Front and back
The circumferential teeth also have geometric parameters such as front, back, rake angle, rear angle, cutting band, etc. Figure 3-26 is a typical circumferential tooth structure. The red line in the enlarged image is the front, which is the only way for the chips to be cut off the workpiece and discharged: the blue dot line is the first back, and the green short line is the second back, which is not a necessary structure for end mills, but it is a structure that many end mills have, which can increase the chip space and reduce the friction between the back and the machined surface. 1) The groove bottom arc in front is the path for the chips to flow out of the curl. In some cases, it is necessary to shorten the contact length between the chip and the front of the tool to increase the deformation of the chip. In this case, the method shown in Figure 3-18b can be used. However, this method increases the diameter of the cutter core and reduces the chip space. Figure 3-27 shows another solution to change the chip outflow state, i.e., the change in the rake face of the circumferential teeth. In this way, the chip is strengthened, the contact length of the knife chip is shortened, and the chip space is guaranteed.
Figure 3-28 shows two different types of rake angles (radial rake angles). The positive rake angle of the circumferential teeth can form a lighter rake angle, which is easy to cut into the material to be machined, and the chips form a bending stress on the front, which is generally recommended for machining materials such as mild steel, aluminum and stainless steel if this bending stress is too large, and it is generally recommended for machining materials such as mild steel, aluminum and stainless steel: the negative rake angle of the circumferential teeth forms a strong cutting edge, and the chips are in front of the tool
The surface produces compressive stress, which is not easy to damage for the tool, and is generally recommended for machining medium carbon steel and hardening pins.
2) The shape behind the perimeter teeth will also have an impact on the use of end milling. In general, there are three basic forms behind the circumferential teeth: planar, concave, and shoveling, as shown in Figure 3-29. (1) The flat type is relatively simple at the back, and it is the most common type when processing non-ferrous materials such as aluminum and copper. It can be used for both circumferential and end teeth, including the first and second rear of the end teeth.
2. The back of the concave type is to create a concave gap behind the cutting edge, this back structure appears very sharp, and the back grinding is very simple, but the large relief angle behind the cutting edge makes the tool fragile and easy to be damaged by chips, therefore, it is usually not recommended, and the manufacturer rarely sells this kind of back milling cutter.
3. The back of the shovel grinding type is also called the back of the shovel back type, which is characterized by a curve on the back (this curve is the Archimedes spiral), as long as the front angle is guaranteed to remain unchanged when the front is re-grinded, the back angle of the milling cutter will not change. This type of back is mainly used for the peripheral tooth relief angle and can form a strong cutting edge. At present, many end mills use this shovel grinding type behind the circumferential radial back, including the first back and the second back, but it can also occasionally be seen that the second back is formed with a flat type.
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Cutting belt
Some milling cutters have a convex star behind the first or second back, and this structure is often referred to as a "ribbed band" or a "edge zone", but the cutting theory of "edge band" defines the trailing angle to 0°, so it is called a "edge band". The two behind the two in Figure 3-26 are on such a "band". Ribs that are too narrow can make the teeth easy to break, while ribs that are too wide can cause excessive friction.
The true 0° "blade belt" has a very strong effect on vibration cancellation, etc. Sumitomo Electric's anti-vibration end mills with unequal teeth and unequal helix angles, as mentioned earlier, have a zero-degree edge belt in the shape of a circular arc, which is very useful for vibration cancellation. The thin white strip inside the red ellipse shown in Figure 3-30 on the right is a cutting edge for machining tasks with long sides, and milling cutters with chip splitting grooves (see Figure 3-31) are also widely used in the roughing range.
Figure 3-32 shows the type of chipping for Walter's roughing cutter with flute. Flutes with round shapes (domed domes) are relatively simple to manufacture, while the top of flutes with flat shapes (flat tops and domes) is done by external cutting. Comparatively, the flat-top chiplet makes the cutting edge of the cutter sharper.
Figure 3-33a is a schematic diagram of the pitch of the chipping groove of a chip-splitting cutter, with different colors representing different cutting edges, and one higher than the other containing the effect of feed. The area between the two cutting edges is the cutting pattern of the cutting edge. It can be seen that this cutting pattern is not only related to the pitch of the chipset, but also to the amount of cutting used. This is somewhat different from the corn cutter discussed in Chapter 4, where the material to be machined left by one cutting edge flute between the flutes of the wavy tooth cannot be completely removed by the latter tooth.
Figure 3-33b shows the effect of different flute pitches on power and wear. Close pitches (small pitches) have lower slotting wear but a high demand for machine power, so fine gears are used for difficult-to-machine materials and small depths of cut, while coarse gears are used for high material removal rates and can be used for low-power machines.
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corner
The corner refers to the transition between the circumference and end teeth of the end mill.
There are two main types of corners for end mills: chamfered and filleted.
Figure 3-34a is a chamfered type. There are two main parameters of the chamfer type: the chamfer width K and the chamfer angle (usually 45°): Figure 3-34b is the rounding type, and the main parameter of the rounding type is the arc radius.
The relief angle of the corner is an independent relief angle for the chamfer type, while the rounding type requires a natural transition from the circumferential corner to the end tooth corner.
It can be a little difficult to achieve a natural transition in front of the corner. Therefore, there are two basic ways to handle the front of the corner: to connect to the front of the circumferential tooth (see Figure 3-34b) and to connect to the front of the end tooth (see Figure 3-34c). Due to the low strength at the corners, the lower value of the two rake angles of the end tooth and the circumferential tooth is connected.
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