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(III) Milling Modes and Reasonable Selection
1. Selection of Milling Methods
Milling is the relative movement between the milling cutter and the workpiece during the machining process.
(1) Peripheral Milling
There are two types of peripheral milling: up-cut milling and down-cut milling. When the cutting tool rotates into the workpiece, the direction of the cutting speed is opposite to the feed direction of the workpiece, which is called up-cut milling. If the direction of the cutting speed is the same as the feed direction, it is referred to as down-cut milling.
In up-cut milling, the cutting thickness gradually increases from zero. When the actual rake angle is negative, the teeth slide and squeeze on the machined surface, causing chips to be difficult to remove. This leads to a more severe chill layer on the workpiece surface. As the next tooth cuts in, it continues to slide over the chilled layer, increasing tool wear and resulting in a higher surface roughness after machining. The longitudinal component force Ff acts opposite to the feed direction, ensuring no gap between the table screw and nut, thus maintaining smooth feed motion. However, the vertical component force FfN changes direction, potentially lifting the workpiece and causing vibration, which affects surface finish.
In down-cut milling, the cutting thickness starts at its maximum, avoiding the issue of sliding and squeezing. The vertical component force FfN always presses the workpiece against the table, leading to a more stable cut and longer tool life. However, the longitudinal component force Ff acts in the same direction as the feed motion. If there is any clearance in the feed mechanism, this can cause the worktable to move unevenly, possibly leading to tool damage. Therefore, down-cut milling should not be used if the machine lacks a mechanism to eliminate such clearance.
(2) Face Milling
Face milling involves three methods: symmetrical face milling, asymmetrical counter milling, and asymmetrical down-cut milling.
1. Symmetrical Milling
As shown in Figure 8-9a, the milling cutter axis remains in the plane of symmetry of the workpiece. The cutting thickness is the same when entering and exiting, resulting in a larger average cutting thickness. This method is commonly used for end mills, especially when machining hardened steel.
2. Asymmetrical Counter Milling
As illustrated in Figure 8-9b, the milling cutter is offset from the symmetry plane. The cutting thickness is smallest when entering and largest when exiting. This method reduces cutting impact, stabilizes the cutting force, and is suitable for machining carbon steel and high-strength low-alloy steel, producing a smoother surface with improved tool life.
3. Asymmetrical Down-Cut Milling
As shown in Figure 8-9c, the milling cutter is also offset from the symmetry plane but the cutting thickness is smallest when exiting. This method is ideal for materials like stainless steel that have medium strength and high plasticity.
2. Selection of Milling Parameters
The principle for selecting milling parameters is: "Under the condition of ensuring machining quality, fully utilize the efficiency of the machine tool and the cutting performance of the tool." Under the constraints of the system rigidity, first select the maximum possible depth of cut (ap) and width of cut (ac), then choose a larger feed per tooth (fz), and finally calculate the cutting speed (vc) based on the selected tool life.
(1) Selection of Depth of Cut (ap) and Width of Cut (ac)
For end mills, the selection of cutting parameters follows these guidelines: if the machining allowance is ≤8 mm, and the system is rigid with sufficient machine power, after setting the semi-finish allowance to 0.5–2 mm, remove as much excess material as possible. If the allowance exceeds 8 mm, divide the operation into multiple passes. The relationship between the milling width (ac) and the end mill diameter (D0) should be maintained as:
D0 = (1.1~1.6) × ac (mm)
For cylindrical cutters, the depth of cut (ap) should be less than the length of the cutter. The selection principle for the width of cut (ac) is similar to that of the end mill.
(2) Selection of Feed Rate
The feed per tooth (fz) is a key factor in determining the efficiency of the milling process. During rough milling, fz is mainly limited by the cutting force. In semi-finish and finish milling, it is mainly limited by surface roughness.
Table 8-1 Recommended Feed per Tooth (fz), mm/z
Workpiece Material | Hardness (HBS) | Carbide | High Speed Steel | Face Milling Cutter | Three-Edge Cutter | Cylindrical Cutter | Milling Cutter | Face Milling Cutter | Three-Edge Cutter
Note: The smaller value in the table is used for fine milling, while the larger value is used for rough milling.
(3) Determination of Cutting Speed (vc)
The cutting speed can be determined by referring to the mechanical processing technology manual, such as Volume I of the Manual of Mechanical Processing Technology.
Milling Cutter Selection
(1) Selection of Milling Cutter Diameter
The diameter of the milling cutter is usually chosen based on the amount of material to be removed. For reference, see Table 8-2 for the selection of some common milling cutters.
Table 8-2 Selection of Cylindrical and End Milling Diameters (Reference) mm
Name | High Speed Steel Cylindrical Cutter | Carbide End Mills | Depth of Cut (ap) | ≤5 | ~8 | ~10 | ≤4 | ~5 | ~6 | ~7 | ~8 | ~10 | Width of Cut (ac) | ≤70 | ~90 | ~100 | ≤60 | ~90 | ~120 | ~180 | ~260 | ~350 | Milling Cutter Diameter (d0) | ≤80 | 80~100 | 100~125 | ≤80 | 100~125 | 160~200 | 200~250 | 320~400 | 400~500
Table 8-3 Selection of Disk and Saw Cutter Diameter mm
Depth of Cut (ap) | ≤8 | ~15 | ~20 | ~30 | ~45 | ~60 | ~80 | Milling Cutter Diameter (d0) | 63 | 80 | 100 | 125 | 160 | 200 | 250
Note: If ap or ac cannot match the values in the table simultaneously, choose a larger cutter diameter based on ap (for cylindrical cutters) or ac (for end mills).
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