Cold heading method for manufacturing mandrel

Abstract

The invention relates to a cold heading method for manufacturing a mandrel. The cold heading method comprises the following steps: cutting a cold heading steel cylinder; a first positioning hole is formed in the upper end face of the cold heading steel cylinder in a downward cold heading mode, a second positioning hole is formed in the lower end face of the cold heading steel cylinder in an upward cold heading mode, and a first cold heading blank is formed; performing cold heading downwards along the first positioning hole of the first cold heading blank body and performing cold heading upwards along the second positioning hole to form a second cold heading blank body; performing cold heading downwards along the first chamfering cavity of the second cold heading blank body to form a third cold heading blank body; performing cold heading downwards along a second chamfering cavity of the third cold heading blank body to form a fourth cold heading blank body; cold heading is conducted downwards along a second step cavity of the fourth cold heading blank, the second step cavity is communicated with the third positioning hole, and a mandrel blank is formed; in the cold heading process and in the process of forming the cold heading steel cylinder to the mandrel blank, the outer diameter does not change and only changes slightly, local extrusion aggregation of a decarburized layer is reduced in the cold heading forming process, it is guaranteed that the decarburized layer can be removed in subsequent machining, and soft spots are avoided.

Description

Technical Field

[0001] The invention relates to the field of core shaft manufacturing, in particular to a cold heading method for manufacturing a core shaft. Background Art

[0002] The core shaft is a key component of the pulley. The core shaft is usually obtained by axially drilling, turning, boring, and turning the end face of the bearing steel bar to obtain the core shaft blank; however, the above steps are relatively many and the process is complicated, and the utilization rate of the bar is low, which results in a high cost for producing the core shaft.

[0003] In order to reduce production costs and improve the performance indicators of the core shaft such as strength and wear resistance, the applicant has made various attempts on the manufacturing process of the core shaft. The applicant tried to use the cold forging process to prepare the core shaft. In the conventional cold forging process, the upsetting ratio is set at about 1.2. When the core shaft blank with an outer diameter of about 29.4mm is formed by cold forging, a standard outer diameter of 24mm cold forging cylinder 01 is used. During the upsetting process of the cold forging cylinder, the decarburization layer 001 on the outer peripheral wall surface of the cold forging cylinder is locally deep, such as Figure 13 As shown. In this way, the core shaft blank 5 with a diameter of 29.4mm is cold-forged and then tempered, ground, and high-frequency quenched. During the grinding process, the decarburized layer 001 on the surface of the core shaft blank 5 cannot be completely ground off, resulting in soft spots, which affects the surface hardness of the material. A larger chip allowance is required for the core shaft blank 5 to completely remove the decarburized layer 001, which increases the grinding time and production cost and reduces the material utilization rate. In addition, when a cold-forged cylinder 01 with an outer diameter of 24mm is used, when the core shaft blank 5 of about 29.4mm is formed by cold forging, cracks appear on the surface of the core shaft blank after forming, because the core shaft blank is usually formed by cold forging and the axial compression and radial extension during the cold forging process are large. Although a spheroidizing pretreatment process is used to remedy this before cold forging, the surface of the core shaft blank 5 is still prone to cracks, resulting in a low yield rate.

[0004] In summary, how to achieve cold heading of the mandrel, improve the yield, reduce production costs, and improve the mechanical properties of the mandrel has become an urgent problem that researchers in this field need to solve. Summary of the Invention

[0005] The technical problem to be solved by the present invention is: how to solve the problem that the core shaft in the pulley is difficult to achieve cold heading forming because it is made of bearing steel. To this end, the present invention aims to provide a cold heading method for manufacturing the core shaft, which can avoid the local accumulation of decarburization layer in the core shaft made of bearing steel during the cold heading process; to inhibit the soft spots that appear during the heat treatment process due to the local accumulation of decarburization layer after cold heading of the core shaft, thereby improving the yield and reducing the grinding allowance of the core shaft, reducing production costs, and improving the mechanical properties of the core shaft.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0007] The present invention is a cold heading method for manufacturing a core shaft, comprising the following steps: step S1: selecting bearing steel suitable for cold heading, and cutting a cold heading steel cylinder of appropriate length according to the specifications of the core shaft through spheroidization; step S2: cold heading a first positioning hole downward on the upper end surface of the cold heading steel cylinder, and cold heading a second positioning hole upward on the lower end surface of the cold heading steel cylinder to form a first cold heading blank; step S3: cold heading downward along the first positioning hole of the first cold heading blank and cold heading upward along the second positioning hole to form a second cold heading blank with a first chamfered cavity and a third positioning hole in the axial direction; step S4: cold heading downward along the first chamfered cavity of the second cold heading blank to form a first chamfered cavity in the axial direction A third cold heading blank having a first chamfering cavity, a first step cavity, a second chamfering cavity, and a third positioning hole; step S5: cold heading downward along the second chamfering cavity of the third cold heading blank to form a fourth cold heading blank having a first chamfering cavity, a first step cavity, a second chamfering cavity, a second step cavity, and a third positioning hole in the axial direction; step S6: cold heading downward along the second step cavity of the fourth cold heading blank to punch out the core material between the second step cavity and the third positioning hole, and then the second step cavity and the third positioning hole are connected to form a core shaft blank having a first chamfering cavity, a first step cavity, a second chamfering cavity, and a through cavity in the axial direction; wherein the outer diameter of the cold heading steel cylinder is 0.3 to 0.6 mm smaller than the core shaft blank.

[0008] Furthermore, in step S1, the axial height of the cold-headed steel cylinder is smaller than the axial height of the core shaft blank.

[0009] Furthermore, in step S2, after the first cold-forged blank is formed by cold heading, a first fillet is formed at the outer diameter of the bottom thereof;

[0010] Furthermore, after the second cold heading blank is formed by cold heading, a second fillet is formed at the outer diameter of its bottom, wherein the radius of the second fillet is smaller than the radius of the first fillet; the third cold heading blank, the fourth cold heading blank, and the core shaft blank all have a second fillet at the outer diameter of the bottom; in step S5, after the fourth cold heading blank is formed by cold heading, a third fillet is formed at the outer diameter of its top; the core shaft blank has a third fillet at the outer diameter of the top.

[0011] Furthermore, in step S2, the height of the cold heading steel cylinder is greater than the height of the first cold heading blank, and the outer diameter of the cold heading steel cylinder is smaller than the outer diameter of the first cold heading blank.

[0012] Further, in step S3, the height of the first cold heading blank is greater than the height of the second cold heading blank, and the outer diameter of the first cold heading blank is smaller than the outer diameter of the second cold heading blank; the diameter of the first positioning hole is greater than the diameter of the first chamfered cavity, and the height of the first positioning hole is smaller than the height of the first chamfered cavity; the diameter of the second positioning hole is greater than the diameter of the third positioning hole, and the height of the second positioning hole is smaller than the height of the third positioning hole.

[0013] Further, in step S4, the height of the second cold heading blank is smaller than the height of the third cold heading blank, and the outer diameter of the second cold heading blank is smaller than the outer diameter of the third cold heading blank; the height and diameter of the third positioning hole of the second cold heading blank are consistent with the third positioning hole of the third cold heading blank; the height of the first chamfered cavity of the second cold heading blank is smaller than the height of the first step cavity.

[0014] Further, in step S5, the height of the third cold heading blank is smaller than the height of the fourth cold heading blank, and the outer diameter of the third cold heading blank is smaller than the outer diameter of the fourth cold heading blank; the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the third cold heading blank are consistent with the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the fourth cold heading blank.

[0015] Further, in step S6, the height of the fourth cold heading blank is equal to the height of the core shaft blank, and the outer diameter of the fourth cold heading blank is larger than the outer diameter of the core shaft blank; the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the fourth cold heading blank are consistent with the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the core shaft blank; the sum of the heights of the second step cavity and the third positioning hole in the fourth cold heading blank is less than the height of the through cavity in the core shaft blank.

[0016] Beneficial effects of the present invention: The present invention is a cold heading method for manufacturing a mandrel, wherein the outer diameter of the cold-headed steel cylinder is 0.3 mm to 0.6 mm smaller than the mandrel blank, so that the outer diameter of the cold-headed steel cylinder changes only slightly during the cold heading process, thereby suppressing the risk of surface cracking of the mandrel blank, reducing local extrusion and aggregation of the decarburized layer during the entire cold heading forming process, and grinding and removing the decarburized layer during the subsequent processing of the mandrel blank, thereby reducing the cutting allowance of the mandrel blank, and the mandrel blank does not have soft spots after heat treatment, thereby improving the yield rate and reducing the production cost; compared with the hole opening processing of the bar, the mandrel blank is processed and formed by the cold heading method, and the waste of the formed mandrel blank is less, and the density of the structure is also improved;

[0017] In addition, the outer diameter of the cold-headed steel cylinder and the cold-headed blank changes slightly during the cold-heading process, which can create a tiny radial gap between the cold-headed steel cylinder or the cold-headed blank and the corresponding mold cavity, thereby preventing the cold-headed steel cylinder or the cold-headed blank from being strained on the inner wall surface of the corresponding mold when placed in the corresponding mold. Moreover, since the outer diameter of the cold-headed steel cylinder or the cold-headed blank remains unchanged during the cold-heading process, the outer diameter of the cold-headed mandrel only changes slightly during the cold-heading process of the present method, thereby compensating for the defect of surface cracking caused by the poor plasticity of the mandrel made of bearing steel. More importantly, due to the radial gap before cold-heading, the mandrel is not only extended axially, but also has space for radial extension, thereby avoiding the problem of surface cracks caused by only axial extension of the mandrel. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] The present invention will be further described below with reference to the accompanying drawings and examples.

[0019] Figure 1 It is a cross-sectional view of a cold-headed steel cylinder;

[0020] Figure 2 It is a cross-sectional view of the first cold heading blank;

[0021] Figure 3 It is a cross-sectional view of the second cold heading blank;

[0022] Figure 4 It is a cross-sectional view of the third cold heading blank;

[0023] Figure 5 It is a cross-sectional view of the fourth cold heading blank;

[0024] Figure 6 It is a cross-sectional view of the mandrel blank;

[0025] Figure 7 It is a cross-sectional view of the cutting die;

[0026] Figure 8 is a cross-sectional view of the first cold heading die;

[0027] Figure 9 is a cross-sectional view of the second cold heading die;

[0028] Figure 10 is a cross-sectional view of the third cold heading die;

[0029] Figure 11 is a cross-sectional view of the fourth cold heading die;

[0030] Figure 12 is a cross-sectional view of the fourth cold heading die;

[0031] Figure 13 It is a schematic diagram of a conventional cold heading steel cylindrical forming mandrel blank;

[0032] Figure 14 This is the microstructure diagram of the mandrel after forming by conventional cold heading process;

[0033] Figure 15 This is a microstructure diagram of the mandrel after forming using the cold heading process of the present invention;

[0034] Figure 16 It is a process flow chart for cold heading of core shaft blank. DETAILED DESCRIPTION

[0035] The present invention will now be described in further detail with reference to the accompanying drawings, which are simplified schematic diagrams that illustrate the basic structure of the present invention in a schematic manner.

[0036] See also Figure 6 , Figure 6 It is a cross-sectional view of the core shaft blank 5. In the figure, the middle part of the core shaft blank 5 is arranged from top to bottom as the first chamfer cavity 21, the first step cavity 31, the second chamfer cavity 32, and the through cavity 52. The above four are arranged to pass through the middle part of the core shaft blank 5 along its axis; the inner diameter of the first step cavity 31 is larger than the inner diameter of the through cavity 52, and the first chamfer cavity 21 connects the top of the core shaft blank 5 and the top of the first step cavity 31, playing a transition role; the second chamfer cavity 32 connects the bottom of the first step cavity 31 and the top of the through cavity 52, and also plays a transition role; a third fillet 42 is provided at the outer diameter of the top of the core shaft blank 5, and a second fillet 23 is provided at the outer diameter of the bottom of the core shaft blank 5. The radians of the second fillet 23 and the third fillet 42 are consistent.

[0037] For cold heading Figure 6 In order to prevent the core shaft blank 5 in the cold heading cylinder 01 from being excessively locally squeezed during the upsetting process, resulting in soft spots in the subsequent heat treatment, the present application provides the following embodiments.

[0038] Example 1

[0039] See also Figure 1 , Figure 1 This is a cross-sectional view of a cold-forged steel cylinder 01. The cold-forged steel cylinder 01 is formed by cutting bearing steel after spheroidization. The outer diameter of the cold-forged steel cylinder 01 is less than the outer diameter of the core shaft blank 5 by 0.3-0.6 mm. 0.3-0.6 mm is the range of the outer diameter of the cold-forged steel cylinder 01 as it is gradually upset to the core shaft blank 5. For example, Figure 6 The outer diameter of the core shaft blank 5 is 29.4mm and the axial length is 39mm. Figure 1The cold-headed steel cylinder 01 that can be used has an outer diameter of 29 mm and an axial length of 32.5 mm; the weight of the cold-headed steel cylinder 01 needs to be greater than the weight of the core shaft blank 5, so that the core 51 formed by the core shaft blank 5 has a margin. The core 51 will be broken by the corresponding punch in step S6. The greater the weight of the cold-headed steel cylinder 01, the more margin the core 51 has, and the impact force on the punch is correspondingly increased. Therefore, a cold-headed steel cylinder 01 with appropriate weight and axial length needs to be selected.

[0040] See also Figure 7 , which is a cross-sectional view of the cutting die for cutting steel into a cold-headed steel cylinder 01. In the figure, the end of the steel is fixed to the cutting die 03 of the cutting die 02, and the cutter 04 moves to cut the steel into the headed steel cylinder 01.

[0041] See also Figure 2 , Figure 2 It is a cross-sectional view of the first cold heading blank 1. In step S2, the upper end surface of the cold heading steel cylinder 01 is cold headed downward by a punch to form a first positioning hole 11, and the lower end surface of the cold heading steel cylinder 01 is cold headed upward by another punch to form a second positioning hole 12. In this way, the first cold heading blank 1 is formed with a first positioning hole 11 and a second positioning hole 12; since the two punches act on the upper and lower end surfaces of the cold heading steel cylinder 01 respectively, the axial height of the first cold heading blank 1 is shorter than that of the cold heading steel cylinder 01, but due to the limitation of the inner diameter of the cold heading die, the outer diameter of the first cold heading blank 1 is slightly expanded compared with the outer diameter of the cold heading steel cylinder 01, that is, the outer diameter of the cold heading steel cylinder 01 expands from 29 mm to the outer diameter of the first cold heading blank 1 of 29.1 mm.

[0042] In addition, during the cold heading process, the outer diameter of the bottom wall of the first cold heading blank 1 is affected by the corresponding cold heading die to form a first fillet 13 .

[0043] See also Figure 8 , which is a cross-sectional view of the first cold heading die 1-1 for forming the first cold heading blank 1. In the figure, the cold heading steel cylinder 01 is placed in the first cavity 1-2 of the first cold heading die 1-1. The first upper punch 1-3 above the cold heading steel cylinder 01 moves downward and acts on the upper end surface of the cold heading steel cylinder 01 to form a first positioning hole 11. The first lower punch 1-4 below the cold heading steel cylinder 01 moves upward and acts on the lower end surface of the cold heading steel cylinder 01 to form a second positioning hole 12. The cold heading steel cylinder 01 is cold headed to form the first cold heading blank 1.

[0044] See also Figure 3 , Figure 3Cross-sectional view of the second cold-headed blank 2. In step S3, the first positioning hole 11 of the first cold-headed blank 1 is cold-headed downwardly by a punch to form a first chamfered cavity 21, and the second positioning hole 12 is cold-headed upwardly by another punch to form a third positioning hole 22. In this way, the second cold-headed blank 2 has the first chamfered cavity 21 and the third positioning hole 22. The function of the third positioning hole 22 is to position the third cold-headed blank 3 and the fourth cold-headed blank 4 with the corresponding cold-heading die in steps S4 and S5, thereby ensuring the coaxiality between the corresponding punch and the third cold-headed blank 3 and the fourth cold-headed blank 4. Figure 3 The height of the first chamfered cavity 21 is greater than Figure 2 The height of the first positioning hole 11, Figure 3 The height of the third positioning hole 22 is greater than Figure 2 The height of the second positioning hole 12 in the middle; similarly, since the two punches act on the upper and lower end surfaces of the second cold heading blank 2 respectively, the axial length of the third cold heading blank 3 will be shorter than that of the second cold heading blank 2, but due to the limitation of the inner diameter of the cold heading die, the outer diameter of the second cold heading blank 2 is slightly expanded compared with the outer diameter of the first cold heading blank 1, that is, the outer diameter of the first cold heading blank 1 is 29.1mm and expands to the outer diameter of the second cold heading blank 2 is 29.15mm.

[0045] In addition, during the cold heading process, the outer diameter of the bottom wall of the second cold heading blank 2 is affected by the corresponding cold heading die, and is formed from the first rounded corner 13 into the second rounded corner 23 .

[0046] See also Figure 9 , is a cross-sectional view of the second cold heading die 2-1 for forming the second cold heading blank 2. In the figure, the first cold heading blank 1 is placed in the second cavity 2-2 of the second cold heading die 2-1, and the second upper punch 2-3 above the first cold heading blank 1 moves downward to upset the first positioning hole 11 of the first cold heading blank 1 to form a first chamfered cavity 21. The diameter of the main body of the second upper punch 2-3 matches the diameter of the first cold heading blank 1; the second lower punch 2-4 below the first cold heading blank 1 moves upward to upset the second positioning hole 12 of the first cold heading blank 1 to form a third positioning hole 22. The diameter of the second lower punch 2-4 is smaller than the diameter of the first cold heading blank 1; in this way, the first cold heading blank 1 is cold-forged to form the second cold heading blank 2.

[0047] See also Figure 4 , Figure 4The cross-sectional view of the third cold heading blank 3 is shown. In step S4, the lower part of the second cold heading blank 2 is kept unchanged, that is, the third positioning hole 22 and the second fillet 23 are kept unchanged. The first chamfered cavity 21 of the second cold heading blank 2 is cold-forged downward by the punch to form a first step cavity 31 and a second chamfered cavity 32. In this way, the third cold heading blank 3 is axially divided from top to bottom into the first chamfered cavity 21, the first step cavity 31, the second chamfered cavity 32, and the third positioning hole 22. The hole 22 is not connected to the second chamfered cavity 32; since the punch deepens the bottom of the first chamfered cavity 21, the height of the third cold heading blank 3 after forming will increase compared with the second cold heading blank 2. Affected by the limitation of the corresponding cold heading die, the outer diameter of the third cold heading blank 3 is slightly expanded compared with the outer diameter of the second cold heading blank 2, that is, the outer diameter of the second cold heading blank 2 is 29.15mm and expands to the outer diameter of the third cold heading blank 3 is 29.2mm.

[0048] See also Figure 10 , is a cross-sectional view of the third cold heading die 3-1 for forming the third cold heading blank 3. In the figure, the second cold heading blank 2 is placed in the third cavity 3-2. The position of the third lower punch 3-4 below the second cold heading blank 2 remains unchanged and is positioned to match the third positioning hole 22. The third upper ram 3-5 is adjusted to above the second cold heading blank 2. The third upper punch 3-3 above the second cold heading blank 2 moves downward to upset the first chamfered cavity 21 to form a first step cavity 31 and a second chamfered cavity 32. The diameter of the third upper punch 3-3 is smaller than the outer diameter of the second cold heading blank. Therefore, during the process of forming the second cold heading blank 2 into the third cold heading blank 3, the axial height will increase until the top of the second cold heading blank 2 is fitted with the third upper ram 3-5.

[0049] See also Figure 5 , Figure 5 It is a cross-sectional view of the fourth cold heading blank 4. In step S5, the lower part of the third cold heading blank 3 is kept unchanged, that is, the third positioning hole 22 and the second fillet 23 are kept unchanged, and the second chamfered cavity 32 is downwardly upset by the punch to form a second step cavity 41. The fourth cold heading blank 4 thus formed is axially arranged from top to bottom as follows: the first chamfered cavity 21, the first step cavity 31, the second chamfered cavity 32, the second step cavity 41, and the third positioning hole 22; the third positioning hole 22 is not connected to the second step cavity 41; the second step cavity 41 The inner diameter of the first step cavity 31 is smaller than the inner diameter of the first step cavity 31, and the height of the first step cavity 31 is smaller than the height of the second step cavity 41; since the punch deepens the bottom of the second chamfered cavity 32, the height of the fourth cold heading blank 4 after forming will increase compared with the third cold heading blank 3. Affected by the limitation of the corresponding cold heading die, the outer diameter of the fourth cold heading blank 4 is slightly expanded compared with the outer diameter of the third cold heading blank 3, that is, the outer diameter of the third cold heading blank 3 expands from 29.2 mm to 29.4 mm of the fourth cold heading blank 4.

[0050] In addition, during the cold heading process, the outer diameter of the top wall of the fourth cold heading blank 4 is affected by the corresponding cold heading die, and a third rounded corner 42 is formed.

[0051] See also Figure 11 , is a cross-sectional view of the fourth cold heading die 4-1 for forming the fourth cold heading blank 4. In the figure, the third cold heading blank 3 is placed in the fourth cavity 4-2. The position of the fourth lower punch 4-4 below the third cold heading blank 3 remains unchanged and is positioned to match the third positioning hole 22. The fourth upper ram 4-5 is adjusted to above the third cold heading blank 3. The fourth upper punch 4-3 above the third cold heading blank 3 moves downward to upset the second chamfered cavity 32 to form a second step cavity 41. The diameter of the fourth upper punch 4-3 is smaller than that of the third upper punch 3-3. Therefore, during the process of forming the third cold heading blank 3 into the fourth cold heading blank 4, the axial height will increase until the top of the third cold heading blank 3 is fitted with the fourth upper ram 4-5.

[0052] See also Figure 6 , Figure 6 It is a cross-sectional view of the core shaft blank 5. The fourth cold heading blank 4 is placed in the corresponding cold heading die. The upper and lower positions of the fourth cold heading blank 4 are limited by two pressing heads. The second step cavity 41 is downwardly upset by the punch until the second step cavity 41 and the second positioning hole 22 are connected to form a through cavity 52. The inner diameter of the through cavity 52 is consistent with the inner diameter of the second step cavity 41. The residual material of the core 51 produced will fall from the bottom of the through cavity 22. In this way, the middle part of the core shaft blank 5 is arranged from top to bottom as the first chamfer cavity 21, the first step cavity 31, the second chamfer cavity 32, and the through cavity 52; in this process, since the axial height of the fourth cold heading blank 4 is limited by the pressing head, the punch plays a breaking role, so the outer contour of the fourth cold heading blank 4 and the outer contour of the core shaft blank 5 do not change, that is, the outer diameter of the fifth core shaft blank 5 is 29.4mm.

[0053] See also Figure 12 , which is a cross-sectional view of the fifth cold heading die 5-1 of the forming core shaft blank 5. In the figure, the fourth cold heading blank 4 is placed in the fifth cavity 5-2. The bottom of the fifth cavity 5-2 is through-set and has a discharge channel 5-4. The fifth upper ram 5-5 is adjusted to abut the top of the fourth cold heading blank 4 and limit the position. The fifth upper punch 5-3 above the fourth cold heading blank 4 moves downward and axially passes through the forming core shaft blank 5, so that the second step cavity 41 and the third positioning hole 22 are upset. The material core 51 between the second step cavity 41 and the third positioning hole 22 falls from the discharge channel 5-4. Therefore, the axial height will not increase during the process of forming the fourth cold heading blank 4 into the core shaft blank 5.

[0054] See also Figure 16In the above process, i.e., during the cold heading process of the steel cylinder 01 to form the core shaft blank 5, the outer diameter changes only slightly, and the depth of the local extrusion of the decarburized layer during the entire cold heading process is relatively shallow (different from the background technology in which the 24mm cold heading cylinder 01 is cold headed to form the core shaft blank 5 by axial compression and radial expansion). After the core shaft blank 5 is ground from the decarburized layer 001 on the outer surface, no decarburized layer 001 appears after heat treatment. 1 has soft spots; for example, when forming a 29.2mm mandrel, the 29mm cold-forged cylinder 01 is cold-forged to a 29.4mm mandrel cold-forged part. During the cold-forging process, the outer diameter of the mandrel only slightly expands and the axial height extends, reducing the degree of local aggregation and extrusion of the decarburized layer. The outer surface of the 29.4mm mandrel cold-forged part is ground 0.2mm to remove the decarburized layer, avoiding the appearance of soft spots on the mandrel surface after heat treatment.

[0055] It should be noted that if the outer diameter of the cold-headed steel cylinder 01 is 0.3mm smaller than the outer diameter of the core shaft blank 5, for example, a 29.2mm cold-headed cylinder 01 is used to cold-head a 29.4mm core shaft blank 5, during the cutting process of the cold-headed cylinder 01, burrs will be generated due to the uneven cutting surface. The burrs will protrude from the end of the cold-headed cylinder, causing the cold-headed cylinder 01 to be loaded into the corresponding mold. The burrs will damage the inner wall of the mold or make it impossible to load the cold-headed cylinder 01 into the mold. In addition, even if the end of the cold-headed cylinder 01 is flat and has no burrs, the cold-headed cylinder 01 can be loaded into the corresponding mold, but the diameter between the cold-headed cylinder 01 and the mold is not uniform. The axial clearance is small, resulting in a large axial flow of the cold-headed cylinder 01 during the cold-heading process. For bearing steel with poor plasticity, the large axial flow of the metal will cause surface cracking, thereby affecting the quality of the finished product; therefore, in the application, the cold-headed steel cylinder is gradually upset during the forming process into the core shaft blank, which can reduce the manufacturing cost of the core shaft and improve the utilization rate of the material; the outer diameter of the cold-headed steel cylinder is 0.3mm~0.6mm smaller than the core shaft blank, which reduces the metal flow to avoid cracking and avoids local aggregation of the decarburized layer, reduces the amount of chips in the later stage, and can also improve the density of the organization and enhance the mechanical properties of the core shaft.

[0056] It should also be noted that if the first positioning hole 11 is directly cold-forged into the first step cavity 31, that is, step S5 is directly executed after step S3, and step S4 is skipped, the punch is easily damaged due to the large impact force required for cold forging. In addition, cracks are easily generated in the process of directly forming the fourth cold-forged blank 4 from the second cold-forged blank 2; therefore, adding step S3 between steps S2 and S4 can increase the service life of the punch, and the second cold-forged blank 2 is less likely to crack.

[0057] See also Figure 14, the microstructure diagram of the core shaft blank 5 with a diameter of cold heading from φ24mm to φ29.4mm. The decarburized layer 001 is darker in color, indicating that the decarburized layer 001 has a large local accumulation. Soft spots appear on the high-frequency quenching surface due to the decarburized layer 001 (sampling 20 pieces); see Figure 15 , the microstructure diagram of the core shaft blank 5 with a diameter from φ29mm to φ29.4mm by cold heading, the decarburization layer 001 is lighter in color, indicating that the local accumulation of the decarburization layer 001 is less, and no soft spots appear on the surface after high-frequency quenching (sampling 20 pieces); the above comparative example shows that the greater the diameter change after cold heading, the more local accumulation of the decarburization layer, resulting in soft spots on the surface; subsequently, the products formed by the two processes were compared, and the test results were the same, further indicating that when the diameter change of the product is greater, the local accumulation of the decarburization layer is more, which is more likely to lead to high-frequency soft spots.

[0058] With the above-described preferred embodiments of the present invention as a guide, and with reference to the above description, relevant personnel are fully capable of making various changes and modifications without departing from the technical scope of this invention. The technical scope of this invention is not limited to the contents of the specification and must be determined according to the scope of the claims.

Claims

1. A cold heading method for manufacturing a mandrel, characterized in that: The steps include: Step S1: selecting bearing steel suitable for cold heading, and cutting the cold heading steel cylinder into a suitable length according to the specifications of the mandrel after spheroidization; Step S2: cold-forging a first positioning hole downward on the upper end surface of the cold-forged steel cylinder, and cold-forging a second positioning hole upward on the lower end surface of the cold-forged steel cylinder to form a first cold-forged blank; Step S3: cold heading downward along the first positioning hole of the first cold heading blank and cold heading upward along the second positioning hole to form a second cold heading blank having a first chamfered cavity and a third positioning hole in the axial direction; Step S4: cold heading downward along the first chamfered cavity of the second cold heading blank to form a third cold heading blank having a first chamfered cavity, a first step cavity, a second chamfered cavity, and a third positioning hole in the axial direction; Step S5: cold heading downward along the second chamfered cavity of the third cold heading blank to form a fourth cold heading blank having a first chamfered cavity, a first step cavity, a second chamfered cavity, a second step cavity, and a third positioning hole in the axial direction; Step S6: cold heading downward along the second step cavity of the fourth cold heading blank, after the core material between the second step cavity and the third positioning hole is punched out, the second step cavity and the third positioning hole are connected, and a core shaft blank is formed with the first chamfered cavity, the first step cavity, the second chamfered cavity, and the through cavity in the axial direction; Wherein, the outer diameter of the cold heading steel cylinder is 0.3 to 0.6 mm smaller than the outer diameter of the core shaft blank.

2. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S1, the axial height of the cold-headed steel cylinder is smaller than the axial height of the core shaft blank.

3. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S2, after the first cold-forged blank is formed by cold heading, a first fillet is formed at the outer diameter of the bottom thereof; In step S3, after the second cold-forged blank is formed by cold heading, a second fillet is formed at the outer diameter of the bottom thereof, wherein the radius of the second fillet is smaller than the radius of the first fillet; The outer diameters of the bottoms of the third cold heading blank, the fourth cold heading blank, and the mandrel blank all have second fillets; In step S5, after the fourth cold-forged blank is formed by cold heading, a third fillet is formed at the outer diameter of the top thereof; The core shaft blank has a third fillet at the outer diameter of the top.

4. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S2, the height of the cold heading steel cylinder is greater than the height of the first cold heading blank, and the outer diameter of the cold heading steel cylinder is smaller than the outer diameter of the first cold heading blank.

5. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S3, the height of the first cold heading blank is greater than the height of the second cold heading blank, and the outer diameter of the first cold heading blank is smaller than the outer diameter of the second cold heading blank; the diameter of the first positioning hole is greater than the diameter of the first chamfered cavity, and the height of the first positioning hole is smaller than the height of the first chamfered cavity; the diameter of the second positioning hole is greater than the diameter of the third positioning hole, and the height of the second positioning hole is smaller than the height of the third positioning hole.

6. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S4, the height of the second cold heading blank is smaller than the height of the third cold heading blank, and the outer diameter of the second cold heading blank is smaller than the outer diameter of the third cold heading blank; the height and diameter of the third positioning hole of the second cold heading blank are consistent with the third positioning hole of the third cold heading blank; the height of the first chamfered cavity of the second cold heading blank is smaller than the height of the first step cavity.

7. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S5, the height of the third cold heading blank is smaller than the height of the fourth cold heading blank, and the outer diameter of the third cold heading blank is smaller than the outer diameter of the fourth cold heading blank; the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the third cold heading blank are consistent with the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the fourth cold heading blank.

8. A cold heading method for manufacturing a mandrel according to claim 1, characterized in that: In step S6, the height of the fourth cold heading blank is equal to the height of the core shaft blank, and the outer diameter of the fourth cold heading blank is larger than the outer diameter of the core shaft blank; the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the fourth cold heading blank are consistent with the diameter and height of the first chamfering cavity, the first step cavity, and the second chamfering cavity in the core shaft blank; the sum of the heights of the second step cavity and the third positioning hole in the fourth cold heading blank is less than the height of the through cavity in the core shaft blank.

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