A multi-station cold heading forming method for gear shafts and its inspection method

Abstract

This invention discloses a multi-station cold heading forming method for gear shafts, comprising the following steps: (a) obtaining raw materials and cutting them into workpieces to be cold-headed according to predetermined dimensions; (b) pre-punching: punching positioning holes on both end faces of the workpiece; (c) upsetting and forming internal thread blanks: upsetting internal thread blanks based on the positioning holes; (d) rough upsetting of the gear blank and the two-end square blanks: preliminary upsetting to form the gear blank and the two-end square blanks; (e) upsetting of the gear blank and the two-end square blanks: finishing the gear blank and the square blank; (f) forming of the gear blank and the two-end square blanks: completing the final forming and controlling the opposite side dimensions of the square blank within the design range; (g) cold upsetting and extrusion forming of the gear: cold extruding to form a high-precision gear shape. This invention also provides a detection method during the multi-station cold heading forming process of gear shafts. This invention offers advantages such as improved material utilization, maintenance of metal fiber continuity, reduced costs, and improved processing efficiency.

Description

Technical Field

[0001] This invention relates to the field of precision cold forging, and in particular to a multi-station cold heading method for gear shafts and its testing method. Background Technology

[0002] As a common automatic door closing device in modern buildings, the machining quality of the core transmission component, the gear shaft, directly affects the service life and operational stability of the door closer. Traditional gear shaft manufacturing processes have significant drawbacks: cutting methods result in material utilization rates of less than 40% and sever metal fibers, severely impacting the mechanical properties of the parts; while hot forging maintains fiber continuity, it causes oxidation and decarburization, leading to a decrease in surface hardness; existing cold forging technology can partially solve these problems, but its detection rate for hidden folding defects generated during the forming process is less than 30%. In particular, the cuboid part connecting the gear shaft and the rocker arm requires extremely high dimensional accuracy and surface quality, making it difficult for traditional machining methods to simultaneously meet the multiple demands of material utilization, mechanical properties, and dimensional accuracy. Furthermore, existing testing methods cannot monitor key parameters throughout the cold heading process, resulting in blind spots in product quality control. Summary of the Invention

[0003] The purpose of this application is to provide a multi-station cold heading forming method for gear shafts and its inspection method, which has the advantages of improving material utilization, maintaining the continuity of metal fibers, reducing costs and improving processing efficiency.

[0004] To achieve the above objectives, the present invention provides the following technical solution:

[0005] This application provides a multi-station cold heading forming method for gear shafts, the technical solution of which includes the following steps in sequence: (a) obtaining raw materials and cutting them into workpieces to be cold-headed according to predetermined dimensions; (b) pre-punching: punching positioning holes on both ends of the workpiece; (c) upsetting and forming internal thread blank holes: upsetting internal thread blank holes based on positioning holes; (d) rough upsetting of gear blanks and two-end square blanks: preliminary upsetting to form gear blanks and two-end square blanks; (e) upsetting of gear blanks and two-end square blanks: finishing of gear blanks and square blanks; (f) forming of gear blanks and two-end square blanks: completing the final forming and controlling the opposite side dimensions of the square within the design range; (g) cold upsetting and extrusion forming of gears: cold extrusion forming of high-precision gear profiles.

[0006] Furthermore, this application also proposes a method for detecting the workpiece obtained by the above method, wherein after step (c), damage detection of the internal thread blank hole is performed, and the steps are as follows: C-100, measure the Vickers hardness at 3 points on the edge of the blank hole; C-200, calculate the damage value, and if the damage value of all measuring points does not exceed 0.62 and the standard deviation is ≤0.03, it is judged as qualified.

[0007] Furthermore, this application also proposes to include rough upsetting streamline testing after step (d), the steps of which are as follows: d-100, cut a sample of the shaft forming section for metallographic etching; d-200, take a microscope to photograph the streamline image of the axial longitudinal section; d-300, if the streamline is continuous and the angle with the axis is ≤10°, and there is no intersection or backflow, it is judged to be qualified.

[0008] Furthermore, this application also proposes that a temperature field scan be performed after step (e), the steps of which are as follows: e-100, the infrared thermal imager scans the workpiece within 0.5 seconds after it leaves the mold; e-200, the analysis area is divided by the image: the analysis area includes the top surface of the cuboid, the transition area of ​​the tooth blank, and the area around the inner hole, and the maximum temperature difference and the proportion of the high temperature area in each area are calculated respectively; if the maximum temperature difference is ≤160℃ and the proportion of the high temperature area is ≤5%, it is deemed qualified.

[0009] Furthermore, this application also proposes that after step (f), dimensional and load testing be performed, the steps of which are as follows: f-100 coordinate measuring machine verifies the opposite sides of the cuboid, and if the dimensions are within 9.08-9.12mm, it is deemed qualified; f-200, collects the maximum load of 6 stations and calculates its coefficient of variation, and if the coefficient of variation is ≤8%, it is determined that the deformation distribution of the stations is reasonable.

[0010] Furthermore, this application proposes to perform a three-stage comprehensive quality test on the tooth profile after step (g), with the following steps: g-100, cut a sample from the tooth root area for metallographic etching, and then take a microscopic image of the tooth root streamline; calculate the tooth root streamline orientation angle based on the image; g-200, select two points each at the tooth root and tooth tip to measure Vickers hardness, and calculate the damage values ​​of the tooth root and tooth tip; g-300, scan the tooth profile with an infrared thermal imager within 0.5 seconds of tooth profile demolding, and calculate the maximum temperature difference and the proportion of the high-temperature zone on the tooth surface; if the tooth root streamline orientation angle is ≤8°; the damage values ​​of the tooth tip and tooth root are both ≤0.62; the maximum temperature difference on the tooth surface is ≤140℃ and the proportion of the high-temperature zone is ≤3%, then it is deemed qualified.

[0011] Furthermore, this application also proposes that a floating die structure be used in steps (e) and (f), and a piezoelectric force sensor be installed on the die base to monitor the forming friction in real time. If the peak value of the friction does not exceed 85% of the theoretical value and the friction fluctuation coefficient is ≤5%, then it is deemed qualified.

[0012] Furthermore, this application also proposes that step (a) includes pretreatment of the raw materials: the raw materials are 25CrMo4 alloy steel and are annealed and softened; the surface is phosphated to form a 5-8μm lubricating layer, and the blank diameter tolerance is controlled within ±0.05mm during cutting.

[0013] Furthermore, this application also proposes that the cold heading equipment adopts a 6-station cold heading machine; the main die stroke accuracy is ±0.01mm; and the forming speed range is 15-25mm / s.

[0014] Furthermore, this application also proposes that the die cavity in the mold material is made of VA70 steel with a hardness ≥62HRC; the punch and punch head are made of SKH51 steel with a hardness ≥60HRC; and the surface roughness Ra of the mold cavity is ≤0.2μm.

[0015] The beneficial effects of this invention are as follows: This application provides a multi-station cold heading forming method for gear shafts and its inspection method. By combining multi-station cold heading forming steps with a whole-process quality inspection system, and employing pre-punching positioning, staged upsetting forming, and precision mold matching, it solves the problems of low material utilization in traditional cutting processes, oxidation and decarburization in hot forging processes, and blind spots in existing cold forging technology. It has the advantages of improving material utilization, maintaining the continuity of metal fibers, achieving ±0.02mm dimensional accuracy control, and whole-process quality monitoring. Attached Figure Description

[0016] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0017] Figure 1 This is a schematic diagram of the process of an embodiment of the present invention. Detailed Implementation

[0018] The following will describe in detail the implementation of this application with reference to the accompanying drawings and embodiments, so that the implementation process of how this application uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly.

[0019] In existing technologies, the connection between the door closer gear shaft and the rocker arm is typically manufactured using machining or hot forging processes. Machining results in a material utilization rate of less than 40% and severs the metal fiber structure, affecting the mechanical properties of the part. While hot forging can improve material utilization, high-temperature processing causes surface oxidation and decarburization, leading to a decrease in hardness. Existing cold forging inspection technologies have an identification rate of only 30% for hidden folding defects, which is insufficient to meet the quality requirements of precision parts.

[0020] To address the aforementioned issues, a forming process is needed that can maintain the continuity of metal fibers while avoiding surface damage. Traditional processes suffer from bottlenecks in positioning accuracy and deformation control, and multi-stage processing easily leads to error accumulation. Analysis of the deformation mechanism of cold forging reveals that staged forming can effectively reduce the single-stage deformation load. Adopting a unified positioning datum strategy can solve the positional deviation problem in multi-station processing.

[0021] Therefore, as Figure 1As shown, this application proposes a multi-station cold upsetting method for gear shafts, which includes the following steps in sequence: obtaining raw materials and cutting them into workpieces to be cold-upset; punching positioning holes on both ends of the workpieces; upsetting internal thread blanks based on the positioning holes; rough upsetting and fine upsetting of the gear blanks and cuboids in stages; controlling the opposite side dimensions of the cuboids; and cold extruding to form high-precision gear profiles.

[0022] Pre-punching refers to machining positioning reference holes on the end face of the workpiece. This can be done simultaneously using a double-headed stamping machine, providing a unified spatial positioning reference for subsequent steps. Upsetting the internal thread blank hole refers to extrusion forming using positioning holes for axial positioning. This can be done in stages using a stepped punch to ensure the accuracy of the internal hole shape. Multi-station cold heading refers to step-by-step forming at different die stations. This can be achieved using a six-station cold heading machine for continuous processing, maintaining the continuity of material flow lines by controlling the deformation at each station.

[0023] Specifically, after precise cutting, the raw material is formed into a standard blank, and the positioning holes at both ends establish a unified coordinate system for subsequent processes. During the internal thread blank forming stage, the positioning holes constrain the material flow direction, preventing eccentric deformation. The rough upsetting process forms the basic outline of the teeth and the cuboid, while the finish upsetting stage corrects the geometry through minor deformation. The cuboid dimensions are controlled through precise fitting of the mold cavity, and the cold-extruded tooth profile utilizes the plastic flow of metal to form a continuous tooth profile.

[0024] Compared to existing technologies, cold heading avoids material waste associated with machining, and the metal fibers are continuously distributed along the part's contour. Multi-station, step-by-step forming replaces traditional hot forging, eliminating surface defects caused by high-temperature oxidation. The unified setting of positioning datums improves the centering of each forming station, ensuring the geometric accuracy of the part.

[0025] Through the above technical solutions, this application achieves efficient utilization of metallic materials while maintaining the integrity of the mechanical properties of the parts. The forming process avoids surface oxidation and decarburization, ensuring that the surface hardness of the parts meets requirements. Staged deformation control effectively reduces the probability of latent defects and improves the product qualification rate.

[0026] This application further proposes to perform damage detection on the internal thread blank hole after the cold heading step. The specific steps include: taking 3 points on the edge of the blank hole to measure the Vickers hardness; calculating the damage value; and judging it as qualified if the damage value of all measuring points does not exceed 0.62 and the standard deviation does not exceed 0.03.

[0027] Among them, measuring Vickers hardness at three points on the edge of the blank hole refers to selecting three equally spaced measurement positions on the circumferential edge of the formed internal thread blank hole. Specifically, this can be achieved by using a microhardness tester to perform indentation testing under 400x magnification. The three-point distribution measurement method can cover the material deformation area in different directions.

[0028] The damage value is a quantitative index calculated by comparing Vickers hardness data with the material's original hardness value. Specifically, it can be expressed using the formula... Calculations show that this indicator can reflect the degree of work hardening of the material during the cold heading process.

[0029] The standard deviation of no more than 0.03 means that the dispersion of the damage values ​​at the three measuring points is limited to within 0.03. This can be achieved by calculating the root mean square error of the damage values ​​at the three measuring points. This index is used to constrain the uniformity of the material properties.

[0030] Specifically, three equally spaced measurement points were selected at the edge of the internal thread blank hole after cold heading, and the Vickers hardness value of each point was obtained using a microhardness tester. The measured hardness was compared with the original hardness of the material in the annealed state to calculate the damage value of each measurement point. When the damage values ​​of all three measurement points did not exceed 0.62 and the numerical fluctuation range was limited to within a standard deviation of 0.03, it indicated that the material had not undergone excessive work hardening and the deformation uniformity met the requirements. The three-point distribution measurement method can effectively capture the deformation differences in different orientations of the hole edge, while the dual criteria setting controls the damage degree of individual points and constrains the uniformity of overall performance.

[0031] Compared to existing technologies, traditional testing methods typically employ single-point hardness sampling or visual inspection, which cannot effectively identify localized hardening differences and latent damage. This proposed solution, however, utilizes a three-point distributed hardness testing network to cover key deformation areas. Combined with a damage value quantification model and standard deviation control indicators, it achieves a comprehensive evaluation of the material's deformation state. Compared to testing methods that only focus on absolute hardness values, this method eliminates the influence of batch-to-batch variations through relative damage value calculations. Furthermore, the standard deviation indicator can provide early warnings of potential uneven deformation problems.

[0032] Through the above technical solution, this application can accurately identify hidden material damage generated during cold heading, avoiding the propagation of microcracks caused by local stress concentration. The combination of three-point measurement and dual criteria significantly improves the reliability of the test results, effectively preventing unqualified blanks from flowing into subsequent processing steps, thereby reducing the defect rate of the finished gear shaft.

[0033] This application further proposes a method for performing streamline inspection after the rough upsetting process, specifically including: taking a sample of the shaft forming section, specifically: performing metallographic etching on the longitudinal section of the shaft substrate; taking images of the streamlines of the axial longitudinal section using a microscope; and judging the forming quality based on the continuity of the streamlines, the range of the angle with the axis, and whether there is a cross-flow phenomenon.

[0034] Among them, the shaft forming section sample refers to the test sample cut from the rough upsetting deformation area of ​​the workpiece shaft. Specifically, a segment containing the complete deformation gradient can be cut along the axial direction by wire cutting to reflect the flow state of the material during the rough upsetting process.

[0035] Metallographic corrosion refers to the selective dissolution of metal grain boundaries by chemical reagents. Specifically, a 4% (v / v) nitric acid alcohol solution can be used to etch the metal for 15-25 seconds, allowing the fibrous structure formed by the metal flow to show a clear microstructure.

[0036] Among them, the axial longitudinal section streamline image refers to the metal fiber distribution image observed after cutting along the workpiece axis. Specifically, a 500x optical microscope can be used to take a complete cross section including the region from the axis to the edge, which is used to quantitatively analyze the material flow direction and uniformity.

[0037] Specifically, after the rough upsetting process, by cutting a sample containing the deformed area and performing metallographic etching, the fibrous structure formed by plastic flow within the material can be fully exposed. When taking axial longitudinal section images under a microscope, the radial and axial metal flow characteristics can be detected simultaneously by selecting an observation plane containing the axis. When the streamlines are continuous and the angle with the axis is controlled within 10°, it indicates that no abnormal shearing or folding occurred in the material during the rough upsetting process; the absence of crossing or backflow phenomena eliminates latent defects caused by disordered metal flow paths. This detection method establishes a correlation between microstructure and macroscopic forming quality, enabling the quantitative determination of latent folding defects in the rough upsetting process.

[0038] Compared to existing technologies, traditional cold forging inspection relies solely on visual inspection or dimensional measurement, failing to identify internal flowline defects in the material. This proposed solution, however, utilizes metallographic corrosion and microscopic image analysis to accurately capture microscopic defects such as fiber fractures and intersections caused by abnormal metal flow, significantly improving detection sensitivity compared to traditional methods. Furthermore, existing technologies lack a correlation standard between flowline angles and folding defects; this solution proposes an angle ≤10° and a criterion of no cross-flow, providing quantifiable technical indicators for evaluating the quality of the rough forging process.

[0039] Through the above technical solution, this application effectively solves the problem that hidden folding defects caused by turbulent metal flow are difficult to detect in a timely manner during the rough upsetting stage. Through microstructure analysis and quantitative evaluation of flow morphology, forming defects can be identified in advance and process parameters can be optimized, thereby avoiding product failure caused by defect expansion in subsequent processes.

[0040] This application further proposes performing a temperature field scan after the step, the steps of which are as follows: the infrared thermal imager scans the workpiece within 0.5 seconds after it leaves the mold; the analysis area is divided by the image, including the top surface of the cuboid, the transition area of ​​the tooth blank, and the area around the inner hole, and the maximum temperature difference and the proportion of the high temperature area in each area are calculated respectively; if the maximum temperature difference does not exceed 160℃ and the proportion of the high temperature area does not exceed 5%, it is deemed qualified.

[0041] The infrared thermal imager's scanning within 0.5 seconds of workpiece ejection refers to the use of non-contact temperature measurement equipment to complete temperature data acquisition within a very short time after the formed workpiece leaves the mold. This can be achieved using an infrared thermal imager equipped with a high-speed data acquisition module. This time window effectively avoids interference from ambient temperature. The image segmentation and analysis region refers to dividing the thermal image into detection areas with different thermal conductivity characteristics based on the workpiece's geometric features. Specifically, image processing algorithms can automatically identify the top surface of the cuboid, the transition zone of the tooth blank, and the boundary around the inner hole. This segmentation method can match the heat distribution patterns of different structural parts. The maximum temperature difference and high-temperature zone proportion refer to the proportion of areas exceeding a set threshold temperature by calculating the highest and lowest temperature differences within each region. This can be achieved using the thermal imager's accompanying analysis software. This combination of indicators can simultaneously reflect local overheating and overall temperature rise.

[0042] Specifically, the dynamic temperature field monitoring mechanism captures the instantaneous heat distribution of the workpiece after the finishing process and performs zoned evaluation of key deformation areas. The top surface of the cuboid, as a high-plasticity deformation zone, reflects the degree of work hardening in terms of temperature change; the temperature gradient in the transition zone of the tooth blank affects the uniformity of metal flow; and the heat accumulation around the inner hole is directly related to the friction effect. By setting a maximum temperature difference threshold, abnormal local grain growth can be suppressed; controlling the area ratio of the high-temperature zone can reduce the risk of residual stress concentration. The dual constraint mechanism of the judgment conditions ensures the stability of the material's microstructure and the consistency of the formed dimensions.

[0043] Compared to existing technologies, traditional cold forging inspection relies on offline sampling metallographic analysis, which cannot monitor temperature anomalies during the forming process in real time. Existing technologies can only detect surface defects through visual inspection or destructive testing, lacking effective means to detect latent damage caused by uneven temperature distribution. This solution achieves comprehensive monitoring of material thermal damage through online temperature field scanning and zonal quantitative assessment.

[0044] Through the above technical solution, this application can effectively identify hidden material damage caused by local overheating during cold heading, and avoid dimensional deviations caused by uneven temperature distribution. By instantly determining the acceptable temperature field status, the scrap rate in subsequent processes can be reduced, ensuring that the formed workpiece has stable mechanical properties and dimensional accuracy.

[0045] This application further proposes to perform dimensional and load inspection after step (f), the steps of which are as follows: a coordinate measuring machine is used to verify whether the opposite side dimensions of the cube are within the set range; the maximum load data of the six stations are collected and their coefficient of variation is calculated, that is, the loads of the six stations are P1, P2, P3, P4, P5, P6; the average load Pm is calculated, and then the standard deviation s is calculated, then the coefficient of variation CV=s / Pm, if CV≤8%, CV≤8% ensures that the mold of each station is uniformly stressed and qualified.

[0046] Among them, the coordinate measuring machine refers to a precision instrument that performs geometric measurements through a three-dimensional spatial coordinate system. Specifically, it can be implemented by using a contact probe in conjunction with measurement software. It is used to verify whether the side dimensions of the cube after cold heading meet the assembly requirements, and to ensure the fitting accuracy between the cube and the rocker arm by setting the tolerance range.

[0047] The coefficient of variation is the ratio of the standard deviation to the mean. Specifically, it can be calculated by collecting load data from each station and then calculating the ratio of the standard deviation to the mean. It is used to evaluate the uniformity of load distribution during multi-station cold heading. When the coefficient of variation is below the threshold, it indicates that the material flow is stable and the mold is subjected to balanced forces.

[0048] Specifically, after cold heading, the dimensions of the cuboid's opposite sides are first inspected using a coordinate measuring machine (CMM) – either 100% or by sampling. For example, the acceptable range is set at 9.08-9.12 mm; workpieces exceeding this range are deemed defective. This directly controls the cuboid's geometric accuracy, preventing loose rocker arm assembly or stress concentration due to dimensional deviations. Subsequently, by collecting the maximum load data from the six stations during the forming process, the coefficient of variation of the load at each station is calculated. For instance, when the coefficient of variation does not exceed 8%, it indicates that the deformation force distribution at each station is within a reasonable range, effectively preventing material folding or abnormal mold wear caused by localized overload. This combination of dimensional inspection and load analysis forms a closed-loop quality control mechanism, ensuring the assembly reliability of the final product while optimizing energy distribution during the forming process.

[0049] Compared to existing technologies, current cold forging inspection relies solely on single-dimensional sampling or visual inspection to identify defects, failing to simultaneously assess the impact of workstation load distribution on latent quality defects. This solution, however, incorporates the coefficient of variation (COP) index to integrate load distribution uniformity into the inspection system. For example, in a six-station setup, real-time monitoring of maximum load fluctuations at each station provides early warning of folding risks caused by mold wear or uneven material flow. Furthermore, combined with high-precision coordinate measuring machine (CMM) measurement, the dimensional tolerances of the cuboid are controlled within a stricter range, such as compressing the tolerance zone to 0.04 mm, significantly improving inspection coverage and defect prevention capabilities.

[0050] Through the above technical solutions, this application can simultaneously solve the assembly failure problem caused by dimensional deviations during cold heading and the hidden folding defects caused by uneven distribution of deformation force at each workstation. The dual detection mechanism ensures that the geometric accuracy of the cuboid meets the design requirements, while optimizing the load distribution at multiple workstations, thereby improving the connection reliability between the door closer gear shaft and the rocker arm and the product service life.

[0051] This application further proposes a three-step comprehensive quality inspection of the tooth profile after the cold heading and extrusion tooth forming process. The steps include: taking a sample from the tooth root area for metallographic etching, then taking a microscopic image of the tooth root streamline and calculating the tooth root streamline orientation angle based on the image; selecting two points each at the tooth root and tooth tip to measure Vickers hardness and calculate the damage value; using an infrared thermal imager to scan and calculate the maximum temperature difference and the proportion of the high-temperature zone on the tooth surface after the tooth profile is demolded; and determining that the tooth is qualified when the tooth root streamline orientation angle, damage value, and temperature parameters meet the preset threshold.

[0052] The tooth root streamline orientation angle refers to the angle between the material flow trajectory and the tooth shaft axis. This angle can be measured using streamline images of the axial longitudinal section of a metallographic specimen. This parameter characterizes whether material flow produces folding defects. The damage value is a plastic deformation damage index calculated based on Vickers hardness test data. It can be calculated using a ratio model of hardness value to the original material hardness, and is used to quantitatively assess the degree of microstructural damage in the tooth tip and root regions. The high-temperature zone proportion refers to the percentage of pixel area exceeding a set temperature threshold in the infrared thermographic image. This can be achieved by segmenting and statistically analyzing the temperature field distribution using image processing software, and is used to identify microstructural abnormalities caused by localized overheating.

[0053] Specifically, by performing metallographic etching on tooth root samples and capturing streamline images, it is possible to determine whether material flow has caused folding defects based on streamline orientation angle parameters. Vickers hardness testing at dual measurement points at the tooth tip and root allows for simultaneous assessment of the degree of plastic deformation damage at different parts of the tooth profile. Rapid scanning of the die tooth surface using an infrared thermal imager, along with analysis of temperature field distribution characteristics, reveals microstructural anomalies caused by localized overheating. These three tests construct quality criteria from three dimensions: material flow trajectory, microstructure, and forming thermal effects. By defining streamline orientation angle, damage threshold, and temperature field parameters, a composite detection mechanism for latent defects in cold-forged tooth profiles is formed.

[0054] Compared to existing technologies, traditional cold forging inspection relies solely on visual inspection of surface defects in a single dimension, failing to effectively identify latent folding defects at the tooth root and lacking quantitative assessment of material damage and temperature anomalies. This proposed solution establishes a three-dimensional quality evaluation system encompassing streamline morphology, material damage, and temperature distribution, enabling simultaneous detection of latent folds, microscopic damage, and thermal anomalies, thus addressing the insufficient coverage of single inspection methods.

[0055] Through the above technical solutions, this application achieves effective identification of hidden folding defects in cold-forged tooth profiles, avoiding the risk of missed detection by traditional visual inspection; by quantitatively evaluating the damage values ​​of the tooth tip and root regions, the degree of plastic deformation of the material can be precisely controlled; through temperature field parameter analysis, local overheating phenomena during the forming process can be detected in a timely manner, ensuring the dimensional accuracy of the tooth profile and the stability of its mechanical properties.

[0056] This application further proposes to use a floating die structure in steps (e) and (f), and to install a piezoelectric force sensor on the die base to monitor the forming friction in real time. If the peak value of the friction does not exceed 85% of the theoretical value and the friction fluctuation coefficient is ≤5%, it is deemed qualified.

[0057] Among them, a floating die structure refers to a die structure that can undergo minute axial displacement during the forming process. This can be achieved using springs or hydraulic buffer mechanisms. By allowing the die to adaptively displace under force, the contact state between the die and the workpiece can be adjusted. A piezoelectric force sensor is a force measurement device based on the piezoelectric effect principle. It can be implemented using a quartz crystal sensor. By converting mechanical stress into an electrical signal, it can collect dynamic friction force data in real time during the forming process. The friction force fluctuation coefficient is the ratio of the standard deviation to the average value of the friction force. It is obtained by continuously collecting friction force data from multiple forming cycles and calculating the standard deviation and average value, and is used to characterize the stability of the friction force.

[0058] Specifically, the floating die structure absorbs impact loads during the forming process through a buffer mechanism, maintaining flexible contact between the die and the workpiece and avoiding localized stress concentration caused by rigid die constraints. A piezoelectric force sensor is installed at the die base to capture the dynamic change curve of friction force in real time during the forming process. When the peak friction force exceeds 85% of the theoretical value, it indicates an abnormal increase in material flow resistance, which may lead to internal cracks; when the friction force fluctuation coefficient exceeds 5%, it indicates abnormal die alignment or lubrication, which may cause folding defects. By simultaneously monitoring these two parameters, abnormal process conditions can be identified in the early stages of defect formation.

[0059] Compared to existing technologies, traditional cold forging inspection methods only detect surface defects through visual inspection or offline testing, failing to monitor the dynamic friction state during the forming process in real time. The fixed die structure used in existing technologies easily leads to localized stress concentration, resulting in uneven material flow. This solution improves material flow uniformity through a floating die structure and establishes process quality control indicators by combining dynamic friction monitoring, enabling the early detection of latent defect formation trends.

[0060] Through the above technical solution, this application can identify friction fluctuations caused by mold eccentricity, lubrication failure, or abnormal material flow during cold heading in real time, and provide early warning of hidden folding defects through dynamic parameter criteria. Compared with traditional offline inspection methods, this solution advances the defect detection point to the forming process stage, which can effectively improve the defect detection rate and reduce the generation of defective products.

[0061] This application further proposes that, in the raw material pretreatment process, 25CrMo4 alloy steel is used as the base material and annealing and softening treatment is carried out, and the surface is phosphated to form a 5-8μm lubricating layer. At the same time, the diameter tolerance of the cut billet is controlled within ±0.05mm.

[0062] Among them, 25CrMo4 alloy steel refers to a medium-carbon alloy steel containing chromium and molybdenum. It can be prepared by vacuum melting process, with its carbon content controlled at 0.22%-0.29%, chromium content at 0.90%-1.20%, and molybdenum content at 0.15%-0.30%. Through optimization of the alloy element ratio, this material can balance strength and plasticity during cold heading.

[0063] Annealing and softening treatment involves heating the material to its austenitizing temperature and then slowly cooling it. Specifically, this can be done in a box-type resistance furnace at 740-760℃ for 2-3 hours, followed by cooling at a rate of ≤30℃ / h to below 500℃ before air cooling. This process refines the grain size within the material, eliminates residual stress, and reduces the risk of cold heading cracking.

[0064] Surface phosphating refers to the chemical conversion that generates a phosphate crystal layer on the surface of steel. Specifically, zinc-based phosphating solution can be used to immerse the steel in the solution at 40-50℃ for 8-12 minutes. The resulting porous structure can adsorb lubricant and reduce the coefficient of friction between the mold and the workpiece during the forming process.

[0065] The blank diameter tolerance control refers to the fluctuation range of the outer diameter of the cylindrical blank after cutting. Specifically, a CNC precision sawing machine combined with an online laser diameter measuring instrument can be used to achieve real-time feedback adjustment to ensure that the geometric accuracy of the blank meets the requirements of multi-station continuous forming.

[0066] Specifically, the synergistic effect of material selection and pretreatment processes is reflected in the following aspects: the alloy composition design of 25CrMo4 alloy steel provides basic mechanical property support for subsequent cold heading; annealing reduces material hardness by adjusting the microstructure; the phosphating layer improves interfacial lubrication conditions through physical adsorption and chemical bonding; and strict control of billet dimensional accuracy avoids fluctuations in forming loads caused by diameter deviations. When these four factors work together, the material's plastic deformation capacity is enhanced, surface defects caused by interfacial friction are suppressed, and the impact of cumulative dimensional errors on the final forming accuracy is eliminated.

[0067] Compared with existing technologies, traditional cold heading processes often use ordinary medium carbon steel for direct cold heading. The lack of annealing treatment leads to a high rate of cracking during forming, and the surface is only coated with mineral oil for lubrication, resulting in severe die adhesion and wear. The blank cutting tolerance typically exceeds ±0.1mm, causing positioning deviations in subsequent workstations. This solution overcomes the limitations of adjusting a single process parameter through multi-dimensional improvements, including alloy composition optimization, microstructure homogenization, interface modification, and precise dimensional control.

[0068] Through the above technical solution, this application solves the problem of microcrack propagation caused by insufficient material ductility during cold heading, avoids surface scratches and abnormal mold wear caused by poor lubrication, and ensures the stability of metal flow during multi-station forming by precise control of blank size, thereby significantly improving the dimensional consistency and surface integrity of cold-headed gear shaft parts.

[0069] This application further proposes that the cold heading equipment adopts a 6-station cold heading machine, the main die stroke accuracy is controlled within ±0.01mm, and the forming speed is controlled within 15-25mm / s.

[0070] The 6-station cold heading machine refers to a cold forming equipment with six continuous processing stations. This can be achieved using a servo drive system and a multi-station mold assembly. By allocating forming processes through continuous stations, the number of workpiece transfers is reduced. The main mold stroke accuracy of ±0.01mm refers to the positional repeatability accuracy when the mold is closed. This can be achieved through a closed-loop control system and a high-precision guide rail structure, ensuring consistent mold alignment. The forming speed range of 15-25mm / s refers to the speed at which the punch extrudes the material. This can be adjusted using a hydraulic system or a variable frequency motor to balance material flow and temperature rise.

[0071] Specifically, the 6-station cold heading machine integrates pre-punching, upsetting of internal thread blanks, and rough upsetting of gear blanks into a single machine through a multi-station continuous forming process, avoiding positioning errors caused by multiple workpiece clamping. The main die stroke accuracy is controlled through closed-loop feedback, correcting die closure position deviations in real time to prevent folding defects caused by uneven material flow due to stroke deviations. The forming speed is controlled within a specific range; for example, a segmented speed control strategy is adopted, using low speed during the material filling stage to ensure full cavity filling, and switching to medium speed during the forming stage to improve efficiency while avoiding localized temperature rises caused by excessive speed, which could lead to material softening and deformation.

[0072] Compared to existing technologies, traditional cold heading equipment typically employs a configuration of four or fewer stations. The dispersed forming processes increase the number of workpiece transfers, making it difficult to eliminate accumulated errors. Existing master die stroke accuracy is generally above ±0.03mm, and die alignment deviations can easily cause material flow disturbances, leading to an increased rate of hidden folding defects. Forming speeds often employ a single high-speed mode, resulting in inadequate material filling or significant localized temperature rise issues. This solution achieves a simultaneous improvement in forming accuracy and efficiency through coordinated optimization of equipment selection and process parameters.

[0073] Through the above technical solutions, this application effectively reduces the rate of hidden folding defects caused by insufficient equipment precision during cold heading, ensuring the integrity of the material's fiber structure. Simultaneously, multi-station continuous processing significantly improves forming efficiency. High-precision control of the main mold stroke avoids abnormal material flow caused by mold misalignment, and the reasonable setting of the forming speed balances cavity filling quality and processing cycle time, thereby achieving mass production while ensuring the forming accuracy of the gear shaft.

[0074] This application further proposes a technical solution in which the die cavity is made of VA70 steel with a hardness of not less than 62HRC, the punch and punch are made of SKH51 steel with a hardness of not less than 60HRC, and the surface roughness of the die cavity is controlled to be no more than 0.2μm on Ra.

[0075] Among them, VA70 steel refers to cold work die steel with high carbon and high vanadium content. Its hardness can be increased through vacuum heat treatment, and its crystal structure stability can resist plastic deformation during cold heading. SKH51 steel refers to high-speed tool steel containing tungsten and molybdenum. Its hardness and toughness can be balanced through graded quenching, and its red hardness can withstand continuous impact loads. A surface roughness Ra of no more than 0.2μm refers to an ultra-smooth cavity surface formed through mirror polishing. This can be achieved through multi-stage polishing with diamond polishing paste; the elimination of micro-grooves reduces material flow resistance.

[0076] Specifically, the high hardness of the VA70 steel used in the die maintains the stability of the cavity dimensions under cold heading impact, preventing workpiece dimensional deviations caused by die deformation. The combination of strength and toughness of the SKH51 steel used in the punch and plunger can withstand high-frequency impact loads, extending the service life of the die in continuous production. The low-roughness surface of the die cavity, formed by precision polishing, reduces frictional resistance during metal flow, allowing for uniform material filling within the cavity and eliminating workpiece scratches caused by surface microcracks. The synergistic effect of these three technical features constructs a high-precision forming system from three dimensions: die material properties, structural stability, and surface quality.

[0077] Compared to existing technologies, traditional cold heading dies often use ordinary tool steel and fail to control surface roughness, resulting in insufficient die hardness causing cavity collapse, and excessive surface roughness causing uneven material flow. This solution overcomes the problem of reduced forming accuracy caused by rapid die wear by selecting high-hardness die steel and precision surface treatment, and solves the quality defects on the workpiece surface caused by unqualified die roughness.

[0078] Through the above technical solution, this application effectively improves the dimensional consistency of cold heading of door closer gear shafts, extends the service life of the mold in continuous production, and improves the surface finish of the workpiece. This technical solution systematically solves the problem of forming quality fluctuations caused by insufficient mold performance by optimizing the mold material system and surface treatment process.

[0079] As used in the specification and claims, certain terms refer to specific components. Those skilled in the art will understand that hardware manufacturers may use different names to refer to the same component. This specification and claims do not distinguish components based on differences in name, but rather on differences in function. The term "comprising" throughout the specification and claims is an open-ended term and should be interpreted as "comprising but not limited to." "Approximately" means that within an acceptable margin of error, those skilled in the art can solve the technical problem and substantially achieve the technical effect within a certain margin of error.

[0080] The foregoing description illustrates and describes several preferred embodiments of the present invention. However, as previously stated, it should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the inventive concept described herein through the foregoing teachings or techniques or knowledge in related fields. Any modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A multi-station cold heading forming method for gear shafts, characterized in that, 依次 include the following steps: (a) Obtain raw materials and cut them into cold heading workpieces according to predetermined dimensions; (b) Pre-punching: Punch positioning holes at both end faces of the workpiece; (c) Upsetting to form an internal thread blank hole: Upset an internal thread blank hole based on the positioning holes; (d) Rough upsetting of the tooth blank and the square blanks at both ends: Rough upset to form a tooth blank and square blanks at both ends; (e) Finishing the tooth blank and the square blanks at both ends: Finish the tooth blank and the square blanks; (f) Forming of the tooth blank and the square blanks at both ends: Complete the final forming and control the dimension of the opposite sides of the square within the design range; (g) Cold heading extrusion to form teeth: Cold extrude to form a high-precision tooth profile; It also includes an inspection step for the manufactured workpiece. After step (c), an internal thread blank hole damage inspection is performed, and the steps are as follows: C-100. Take 3 points at the edge of the blank hole to measure the Vickers hardness; C-200. Calculate the damage value, and if the damage values of all measured points do not exceed 0.62 and the standard deviation ≤ 0.03, it is judged as qualified.

2. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, It also includes a rough upset flow line inspection after step (d), and the steps are as follows: d-100.截取 the shaft forming section sample for metallographic corrosion; d-200. Microscope photograph the flow line image of the axial longitudinal section; d-300. If the flow line is continuous and the angle with the axis ≤ 10°, without crossing or backflow, it is judged as qualified.

3. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, A temperature field scan is performed after step (e), and the steps are as follows: e-100. Infrared thermal imager scans within 0.5 seconds after the workpiece exits the die; e-200. Analyze the region through image division: The analysis region includes the top surface of the square, the tooth blank transition region, and the surrounding of the inner hole, and calculate the maximum temperature difference and the proportion of the high-temperature region in each region respectively; If the maximum temperature difference ≤ 160°C and the proportion of the high-temperature region area ≤ 5%, it is judged as qualified.

4. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, A dimension and load inspection is performed after step (f), and the steps are as follows: [[ID= ​ 5. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, ​ ​ ​ ​ ​ 6. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, ​ 7. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, Step (a) also includes the pretreatment of raw materials: the raw materials are 25CrMo4 alloy steel and are annealed and softened; the surface is phosphated to form a 5-8μm lubricating layer, and the blank diameter tolerance is controlled within ±0.05mm during cutting.

8. The multi-station cold heading method for a gear shaft according to claim 1, characterized in that, The cold heading equipment adopts a 6-station cold heading machine; the main die stroke accuracy is ±0.01mm; the forming speed range is 15-25mm / s.

9. A multi-station cold heading method for a gear shaft according to claim 1, characterized in that, The die cavity is made of VA70 steel with a hardness ≥62HRC; the punch and punch head are made of SKH51 steel with a hardness ≥60HRC; the surface roughness of the die cavity Ra≤0.2μm.

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