The front axle of a vehicle is a critical load-bearing component that connects the wheels to the frame or body through the suspension system. It is responsible for transmitting vertical, longitudinal, and lateral forces between the frame and the suspension. Additionally, it must withstand braking and lateral forces from the suspension, which makes it essential for high strength, fatigue resistance, and rigidity.
Currently, many automotive front axles use a lock pin structure for the main pin hole. This design offers advantages such as ease of assembly and replacement. However, during operation, the main pin hole and leaf spring seat experience the highest stress. In case of failure, the entire axle can be compromised, leading to serious consequences. According to bench test data, 50% of failures start at the lock pin hole. The lock pin structure can reduce the overall strength of the axle, and machining the intersection of these holes is challenging. Some models instead use an interference fit with press or freeze fitting of the kingpin, which improves strength but increases demands on precision and complicates part replacement.
To address these issues, a tapered main pin hole design was developed (see Figure 1). This design maintains the structural integrity of the axle while allowing easier part replacement. It requires a contact area of at least 70%, with surface roughness Ra ≤ 2.5μm and a maximum deviation of 0.5mm. This paper outlines the processing method for the tapered main pin hole.
**Equipment Selection**
The primary equipment used for processing the main pin hole includes a horizontal special boring machine (GHC-U200046) and a vertical machining center (VB-825A). However, due to the aging of the horizontal machine, integrating a CNC flat suspension device proved incompatible, making it unsuitable for precise taper hole machining. The vertical machining center, on the other hand, offers versatility in milling, drilling, and reaming, along with easy clamping, operation, and observation. It has X, Y, Z travel distances of 1400mm, 825mm, and 700mm, with a spindle speed of 4000rpm and a feed rate range of 1–5000mm/min. Therefore, the VB-825A was selected for this task.
**Tapered Main Pin Hole Processing Scheme**
Based on the VB-825A’s capabilities, a 1:10 taper reamer was chosen for the main pin hole. The process involves four stages: rough drilling, semi-finishing, finishing, and straight-hole reaming. After initial reaming, it was found that the surface roughness of the φ47+0.5 0mm hole did not meet the required standards, so a straight-hole process was added to ensure quality.
**Clamping Methods and Fixtures**
During machining, a double-pin positioning method was used, involving the leaf spring surface and two positioning holes. A hydraulic clamp pressed the back of the leaf spring, and an angle panel was installed to meet the rake and camber angles. To ensure coaxiality and avoid unevenness, the fixture was clamped once, and the workpiece was fixed on the machine table. The main pin hole and the end face were processed together to maintain verticality.
Due to the length of the first end face (350mm), large cutting forces during reaming could cause vibration. To improve surface quality, a V-shaped auxiliary support was added. Additionally, axial forces during reaming could affect roundness and contact area, so an auxiliary support was placed at the front of the first end to stabilize the process.
**Tool Requirements**
1. The roughing cutter must have sufficient hardness to prevent hard layers and ensure uniform residual material, with a surface roughness Ra < 6.3μm. The precision reamer must match the kingpin hole taper to meet pattern requirements. Both rough and fine reamers require tool life management, with replacements after 10 and 12 parts respectively.
2. The reamer should have chip grooves for efficient chip removal. With a flute spacing of 140mm and three teeth, the flutes are left-handed with a 12mm pitch and staggered arrangement. The cutting edge is also left-handed at a 30° angle.
3. To manage high cutting forces, the tool includes five cooling holes to ensure adequate cooling and improve machining quality.
**Cutting Parameters**
1. Coaxiality of the reamed hole was ensured by setting a 2mm allowance for rough drilling and 0.5mm for semi-finishing.
2. For rough drilling, an HDD1-145 U drill was used at 650rpm with a feed rate of 22mm/r.
3. Semi-finishing used a K23D1-280 reamer at 60rpm and 3mm/r.
4. Final reaming employed an HDD1-141 reamer at 60rpm and 0.5mm/r.
**Precautions**
1. The first three parts were inspected using coloring to check the contact area.
2. Clamping required clean surfaces and accurate alignment of positioning holes.
3. Strict adherence to process parameters and a three-check system was enforced.
4. Worn locating pins were replaced promptly, and defective parts were isolated.
5. Continuous supply of cutting fluid was necessary to remove chips and enhance surface finish.
**Conclusion**
By implementing the above methods, our company achieved a 99.85% qualification rate for the tapered main pin holes of Volvo axles. The coloring rate ranged from 83% to 95%, meeting customer expectations and establishing a strong reputation for quality and cost-effectiveness.
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