Technological method for controlling size precision of connecting rod big end hole

Abstract

According to the technological method for controlling the size precision of the connecting rod large-head hole, the technological steps of annealing stress relief, large-head hole and small-head hole semi-fine boring for controlling the fine machining allowance, connecting rod disassembling and secondary assembling stress relief are introduced, and the initial stress generated by metal material cutting and large-head hole deformation caused by large fine boring allowance are effectively removed; by increasing the lower deviation of the pin hole I and the pin hole II, reducing the upper deviation of the positioning pin and increasing the fit clearance, the stress state of the connecting rod is improved, so that the deformation phenomenon caused by stress is reduced, and the problem of deformation after assembly caused by positioning fit interference and residual stress accumulation of a connecting rod big end hole in the prior art is solved; and the application range is wide, the operability is high, the interference problem of assembly pins is reduced, the connecting rod assembly is free of interference, the size precision of the large-head hole is stable, and the size precision requirement of the large-head hole machined by the connecting rod is met.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of diesel engine connecting rod processing, and particularly relates to a process method for controlling the size precision of a connecting rod big-end hole. BACKGROUND

[0002] The connecting rod of a diesel engine is a core transmission component of an engine, and the machining precision thereof directly affects the power output, running stability and service life of the engine. The big-end hole of the connecting rod is a key matching structure formed by assembling a rod body and a rod cover through bolts and positioning pins, and needs to meet strict size precision and shape and position tolerance requirements. Taking a certain diesel engine connecting rod as an example, the size precision requirement of the big-end hole thereof is φ253H6 (+0.032 0), and the cylindricity tolerance is 0.016.

[0003] In the existing process, the rod body and the rod cover are directly precisely bored after being assembled according to a set assembly force, and although the machining can meet the drawing requirements, after being disassembled and reassembled, the big-end hole is prone to present an oval deformation with an axial size smaller than a width direction, resulting in size and cylindricity out-of-tolerance, and failing to meet the assembly and use requirements. Through analysis, the core reasons for the deformation include two aspects: 1) positioning interference: the positioning pin and the rod cover pin hole are in interference fit, and the rod body pin hole is in clearance fit, but there are manufacturing errors between the rod cover pin spacing and the rod body pin hole spacing, the errors are accumulated to make the pin and the pin hole have no clearance or even interference, and an additional force is generated during assembly, causing deformation; 2) residual stress accumulation: when the connecting rod is cut from a whole blank to form the rod body and the rod cover, initial stress is generated, and subsequent multiple machining processes further introduce machining stress; and in addition, the one-side machining allowance of the traditional precise boring process is 1mm, too much material is removed, and besides, the structure around the big-end hole is non-uniform, causing uneven stress release and causing micro-deformation; therefore, it is necessary to improve the big-end hole deformation caused by the above machining residual stress and positioning interference. SUMMARY

[0004] The application solves the technical problem of providing a process method for controlling the size precision of a connecting rod big-end hole, by introducing annealing stress relief, half-precision boring big-end and small-end holes to control the precision machining allowance, disassembling the connecting rod, and releasing stress through secondary assembly, effectively removing the initial stress generated by cutting the metal material and the big-end hole deformation caused by the precision boring hole allowance, and by increasing the lower deviation of the pin hole one and the pin hole two and reducing the upper deviation of the positioning pin, increasing the fitting clearance, which is beneficial to improve the stress state of the connecting rod, thereby reducing the deformation phenomenon caused by stress, solving the deformation problem of the connecting rod big-end hole caused by positioning interference and residual stress accumulation in the existing process, ensuring that the size precision and cylindricity of the big-end hole meet the design requirements, having a wide application range, strong operability, reducing the assembly pin interference problem, making the connecting rod assembly have no interference, the size precision of the big-end hole is stable, and meeting the size precision requirements of the big-end hole of the connecting rod.

[0005] The technical scheme adopted by the present application is a process method for controlling the size precision of a big-end hole of a connecting rod, comprising the following steps: 1) fine milling of an upper plane: after positioning and fixing the connecting rod with the lower end face of the connecting rod and the big-end hole and small-end hole of the connecting rod as positioning references, fine milling of the upper plane of the connecting rod is performed; 2) fine milling of a lower plane and processing of reference holes: after positioning and fixing the connecting rod with the fine-milled upper plane of the connecting rod and the big-end hole and small-end hole of the connecting rod as positioning references, fine milling of the lower plane of the connecting rod is performed, and a plurality of process positioning holes distributed on the rod body and the rod cover are processed as reference holes on the lower plane of the connecting rod; 3) processing of big-end faces and process holes: after positioning and fixing the connecting rod, shoulder faces and process holes are respectively processed on the big-end and small-end faces of the connecting rod, and the connecting rod clamp is removed after the processing is completed; 4) processing of arc faces: after positioning and fixing the connecting rod with the small-end of the connecting rod and the shoulder face of the big-end as positioning references, two arc faces of the connecting rod are processed; 5) cutting: the connecting rod is cut along a cutting line to form a rod body and a rod cover; 6) annealing treatment: the cut rod body and rod cover are subjected to annealing treatment to remove initial stress generated in the metal material during the cutting; 7) drilling of deep holes: after positioning and fixing the annealed rod body, deep holes are drilled on the rod body along the joint surface; 8) semi-fine boring of the rod body hole: the rod body hole is semi-finely bored; 9) milling of the rod body joint surface: after milling the rod body joint surface according to the requirements of the drawing, bolt holes and a pin hole one with increased lower deviation are processed on the rod body joint surface; 10) semi-fine boring of the rod cover hole: after positioning and fixing the annealed rod cover, the rod cover hole is semi-finely bored; 11) milling of the rod cover joint surface: after milling the rod cover joint surface according to the requirements of the drawing, screw holes and a pin hole two corresponding to the positions of the bolt holes and the pin hole one on the rod body joint surface are processed on the rod cover joint surface; 12) milling of the rod cover shoulder face: the shoulder face on the end face of the rod cover is milled; 13) assembly: the joint surfaces of the rod cover and the rod body are matched and aligned, a positioning pin with reduced upper deviation is matched and positioned with the pin hole one on the rod body joint surface and the pin hole two on the rod cover joint surface, and the rod cover and the rod body are fixed and connected into one body by a screw rod matched and connected with the bolt holes on the rod body joint surface and the screw holes on the rod cover and a nut matched and connected with the screw rod, wherein the assembly force of the rod body and the rod cover is the upper deviation in the tolerance range; 14) semi-fine boring of the big-end hole and the small-end hole: after positioning and fixing the assembled connecting rod, the big-end hole and the small-end hole of the connecting rod are semi-finely bored, wherein the processing amount of the semi-fine boring of the big-end hole and the small-end hole is 1 / 2 of the single-side processing allowance of the big-end hole and the small-end hole; 15) disassemble the connecting rod, secondary assembly: the connecting rod cover and the screw and nut of the rod body are disassembled, and after the processing stress is released, the rod cover and the rod body are secondarily assembled and fixed through the screw and the nut, wherein the assembly force of the rod cover and the rod body during the secondary assembly and fixation is the lower deviation of the assembly force tolerance range; 16) fine boring: the connecting rod after the secondary assembly is positioned and fixed, and the big end hole and the small end hole on the connecting rod are fine bored.

[0006] In the above step 14), the one-sided processing allowance of the big end hole and the small end hole is 1mm.

[0007] Compared with the prior art, the advantages of the present application are: 1, the technical scheme adds annealing treatment after cutting the connecting rod, effectively removes the initial stress generated by cutting the metal material; through the step-by-step processing procedure of releasing stress after half-fine boring of the big end hole and the small end hole, and fine boring after secondary assembly, the stress accumulation caused by removing too much material at one time is avoided, and the deformation risk is reduced; 2, the technical scheme increases the lower deviation of the pin hole one and the pin hole two, reduces the upper deviation of the positioning pin, increases the fitting clearance, so that the assembly has no interference, the rod body and the rod cover are not affected by external force and are in a free state, which is beneficial to improve the stress state of the connecting rod, thereby reducing the deformation phenomenon caused by stress 3, the technical scheme utilizes the assembly force tolerance, the assembly force of the rod body and the rod cover during the primary assembly is the upper deviation of the tolerance range, and the assembly force of the rod cover and the rod body during the secondary assembly is the lower deviation of the assembly force tolerance range, the micro-deformation of the big end hole of the connecting rod is offset by the method of reverse deformation, so that the big end hole finally presents a circular state, and the size tolerance and the cylindricity requirement of the drawing are met; 4, the technical scheme solves the deformation problem of the big end hole of the connecting rod caused by positioning and fitting interference and residual stress accumulation in the existing process, ensures that the size precision and cylindricity of the big end hole meet the design requirements, has wide application range and strong operability, reduces the interference problem of the assembly pin, so that the connecting rod assembly has no interference, the size precision of the big end hole is stable, and the size precision requirement of the big end hole of the connecting rod is met. BRIEF DESCRIPTION OF DRAWINGS

[0008] Figure 1 The connecting rod processing flowchart of the present application. DETAILED DESCRIPTION

[0009] The specific embodiments of the present application will be described below. Figure 1The technical solutions in the embodiments of the present invention are clearly and completely described herein. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0010] It should be noted that, unless otherwise stated herein, the terms "center," "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," and "outer," etc., indicating orientations or positional relationships based on the orientations or positional relationships shown in the accompanying drawings, are used only for the convenience of describing the invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation, and therefore should not be construed as a limitation of the invention. Furthermore, the terms "first," "second," and "third," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0011] In this document, the terms "comprising," "including," or any other variations thereof are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0012] Taking the machining of connecting rod 1 with a large end hole 2 having a dimensional accuracy requirement of φ253H6 (+0.032 0) and a cylindricity of 0.016 as an example, the process method for controlling the dimensional accuracy of the connecting rod's large end hole is adopted, such as... Figure 1 As shown, it includes the following steps: 1) Finish milling the upper plane: After positioning and fixing the connecting rod 1 with the lower end face of the connecting rod 1 and the large end hole 2 and small end hole 3 of the connecting rod 1 as positioning references, finish mill the upper plane of the connecting rod 1. 2) Precision milling of the lower plane and machining of reference holes: Using the upper plane of the connecting rod 1 after precision milling and the large end hole 2 and small end hole 3 of the connecting rod 1 as positioning references, after positioning and fixing the connecting rod 1, precision mill the lower plane of the connecting rod 1, and machine multiple process positioning holes 4 distributed on the rod body 1-1 and the rod cover 1-2 on the lower plane of the connecting rod 1 as reference holes, which will serve as positioning reference holes for subsequent processes. 3) Machining the big end face and process hole: After the connecting rod 1 is positioned and fixed by the connecting rod clamp which is connected with the lower plane of the connecting rod 1 and the two process positioning holes 4 on the rod body 1-1, the shoulder surface 6 and the process hole 5 are machined on the big end and the small end of the connecting rod 1 respectively, and then the connecting rod clamp is removed. The connecting rod clamp is a structure disclosed in the prior art, and thus will not be described here; 4) Machining the arc surface: After the small end of the connecting rod 1 and the shoulder surface 6 of the big end are used as positioning references to fix the connecting rod 1, the two arc surfaces of the connecting rod 1 are machined; 5) Cutting: The connecting rod 1 is cut along the cutting line to form the rod body 1-1 and the rod cover 1-2; 6) Annealing treatment: The rod body 1-1 and the rod cover 1-2 are subjected to annealing treatment to remove the initial stress generated in the metal material during cutting; After the connecting rod 1 is cut, a large structural stress is generated. In order to release the stress as much as possible after the connecting rod 1 is cut, a large amount of excess amount is removed before cutting, including the excess amount in the end face of the connecting rod 1 and the big end hole, so as to reduce the subsequent machining allowance and increase the annealing stress relief treatment process, and to reduce the influence of the stress generated due to the removal of a large amount of metal machining amount in the previous process on the deformation of the big end hole 2 as much as possible; 7) Drilling the deep hole: After the rod body 1-1 is positioned and fixed by the connecting rod clamp which is connected with the lower plane of the rod body 1-1 and the process positioning holes 4 at both ends of the rod body 1-1, the deep hole 7 is machined on the rod body 1-1 along the joint surface thereof; 8) Semi-precision boring of the rod body hole: The rod body hole is semi-precision bored on the rod body 1-1; 9) Milling the rod body joint surface: After the joint surface of the rod body 1-1 is milled according to the requirements of the drawing, the bolt hole and the pin hole one with increased lower deviation are machined on the joint surface of the rod body 1-1, and then the connecting rod clamp is removed. The pin hole one is changed from to ; 10) Semi-precision boring of the rod cover hole: After the rod cover 1-2 is subjected to annealing treatment, the rod cover 1-2 is positioned and fixed, that is, the rod cover clamp which is connected with the lower end face of the rod cover 1-2 and the two process positioning holes 4 on the rod cover 1-2 is used to position and fix the rod cover 1-2, and then the rod cover hole is semi-precision bored; 11) Milling the joint surface of the rod cover: After the joint surface of the rod cover 1-2 is milled according to the requirements of the drawing, the screw hole and the pin hole two corresponding to the positions of the bolt hole and the pin hole one on the joint surface of the rod body 1-1 are machined on the joint surface of the rod cover 1-2. The size tolerance range of the pin hole two is unchanged, and the positioning pin is in interference fit; 12) Milling the shoulder surface of the rod cover: After the shoulder surface 6 on the end face of the rod cover 1-2 is milled, the rod cover clamp is removed; 13) Assembly: the joint surface of the rod cover 1-2 and the rod body 1-1 is matched and aligned, and the positioning pin with reduced upper deviation is matched and positioned with the pin hole one on the joint surface of the rod body 1-1 and the pin hole two on the joint surface of the rod cover 1-2, that is, the positioning pin with reduced upper deviation is ), and the rod cover 1-2 and the rod body 1-1 are fixed and connected into one by the screw rod matched and connected with the bolt hole on the joint surface of the rod body 1-1 and the screw hole on the rod cover 1-2 and the nut matched and connected with the screw rod, wherein the assembly force of the rod body 1-1 and the rod cover 1-2 is the upper deviation of the tolerance range, that is, the assembly force of the rod body 1-1 and the rod cover 1-2 is controlled at the upper deviation 135MPa; 14) Semi-precision boring large and small head holes: after the assembled connecting rod 1 is positioned and fixed, the large head hole 2 and the small head hole 3 on the connecting rod 1 are semi-precision bored, wherein the machining amount of the semi-precision boring of the large head hole 2 and the small head hole 3 is 1 / 2 of the single-sided machining allowance of the large head hole 2 and the small head hole 3; Specifically, the single-sided machining allowance of the large head hole 2 and the small head hole 3 is 1mm; The machining allowance of the precision bored hole is 1mm on a single side. Since the allowance is large, machining stress will be generated after machining, resulting in the large head hole 2 being out of round. In order to ensure the precision of the precision bored hole, the process of increasing the semi-precision boring of the large and small head holes is adopted, the allowance is removed by 0.5mm on a single side in advance, and the allowance is left by 0.5mm on a single side when the precision bored hole is bored, so as to avoid stress accumulation caused by removing too much material at a time and reduce the risk of deformation; Disassemble the connecting rod and reassemble: remove the screw rod and nut connecting the rod cover 1-2 and the rod body 1-1, release the machining stress, and then reassemble and fix the rod cover 1-2 and the rod body 1-1 through the screw rod and the nut under the matching and positioning of the positioning pin, wherein the assembly force of the rod cover 1-2 and the rod body 1-1 during the reassembly and fixation is the lower deviation of the assembly force tolerance range, that is, the assembly force (i.e. the bolt pre-tightening force) is controlled at the lower deviation 130MPa when the rod cover 1-2 and the rod body 1-1 are reassembled; The big head hole 2 of the connecting rod 1 is in an elliptical state when the connecting rod 1 is assembled without applying an assembly force, that is, the long axis of the ellipse is along the length direction of the connecting rod 1, and the short axis of the ellipse is along the width direction of the connecting rod 1, and the big head hole 2 is in a circular state only after being assembled according to the assembly force required by the drawing; when the connecting rod 1 is assembled, for example, the assembly force and the upper and lower deviations required by the drawing are 130 (+ 50) MPa; the assembly force varies within the tolerance range, which will cause a small deformation of the big head hole 2; in order to correct the non-circular state of the big head hole 2 of the connecting rod 1, by using the characteristics of the big head hole 2, the assembly force of the connecting rod 1 is controlled at the upper deviation of 135 MPa when the connecting rod 1 is assembled before the semi-precision boring of the big head hole, and the assembly force of the connecting rod 1 is controlled at the lower deviation of 130 MPa when the connecting rod 1 is reassembled, so as to offset the micro-deformation of the big head hole of the connecting rod, and finally the big head hole is in a circular state, which meets the size tolerance and the cylindricality requirement required by the drawing; after the semi-precision boring of the big head hole, the rod body 1-1 and the rod cover 1-2 are separated to release the machining stress, and then the rod body 1-1 and the rod cover 1-2 are assembled to precision bore the hole, so that the precision boring allowance or the deformation of the big head hole 2 caused by the big head hole is reduced as much as possible; 16) Precision boring: after the assembled connecting rod 1 is positioned and fixed by using a connecting rod clamp which is adapted to the process positioning hole 4 on the upper end surface of the rod body 1-1 and the two ends of the rod body 1-1, the big head hole 2 and the small head hole 3 of the connecting rod 1 are precision bored to meet the design requirements of size accuracy and cylindricality.

[0013] In order to improve the interference between the pin hole one machined on the joint surface of the positioning pin and the rod body 1-1 and the pin hole two machined on the joint surface of the rod cover 1-2, the lower deviation of the pin hole one and the pin hole two is increased and the upper deviation of the positioning pin is reduced by compressing the tolerance of the pin hole and the pin, so as to increase the fitting clearance, so that the assembly is not overfilled, the rod body 1-1 and the rod cover 1-2 are not affected by external force and are in a free state, which is beneficial to improve the stress state of the connecting rod 1, thereby reducing the deformation phenomenon caused by stress; it should be noted that when the positioning pin is machined, the tolerance range is compressed as much as possible under the premise of normal machining, so as to ensure that there is a small gap between the positioning pin and the pin hole one after assembly, and the interference phenomenon is avoided.

[0014] After detection, the connecting rod 1 processed by the process of the embodiment has no oval deformation of the big head hole 2 after secondary assembly, the size accuracy and the shape and position tolerance meet the design requirements, and the qualified rate is greatly improved compared with the traditional process.

[0015] The technical solution solves the deformation problem of the big head hole of the connecting rod 1 after assembly caused by the interference of the positioning and fitting and the accumulation of residual stress in the existing process, ensures that the size accuracy and the cylindricality of the big head hole 2 meet the design requirements, has a wide application range and strong operability, reduces the interference problem of the assembly pin, makes the connecting rod 1 assembled without interference, and the size accuracy of the big head hole is stable, which meets the size accuracy requirement of the big head hole 2 of the connecting rod 1.

[0016] It will be obvious to a person skilled in the art that the application is not limited to the details of the foregoing exemplary embodiments and can be implemented in other concrete forms without departing from the spirit or essential characteristics of the application. The embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the application being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein. No reference signs in the claims should be considered as limiting the scope of the claims to the identity of the reference signs therein.

[0017] Furthermore, it should be understood that although the description is made on the basis of the embodiments, not every embodiment contains only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that those skilled in the art can understand.

Claims

1. A process for controlling the dimensional accuracy of a big end bore of a connecting rod, characterized by The method comprises the following steps: 1) fine milling of the upper plane: after positioning and fixing the connecting rod (1) with the lower end surface of the connecting rod (1) and the large head hole (2) and the small head hole (3) of the connecting rod (1) as the positioning reference, fine milling of the upper plane of the connecting rod (1) is performed; 2) fine milling of the lower plane and processing of the reference hole: after positioning and fixing the connecting rod (1) with the upper plane of the connecting rod (1) processed by fine milling and the large head hole (2) and the small head hole (3) of the connecting rod (1) as the positioning reference, fine milling of the lower plane of the connecting rod (1) is performed, and a plurality of process positioning holes (4) distributed on the rod body (1-1) and the rod cover (1-2) are processed on the lower plane of the connecting rod (1) as the reference hole; 3) processing of the large end surface and the process hole: after positioning and fixing the connecting rod (1), the shoulder surface (6) and the process hole (5) are processed on the large head end and the small head end of the connecting rod (1), respectively, and then the connecting rod clamp is removed after the processing is completed; 4) processing of the circular arc surface: after positioning and fixing the connecting rod (1) with the small head end of the connecting rod (1) and the shoulder surface (6) of the large head end as the positioning reference, the two circular arc surfaces of the connecting rod (1) are processed; 5) cutting: the connecting rod (1) is cut along the cutting line to form the rod body (1-1) and the rod cover (1-2); 6) annealing treatment: the rod body (1-1) and the rod cover (1-2) cut are subjected to annealing treatment to remove the initial stress generated in the metal material during cutting; 7) drilling of the deep hole: after positioning and fixing the rod body (1-1) subjected to the annealing treatment, the deep hole (7) is processed on the rod body (1-1) along the bonding surface thereof; 8) semi-fine boring of the rod body hole: semi-fine boring of the rod body hole is performed on the rod body (1-1); 9) milling of the rod body bonding surface: after milling of the bonding surface of the rod body (1-1) according to the requirements of the drawing, the bolt hole and the lower deviation increased pin hole one are processed on the bonding surface of the rod body (1-1); 10) semi-fine boring of the rod cover hole: after positioning and fixing the rod cover (1-2) subjected to the annealing treatment, semi-fine boring of the rod cover hole is performed; 11) milling of the rod cover bonding surface: after milling of the bonding surface of the rod cover (1-2) according to the requirements of the drawing, the screw hole and the pin hole two corresponding to the positions of the bolt hole and the pin hole one on the bonding surface of the rod body (1-1) are processed on the bonding surface of the rod cover (1-2); 12) milling of the rod cover shoulder surface: the shoulder surface (6) on the end surface of the rod cover (1-2) is milled; 13) assembly: the bonding surfaces of the rod cover (1-2) and the rod body (1-1) are matched and aligned, the positioning pins with the upper deviation reduced are matched and positioned with the pin hole one on the bonding surface of the rod body (1-1) and the pin hole two on the bonding surface of the rod cover (1-2), respectively, and the rod cover (1-2) and the rod body (1-1) are fixed and connected into one body by the screw rod matched and connected with the bolt hole on the bonding surface of the rod body (1-1) and the screw hole on the rod cover (1-2) and the nut matched and connected with the screw rod, wherein the assembly force of the rod body (1-1) and the rod cover (1-2) is the upper deviation in the tolerance range. 14) Semi-finishing the big and small head holes: after the assembled connecting rod (1) is positioned and fixed, the big head hole (2) and the small head hole (3) on the connecting rod (1) are semi-finished, wherein the semi-finishing machining amount of the big head hole (2) and the small head hole (3) is 1 / 2 of the single-side machining allowance of the big head hole (2) and the small head hole (3); 15) Disassembling the connecting rod and secondary assembling: the screw rod and the screw nut of the connecting rod cover (1-2) and the rod body (1-1) are disassembled, the processing stress is released, and then the rod cover (1-2) and the rod body (1-1) are secondary assembled and fixed through the screw rod and the screw nut, wherein the assembling force of the secondary assembled and fixed rod cover (1-2) and the rod body (1-1) is the lower deviation of the assembling force tolerance range; 16) Finishing the holes: the secondary assembled connecting rod (1) is positioned and fixed, and the big head hole (2) and the small head hole (3) on the connecting rod (1) are finished.

2. The process of controlling the dimensional accuracy of the big end bore of a connecting rod as set forth in claim 1, wherein: In the above step 14), the single-side machining allowance of the big head hole (2) and the small head hole (3) is 1mm.

Images