Number of teeth
There is another important parameter on the end mill, which can also be said to be mainly reflected in the end face view, that is, the number of teeth of the end mill.
There are several combinations of the total number of teeth and the number of teeth that cross the center of the end mill, as shown in Figure 3-14 from left to right: single tooth mill, 2 tooth mill - 2 tooth undercenter, 2 tooth mill - 1 tooth undercenter, 3 tooth mill - 1 tooth undercenter, 4 tooth mill - 2 tooth overcenter, and multi-tooth mill - 0 tooth undercenter. The number of cutter teeth of the milling cutter is related to the milling efficiency, and the rigidity of the milling cutter is related to the diameter of the core of the milling cutter. Figure 3-15 is a simplified diagram of the relationship between the number of cogging teeth of the milling cutter and the rigidity and chip capacity of the milling cutter.
The 2-tooth (slot) milling cutter is characterized by a large chip removal space and insufficient rigidity, which is suitable for long-chip materials.
The 3-tooth (slot) milling cutter is characterized by large chip space, good rigidity, high cutting efficiency and good versatility.
The 4-tooth (slot) milling cutter is characterized by a slight lack of chip removal space, but the milling cutter has good rigidity, which is suitable for efficient finishing and good surface quality of the workpiece.
The 6-tooth (slot) milling cutter is characterized by very small chip removal space, but the milling cutter has excellent rigidity, this milling cutter is very suitable for finishing, efficient machining, high hardness machining, and the machining surface quality is very good.
Of course, it is possible to increase the chip space with the same number of teeth, but this will result in a decrease in rigidity. This geometry (see Figure 3-16) is suitable for machining non-ferrous materials with low strength, such as aluminum and copper. On the one hand, because the strength of this kind of metal is low, the cutting force of the tool is small, and the force required by the tool is also small, and the lower strength is still competent for such a milling task; On the other hand, this type of material has a low cutting heat due to its low cutting force.
However, it is precisely because the cutting force and cutting heat of this kind of material are low, and the cutting amount can be increased after the chip holding capacity is increased, but the increased cutting amount increases the cutting force, so that the rigidity of the tool needs to be improved, so the end mill with a double core diameter as shown in Figure 3-17 needs to be used. The milling cutter shown here is Jabro-Solid from Seco Tools in colour, while the Proto·max TM tG from Walter Tools is shown in grey. The design of the double core diameter provides some balance between chip holding capacity and tool rigidity.
Figure 3-18 is a schematic diagram of the groove bottom of a specially modified milling cutter. In this case, the rigidity of the modified milling cutter is much higher than that of the normal default groove bottom, and the deformation of the chips during discharge is intensified, and the chips are tighter.
There is a different structure for the same number of teeth, that is, unequal teeth. Figure 3-19 is a schematic diagram of two types of unequal milling cutters. The unequal cutter teeth can produce alternating cutting frequencies during cutting, which is not easy to resonate with the machine tool and suppresses the tool vibration during milling.
In addition to the number of teeth, the chip capacity of the milling cutter is also related to the geometric parameters of the circumferential teeth, and the circumferential teeth of the milling cutter are discussed below.
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Circumferential teeth
The cutter teeth on the outer circle of the end mill are called circumferential teeth. The circumferential tooth is the main part of the end mill engaged in sidewall milling.
◆ Helix angle
The first parameter of the circumference tooth to be discussed is the helix angle, which is the angle between the tangent line of the helical cutting edge of the milling cutter and the axis of the milling cutter, as shown in Figure 3-20.
In cutting theory, the helix angle is also the axial rake angle at the outer circle of the tool (please refer to Figure 1-33 for the axial rake angle and related text).
The main effects of different helix angles of end mills on cutting performance are shown in Figure 3-21. As you can see from the figure, the straight flute end mill (helix angle 8-0°) on the right side has zero axial cutting force due to the zero axial rake angle, and all the cutting force is in the radial direction with the weakest rigidity, so it is prone to chatter. On the other hand, the left and middle spiral flute cutters are divided into axial directions due to a part of the cutting force (the axial direction is the direction with the best rigidity of the milling cutter), and the radial load is reduced, and the chatter is not easy to occur.
On the other hand, the chip flow of the straight groove milling cutter is transverse, which is easy to be interfered by the cutting area of the workpiece and form a secondary cut, and the chip removal performance is poor. The chips of the spiral flute cutter are discharged from the cutting zone perpendicular to the cutting edge, and the chip evacuation performance is greatly improved.
Figure 3-22 shows the effect of the number of cutter teeth and helix angle on the axial component of the total cut length. For the cutting task of a 10mm diameter milling cutter with a cutting width (also known as "radial depth of cut") of 10 mm and a cutting depth (also known as "axial depth of cut") of 15 mm, the axial projection of the total contact edge length of the milling cutter with 2 slots and 30° helix angle is about 17 mm; When using a 3-groove 30° helix cutter, the axial projection of the total contact edge length increases to about 25 mm. When a 4-groove 30° helix angle milling cutter is used, the axial projection of the total contact edge length is increased to about 30 mm, and finally when a 6-groove 60° helix angle milling cutter is used, the axial projection of the total contact edge length can be increased to about 47 mm. These data show that with the increase of the number of milling cutter teeth, the number of cutting edges in contact with the workpiece also increases, the axial projection of the total contact edge length increases, and the effect of increasing the helix angle is similar. With the increase of the axial projection of the total contact edge length, the load per unit tooth length is reduced, and the cutting efficiency can be improved under the premise that the tooth load remains the same.
Figure 3-23 shows four combinations of different cutting directions and spiral groove rotation directions, the common one is the right helical tooth right cutting direction, generally speaking, the cutting direction of the milling cutter is mainly determined by the spindle rotation direction of the milling machine, and after the cutting direction is determined, the helix determines the direction of the axial cutting force.
Figure 3-24 shows a JS840 milling cutter with a double helix direction. This milling cutter is used to machine the side edges of carbon fiber composite panels. Since carbon fiber composite panels are made up of several different materials, it is difficult to avoid delamination with conventional milling cutters. The advantages of the JS840 milling cutter are: the cutting force in the opposite direction is divided into downward pressure and central force: the chip space is large, which is conducive to chip removal: the cutting contact area is small, which produces less cutting heat and cutting force: only the shear force is generated on the fiber, and there is no torsion to the middle.
Figure 3-25 shows Sumitomo Electric's GSXVL type anti-vibration end mill. This end mill not only uses unequal teeth like those shown in Figure 3-19, but also improves vibration protection when machining on the side with unequal helix angles.
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