JP6039375B2 - Manufacturing method of connecting rod and semi-finished product of connecting rod - Google Patents

Description

  The present invention relates to a manufacturing technique of a connecting rod having a large end portion that is divided by breaking.

  The engine incorporates a connecting rod (hereinafter referred to as a connecting rod) that connects the crankshaft and the piston. In order to mount the crank pin of the crankshaft on the large end portion of the connecting rod, the large end portion of the connecting rod is divided into a main body portion and a cap portion. Generally, a knock pin or the like is provided between the main body portion and the cap portion in order to position the divided main body portion and cap portion. However, positioning the main body portion and the cap portion using the knock pin has been a factor in increasing the cost of the connecting rod. Therefore, a so-called cracking method for breaking and dividing the large end of the connecting rod has been developed (see Patent Document 1). By this cracking manufacturing method, moderate irregularities can be formed on the fracture surface of the main body portion and the cap portion, so that the main body portion and the cap portion can be positioned with high accuracy without using a knock pin.

JP 2005-188741 A

  By the way, in the cracking manufacturing method in which the large end portion is broken and divided, it is important to suppress the occurrence of poor quality on the fracture surface of the main body portion and the cap portion, that is, the occurrence of chipping on the fracture surface. However, it has been extremely difficult to control the quality of the fracture surface, which is the cleavage fracture surface, that is, to suppress the occurrence of chipping on the fracture surface.

  An object of the present invention is to suppress the occurrence of chipping on the fracture surface.

A method for manufacturing a connecting rod according to the present invention is a method for manufacturing a connecting rod having a large end portion that is divided into a main body portion and a cap portion, and includes a large end hole provided in a central portion of the large end portion. A notch forming step for forming a notch on the inner peripheral surface, a stress concentrated portion processing step for forming a stress concentrated portion on the inner peripheral surface of the bolt hole provided at both ends of the large end, and the large end A split splitting step of splitting the large end by splitting the large end by generating a crack from the notch by expanding the hole in the radial direction, and the bolt hole has a large diameter hole and the large diameter A small-diameter hole portion connected to the hole portion and an internal thread portion connected to the small-diameter hole portion, and the stress concentration portion is an annular step formed at a boundary portion between the small-diameter hole portion and the large-diameter hole portion. a biasing unit in the fracture splitting process, a crack the Ri to advance through the stress concentration portion from outs, characterized in that. The connecting rod manufacturing method of the present invention is a connecting rod manufacturing method having a large end portion that is divided into a main body portion and a cap portion, and is provided at a central portion of the large end portion. A notch forming step of forming a first notch on the inner peripheral surface of the hole and forming a second notch on the outer peripheral portion of the large end, and an inner periphery of the bolt hole provided at both ends of the large end A stress concentration portion machining step for forming a stress concentration portion on the surface, and a fracture split that splits the large end portion by generating a crack from the first notch by expanding the large end hole in the radial direction. And the stress concentration portion is an annular stepped portion formed on the inner peripheral surface of the bolt hole, and in the fracture splitting step, the stress is concentrated from the first notch to the stress concentration portion. To the second notch through the part. To.

A semi-finished product of a connecting rod according to the present invention is a semi-finished product of a connecting rod, which is an intermediate product of a connecting rod having a fractured split type large end, and has an inner end of a large end hole provided in the center of the large end. Formed on the peripheral surface, the notch that becomes the starting point of cracks at the time of fracture division, and the inner circumferential surface of the bolt holes provided at both ends of the large end, the crack propagates and passes at the time of fracture division a stress concentrator for the possess, the bolt hole includes a large-diameter hole portion, and a small-diameter hole portion continuous to the large-diameter hole portion, and a female threaded portion communicating with the small-diameter hole portion is constituted by the stress concentration The portion is an annular stepped portion formed at a boundary portion between the small diameter hole portion and the large diameter hole portion . The semi-finished product of the connecting rod according to the present invention is a semi-finished product of a connecting rod that is an intermediate product of a connecting rod having a large end portion of a split split type, and has a large end hole provided in a central portion of the large end portion. Formed on the inner circumferential surface of the first notch, which is a starting point of crack generation at the time of fracture division, and at the inner circumferential surface of the bolt holes provided at both ends of the large end portion. A stress concentration portion that propagates and passes, and a second notch that is formed on the outer peripheral portion of the large end portion and that reaches a crack that has passed through the stress concentration portion at the time of fracture division. Is an annular stepped portion formed on the inner peripheral surface of the bolt hole.

  According to the present invention, a notch is formed on the inner peripheral surface of the large end hole, and a stress concentration portion is formed on the inner peripheral surface of the bolt hole. The cracks are developed. Thereby, it is possible to suppress the passing of cracks and to suppress the occurrence of chipping on the fracture surface.

(a) is a front view which shows a connecting rod, (b) is a front view which shows the disassembled connecting rod. It is explanatory drawing which shows an example of the manufacturing process of a connecting rod. It is the schematic which shows an example of the fracture | rupture apparatus used in a fracture | rupture division | segmentation process. (a) And (b) is explanatory drawing which shows the process condition of the workpiece | work in a fracture | rupture division | segmentation process. (a) is a front view which shows a workpiece | work, (b) is sectional drawing which shows a workpiece | work along the AA line of Fig.5 (a). (a) is a side view which shows a workpiece | work, (b) is sectional drawing which shows a workpiece | work along the AA line of Fig.6 (a). (a)-(c) is explanatory drawing which shows the fracture | rupture process of the big end part in a fracture | rupture division | segmentation process. It is sectional drawing which shows a workpiece | work along the BB line of Fig.5 (a). (a) is an image figure which shows the progress condition of the tear in case it does not have a stepped part, (b) is an image figure which shows the progress condition of the tear in the case of having a step part. It is sectional drawing which shows a part of semi-finished product of the connecting rod which is other embodiment of this invention. It is sectional drawing which shows a part of semi-finished product of the connecting rod which is other embodiment of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1A is a front view showing a connecting rod 10 (hereinafter referred to as a connecting rod). FIG. 1B is a front view showing the disassembled connecting rod 10. As shown in FIG. 1A, the connecting rod 10 includes a small end portion 12 having a piston pin hole 11, a large end portion 14 having a crank pin hole (large end hole) 13, a small end portion 12 and a large end. It has a rod portion 15 that connects the portion 14. A piston pin of a piston (not shown) is assembled to the small end portion 12, and a crank pin of a crankshaft (not shown) is assembled to the large end portion 14. Further, as shown in FIGS. 1A and 1B, the large end portion 14 of the connecting rod 10 is divided into a main body portion 16 and a cap portion 17. When assembling the connecting rod 10 to the crank pin, the crank pin is sandwiched between the divided main body portion 16 and the cap portion 17, and then the main body portion 16 is capped using the connecting rod bolt 19 inserted into the bolt hole 18. Part 17 is fastened.

  When the large end portion 14 of the connecting rod 10 is divided, in order to improve productivity and positioning accuracy, a so-called cracking manufacturing method is used in which the inner end of the large end portion 14 is expanded and broken and divided. By using this cracking manufacturing method, it is possible to form irregularities having an appropriate roughness on the fracture surfaces 20 and 21 of the main body portion 16 and the cap portion 17. Thereby, when assembling the cap part 17 with respect to the main body part 16, the unevenness | corrugation of the torn surfaces 20 and 21 can be meshed | engaged with each other, and the positioning accuracy of the main body part 16 and the cap part 17 can be improved. It becomes. Moreover, the knock pin for positioning etc. can be reduced from the connecting rod 10, and it becomes possible to improve the productivity of the connecting rod 10. FIG.

  Next, a method for manufacturing the connecting rod 10 will be described. FIG. 2 is an explanatory view showing an example of a manufacturing process of the connecting rod 10. As shown in FIG. 2, in the material forging step S <b> 1, a work in which the small end portion 12, the rod portion 15, and the large end portion 14 are integrated by subjecting the cut carbon steel material or titanium alloy material to hot forging or the like. Is manufactured. In the grinding step S2, side grinding of the small end portion 12 and the large end portion 14 is performed using a grinding machine. Subsequently, in the small end hole processing step S <b> 3, the piston pin hole 11 is processed in the small end portion 12 by performing drilling or reamer processing on the small end portion 12. In the subsequent large end hole processing step S4, the crank pin hole 13 is processed in the center of the large end portion 14 by drilling or reaming the large end portion 14. Further, in the bolt hole machining step (stress concentration portion machining step) S5, the bolt holes 18 are machined at both ends of the large end portion 14 by drilling or reaming the large end portion 14. In this bolt hole machining step S5, as will be described later, an annular stepped portion (stress concentration portion) 23 is formed on the inner peripheral surface 22 of the bolt hole 18. Subsequently, in a notch processing step (notch processing step) S6, a pair of notches (notches) 23 and 24 facing each other are formed on the inner peripheral surface 24 of the crankpin hole 13, as will be described later. In the notch machining step S6, the notches 25 and 26 are formed by using cutting or electric discharge machining. In the breaking and dividing step S7, the inner diameter of the large end portion 14 is expanded using a breaking device 30 described later, and the large end portion 14 is broken and divided into the main body portion 16 and the cap portion 17 starting from the notches 25 and 26. The

  In the subsequent bolt tightening step S <b> 8, the divided cap portion 17 is assembled to the main body portion 16 by tightening the connecting rod bolt 19. Also, in the finish grinding step S9, a grinder is used to remove the level difference of the large end portion 14 after the fracture division and to ensure the flatness and parallelism of the small end portion 12 and the large end portion 14. Side grinding of the small end portion 12 and the large end portion 14 is performed. Further, in the oil hole machining step S <b> 10, the oil holes for lubrication are machined in the small end portions 12 and the large end portions 14 by drilling the small end portions 12 and the large end portions 14. Subsequently, in the metal groove processing step S <b> 11, a metal groove for fixing the bearing metal to the inner peripheral surface 24 of the large end portion 14 is processed by cutting the large end portion 14. In the honing step S12, honing is performed on the small end portion 12 and the large end portion 14 in order to ensure the processing accuracy of the piston pin hole 11 and the crank pin hole 13. Then, after removing chips and the like adhering to the connecting rod 10 in the cleaning step S13, a completion inspection of the connecting rod 10 is performed in the inspection step S14.

  FIG. 3 is a schematic view showing an example of the breaking device 30 used in the breaking division step S7. 4 (a) and 4 (b) are explanatory diagrams showing the machining status of the workpiece X in the fracture division step S7. The workpiece X shown in FIG. 4 (a) is the workpiece X that is put into the fracture dividing step S7, and is a semi-finished product of the connecting rod according to one embodiment of the present invention, that is, an intermediate product in the manufacturing process. Yes.

  As shown in FIG. 3, the breaking device 30 includes a base member 31 and a pair of sliders 32 and 33 that are movably assembled to the base member 31. One slider 32 is provided with a positioning pin 34 inserted into the piston pin hole 11 of the small end portion 12 and a pressurizing jig 35 inserted into the crank pin hole 13 of the large end portion 14. The other slider 33 is provided with a pressure jig 36 inserted into the crank pin hole 13 of the large end portion 14. Further, the breaking device 30 has a wedge 38 that is driven up and down by a hydraulic cylinder 37. The wedge 38 is movable to a retracted position where it is lifted away from the pressurizing jigs 35, 36 and a pressurizing position where it is lowered and inserted between the pressurizing jigs 35, 36. At the time of breaking and dividing, as shown in FIG. 4A, the small end portion 12 of the work X is attached to the positioning pin 34 with the sliders 32 and 33 of the breaking device 30 close to each other, and both pressures are applied. The large end portion 14 of the work X is attached to the jigs 35 and 36. Then, as shown by black arrows in FIGS. 3 and 4B, the wedge 38 is lowered toward the pressing position, and the pair of pressing jigs 35 and 36 are moved away from each other. In this way, by expanding the inner diameter of the large end portion 14 of the workpiece X using the pressurizing jigs 35 and 36, the large end portion 14 is connected to the main body portion 16 and the cap portion 17 starting from the notches 25 and 26. Break and split. That is, the intake side connecting portion 39 of the large end portion 14 is broken starting from the notch 25, and the exhaust side connecting portion 40 of the large end portion 14 is broken starting from the notch 26. The intake side connecting portion 39 is a portion located on the intake port side when the connecting rod 10 is assembled to the engine. Further, the exhaust side connecting part 40 is a part located on the exhaust port side when the connecting rod 10 is assembled to the engine.

  Hereinafter, a work structure for suppressing the occurrence of chipping in the fracture surfaces 20 and 21 will be described. FIG. 5A is a front view showing the workpiece X, and FIG. 5B is a cross-sectional view showing the workpiece X along the line AA in FIG. 5A. 6A is a side view showing the workpiece X, and FIG. 6B is a cross-sectional view showing the workpiece X along the line AA in FIG. 6A.

As shown in FIGS. 5A and 5B , notches 25 and 26 that are first cutouts face each other on the inner peripheral surface 24 of the crankpin hole 13 provided in the large end portion 14 of the workpiece X. Formed in position. These notches 25 and 26 are formed in the inner peripheral surface 24 of the crankpin hole 13 so as to divide the inner peripheral surface 24 in the width direction, that is, so as to cross the large end portion 14 in the thickness direction. Further, the notch 25 and the notch 26 are formed at positions shifted by 180 ° on the inner peripheral surface, and are formed at positions overlapping the plane S including the center line C1 of the crankpin hole 13. Thus, the plane S that overlaps both the notches 25 and 26 is a rough planned fracture surface that is fractured in the fracture division step S7. Further, since the connecting rod 10 manufactured using the illustrated workpiece X is a symmetric connecting rod that is symmetric with respect to the center line C2, the plane S is perpendicular to the center line C2. In addition, the code | symbol C3 shown in FIG. Subsequently, as shown in FIG. 6B, an annular stepped portion 23 is formed as a stress concentration portion on the inner peripheral surface 22 of the bolt hole 18 provided in the large end portion 14 of the workpiece X. The bolt hole 18 includes a small-diameter hole portion 51 having a female screw portion 50 and a large-diameter hole portion 52 arranged concentrically therewith. The large diameter hole portion 52 has a larger diameter than the small diameter hole portion 51, and a boundary portion between the small diameter hole portion 51 and the large diameter hole portion 52 appears as an annular stepped portion 23. The stepped portion 23 of the bolt hole 18 is formed so as to be positioned in the vicinity of the plane S. The connecting rod bolt 19 can be easily inserted into the small diameter hole 51 by providing the large diameter hole 52 in the bolt hole 18. That is, the large diameter hole portion 52 of the bolt hole 18 functions as a guide portion of the connecting rod bolt 19.

  Subsequently, the breaking process of the large end portion 14 in the breaking division step S7 will be described. FIGS. 7A to 7C are explanatory views showing a breaking process of the large end portion 14 in the breaking division step S7. 7 (a) to 7 (c) show the part indicated by the symbol α in FIG. 4 (a). 7 and 8, the rupture process of the intake side connecting portion 39 will be described. The exhaust side linking portion 40 is also broken in the same rupture process as the intake side connecting portion 39.

  As shown in FIG. 7A, when the inner diameter of the large end portion 14 of the workpiece X is expanded using the pressurizing jigs 35 and 36, a crack C (hereinafter referred to as a crack) is generated from the notch 25 as a starting point. However, as indicated by the arrow A <b> 1, the crevice C progresses outward in the radial direction of the crankpin hole 13. Subsequently, as shown by an arrow A2 in FIG. 7 (b), after the tear C has progressed toward the stepped portion 23 which is a stress concentration portion, the tear C is formed at the stepped portion 23 as shown by an arrow A3. It progresses toward the radial direction outward of the crankpin hole 13 along. As shown in FIG. 7C, the intake side connecting portion 39 of the large end portion 14 is divided into a main body portion 16 and a cap portion 17 by breaking. Thus, by forming the stepped portion 23 where the stress is concentrated in the bolt hole 18, it is possible to guide the tear C to the stepped portion 23. Thereby, since the crevice C can be advanced along the step part 23, the height position of the crevice C to merge can be made to correspond and it becomes possible to suppress the chipping of the torn surfaces 20 and 21. .

  Here, FIG. 8 is a cross-sectional view showing the workpiece X along the line BB in FIG. 5A, and FIG. 8 shows the progress of the tear using arrows. FIG. 9A is an image diagram showing the progress of the tear when the stepped portion 23 is not provided, and FIG. 9B is an image diagram showing the progress of the split when the stepped portion 23 is provided. It is. As shown by a plurality of arrows in FIG. 8, the split extending from the notch 25 is split into two hands so as to avoid the bolt hole 18, and then merges at a site indicated by the symbol α. Here, as shown in FIG. 9 (a), when the height position of the rift extending at the arrow A1 and the rift extending at the arrow A2 are misaligned, as shown by the symbol α, The material will be lost between. On the other hand, as shown in FIG. 9B, by forming the stepped portion 23 where the stress is concentrated in the bolt hole 18, the crack that develops at the time of fracture division passes through the stepped portion 23 that is easily broken. become. As a result, as shown in FIG. 9 (b), the height position of the tear that propagates in the arrow A3 and the tear that progresses in the arrow A4 can be matched. It becomes.

  In the above description, the annular stepped portion 23 is cited as the stress concentration portion formed in the bolt hole 18, but the present invention is not limited to this, and any shape can be used as long as stress is concentrated compared to the peripheral portion. It may be a shape. For example, an annular notch may be formed as the stress concentration portion, or a stepped portion or a notch may be formed on a part of the inner peripheral surface 22 of the bolt hole 18. Here, FIG. 10 is a sectional view showing a part of a semi-finished product (workpiece Xa) of a connecting rod according to another embodiment of the present invention. FIG. 10 shows the same part as in FIG. 10, parts that are the same as the parts shown in FIG. 8 are given the same reference numerals, and descriptions thereof are omitted. As shown in FIG. 10, in the workpiece Xa, a notch (stress concentration portion) 53 is formed in a part of the inner peripheral surface 22 of the bolt hole 18. As described above, when the notch 53 is formed in a part of the inner peripheral surface 22 of the bolt hole 18, the notch 53 is formed in a portion of the inner peripheral surface 22 of the bolt hole 18 opposite to the notch 25 side. Will be. That is, by forming the notch 53 which is a stress concentration portion at the joining part (symbol α in FIG. 8) of the above-described tear, the tear can be joined along the notch 53, and therefore the fracture surface 20 , 21 can be suppressed.

In the above description, the stepped portion 23 and the notch 53 are formed on the inner peripheral surface 22 of the bolt hole 18 in order to suppress the chipping of the fracture surfaces 20 and 21, but the large end portion 14 is further formed. A notch or the like may be formed at the end in the width direction. Here, FIG. 11 is a sectional view showing a part of a semi-finished product (workpiece Xb) of a connecting rod according to another embodiment of the present invention. FIG. 11 shows the same part as in FIG. In FIG. 11, the same parts as those shown in FIG. 8 are denoted by the same reference numerals, and the description thereof is omitted. As shown in FIG. 11, the work Xb has a second notch 55 formed at the end 54 in the width direction of the large end 14. Thus, by providing the notch 55, it is possible to guide to the end portion 54 in the width direction of the large end portion 14 without re-separating the tear that has joined along the stepped portion 23. , 21 can be further suppressed.

  It goes without saying that the present invention is not limited to the above-described embodiment, and various modifications can be made without departing from the scope of the invention. In the case shown in FIG. 6B, the stepped portion 23 is formed on the lower side of the plane S. However, the present invention is not limited to this, and the stress concentration of the stepped portion 23 or the like is on the crack propagation path. The part should just be formed. For example, a stress concentration portion such as the stepped portion 23 may be formed on the upper side of the plane S, or a stress concentration portion such as the stepped portion 23 may be formed on the plane S. It is desirable to form a stress concentration portion such as a stepped portion 23 on the upper side of the plane S for a workpiece whose tear tends to progress upward, and for a workpiece whose tear tends to progress in the horizontal direction. It is desirable to form a stress concentration portion such as the stepped portion 23 on the plane S.

  In the above description, in the bolt hole machining step S5, the stepped portion 23 is formed in conjunction with the drilling of the bolt hole 18, but the present invention is not limited to this, and the bolt hole 18 is formed in other steps. A stepped portion 23 may be formed. Further, in the above description, the notch machining step S6 is provided after the bolt hole machining step S5. However, the present invention is not limited to this, and the bolt hole machining step S5 may be provided after the notch machining step S6. That is, the notches 23 and 24 may be formed on the inner peripheral surface 24 of the crankpin hole 13 and then the stepped portion 23 may be formed on the inner peripheral surface 22 of the bolt hole 18. The connecting rod 10 shown in the figure is a symmetric connecting rod having a symmetric shape with respect to the center line C2, but is not limited thereto. For example, the manufacturing technique of the present invention can be effectively applied even to an asymmetric connecting rod having an asymmetric shape with respect to the center line C2.

10 Connecting rod (Connecting rod)
13 Crank pin hole (large end hole)
14 Large end portion 16 Body portion 17 Cap portion 18 Bolt hole 22 Inner peripheral surface 23 Stepped portion (stress concentration portion)
24 inner peripheral surface 25, 26 notch (notch)
53 Notch (stress concentration part)
S5 Bolt hole machining process (stress concentrated part machining process)
S6 Notching process (notch process)
S7 Break splitting process C Crack (crack)
X Workpiece (Connecting rod semi-finished product)
Xa work (connecting rod semi-finished product)
Xb Workpiece (Semi-finished product of connecting rod)

Claims (6)

A method for manufacturing a connecting rod having a large end portion that is split into a main body portion and a cap portion,
A notch processing step of forming a notch on the inner peripheral surface of the large end hole provided in the center of the large end; and
A stress concentration portion machining step for forming a stress concentration portion on the inner peripheral surface of the bolt hole provided at both ends of the large end portion;
Wherein by expanding the large end hole in the radial direction, has a fracture splitting step of fracturing the large end to generate the cutout Karaki cleft,
The bolt hole includes a large-diameter hole, a small-diameter hole continuous with the large-diameter hole, and an internal thread connected with the small-diameter hole,
The stress concentration part is an annular stepped part formed at a boundary part between the small diameter hole part and the large diameter hole part,
The fracture in the dividing step, to advance through the stress concentration portion from lack crack the cutting method of a connecting rod, characterized in that. A method for manufacturing a connecting rod having a large end portion that is split into a main body portion and a cap portion,
A notch processing step of forming a first notch on an inner peripheral surface of a large end hole provided in a central portion of the large end portion, and forming a second notch on an outer peripheral portion of the large end portion ;
A stress concentration portion machining step for forming a stress concentration portion on the inner peripheral surface of the bolt hole provided at both ends of the large end portion;
Wherein by expanding the large end hole in the radial direction, has a fracture splitting step of fracturing the large end to generate the first notch Karaki cleft,
The stress concentration portion is an annular stepped portion formed on the inner peripheral surface of the bolt hole,
Wherein the fracture splitting process, to advance to the lack crack the second cut through the stress concentration portion because lacks the first cut, a manufacturing method of a connecting rod, characterized in that. In the manufacturing method of the connecting rod of Claim 1,
In the notch processing step, a first notch is formed as the notch on the inner peripheral surface of the large end hole, and a second notch is formed on the outer peripheral portion of the large end portion,
The method of manufacturing a connecting rod according to claim 1, wherein, in the fracture splitting step, a crack is propagated from the first notch to the second notch through the stress concentration portion . A semi-finished product of a connecting rod, which is an intermediate product of a connecting rod having a fractured split type large end ,
Formed on the inner peripheral surface of the large end hole provided in the central portion of the large end portion, and a notch serving as a starting point of cracks during fracture division;
Wherein formed on the inner peripheral surface of the bolt holes provided at both ends of the large end portion, it possesses cracks during fracture splitting and the stress concentration portion passing progress, and
The bolt hole includes a large-diameter hole, a small-diameter hole continuous with the large-diameter hole, and an internal thread connected with the small-diameter hole,
The connecting rod semi-finished product , wherein the stress concentration portion is an annular stepped portion formed at a boundary portion between the small diameter hole portion and the large diameter hole portion . A semi-finished product of a connecting rod, which is an intermediate product of a connecting rod having a fractured split type large end ,
A first notch formed on an inner peripheral surface of a large end hole provided at a central portion of the large end portion, and serving as a starting point of a crack during fracture division;
Formed on the inner peripheral surface of the bolt hole provided at both ends of the large end portion, and a stress concentration portion through which a crack propagates and passes at the time of fracture division ;
Wherein formed on the outer peripheral portion of the large end portion, the crack passes through the stress concentration portion is closed and a lack second cut reaching the time fracture splitting,
The connecting rod semi-finished product , wherein the stress concentration portion is an annular stepped portion formed on an inner peripheral surface of the bolt hole . The connecting rod semi-finished product according to claim 4,
A first notch as the notch formed on the inner peripheral surface of the large end hole provided in the central portion of the large end, and serving as a starting point of cracks during fracture division;
A semi-finished product of a connecting rod , comprising: a second notch formed on an outer peripheral portion of the large end portion and reaching a crack that has passed through the stress concentrating portion at the time of fracture division .

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