n represents the milling cutter speed, in r/min.

The depth of cut (aₚ) in milling is the perpendicular distance between the workpiece and the machined surface, i.e., the depth to which the milling cutter penetrates the workpiece.

The feed rate (vᵢ) refers to the relative displacement between the workpiece and the milling cutter along the feed direction per unit time, measured in mm/min. It is related to the milling cutter speed n, the number of teeth z, and the feed per tooth f_z (in mm/z).

The formula for calculating the feed rate is:

In the formula, the feed per tooth f mainly depends on factors such as the mechanical properties of the workpiece and tool materials, and the surface roughness requirements of the workpiece. When the workpiece material has high strength and hardness, high surface roughness requirements, poor workpiece rigidity, or low tool strength, f should be a smaller value. The feed per tooth of carbide end mills is higher than that of similar high-speed steel end mills. The reference table for selecting the feed per tooth is shown in Table 3-4.

Table 3-4 Reference Table for Feed per Tooth of Milling Cutters

Unit: mm

The milling speed (vₙ) of a milling cutter is inversely proportional to tool life T, feed per tooth f_z, depth of cut aₚ, depth of cut aₑ, and the number of teeth z, and directly proportional to the cutter diameter d. This is because as f_z, aₚ, aₑ, and z increase, the number of teeth working simultaneously increases, leading to increased cutting edge load and heat, accelerating tool wear. Therefore, tool life limits the increase in cutting speed. Increasing the cutter diameter improves heat dissipation, thus increasing the milling speed accordingly. Table 3-5 lists reference values ​​for milling speeds.

Table 3-5 Reference Values of Milling Cutter Speed

Unit: m/min