Die design and process experimental research of op

Abstract: Aiming at the square spline spindle of 125cc motorcycle gearbox, the die structure characteristics of spline open die extrusion and the die springback problem affecting spline forming are discussed. It is proposed that selecting the die angle within the formability range of open die extrusion can avoid the local upsetting of the die inlet during extrusion

key words: spline shaft; Open die extrusion; Mold; Local upsetting

I. Introduction

at present, the method of forming spline shaft by open die cold extrusion has been widely valued at home and abroad. This advanced process has the characteristics of material saving, high quality and high efficiency. After continuous improvement and development, this technology will replace the traditional cutting methods. During spline open die extrusion, the die structure and the shape of the die entrance have a great influence on its forming. In addition, during open die extrusion, it is easy to cause local upsetting of the blank at the die entrance, which makes the extrusion impossible. At present, there is no report on the discussion of this local upsetting problem. Trial and error methods are often used in factories to solve it by adjusting die parameters. In this paper, experimental research is carried out to solve the above problems, and the formability range of open die extrusion spline is proposed to measure whether local upsetting occurs

II. Die structure of spline shaft open die extrusion [1, 2]

Figure 1 is the part drawing of spline shaft used in the experiment. There are rectangular splines with the same size at both ends of the shaft, and the middle step is gear

Fig. 1 spline shaft part drawing

in the experiment, two processes are used to complete the free upsetting of the middle step and the extrusion of the splines at both ends respectively. In order to ensure the coaxiality of the upper and lower splines, the method of simultaneous extrusion at both ends is adopted. When the die is installed between the upper and lower templates, the first stage guidance of the guide post and the guide sleeve is used, and then the second stage guidance of the die opening of the upper and lower die sleeves is used, so as to ensure the guidance accuracy during forming. See Fig. 2 for the structure of spline forming die

Figure 2 spline forming die structure

1 Structural features of spline die

as shown in Figure 2, the spline forming die must have rounded corners at the beginning of deformation. If it is not designed as a rounded corner, the tooth width will become narrow, and the tooth top will not be fully filled. In addition, if the fillet of each tooth is uneven, it will cause tooth profile error and bending in the extruded part. There are three shapes of flower keying die: bow shape, pointed shape and trapezoid shape. Among the three shapes, when the sharp head is used to form the shape, the phosphating film is easy to be scratched, resulting in poor lubrication. The bow shape and trapezoid will not have the above problems, but the trapezoid extrusion force is larger than the bow shape. No matter which method is adopted, the corners and transition surfaces should be processed into rounded corners to make the metal flow smoothly. In this experiment, trapezoidal die entrance is used

2. Mold springback

during cold extrusion, due to the high unit extrusion pressure on the mold, the mold has elastic expansion, which has a serious impact on the dimensional accuracy of products, especially after cold extrusion, the surface is no longer machined. There is no accurate formula for the calculation of die springback when "gold" belongs to volume forming, which is limited to empirical values. In this experiment, because the unit extrusion force is small, the recoil amount is 0.01mm, and the actual measurement results are shown in Table 1. It can be seen from the table that the actual rebound amount is larger than the value taken in the experiment. Because the lower limit of the part size is selected in the design of the mold cavity size, the size of the test piece does not exceed the tolerance range of the part size, but this will reduce the wear amount and affect the service life of the mold. From this, we can see that the accurate selection of springback has a significant impact on the size of the workpiece and the service life of the die. Table 1 measurement data of die springback

- root circle diameter (mm) addendum circle diameter (mm) tooth width (mm) die size 16.63520.2354.835 extrusion size 16.683 ~ 16.68920.285 ~ 20.3014.845 ~ 4.850 die elastic deformation 0.048 ~ 0.0540.050 ~ 0.0660.010 ~ 0.015

3 Hardness change of extruded specimen

the hardness of the blank will change after cold extrusion. Cold work hardening increases the hardness of the blank. When the deformation degree is different, the hardness will be different. Where the deformation is large, the hardness value is high. In the process of spline die opening extrusion, the hardness of the blank before extruding the spline is a very important parameter, so it is very necessary to make great progress and breakthrough in the measurement of hardness in China's plastic extruder market. Figure 3 shows the hardness distribution of the blank after annealing heat treatment and the test pieces in the two processes in this experiment. Because the test pieces are symmetrical, only one quarter is taken for measurement, and the section taken is the same section in different processes of the shaft and rod of the part. Because the first process of the blank is local upsetting, the deformation of the non upsetting part of the rod is very small, so the hardness value changes little. However, the hardness of the upsetting step increases a lot due to cold work hardening, as shown in Figure 3C. The hardness distribution of the cross-section of the spline after forming is shown in Figure 3D. The hardness value increases most obviously at the fillet transition of the tooth root due to its large deformation, but the hardness of the tooth top changes little. This is because the blank radius r0 is very close to the tooth top radius R1, and there is little deformation here

Fig. 3 changes in hardness during extrusion

(a) section of blank after spheroidizing annealing (b) section of shaft and rod after upsetting

(c) section of step after upsetting (d) section of spline after extrusion

III. formability experiment of spline die opening extrusion

1 Proposal and calculation of formability

during spline die opening extrusion, local upsetting is easy to occur at the die entrance, because the extrusion force P per unit area of the blank is greater than the yield strength of the material σ S. The main factor affecting P is the half angle of molding α， Friction coefficient m, spline section reduction rate R.A%, blank radius r0, of which the main variable influence parameters are α。 In addition, σ S value changes with the change of hardness. In this way, P ≤ σ S. There is no local upsetting phenomenon. When the hardness and other conditions are certain, α There will be a critical value α Cr, only half angle of molding α Normal open die extrusion can be carried out only when it is within the critical value range

about α The CR value can be calculated by the upper bound method [3], and the spline extrusion can be calculated by using the allowable velocity field of 3D extrusion kinematics of any section proposed by Yang [4]. For the spline in Figure 1, the calculation result is α cr1＝4.48°， α CR2 = 26.68 °, that is, when α cr1≤ α ≤ α CR2, normal die opening extrusion can be carried out without local upsetting

2. Formability experiment

in this paper, the open die extrusion experiment is carried out on the parts shown in Figure 1. The material is 20CrMnTi steel, Hb = 167 after spheroidizing annealing, σ S ＝ 375.2mpa, which is the value of test calculation. In the experiment, the half angle of short end spline molding is taken as α＝ 30 °, half angle of long end spline molding α＝ 25 °, the blank is subject to phosphorus saponification treatment, and the extrusion equipment is yb32-100a four column hydraulic press. The test piece after extrusion is shown in Figure 4. Observe the test piece, α＝ The lubrication film at the 25 ° end at the die entrance is intact without brightening, indicating that there is no local bulge, and the spline shape is intact, which is consistent with the calculation results. α＝ At the 30 ° end, there is a bright area at the upper end of the die inlet, and the blank has a slight bulge. Because the right side of the spline is limited by the straight wall die of the shaft rod, the bulge part is stuck to the mold, so that the lubricating film falls off and shines, but the spline can still be well formed, which can be seen from it α＝ It is critical at 30 °. The calculated critical value is α CR2 ＝ 26.68 °, actual α＝ Open die extrusion can also be completed at 30 °, and there is a certain error between the calculated value and the experimental results. The brush clip aligns and clamps the remaining hair bundle difference, which is mainly caused by the large P value calculated by the upper limit method α CR2 is too small. It can be seen that the upper bound method is reliable in engineering design

Figure 4 spline shaft cold extrusion part (two processes)

IV. conclusion

(1) the guiding accuracy of the die is very important for spline die extrusion. The shape of the die entrance directly affects the spline forming quality. The use of transition fillet and bow shaped or trapezoidal die entrance is very beneficial to spline forming

(2) the elastic deformation of the die directly affects the dimensional accuracy of the product and the life of the die. How to accurately select the springback is an urgent problem to be solved

(3) due to the effect of strain hardening, the hardness of the spline changes after extrusion. Where the deformation is large, the deformation is intense, and the hardness value increases significantly

(4) the formability range of spline during die opening extrusion is proposed, within which local upsetting at the die inlet can be avoided, and the rationality of the proposed formability range and the reliability of the upper bound method are verified by experiments

author unit: Huang Chaohui, Beijing Institute of Aeronautical Materials (100095)

Fu Peifu, Jilin industry 3:5, no equipment is allowed to operate under overload; It is not allowed to dismantle, install or change randomly; It is not allowed to cancel the safety device casually; It is not allowed for unqualified personnel to work and engage in protection work independently; It is not allowed for unlicensed and inexperienced personnel to operate and inspect equipment. University

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2 Yang Changshun Cold extrusion die design Beijing: National Defense Industry Press, 1994

3 Fu Peifu, Huang Zhaohui, etc Study on forming conditions of spline shaft by open die cold extrusion Journal of agricultural machinery, 1997, 28:129 ~ 133

4D y Yang, C H han A generalized method for analysis of three－dimensional extrusion of arbitrary shaped i.,1986,28(8):517～534(end)