Quality Enhancement in Small Lock Pin Fabrication Processes
We analyzed the causes of inconsistent quality and low production efficiency in small load-bearing lock pins. To address these issues, we developed a composite milling-turning machining solution. By simplifying the process, adjusting the method used for machining spiral grooves, and implementing a slotted assembly and clamping tool, we significantly improved both the quality of the pins and the processing efficiency. These changes eliminated the factors that previously contributed to low efficiency in the assembly and commissioning of small load-bearing locks.
01 Introduction
Small load-bearing locks are commonly used to secure aircraft cabin door covers. These locks are composed of several components: a lock housing, a lock washer, a pin, a bushing, a spring, and a cover. They offer several advantages, including easy opening, reliable vibration resistance, a high load-bearing capacity, and excellent sealing performance.
The pin is a crucial part of these locks; by rotating it, the lock can be opened and closed. However, currently, the pins are manufactured using traditional methods, which results in inconsistent quality and low production efficiency. This significantly affects the assembly and commissioning of small load-bearing locks. As the demand for these locks increases, traditional pin processing techniques are increasingly unable to meet the assembly and supply needs for small load-bearing locks.
02 Problem Analysis
The small load-bearing lock pin is constructed from high-strength stainless steel, specifically 1Cr15Ni4Mo3N. Figure 1 illustrates the pin’s dimensions and spiral groove structure. The pin functions as a rotating body, characterized by a flat tail featuring double spiral grooves and a straight head. The dimensions of the pin’s components are as follows: the center rod has a diameter of 5 mm, the tail has a diameter of 9 mm, and the flat tail measures 5 mm in width. Additionally, the tail contains two spiral grooves that form a 90° angle and allow for a travel distance of 12 mm with a 6 mm pitch.
The traditional process for lock nail processing is: material inspection → group cutting → turning the nail body (nail head, intermediate shank, and tail ball) → lathe cutting to ensure the total length → milling the two flat surfaces of the tail → milling by a fitter → milling of the slot → milling by a fitter → filing of the two spiral grooves on the tail → filing of the four edges → cleaning → heat treatment → sandblasting → magnetic particle inspection → surface treatment → finished product inspection → storage.
The following are the problems and causes of this processing:
1) External machining is carried out using standard equipment such as hexagonal lathes and horizontal milling machines. These processes are fragmented, the machining is straightforward, and overall efficiency is low. Additionally, multiple clamping operations can easily result in large geometric tolerances, which can hinder the assembly of small load-bearing locks.
2) The two spiral grooves on the tail are created manually, making the quality of the processing highly dependent on the operator’s experience. Since a large number of lock nails are produced in batches, the depth of the spiral grooves can vary significantly, with one side being deeper than the other. When assembling small load-bearing locks, if the spiral groove in the locking pin is shallow, it increases the torque required to open and close the lock. This higher torque necessitates greater force during load-bearing lock testing. In some cases, additional measures may be required to reduce the opening and closing torque to comply with specified requirements, which can negatively affect the efficiency of lock assembly and commissioning.
3) Slotted slots are produced using traditional single-piece processing methods, resulting in low processing efficiency.
03 Process Improvement and Optimization Measures
To improve quality consistency and efficiency in lock nail processing, we implemented various process enhancements and optimizations. These included improving the process flow, refining spiral groove processing methods, and enhancing the design of slotted slot assembly and clamping fixtures. These changes aimed to boost product quality and processing efficiency.
3.1 Simplified Process Flow
Two CNC lathes equipped with milling capabilities have been introduced. By utilizing their combined turning and milling functions, these machines can process two flat surfaces, create two spiral grooves, round four corners, and manufacture the lock nail body all in a single assembly.
The improved process flow is as follows: Material inspection → Group cutting → Milling and turning of the outer shape (nail head, shank, ball head, two flat surfaces, two spiral grooves, and four rounded corners) → Lathe cutting to ensure total length → Milling deburring by benchwork → Milling of slotted slots → Milling deburring by benchwork → Cleaning → Heat treatment → Sand blasting → Magnetic particle inspection → Surface treatment → Finished product inspection → Warehousing.
3.2 Spiral groove processing method
A spiral groove is a helical structure machined onto the surface of a cylindrical workpiece. Due to its complex shape, it cannot be processed using conventional three-axis CNC machine tools; instead, it is typically manufactured using a four-axis linkage CNC machining center.
CNC lathes equipped with milling capabilities are adaptations of traditional three-axis lathes that include an auxiliary power head, allowing them to perform both turning and milling operations. The milling motion for creating a spiral groove involves several coordinated movements: the power head rotates the milling cutter, the spindle rotates the workpiece, and the milling cutter moves along the axis of the workpiece. It is crucial for the workpiece’s rotation and the axial movement of the milling cutter to be synchronized according to the specifications of the spiral groove.
For this process, a solid carbide three-edge end mill with a diameter of 3 mm is used, clamped onto the power head. The locking pin rotates 90° while the end mill advances 3 mm in its axial movement. The domestic ETC3650h CNC lathe is selected for this task, operating with a power head speed of 2000 rpm and a feed rate of 0.05 mm/revolution. The state of the spiral groove processing is illustrated in Figure 2.
3.3 Slotted Slot Assembly Clamping Fixture
To enhance the efficiency of slotted slot machining on the heads of locking pins, a group processing method was implemented. A specialized clamping fixture, designed for slotted slot assembly (as shown in Figure 3), can clamp up to 20 locking pins at once. The fixture is designed to accommodate the shape of the locking pin head, allowing the flat structure at the tail of the locking pin to be inserted through the fixture slot, thereby controlling the direction of the locking pin.
Two pressure plates are used to secure the upper end of each locking pin in place, with these plates fastened to the fixture body using hexagon socket head screws. An electric torque wrench is utilized to quickly tighten and remove these screws, significantly reducing installation time. The fixture includes three pressure plates, with two adjacent plates working together to clamp the locking pins. This arrangement creates two processing lines, each capable of accommodating 10 locking pins.
When 4 axis machining on a conventional milling machine, two saw blades are placed in the toolholder simultaneously. The distance between the saw blades and the cutters is adjusted to the appropriate position, allowing for the simultaneous milling of all slotted slots on the locking pins.
04 Process Improvement Results
To address the issues of “poor product quality consistency” and “low production and processing efficiency” for lock nails, we implemented optimizations and improvements in three key areas: the process flow, the spiral groove machining method, and the assembly and clamping fixture for the slotted screws.
Significant results were achieved as a result of these improvements. The original four contour machining steps—turning the nail body, milling the two flat surfaces at the tail, filing the two spiral grooves at the tail, and filing the four edge fillets—were combined into a single process. This change reduced the contour machining time (excluding the slotted screws) from 63.4 hours to 35 hours for a production batch of 1,000 pieces, resulting in a 45% increase in efficiency.
Additionally, the machining time for the slotted screws was reduced from 16.7 hours to 12.5 hours for the same production batch, reflecting a 25% improvement in efficiency. The consistency of the spiral groove depth was also enhanced, with a deviation of only 0.02 mm. The spiral groove depth dimension was machined to the maximum possible depth, which significantly reduced torque during the assembly and commissioning of small load-bearing locks, leading to smoother opening and closing motions.
A comparison of the results before and after the process improvements can be found in Table 1, while Figure 4 illustrates the processing effects of the locking screw spiral groove before and after the improvements.
05 Conclusion
We analyzed the causes of existing issues in the machining of small bearing lock pins and implemented process improvements to address them. By simplifying the process, adjusting the machining method for the spiral groove, and using a slotted assembly fixture, we achieved outstanding results. This involved turning the pin’s rotating surface, milling the tail surface, spiral groove, and edge radius in a single clamping operation on a CNC lathe with milling capabilities. These changes effectively resolved problems related to poor quality consistency and low machining efficiency, and they eliminated factors that contributed to the inefficiencies in the assembly and commissioning of small bearing locks.
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