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Crimp form of chip and chip breaking method

in metal cutting, adverse chip shape will seriously affect operation safety, processing quality, tool life, machine tool accuracy and productivity. Therefore, it is necessary to study the curl form and chip breaking method of chip in order to effectively control the chip shape

Based on two force columns, measure with a dial indicator

1 Chip curl form

in the process of plastic metal cutting, due to the different degrees of upward curl and transverse curl of chips, the chip morphology is also different. In order to analyze the form of chip curl, chips can be divided into three categories: upward curl, compound curl and transverse curl. In brittle metal cutting, it is easy to produce granular chips and needle shaped chips. Only in high-speed cutting, large tool rake angle and small cutting thickness, the curl support batch experiment direction of such chips is slightly different from the general situation

when cutting plastic metals, if the inclination of the tool edge is 0 °, there is a chip curling groove and the cutting width is large, most of the chips curl upward. In other cases, the chips are mostly laterally curled. For example, in the cylindrical turning process, when the feed rate is large compared with the back feed rate, and the rake angle of the tool is 0 °, the chip is easy to curl laterally into a cushion ring shape (see Figure 1). This is because the two ends of the chip become wider in the transverse direction, while the volume of the chip remains the same, and the thickness of the transverse widening part will inevitably become thinner. If the length is not shortened, it will inevitably produce transverse curl; In addition, if the transition edge is worn on the turning tool, combined with the role of the tool tip and auxiliary cutting edge, the shear angle in the direction of chip width changes, and the chip can also be laterally bent to form a cushion ring

Figure 1 cushion shaped chip

under normal circumstances, the chip cannot only curl upward or horizontally, but also produce lateral curl while curling upward. The formation of long and tight curls and spiral curls is the result of the upward and horizontal curling of chips at the same time (as shown in Figure 2)

Figure 2 long spiral curls during fine turning

2 Chip breaking method

in plastic metal cutting, straight strip chips and spiral chips are not popular; In brittle metal cutting, continuous chips are expected. Generally, chip shape can be controlled by changing cutting parameters or tool geometric parameters. When processing plastic metal with a certain cutting amount, chip breaking table and chip curling groove are mostly used to control chip shape. This paper mainly discusses the calculation of basic parameters of chip curling groove

Figure 3 shows the basic forms of three chip curling grooves: linear, linear arc and arc. The main parameters are as follows:

Figure 3 basic form and parameters of chip curling groove

(1) contact length L

in Figure 3, the contact length of chips on the rake face can be obtained by the following formula

L = kmachsin( φ+β-γ o)/sin φ cos β (1)

in the formula, KM -- the correction coefficient of the contact length between the chip and the rake face, generally taking about 1.6

ach -- chip thickness

(2) chip curling groove radius R2

according to the fracture theory, the chip breaking condition of plastic metal is

ε f≥ ε FC (2)

where ε F -- chip curl strain

ε FC -- critical breaking strain

Fig. 4 breaking condition of upward curling chip

for upward curling chip, its breaking condition is shown in Fig. 4. Suppose it is stretched on the outer surface of the chip Δ After L, the fracture limit is reached, and

Δ L=(R1+y)d θ- R1d θ (3)

where Δ L -- the elongation of the broken surface when the chip breaks

r1 -- the curl radius when the chip breaks

y -- the normal distance from the neutral layer of the chip to the broken surface

because the bending strain is

ε=Δ L/L

where l - chip length on the neutral layer

so the fracture strain is

ε fc=[(R1+y)d θ- R1d θ]/R1d θ= Y/r1 (4)

known from plastic mechanics

σ b=E ε FC (5)

from equations (4) and (5), it can be obtained that the curl radius of upward curl chip when it is broken is

r1 = ey/σ B (6)

because the short ear chip is similar to the arc chip, its theoretical model can refer to the arc chip (omitted). For the long ear shaped chip, it breaks and stops because it collides with the back face of the tool: when the injection molding cycle is interrupted, its theoretical model is shown in Figure 5. The strain can be calculated from the following formula

ε Fc=0.5ach (1/r2+1/rl) (7)

where ACh -- chip thickness

r2 -- chip curling groove radius

rl -- curling radius when long ear chip breaks

Figure 5 curling and breaking conditions of long ear chip

from formula (5) and formula (7), the chip curling groove radius is

r2 = 1/(2) σ B/each+1/rl) (8)

taking cutting 45 steel (quenching and tempering) as an example, its elastic modulus E = 206gpa, strength limit σ B=650gpa, the measured chip thickness is 0.2mm

r1=1.7mm from formula (6) and R2 < 31.7mm from formula (8)

in order to ensure chip breaking, according to different materials, the chip breaking coefficient can be selected as 8 ~ 12. At this time, the Maximum Curling radius of the chip curling groove is 2.64 ~ 3.96mm, while the curling radius of the chip curling groove obtained from the test statistics is about 3mm. Through analysis, it is found that after plastic deformation, the hardness and strength of metal are greatly improved, while the plasticity and toughness are significantly reduced, which is the main reason why the theoretical value is much higher than the experimental value. Therefore, when designing the chip curling groove, the ratio of theoretical value to experimental value can be selected for the chip breaking coefficient. The final calculation formula of chip curling groove radius is

r = r1/n (9)

where R - chip curling radius of chip curling groove

n - chip breaking coefficient. General precautions: Take 8 ~ 12

(3) in the case of medium cutting depth, it can generally be selected θ= 110 ° ~ 120 ° (see Figure 3). θ If the angle is too small, the chips will be blocked in the groove, resulting in cutting; θ If the angle is too large, the chip curl radius will be too large, and the chip will not break due to small deformation

(4) the inclination of the chip curling groove and the main cutting edge can be divided into external inclined type, parallel type and internal inclined type. The outward inclined chip curling groove is easy to cause the chips to overturn to the rear face of the turning tool and get C-shaped chips (short ear shaped or long ear shaped chips); The chips in the parallel chip curling groove are mostly broken when they touch the workpiece processing surface; The chips in the inner inclined chip curling groove are easy to form continuous long and tight chips. When cutting medium carbon steel, the inclined angle of inner inclined and outer inclined chip curling grooves is usually 8 ° ~ 10 °

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