Introduction
In various fields such as automobiles, machinery, and aerospace, gear shafts are an integral component of many manufacturing equipment. However, in smaller-scale production enterprises, due to the decentralization of machinery and equipment used and lower productivity, there are often process defects in gear honing. Therefore, it is necessary to improve and innovate the processing technology according to design requirements to enhance honing quality, avoid situations such as unqualified pre-inspection and grinding wheel cutting, improve the overall processing level, and achieve desired process application results. This article analyzes the technical principles of automobile gear honing, introduces the characteristics of gear honing technology combined with practical cases, and proposes two process improvement methods to address common defects in automobile gear honing.
1. Technical Principles of Automobile Gear Honing
In the automobile manufacturing industry, the application of powerful gear honing is relatively widespread. Taking the MQ and DQ series transmissions of Volkswagen Germany as examples, their high-speed gears mostly use a combination of honing and grinding for transmission. The transmission is one of the main components in an automobile, and its transmission performance has a direct impact on the overall movement effect of the vehicle. If the transmission has good transmission performance, and the transmission as a whole has characteristics such as low noise, strong impact resistance, and high transmission speed, then gears with high precision are required. At this stage, powerful gear honing and worm wheel grinding processes are mostly used to improve transmission performance. However, due to the complex structure of individual gears and the economic disadvantages of worm wheel grinding, the application frequency of powerful gear honing is higher. The main principle of its processing technology is internal meshing of staggered axes gears. During processing, the honing wheel is installed on the outside, positioned at the honing head of the machine tool, and the workpiece being processed is installed on the inside, positioned on the spindle. A certain angle, known as the axis intersection angle, is reserved between the center of the honing wheel and the center of the workpiece.
It can be seen that the axis intersection angle is the sum of β1 and β2, where β1 and β2 are the helix angles of the workpiece and the honing wheel, respectively. The gear of the part and the grinding wheel not only have radial cutting forces but also the grinding wheel can move in a straight line cyclically along the axial direction, making axial and radial cutting cooperate with each other to form a complex staggered network surface, which makes the gear meshing more stable and extends the service life of the component.
2. Causes and Handling Methods of Defects in Automobile Gear Honing
2.1 Unqualified Pre-inspection
Pre-inspection has high requirements for the accuracy of workpieces to be processed. However, during the processing of workpieces, some impurities inevitably adhere to the surface and sometimes stick to the inspection wheel. In this case, high-pressure air guns can be used for cleaning. If the inspection wheel is severely worn, it will also affect the pre-inspection results. This issue can be resolved by replacing the inspection wheel. Additionally, due to the accumulation of deviations in the previous few processes, some workpieces to be processed have poor quality, which can also lead to unqualified pre-inspection. In this situation, whether the workpiece can be used can be judged based on the cross-bar distance size and pre-inspection runout value. If the radial runout value exceeds the set standard or is close to the lower limit, it should be scrapped. Otherwise, it can be processed and improved before continuing to be used. For workpieces with thinner tooth thickness, such workpieces are prone to becoming waste products and basically have no processing significance, so they can be scrapped directly. If the tooth thickness of the workpiece is thicker, the honing tooth top position may interfere with the tooth root during processing, thereby increasing the risk of honing wheel tooth breaking. Therefore, these also need to be scrapped.
2.2 Honing Wheel Tooth Breaking
The material of the honing wheel is relatively fragile and prone to tooth breaking during processing, mainly in the form of whole-circle tooth breaking. The main reasons are as follows. On the one hand, the hob design is not reasonable. Hobbing is carried out before honing. At this time, if the product processed by hobbing does not match the honing process, such as too small convex corner design, a large step will be generated at the root of the involute of the honed workpiece, leading to honing wheel tooth breaking. To address this, optimizations can be made during the hob drawing design stage, such as appropriately enlarging the hob convex corner. On the other hand, the grinding wheel parameters are not set accurately. During the honing wheel correction process, if the corrected tooth length is too long, the tooth top will inevitably contact the product root during processing, causing tooth breaking. This can be resolved by adjusting the tooth surface parameters and appropriately reducing the value of “tooth length correction” in the correction tool.
3. Case Analysis of Automobile Gear Honing Process and Defect Improvement
3.1 Process Characteristics
Taking the transmission gears of MQ200 and MQ250 as examples, the processing forms of the two are basically the same, mainly involving hobbing and chamfering, but there are differences in specific processing techniques. For example, MQ200 uses integrated hobbing and chamfering technology, which effectively eliminates high points on the chamfer through coarse hobbing, chamfering, and finish hobbing, thereby improving processing efficiency. MQ250 adopts a welding process before heat treatment, combining the gear, gear seat, and gear ring to form the entire shift gear. The advantage of the process adopted by MQ250 lies in its lower cost, but the disadvantage is that the combined gear ring has no subsequent processing, which can easily lead to deformation after heat treatment. The characteristic of the honing process is the addition of a “hard turning” process, which means that after heat treatment, the mating surface is turned to meet the accuracy requirements. The advantage of this method is that the gear ring does not deform under the action of heat treatment, allowing the gear seat and gear ring to fully mate. The disadvantage is that the use of this process increases the input cost.
3.2 Defect Issues
In the field of precision processing of automobile gears, gear honing is one of the key links, and it has strict requirements on the net size of parts before honing. Therefore, before formal processing, a pre-inspection system is used to conduct bidirectional rolling tests on gear parts and pre-inspection wheels, relying on the center distance to predict whether eccentric radial runout, high points, etc., are within a reasonable range. During mass production, compared to gear grinding, the incidence of defects in gear honing is higher, such as unqualified pre-inspection, grinding wheel tooth breaking, etc. The specific causes are as follows.
3.2.1 Unqualified Pre-inspection
The main reason for this issue is that there are high points on the tooth surface, which deform after heat treatment, thereby increasing the radial runout and causing excessive or insufficient processing allowance. The specific manifestations are that the unqualified rates of MQ200 and MQ250 are 0.2% and 30%, respectively.
3.2.2 Grinding Wheel Cutting
The main reasons for this issue are excessive cutting forces during processing, low equipment accuracy, and quality issues with cutting tools. The specific manifestation is abnormal damage to the grinding wheel.
3.2.3 Dark Skin on Tooth Surface
The main reasons for this issue are insufficient processing allowance, low equipment accuracy, and uneven deformation after heat treatment. The proportion of defects caused by this issue is approximately 0.1%.
3.2.4 Non-standard Dimensions
The main reasons for this issue are low equipment accuracy, quality defects in cutting tools, and uneven deformation after heat treatment. The proportion of defects caused by this issue is approximately 0.1%.
3.3 Improvement Methods
According to the above, the main deficiency of the MQ250 transmission lies in unqualified pre-inspection, with a defect unqualified rate of around 30%. The main cause is high points on the tooth surface, which leads to inevitable formation of high points on the end face and tooth surface during extrusion chamfering. For high points on the end face, a burr cutter can be used to remove them, but it is still difficult to remove high points on the tooth surface. Generally, during mass production, for unqualified pre-inspection, repair measures need to be taken. For more severe high points, technicians can manually remove them. For less severe high points, the pre-inspection range can be appropriately increased to make unqualified parts meet the pre-inspection standards, but this may lead to the situation of honing wheel tooth breaking. In response, this article proposes two improvement solutions to effectively eliminate the drawbacks of this process.
3.3.1 Method One: Pre-heat Treatment Tooth Shaving to Assist in Removing High Spots
For the pre-inspection failure issue caused by high spots on the tooth surfaces of MQ250 transmissions, the first improvement method involves using a tooth shaving device to assist in removing these high spots before heat treatment. The specific operational steps are as follows:
- Hobbing and Chamfering: Initially, hobbing and chamfering processes are carried out to form the preliminary gear shape.
- Tooth Shaving to Remove High Spots: Subsequently, a tooth shaving machine is utilized to further process the tooth surfaces, slowly removing the burr high spots on both sides of the teeth resulting from the chamfering process. This step is not completely equivalent to traditional tooth shaving but serves as a pre-treatment means before finishing.
- Pre-inspection and Heat Treatment: After tooth shaving, the parts are inspected to ensure that the high spots on the tooth surfaces have been completely removed. Following this, heat treatment is performed to strengthen the gear material.
- Honing: Finally, honing is conducted to achieve the required precision and surface quality.
Practical results indicate that after adopting this method, the pre-inspection failure rate of MQ250 transmissions significantly decreased from the original 30% to 1%. This fully demonstrates the effectiveness of tooth shaving equipment in removing high spots on tooth surfaces.
3.3.2 Method Two: Pre-heat Treatment Tooth Shaving Rough Machining to Optimize Blank Quality
The second improvement method further optimizes the blank quality and machining allowance before honing. Specific measures include:
- Tooth Shaving Rough Machining: Prior to heat treatment, tooth shaving is employed to roughly machine the tooth surfaces, removing most of the allowance and initially forming the gear shape.
- Hobbing, Chamfering, and Heat Treatment: Following this, hobbing and chamfering processes are carried out, followed by heat treatment to further strengthen the gear material.
- Honing Finishing: Lastly, honing finishing is conducted to meet the required precision and surface quality.
Compared to Method One, Method Two makes greater efforts in optimizing the honing allowance and improving blank quality. Through tooth shaving rough machining, the machining allowance during honing can be effectively reduced, thereby lowering the risk of honing wheel breakage. Additionally, since the tooth surfaces are relatively smooth after tooth shaving, the cutting force and cutting time during honing can be further reduced, enhancing production efficiency.
Furthermore, Method Two also improves processing quality by optimizing gear parameters. Issues such as tooth profile angle, crown amount, and tooth orientation angle deviation can be corrected through heat treatment deformation. This approach not only enhances the quality of the pre-honing blank but also reduces variations in cutting force and abnormal pre-inspection results caused by deformation.
Comparison Between the Two Methods
A comparison of the two methods in terms of quality, efficiency, and cost is as follows:
- Quality: Both methods can effectively address the pre-inspection failure issue caused by high spots on tooth surfaces and improve the quality of honing processing. However, Method Two performs better in optimizing blank quality and reducing the risk of honing wheel breakage.
- Efficiency: After using tooth shaving to assist in removing high spots in Method One, the honing efficiency is not significantly affected. In contrast, Method Two further improves production efficiency by reducing the honing allowance and cutting time.
- Cost: Method One can be implemented using old tooth shaving equipment, resulting in lower tooling costs. While Method Two requires additional tooth shaving equipment and tooling costs, considering the benefits of increased production efficiency and reduced honing wheel breakage, the overall cost remains manageable.
In summary, although both methods can effectively address the pre-inspection failure issue in MQ250 transmissions, Method Two exhibits superior performance in terms of quality, efficiency, and cost. Therefore, in actual production, enterprises can select the appropriate method based on their own circumstances and needs for implementation.