Optimized Turn-Mill Processes for Stainless Steel Connector Production
This research focuses on the automated production of pipe fittings, with a particular emphasis on the functional design of automated production equipment tailored to the characteristics of the products. A clear operational flow for the production unit was developed, and orthogonal experiments were conducted to continuously optimize the production process parameters, resulting in efficient manufacturing of pipe fittings. Additionally, a rapid changeover and line-switching mechanism was established to enable the production unit to handle multiple product applications. This approach provides valuable production insights for the automated manufacturing of spacecraft components, which are typically characterized by a high variety of products, small batch sizes, and stringent precision requirements.
1. Introduction
Pipe fittings are essential components that connect conduit assemblies in various spacecraft systems. Their quality and performance are crucial for ensuring the functionality, sealing, and safety of each system. As the aerospace industry continues to evolve, the demand for diverse and numerous pipe fittings has significantly increased. Traditional discrete CNC machining methods can no longer meet the varied and fast-paced requirements for these products, which makes flexible automated production a necessary option.
2. Analysis of Pipe Fitting Product Characteristics
There are 37 types of plunger fittings, which have diameters ranging from 16 mm to 32 mm and lengths between 32 mm and 60 mm. Additionally, there are 8 types of threaded fittings, with diameters from 28 mm to 35 mm and lengths ranging from 50 mm to 59 mm. Finally, there are 36 types of outer sleeve nuts, with diameters varying from 28 mm to 37 mm and lengths from 19 mm to 23.5 mm. An example of a typical pipe fitting product is illustrated in Figure 1.
This article uses a standard plunger connector as an example. The product specifications require an outer diameter of either ϕ13mm or ϕ8mm. Additionally, the sealing groove dimensions should be ϕ10×2.7mm, the parallel groove dimensions must be 6mm×9mm, and the through-hole size should be ϕ6×58mm. It is important to note that the product has high precision requirements. A depiction of the plunger connector can be found in Figure 2.
3. Product Development Process
The product has stringent requirements for raw materials and dimensional accuracy. To ensure the quality of the raw materials, flaw detection processes are conducted both when materials arrive and after they are unloaded. Additionally, to control product deformation, both roughing and finishing processes are utilized. After processing, an endoscopic inspection of the inner wall of the product is performed. A typical process flow for pipe fitting production is illustrated in Figure 3.
Based on the product processing elements, the processing schemes for each size of the product are refined. The outer circle and sealing groove are machined by turning, the parallel groove by milling, and the through holes by drilling and reaming.
4. Functional Design of Automated Production Equipment
The pipe fittings share similar dimensions and specifications and fall under the category of small shaft-type metal components. The primary manufacturing process involves turning, with milling used as a secondary operation. The raw materials consist of slender bars that are well-suited for automated processing, utilizing a top-loading mechanism for feeding, in-machine cutting, and reversing clamping.
The equipment features a dual-spindle design, which enhances processing efficiency and enables automatic clamping within the machine. By incorporating automatic feeding and receiving devices, the system allows for seamless loading and unloading of products. A standardized procedure and automated CNC program have been established to facilitate an efficient processing flow, which includes the following steps: “feeding → processing at station 1 → deburring → butt clamping (double clamping) → cutting → processing at station 2 → deburring → unloading.”
The automated production unit is equipped with a central control system that manages production tasks and automatically issues and retrieves CNC programs. The external appearance of the equipment can be seen in Figure 4.
The machining process for this part involves several operations: turning, milling, drilling, reaming, and boring. It is important to analyze the functional requirements of the automated production equipment in relation to the size specifications of the product. This analysis is essential for the design and construction of the production equipment. A detailed overview of the functional analysis of the automated production equipment can be found in Table 1.
5. Automated Production Process Design
The automated production process begins with an automatic feeding device that fills the equipment with materials. Inside the equipment, automatic clamping, processing, and cutting are carried out under program control. After the processing is complete, the product is automatically unloaded, and the cycle repeats. The quantity of material to be processed is set as a variable in the CNC program, while the number of program cycles is determined based on the production plan. The internal flow of the automated production cycle is illustrated in Figure 5.
The automated production process for the plunger joint involves the automatic feeding of parts in the logistics module and the unloading of finished components. In Channel 1, parts are fed to the spindle where several operations take place, including rough and finish turning of the front end, turning of the sealing groove, milling of the outer cylindrical groove, drilling of the front end hole, and rough turning of the rear end.
In Channel 2, rough and finish turning of the end is performed, followed by drilling, reaming, chamfering of the hole opening, and then automatic unloading and spindle cleaning. The use of dual spindles and dual channels allows for automatic part docking and simultaneous processing in both channels, thereby overcoming automation challenges related to workpiece turning and secondary clamping. Channels 1 and 2 process different prototype parts independently, significantly enhancing overall processing efficiency. The processing flow for the plunger joint is illustrated in Figure 6.
6. Optimization of Automated Production Parameters
6.1 Material Property Analysis
The pipe fitting material is 1Cr18Ni9Ti, which has a machinability rating of only 50% to 70% compared to that of 45 steel. This makes it a challenging material to machine, primarily due to the difficulties in breaking chips, which tend to become entangled and significantly hinder automated machining. Additionally, cutting tools wear out quickly when working with this stainless steel. The contact length between the chip and the rake face is approximately 65% to 70% that of carbon steel, leading to stress concentration near the cutting edge, which can easily result in tool breakage. Moreover, the cutting heat is also concentrated at the cutting edge, causing high temperatures, accelerated tool wear, and increased cutting forces.
6.2 Orthogonal Experimental Design of Cutting Parameters
Based on the machinability characteristics of stainless steel and the requirements for automated machining, cutting parameters for the plunger fitting have been established in three key areas: cutting time, chip morphology, and machine tool status. Cutting time is directly related to machining efficiency, chip morphology is essential for ensuring effective automated machining, and stable machine tool operation is crucial for maintaining dimensional accuracy and surface quality.
The design of cutting process experiments is typically categorized into two types: single-factor experiments and orthogonal experiments. Single-factor experiments change only one variable at a time while keeping others constant. While this approach is straightforward, it does not account for interactions between factors, resulting in independent outcomes. In contrast, orthogonal experiments enable multi-factor and multi-level analyses. This scientific and efficient method considers the interactions among factors, reducing the overall number of experiments needed while facilitating data processing and interpretation. For the study of cutting parameters related to a plunger joint, an orthogonal experimental design was implemented to explore the relationships and effects of cutting parameters on cutting time and machining outcomes.
The research analyzed how cutting parameters influence product cutting time, chip morphology, and machine tool condition. The selected influencing factors included cutting speed, feed rate, depth of cut, and dwell time. Drawing from commonly used stainless steel cutting parameters and consulting cutting parameter manuals for engineering applications, the following specifications were designed: cutting speed (v) = 40–60 m/min, feed rate (f) = 0.2–0.3 mm/rev, depth of cut (ap) = 0.7–1.2 mm, and dwell time (t0) = 0.2–0.3 seconds. An orthogonal experimental design was adopted, with the experimental scheme and results summarized in Table 2.
6.3 Orthogonal Experimental Analysis of Cutting Parameters
The orthogonal experiment conducted on the plunger joint examined the operational status of the machine tool, the morphology of chips produced, and the machining time. In automated production, it is essential to meet the requirements for fully automated machining. If chip entanglement or machine tool vibration occurs during the machining process, these requirements cannot be fulfilled.
Initially, machining parameters from groups 3, 4, 5, 7, and 9 were excluded from consideration. Subsequently, an optimal level and range analysis of the orthogonal experiment was performed. This range analysis helps identify the degree and nature of influence that each factor has on the target value.
Let Yjm represent the sum of the experimental indicators for level m of the j-th factor, while Kjm indicates the average value of Yjm. The range, denoted as R, is calculated as R = Kjmmax – Kjmmin. A larger range signifies a greater influence of the corresponding factor on the target value. Using the values of Kjm and the range R, the optimal level and combination for the j-th factor were determined. The results of the cutting time range analysis are presented in Table 3.
A range analysis revealed that the factors influencing cutting time, ranked from largest to smallest, are: depth of cut, cutting speed, feed rate, and dwell time. The optimal conditions for achieving the shortest machining time are as follows: cutting speed of 60 m/min, feed rate of 0.3 mm/r, depth of cut of 1.2 mm, and dwell time of 0.2 seconds.
This analysis shows a clear relationship among minimum cutting time, stable machine tool processing, and optimal chip condition. It demonstrates that optimizing a single factor alone is insufficient to meet actual processing demands, highlighting the need for multi-objective optimization of parameter combinations.
Firstly, optimizing the depth of cut to 1 mm is advantageous for achieving both optimal cutting time and favorable machine tool conditions. Secondly, setting the cutting speed to 40 m/min is crucial for machining stainless steel products, as excessively high speeds can negatively affect processing stability. Maintaining the feed rate at 0.3 mm/r and adjusting the dwell time to 0.3 seconds can enhance feed rate and improve chip breaking during the machining of stainless steel, ensuring stable automated operations.
By constructing automated production units and optimizing the process flow, while replacing manual operations with automated feeding and unloading, production efficiency has been significantly enhanced. Table 4 presents a comparison of pipe fitting production efficiency.
The calculations indicate that the automated unit is capable of automatic feeding, continuous turning, and milling, which reduces the need for clamping and turnaround steps. As a result, the processing time for each product is cut down from 115 minutes to just 57 minutes, effectively doubling production efficiency. Additionally, one operator can manage three machine tools at the same time, boosting processing efficiency by over five times.
7. Conclusion
1) A product development process flow was established by focusing on the characteristics of the materials, structure, and dimensional accuracy of pipe fitting products. The functional design and construction of automated production equipment were also completed. Additionally, a unit operation flow for product processing was created, allowing for the processing to be completed in a single clamping operation.
2) Orthogonal experiments were conducted during production to continuously optimize process parameters, resulting in the efficient production of pipe fitting products. Compared to traditional processing methods, the combined turning and milling automated machining significantly increases processing efficiency by more than five times.
3) Actual production results indicate that the combined turning and milling automated machining approach for producing complex, high-precision pipe fitting parts is feasible. This scheme can serve as a reference for automated production enterprises focused on aerospace products, which are characterized by small-batch, multi-variety, and high-precision manufacturing.
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