(III) Milling Modes and Reasonable Selection
1. Selection of milling methods
Milling refers to the movement of the milling cutter relative to the workpiece during milling.
(1) Perimeter milling (peripheral milling)
There are two ways to mill a workpiece in the circumferential milling method, ie, up-cut milling and crush milling. If milling cutter rotates and cuts into the workpiece, the direction of cutting speed is opposite to that of the workpiece. It is called up milling, otherwise it is called milling.
1 Reverse Milling As shown in Figure 8-8a, the cutting thickness gradually increases from zero. When the actual rake angle is negative, the teeth are squeezed and glide on the machining surface, and the chips cannot be removed. The wear of the surface also causes the surface of the workpiece to produce a more severe chill layer. When the next blade cuts in, it squeezes and slides on the surface of the chilled layer, further exacerbating the wear of the milling cutter, and at the same time, the surface roughness value after processing the workpiece is also large. In up milling, the longitudinal component Ff acting on the workpiece by the milling cutter is always opposite to the feed direction of the table, so that there is no gap between the table screw and the nut, and always maintain good contact, so that the feed motion Smooth; however, the direction and size of the vertical component force FfN change, and when the cutting teeth cut away from the workpiece, FfN upward, there is a tendency to provoke the workpiece, causing the table vibration, affecting the surface roughness of the workpiece.
2 Climbing As shown in Fig. 8-8b, the cutting thickness of the cutting tooth starts from the maximum, avoiding the phenomenon of extrusion and gliding; and the vertical component force FfN is always pressed against the table, so that the cutting is stable and the durability of the cutting tool is improved. The quality of the machined surface; but the longitudinal component force Ff is the same as the direction of the feed motion. If there is a gap between the screw and the nut of the milling machine table, this will cause the worktable to move, causing the milling feed to be uneven, and in severe cases the knife will be hit. . Therefore, if there is no screw and nut to eliminate the gap mechanism in the milling machine feed mechanism, then chopper milling cannot be used.
(2) End milling
There are three ways of end mills: symmetrical end milling, asymmetrical counter milling and asymmetrical milling.
1 Symmetrical Milling As shown in Fig. 8-9a, the milling cutter axis is always in the plane of symmetry of the workpiece. When it cuts in and cuts out, the cutting thickness is the same and there is a greater average cutting thickness. General end mills use this kind of milling method, especially for milling hardened steel.
2 Asymmetrical counter milling As shown in Fig. 8-9b, the milling cutter is offset on one side of the symmetry plane of the workpiece. When it is cut in, the cutting thickness is the smallest and the cutting thickness is the largest. This kind of processing method has the advantages of small cutting impact, small change of cutting force and stable cutting process. It is suitable for milling common carbon steel and high-strength low-alloy steel, and has small surface roughness and high tool durability.
3 Asymmetrical Climbing As shown in Fig. 8-9c, the milling cutter is offset on one side of the symmetry plane of the workpiece. When it is cut out, the cutting thickness is minimal. This milling method is suitable for processing medium-strength and high-plastic materials such as stainless steel.
2. Selection of milling amount
The selection principle of the milling amount is: "Under the premise of guaranteeing the processing quality, give full play to the working efficiency of the machine tool and the cutting performance of the tool." Under the conditions allowed by the rigidity of the process system, firstly, a large milling depth ap and a milling width ac should be selected as far as possible; secondly, a larger feed per tooth fz is selected; finally, the milling speed is calculated according to the selected durability. .
(1) Selection of milling depth ap and milling width ac
For end mills, the principle of choosing the amount of knife is: When the machining allowance is ≤8mm, and the process system is rigid, and the power of the machine tool is sufficient, after the half-finishing allowance is set to 0.5~2mm, excess excess should be removed as much as possible. Quantity; When the margin is >8mm, it can be divided into two or more passes. Milling width and end mill diameter should maintain the following relationship:
D0=(ll~1.6) ac(mm)
For cylindrical cutters, the milling depth ap should be less than the milling cutter length. The selection principle of the milling width ac is the same as the selection principle of the milling depth of the end mill.
(2) Selection of feed amount
The feed per tooth fz is an important indicator of the efficiency of the milling process. The fz is mainly limited by the cutting force during rough milling. When semi-finishing and finishing, fz is mainly limited by the surface roughness.
Table 8-1 Recommended feed per tooth fz, mm/z
Workpiece material
Hardness of workpiece
HBS
Carbide
High speed steel
Face milling cutter
Three-edge cutter
Cylindrical cutter
Milling cutter
Face milling cutter
Three-edge cutter
Low-carbon steel
<150
150～200
0.20 to 0.40
0.20 to 0.35
0.15 to 0.30
0.12 to 0.25
0.12 to 0.20
0.12 to 0.20
0.04 to 0.20
0.03 to 0.18
0.15 to 0.30
0.15 to 0.30
0.12 to 0.20
0.10 to 0.15
Medium and high carbon steel
120～180
180～220
220～300
0.15 to 0.50
0.15 to 0.40
0.12 to 0.25
0.15 to 0.30
0.12 to 0.25
0.07 to 0.20
0.12 to 0.20
0.12 to 0.20
0.07 to 0.15
0.05 to 0.20
0.04 to 0.20
0.03 to 0.15
0.15 to 0.30
0.15 to 0.25
0.10 to 0.20
0.12 to 0.20
0.07 to 0.15
0.05 to 0.12
grey cast iron
150～180
180～220
220～300
0.20 to 0.50
0.20 to 0.40
0.15 to 0.30
0.12 to 0.30
0.12 to 0.25
0.10 to 0.20
0.20 to 0.30
0.15 to 0.25
0.10 to 0.20
0.07 to 0.18
0.05 to 0.15
0.03 to 0.10
0.20 to 0.35
0.15 to 0.30
0.10 to 0.15
0.15 to 0.25
0.12 to 0.20
0.07 to 0.12
Malleable cast iron
110~160
160～200
200 to 240
240～280
0.20 to 0.50
0.20 to 0.40
0.15 to 0.30
0.10 to 0.30
0.10 to 0.30
0.10 to 0.25
0.10 to 0.20
0.10 to 0.15
0. 20 to 0.35
0.20 to 0.30
0.12 to 0.25
0.10 to 0.20
0.08 to 0.20
0.07 to 0.20
0.05 to 0.15
0.02 to 0.08
0.20 to 0.40
0.20 to 0.35
0.15 to 0.30
0.10 to 0.20
0.15 to 0.25
0.15 to 0.20
0.10 to 0.20
0.07 to 0.12
Carbon content
<0.3%
alloy steel
125～170
170～220
220～280
280～300
0.15 to 0.50
0.15 to 0.40
0.10 to 0.30
0.08 to 0.20
0.12 to 0.30
0.12 to 0.25
0.08 to 0.20
0.05 to 0.15
0.12 to 0.20
0.10 to 0.20
0.07-0.12
0.05-0.10
0.05 to 0.20
0.05 to 0.10
0.03 to 0.08
0.025 to 0.05
0.15 to 0.30
0.15 to 0.25
0.12 to 0.20
0.07 to 0.12
0.12 to 0.20
0.07 to 0.15
0.07 to 0.12
0.05 to 0.10
Carbon content
>0.3%
alloy steel
170～220
220～280
280~320
320～380
0.125 to 0.40
0.10 to 0.30
0.08 to 0.20
0.06 to 0.15
0.12 to 0.30
0.08 to 0.20
0.05 to 0.15
0.05 to 0.12
0.12 to 0.20
0.07 to 0.15
0.05 to 0.12
0.05 to 0.10
0.12 to 0.20
0.07 to 0.15
0.15 to 0.12
0.05 to 0.10
0.15 to 0.25
0.12 to 0.20
0.07 to 0.12
0.05 to 0.10
0.07 to 0.15
0.07 to 0.20
0.05 to 0.10
0.05 to 0.10
Tool Steel
Annealed state
36HRC
46HRC
56HRC
0.15 to 0.50
0.12 to 0.25
0.10 to 0.20
0.07 to 0.10
0.12 to 0.30
0.08 to 0.15
0.06 to 0.12
0.05 to 0.10
0.07 to 0.15
0.05 to 0.10
0.05 to 0.10
0.03 to 0.08
0.12 to 0.20
0.07 to 0.12
0.07 to 0.15
0.05 to 0.10
Aluminum-magnesium alloy
95～100
0.15 to 0.38
0.125 to 0.30
0.15 to 0.20
0.05-0.15
0.20 to 0.30
0.07 to 0.20
Note: The small value in the table is used for fine milling, and the large value is used for rough milling
(3) Determination of the milling speed vc
The milling speed can be determined by checking the milling volume manual, such as Volume I of Mechanical Processing Technology Manual.
Milling cutter selection
(1) Selection of milling cutter diameter
The milling cutter diameter is usually selected according to the amount of milling. 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
Milling depth ap
≤5
~8
~10
≤4
~5
~6
~7
~8
~10
Milling width 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, ac can not be unified with the values ​​in the table at the same time, and ap (cylindrical cutter) or ac (end mill) choose a larger cutter, mainly based on ap (cylindrical cutter) or ac (end milling Knife) Select the cutter diameter.
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