35CD4 high-strength steel is widely used in precision cylindrical parts of aerospace due to its excellent performance, but it has technical difficulties such as high hardness, high cutting difficulty, easy deformation of thin-walled parts, and strict tolerance requirements. In response to these technical difficulties, a lean machining scheme covering multiple processes was formulated by combining the material characteristics with the precision dimensions and special process requirements of the parts. By optimizing the parameters and tools of key links such as external turning, internal boring, oblique hole and bearing hole machining in the machining process, and adjusting the sequence of shot peening and honing, problems such as machining deformation and dimensional deviation were solved. Finally, stable mass production of parts was achieved, and the relevant process can provide a reference for the machining of precision parts of difficult-to-cut materials.
PART 1
Introduction
The performance breakthrough of aerospace equipment is highly dependent on the machining accuracy and reliability of core components[1]. High-strength steel 35CD4 has become an ideal material for precision cylindrical parts of aerospace due to its excellent characteristics such as high strength and high hardenability. However, the high cutting force and significant work hardening tendency of this material, combined with the irregular shape and easily deformable, and the stringent precision dimensional tolerances of the cylindrical parts, pose a significant challenge to traditional machining processes.
To overcome this production challenge, this paper focuses on the material properties of 35CD4 and the process requirements of cylindrical parts, and conducts research on lean machining processes. Through a series of measures, including optimizing process design, improving tool selection, and adjusting machining parameters, key issues such as machining deformation and dimensional deviations were systematically resolved, resulting in a stable and reliable mass production solution.
This paper systematically reviews the research and development ideas and verification process of this technology, aiming to provide technical reference for the machining of similar aerospace precision parts.
PART 2
Process Characteristics of Aerospace Precision Cylindrical Parts
A certain aerospace precision cylindrical part manufactured by our company is made of 35CD4, and its shape is shown in Figure 1. This part is an irregularly shaped part, its special structure is prone to deformation, and its wall thickness is relatively thin. The tolerances for the inner hole and outer circle are strictly required, the bearing hole at the lug is a precision hole, and the part has oblique holes inside, making the control of related dimensional tolerances difficult. The technological characteristics of this cylindrical part are mainly reflected in the following aspects.
Figure 1. External shape of the cylindrical part
2.1 Material characteristics
(1) Metallic characteristics
35CD4 high-strength alloy steel is a commonly used medium-carbon nickel-chromium-molybdenum steel in the aerospace industry. Its chemical composition is shown in Table 1. Compared with other metals, this steel has unique advantages in chemical, physical and mechanical properties.
Table 1. Alloy elements and parameters of 35CD4 (chemical composition)
(2) Material characteristics of 35CD4
Mainly include the following four points:
① High strength. Because it is a low-carbon, low-alloy steel, it has higher strength than non-alloy steel, hence it is called "low-alloy high-strength steel".
② High yield strength. Its yield point is higher than that of non-alloy steel. Under the same load, the weight of parts made of this material can be reduced by 20% to 30%.
③ Good plasticity and toughness. The proportion of alloy elements is relatively low, giving the material both good plasticity and toughness. ④ High hardenability. The alloy composition contains elements such as nickel, chromium, and molybdenum, which can keep the supercooled austenite of the steel highly stable. Martensite and bainite structures can be obtained by air quenching.
(3) Material processing characteristics. This material generally has two heat treatment states: normalizing + tempering and quenching + tempering. The hardness corresponding to these two states is shown in Table 2.
Table 2 Material Hardness
As can be seen from Table 2, the hardness and tensile strength of this material are both at a high level. It is also rich in elements such as chromium and manganese. These factors together make it difficult to process, and it is a typical difficult-to-machine material. The specific processing difficulties are as follows:
1) High cutting force. The material has high hardness and strength, high atomic density and bonding force, and excellent fracture toughness and long-term plasticity. During the cutting process, not only is the cutting force large, but the cutting force also fluctuates significantly [2].
2) High cutting temperature. The cutting deformation consumes a lot of power and generates a lot of heat. A large amount of cutting heat is concentrated in the cutting zone, forming a high-temperature environment.
3) High tendency for work hardening. The material exhibits good plasticity and toughness, with a high strengthening coefficient. Under the combined effects of cutting force and cutting heat, it undergoes severe plastic deformation, leading to work hardening. Simultaneously, cutting heat causes the material to absorb hydrogen, oxygen, and nitrogen atoms from the surrounding medium, forming a hard and brittle surface layer, further increasing the difficulty of cutting.
4) Severe tool wear. High cutting forces and temperatures during cutting intensify friction between the tool and chips, and the tool material is prone to affinity interactions with the workpiece material. Combined with the presence of hard particles in the material and severe work hardening, the tool is highly susceptible to adhesive wear, diffusion wear, abrasive wear, boundary wear, and grooved wear during cutting, ultimately losing its cutting ability.
5) Difficult chip handling. The material's high strength, plasticity, and toughness result in ribbon-like, entangled chips during cutting, posing safety hazards, hindering the cutting process, and causing scratches on the part surface, making it difficult to meet surface roughness requirements.
6) Large cutting deformation. High cutting temperatures and high material plasticity during machining easily lead to thermal deformation, making it difficult to guarantee precise dimensions and shapes.
(4) Material Processing Scheme To ensure the raw materials can fully complete the quenching and tempering treatment, the forging blank needs to be rough-machined before heat treatment to ensure that the wall thickness of each cross-section is <30mm. Simultaneously, a datum is machined on the rough-machined blank to facilitate the connection between rough and finish machining processes. The blank and the rough-machined shape of the part are shown in Figures 2 and 3, respectively.
Figure 2: Blank
Figure 3: Rough-machined shape
2.2 Process Characteristics This part has several high-precision dimensional requirements. A stable and reliable process flow must be designed by comprehensively considering all precision dimensional parameters to ensure product quality. The main precision dimensions of the part are shown in Figure 4.
Figure 4: Main Precision Dimensions of the Part
(1) Precision Outer Diameter The precision outer diameter at the port is mm, with a tolerance zone width of only 0.022mm; the inner hole size is φ83.37mm, corresponding to a wall thickness of 2.81mm. This dimensional tolerance requirement is strict, the part wall is thin, and the machining difficulty is extremely high. (2) Precision Inner Hole: The precision inner hole has a diameter of φ mm and a tolerance of 0.04 mm. The coaxiality requirement of this inner hole relative to the outer circle is φ0.04 mm, the cylindricity requirement is φ0.02 mm, and the hole depth is 287.5 mm.
(3) Extended Slanted Hole: As shown in Figure 5, the slanted hole located at the ear position of the part has a diameter of mm and an angle of 45° with the part's axis. When machining this hole, the machine tool spindle needs to be adjusted to a 45° angle with the part. To avoid interference between the tool and the fixture, a long overhang structure is required for the machining tool, which is a major challenge in machining this part.
Figure 5: Extended Slanted Hole and Bearing Hole
(4) Bearing Hole: As shown in Figure 5, the bearing hole at the ear has a diameter of mm. This hole can only be machined from the left end face of the part. The tool overhang is large, and the diameter tolerance requirement is strict, making it difficult to guarantee dimensional accuracy. 2.3 Special Processes
In addition to the above processing requirements, this part requires several special processing steps, as follows:
1) Shot peening. The shot peening area is the outer diameter of the part, but the outer diameter portion with a diameter of φmm must be avoided. Shot peening of the inner hole is strictly prohibited.
2) Chromium plating. The chromium plating area is the inner hole with a diameter of φmm. The chromium layer thickness must be controlled between 20 and 25 μm.
3) Cadmium plating. The cadmium plating area is all surfaces except the chromium plating area.
4) Painting. The painting area is the outer diameter of the part, but the outer diameter portion with a diameter of φmm must be avoided.
PART 3
Aircraft Precision Cylinder Machining Process Design
Based on the analysis of the material properties, precision dimensions, and special processes of this cylinder part, the process plan is shown in Table 3.
Table 3 Process Plan
PART 4
Aircraft Precision Cylinder Machining Process Verification
4.1 Finishing of the Outer Diameter of the Ear
35CD4 material has strong adhesion, making chip breakage difficult during machining. When using lower machining parameters, deep cutting marks appear on the end face of the part, resulting in a torn appearance of the surface material. To solve this problem, the external turning machining parameters need to be adjusted. Specific optimization parameters are shown in Table 4. Through parameter optimization, the cutting marks on the end face of the part are eliminated, the machined end face is smooth, and the machining quality remains consistent from the center to the edge. The optimized end face state is shown in Figure 6.
Table 4 End Face Turning Parameter Optimization
Figure 6 Part State After End Face Finish Turning
4.2 Precision External Diameter Turning
The 14th CNC turning operation is a key process in machining this part. This operation uses a self-centering chuck and a center rest to support the external diameter. Specific machining requirements are shown in Figure 7. The precision external diameter tolerance of the part is 0.024mm, and machining needs to be performed in three stages. Specific machining parameters are shown in Table 5. Due to the high viscosity of the material, a sharp external turning insert is required. The preferred insert tip radius is R0.2mm. If the insert tip radius is too large, the surface roughness requirements of the part cannot be guaranteed.
Figure 7 Requirements for CNC turning process Table 5 Optimization of external turning parameters After finishing, the outer diameter of the part is 88.85~88.89mm, which exceeds the requirements of the drawing. Before finding the root cause, the only way to adjust the size is to leave a margin on the outer diameter and then manually polish it. After analysis, the roundness of the outer diameter supported by the center rest during clamping will have a significant impact on the rotation runout of the part [3]. Under the high precision tolerance requirements, this impact is particularly prominent. The clamping outer diameter of the out-of-tolerance part was measured, and its roundness change was 0.020~0.035mm, which is basically consistent with the out-of-tolerance fluctuation of the outer diameter of the part. To solve this problem, it is necessary to optimize and correct the technical requirements of the reference process. In the external turning process of the 11th process, it is clearly required that the roundness of the outer diameter be controlled within 0.01mm. The specific improvement requirements are shown in Figure 8. After adding roundness control requirements to the 11th process, the dimensional stability of the outer diameter turning in the 14th process was significantly improved. The fluctuation range of the outer diameter was reduced to 88.865–88.875 mm, and the deformation could be controlled within 0.01 mm. No local adjustments by the operator were required to ensure that the outer diameter tolerance met the requirements. Stability tests verified that a single cutting tool tip could stably machine four parts; however, dimensional accuracy could not be guaranteed after tool wear.
Figure 8: Improvement Requirements for the Baseline Process
4.3 Lean Improvement of the Bottom Surface of the Inner Hole
During the CNC turning process in the 14th process, a machining defect appeared at the bottom of the part, manifested as a residual boss with a diameter of 1 mm and a height of 0.3 mm, as shown in Figure 9. The initial machining plan was to use a flat-end mill to remove the bottom excess, then use a large-diameter boring tool to bore the hole, and finally align the bottom surface. When removing the bottom allowance using a milling cutter, the tool runout must be controlled within 0.01mm. However, due to the small bevel at the bottom of the flat-end mill, the bottom of the part cannot be machined to a flat state. The flat-end mill is shown in Figure 10.
Figure 9: Bottom Defect
Figure 10: Flat-end Mill
To solve this problem, the machining scheme was optimized by replacing the bottom with a small-diameter boring cutter, as shown in Figure 11. Machining tests proved that using a boring cutter to bore the bottom completely eliminated the boss defect.
Figure 11: Added Boring Cutter
4.4 Internal Hole Turning Optimization
During the 14th CNC turning operation, the internal hole boring leaves a 0.05mm allowance on one side for subsequent honing. However, after boring, various quality defects appeared in the internal hole. The defect types and proportions are shown in Table 6. These defects pose a significant risk to the subsequent honing process.
Table 6. Defect Types and Percentages
Analysis revealed three main causes of the aforementioned defects: First, the diameter of the internal boring tool was too small, resulting in insufficient tool rigidity and unstable hole dimensions after boring. Second, the material had high viscosity, leading to unstable cutting and poor chip breaking when using traditional finishing parameters, with chips swirling around and scratching the hole wall. Third, the cutting speed was too low, failing to meet the part's dimensional accuracy and surface roughness requirements.
To address these issues, the following optimization measures were implemented: First, replace the internal boring tool with a larger diameter one to improve tool rigidity; the optimized boring tool is shown in Figure 12. Second, increase the cutting speed to increase the depth of cut in finishing. Third, optimize the internal cooling system to allow the cutting fluid to directly act on the boring tool tip. A comparison of tool optimizations is shown in Table 7.
Figure 12 Optimized Boring Tool
Table 7 Tool Optimization Comparison
4.5 Optimization of Extended Slanted Hole
In the 13th operation, horizontal milling of the extended slanted hole, the initial plan was: using a φ6mm extended milling cutter to enlarge the hole to φ7.3mm, the actual hole diameter after machining was 7.1~7.2mm, with a dimensional fluctuation of 0.1mm; after adjusting the program to mill to φ7.5mm, the hole diameter was 7.35~7.42mm, and the fluctuation was reduced to 0.07mm; finally, a straight shank reamer was used for finishing, the hole diameter was 7.62~7.69mm, and the dimensions were extremely unstable.
In the first optimization, the reamer was replaced with a boring tool, and the hole diameter after machining was 7.60~7.72mm, exceeding the tolerance requirements and causing the part to be scrapped. The final optimized plan was: using a 7mm diameter variable-diameter milling cutter for slanted hole milling, the pass rate of the machined parts reached 100%. The tool for extended slanted hole machining is shown in Figure 13.
a) Straight shank end mill b) Straight shank reamer c) Boring cutter d) Variable diameter end mill Figure 13 Extended oblique hole machining tools
4.6 Improvement of bearing hole machining
In the 13th machining process of the bearing hole, the hole diameter requirement is mm. The initial plan was: use a φ20mm end mill to mill to φ34.4mm, and then use a boring cutter to bore to the finished size. However, after machining, vibration marks appeared on the hole wall, which could not be resolved after multiple parameter adjustments. The machining tools are shown in Figure 14.
a) Boring cutter b) End mill Figure 14 Bearing hole machining tools
4.7 Impact of shot peening on parts
The initial precision hole machining plan was: CNC turning precision boring of the inner hole with a 0.05mm allowance on one side, honing to ensure a hole diameter tolerance of 0.02mm and a cylindricity of 0.02mm, and finally shot peening of the outer diameter of the part. However, shot peening caused deformation of the inner hole of the part, with the diameter increasing by 0.015–0.025 mm at the axial center position. Some parts exceeded the upper tolerance limit and were scrapped.
To solve this problem, the process sequence was adjusted after consultation with the customer, placing honing after shot peening. The deformation of the inner hole after shot peening was approximately 0.025 mm, which was completely covered by the 0.05 mm single-sided allowance reserved for honing. After the adjustment, the inner hole diameter tolerance after honing could be controlled within 0.010 mm, and the cylindricity within 0.015 mm, meeting the drawing requirements. The impact of shot peening on the inner hole is shown in Table 8.
Table 8 Impact of Shot Peening on Internal Hole Control (Unit: mm)
PART 5
Conclusion
Through multiple rounds of lean optimization and improvement during the processing verification stage, this aerospace precision cylindrical part finally achieved stable mass production. Traditional processing experience is no longer applicable to the processing difficulties of 35CD4 material. Multiple measures, such as process optimization, tool selection optimization, and processing parameter optimization, are needed to overcome the processing challenges [4]. 35CD4 high-strength steel has excellent performance, but it also brings higher processing challenges. In actual production, it is necessary to select tool models specifically and formulate reasonable processing parameters. With the development of emerging processing technologies, the processing difficulty of such difficult-to-machine materials will gradually decrease, and relevant processing experience needs to be continuously summarized and accumulated. In addition, the shot peening process has an irreversible impact on the precision dimensions of parts, which indicates that process design is a systematic project. The process arrangement needs to follow logical rules and fully consider the mutual influence between each process in order to formulate a scientific and reasonable processing plan [5], and achieve efficient production while ensuring product quality.
