1.What is Planetary Gear Ring Gear
Planetary gear ring gear is one of the core components in automatic transmission AT, dual-clutch transmission DCT, hybrid/electric vehicle reduction mechanism, and belongs to the key components of planetary gear set. It together with the sun wheel and planetary wheel constitute a planetary gear mechanism, which realizes multi-gear speed variation, torque amplification or power diversion, which directly affects the transmission efficiency, smoothness and reliability of the gearbox.
2.Pain Points of Ring Broaching Processing
Combined with the structural characteristics of the planetary gear ring of the automobile gearbox (internal teeth, thin wall, high precision requirements) and the characteristics of the broaching process, the following core pain points are sorted out through on-site production investigation, and the targeted basis for scheme design is provided:
2.1 Quality pain points
- Tooth shape error exceedance: mostly caused by unreasonable tooth shape design of broach, tool vibration during broaching, workpiece clamping eccentricity, manifested as tooth shape angle deviation and uneven tooth thickness, affecting the meshing accuracy of planetary wheels and gear rings, which in turn causes the noise of the gearbox to increase and the transmission efficiency decreases;
- Tooth surface strain and scratches: The tooth ring material is highly plastic, the cutting temperature is too high during broaching, the chips are bonded to the tooth surface, or the broach edge is worn and there are accumulated edges, resulting in strain and scratches on the tooth surface, reducing the wear resistance and fatigue life of the tooth surface.
- Burrs exceed the standard: In the broaching finishing stage, the roots and tops of the teeth are prone to burrs, and if the subsequent cleaning is not thorough, the burrs are prone to fall off after assembly, resulting in gear jamming and aggravated
2.2 Efficiency and cost pain points
- Too fast broach wear: The ring material (such as 20CrMnTi) has high hardness (HRC28-32) after quenching and tempering, and the cutting resistance is high during broaching, and the cutting edge of the tool is easy to wear and crumble, resulting in frequent replacement of broaches, increasing tool costs, and affecting production continuity.
- Low processing efficiency: traditional broaching parameters are conservative, and the tooth lift and broaching speed are not reasonably set, resulting in a single piece processing time that is too long to meet the needs of mass production;
- High scrap rate: Due to the ineffective solution of quality pain points and insufficient process control, some gear rings are scrapped due to problems such as tooth shape deviation and tooth surface strain, increasing production costs.
2.3 Equipment and operation pain points
- Decrease in equipment accuracy: After long-term use of the broaching machine, the guide rail wears out and the spindle runout increases, resulting in the relative position shift of the tool and the workpiece during the broaching process, affecting the machining accuracy.
- Inaccurate clamping and positioning: Existing clamping methods (such as chuck clamping) are prone to workpiece eccentricity, positioning reference deviation, and the clamping force cannot be accurately controlled, resulting in poor machining consistency.
- Uneven operator skills: Some operators lack skills in broaching parameter adjustment, tool maintenance, and exception handling, which can easily lead to quality problems and safety hazards due to operational errors.
3.Full-process Broaching Solution
The solution is a CNC pull-up machine tool for machining internal spiral splines and internal helical gears. The main motion servo motor drives the main slider through a high-precision ball screw to achieve the workpiece+linear motion, and the tool rotation motion servo motor realizes the spiral broach rotation motion through the heavy-duty high-precision turbo worm pair.
The parameters of the main movement and tool rotation are set through CNC programming, which can easily adapt to the processing of parts such as internal spiral splines and internal helical gears with different leads and different rotation directions.
3.1 Optimized workpiece pretreatment
The pretreatment quality before ring broaching directly affects the broaching effect, and it is necessary to focus on controlling the following links to reduce the hidden dangers of subsequent processing:
- Material pretreatment: For alloy structural steel gear rings such as 20CrMnTi, optimize the quenching and tempering treatment process to ensure uniform hardness of the gear ring (HRC28-32) and hardness deviation ≤ 2HRC, so as to avoid excessive hardness increasing broaching resistance, aggravating tool wear, or low hardness leading to easy strain and deformation of the tooth surface; At the same time, the grains are refined through normalization treatment to improve the cutting performance of the material and reduce the plastic deformation during the broaching process.
- Reference surface processing: Before broaching, the end face and internal hole positioning reference of the gear ring should be accurately machined to ensure that the verticality of the end face and the inner hole is ≤ 0.01mm, the inner hole size tolerance is controlled at H7 level, and the surface roughness Ra≤ is 1.6μm to avoid clamping eccentricity caused by positioning reference deviation; After the datum is processed, iron filings and burrs should be cleaned to prevent iron filings from affecting the clamping accuracy;
- Workpiece cleaning: Before broaching, use anhydrous ethanol to wipe the positioning surface and inner hole of the gear ring to remove surface oil and dust, avoid clamping slippage and inaccurate positioning caused by oil and dust, and reduce the bonding between chips and tooth surfaces, reducing the risk of tooth surface strain.
3.2 Optimization of broaching process parameters (core link)
Combined with the material, specification and machining accuracy requirements of the gear ring, the broaching parameters are optimized through orthogonal tests, and the balance point of “precision-efficiency-cost” is found to replace the traditional conservative parameters, and the specific optimization measures are as follows:
3.2.1 Broaching speed optimization
The broaching speed directly affects the cutting temperature, tool wear and processing efficiency: if the speed is too fast, the cutting temperature will rise sharply, which is easy to cause edge accumulation and tooth surface strain, which will aggravate tool wear. The speed is too slow, the processing efficiency is low, and the chips are easy to bond to the cutting edge.
Optimization scheme: adjust the broaching speed according to the gear ring material, the specific parameters are as follows:
- 20CrMnTi (HRC28-32): The broaching speed has been optimized from the traditional 3-5m/min to 5-7m/min, which not only improves the machining efficiency but also avoids excessive cutting temperature;
- 45# steel (HRC22-26): The broaching speed is optimized to 6-8m/min, which takes advantage of the good cutting performance of the material to further improve efficiency;
- Thin-walled ring (wall thickness ≤5mm): The broaching speed is appropriately reduced, controlled at 4-6m/min, reducing broaching force and vibration, and avoiding workpiece deformation.
3.2.1 Broaching speed optimization
The broaching speed directly affects the cutting temperature, tool wear and processing efficiency: if the speed is too fast, the cutting temperature will rise sharply, which is easy to cause edge accumulation and tooth surface strain, which will aggravate tool wear. The speed is too slow, the processing efficiency is low, and the chips are easy to bond to the cutting edge.
Optimization scheme: adjust the broaching speed according to the gear ring material, the specific parameters are as follows:
- 20CrMnTi (HRC28-32): The broaching speed has been optimized from the traditional 3-5m/min to 5-7m/min, which not only improves the machining efficiency but also avoids excessive cutting temperature;
- 45# steel (HRC22-26): The broaching speed is optimized to 6-8m/min, which takes advantage of the good cutting performance of the material to further improve efficiency;
- Thin-walled ring (wall thickness ≤5mm): The broaching speed is appropriately reduced, controlled at 4-6m/min, reducing broaching force and vibration, and avoiding workpiece deformation.
3.3 Tool optimization (improving machining accuracy and tool life)
Broach is the core tool of ring broaching, the tooth shape design, material selection, edge treatment of the tool directly affect the processing quality and tool life, for the traditional broach existing problems such as fast wear and low tooth shape accuracy, the following optimizations are carried out:
3.3.1 Optimization of broach materials and coatings
- Material selection: Traditional broaches are mostly made of high-speed steel (W18Cr4V), which is optimized as high-performance high-speed steel (W6Mo5Cr4V2Al) for the characteristics of high hardness of the gear ring material, which can reach HRC64-66, and the wear resistance and toughness are 20%-30% higher than that of traditional high-speed steel, which can effectively reduce tool wear and chipping. For mass production scenarios, coated carbide broaches (TiAlN coating) are used, and the tool life can reach 5-8 times that of high-speed steel broaches, further reducing the cost of tool consumption.
- Coating treatment: The coating thickness is controlled at 3-5μm, and the coating hardness is ≥ HV2000, which can significantly improve the wear resistance and lubricity of the tool surface, reduce the bonding between chips and the tool, reduce the risk of tooth surface strain, and extend the tool life by more than 30%.
3.3.2 Tooth shape and structure optimization of broach
- Tooth shape design: according to the tooth shape requirements of the gear ring (involute tooth shape), optimize the tooth shape angle and tooth top arc radius of the broach, the tooth shape angle deviation is controlled at ±0.005°, and the tooth top arc radius matches the tooth root arc radius of the tooth ring (reserve a gap of 0.01-0.02mm) to avoid root stress concentration and reduce the burr generation of tooth top; The tooth surface of the fine broach is polished, and the surface roughness Ra≤2μm is reduced to reduce the friction between the tool and the tooth surface and avoid tooth surface strain;
- Edge treatment: The cutting edge of the broach is chamfered (chamfer width 0.02-0.03mm, angle 15°-20°) to avoid edge chipping and reduce the occurrence of edges; The edge edge of the rough broaching cutter adopts a negative rake angle (-5°–3°) to improve the rigidity of the tool and reduce wear; The cutting edge of the fine drawing tool adopts a positive rake angle (3°-5°) to enhance the cutting sharpness and improve the quality of the tooth surface;
- Structure Optimization: For thin-walled ring broaching, optimize the diameter and number of teeth of the broach to increase the rigidity of the broaching tool and reduce tool vibration during broaching. A guide mechanism is set at the end of the broach to ensure that the tool is coaxial with the gear ring during the broaching process to avoid tooth direction error; Internal broaching adopts an integral broach to ensure the rigidity of the tool body and reduce the impact of vibration on machining accuracy.
4.Process Advantages
- Ultra-high efficiency: Processing a complete cagger in a single stroke, with high-speed indexing, extremely short cycle time, is the first choice for high-volume production (annual output of hundreds of thousands of pieces).
- Superior consistency: All coggings machined with the same broach have a high degree of consistency, guaranteeing the NVH performance of the gearbox.
- Good accuracy: Stable accuracy classes required for automotive gears (e.g. DIN 7-8).
- Excellent surface quality: Fine cutting and calibration teeth provide a smooth tooth surface, which is beneficial for subsequent break-in and reduced noise.
- Suitable for complex tooth profiles: Shaped teeth (drum teeth, tooth end thinning) can be machined with broach design to optimize engagement and noise.
- Mature and reliable process: It has been used in the automotive industry for many years, and the equipment, tools, and process control are mature.
5.Implementation Effect Prediction
Through the full implementation of this plan, the whole process is optimized for the core pain points of ring broaching processing, and it is expected to achieve the following implementation effects to meet the high-precision and high-efficiency production requirements of planetary gear rings of automobile gearboxes:
5.1 Quality effect
- Product qualification rate: from the existing 95% to more than 99.5%, reducing scrap loss and production costs every year;
- Machining accuracy: tooth shape error ≤0.015mm, tooth direction error ≤0.02mm, tooth surface surface roughness Ra≤1.6μm, burr height ≤0.03mm, fully in line with the technical requirements of automobile gearbox gear ring, improve the meshing accuracy of gear ring and planetary wheel, reduce gearbox noise, and improve transmission efficiency;
- Surface Quality: Completely solve the problems of tooth surface strain, scratches, etc., the tooth surface is smooth and uniform, improve the wear resistance and fatigue life of the gear ring, and extend the service life of the gearbox.
5.2 Efficiency and cost effect
- Machining efficiency: shorten the single-piece broaching time by 10%-15%, and increase the mass production efficiency by more than 10%, which can meet the needs of mass production and delivery;
- Tool life: The service life of broaches is increased by more than 30%, and the cost of tool consumption is reduced by 25%-30%, saving tool procurement costs every year.
- Comprehensive Cost: Reduced scrap rates, increased efficiency, and reduced tool consumption are expected to reduce comprehensive production costs by 15%-20% per year.
5.3 Stability and safety effects
- Production stability: the number of abnormal downtime during processing is reduced by more than 60%, and the quality fluctuation of mass production is controlled within ±0.005mm, ensuring product consistency;
- Operational safety: Standardized operating procedures and protective measures can completely prevent potential safety hazards such as workpiece flying out and tool collapse, and achieve safe production without safety accidents.
- Personnel skills: The professional skills and sense of responsibility of operators have been significantly improved, and they can independently complete parameter adjustment, tool maintenance, exception handling, etc., reducing the dependence on technicians.
Epilogue
The core difficulty of the broaching process of the planetary gear ring of the automobile gearbox lies in the balance of “high precision, high efficiency and low loss”, and this scheme is aimed at the common quality, efficiency and cost pain points in the industry, from the preliminary preparation, process parameters, tools, clamping positioning, quality control, operation specifications, personnel training and other whole processes to systematically optimize, combined with the existing production conditions, put forward practical solutions, without large-scale transformation of equipment, can be quickly implemented.
After the implementation of the scheme, it can significantly improve the machining accuracy and product qualification rate of ring broaching, shorten the machining cycle, reduce tool consumption and comprehensive production costs, and ensure production safety and stability, providing strong support for the high-quality production of automotive gearboxes. In the later stage, through continuous maintenance and optimization, it can further adapt to the iterative needs of products, enhance the core competitiveness of enterprises, and help enterprises achieve the goals of cost reduction, efficiency increase and high-quality development.
