Precision part machining stands at the core of aerospace component manufacturing. 35CD4 high‑strength steel features high strength and excellent hardenability, making it a preferred material for aerospace precision cylinder parts. Nevertheless, the material comes with prominent machining difficulties such as high hardness, large cutting force and severe work hardening. Meanwhile, the cylinder belongs to irregular thin‑walled structure with strict dimensional tolerance and complex features including inclined holes and bearing holes. Traditional machining methods can hardly balance machining accuracy and operational stability.

To tackle the above technical difficulties, researchers formulated a complete lean machining scheme according to material characteristics and part process requirements. Through systematic optimization of cutting tools, machining parameters and process sequence, common problems such as machining deformation and dimensional out‑of‑tolerance were effectively solved. Stable mass production was finally realized, offering reliable reference for precision part machining of other difficult‑to‑cut materials.

The aerospace precision cylinder component is made of 35CD4 high‑strength steel, with its overall structure shown in Figure 1. The part is prone to structural deformation, and manufacturing faces dual constraints from material performance and precision process requirements.

Figure 1 Outline of cylindrical part

Table 1 Chemical Composition of 35CD4（%）

As a medium‑carbon nickel‑chromium‑molybdenum alloy steel, 35CD4 is categorized as low‑alloy high‑strength steel. It possesses high yield strength, favorable plasticity, toughness and outstanding hardenability. Components made of this material can reduce overall weight by 20%~30% while maintaining structural performance. Its superior physical properties also bring obvious machining limitations. High hardness and tensile strength raise cutting force and cutting temperature dramatically. Severe plastic deformation generates hard and brittle surface layer during processing, which accelerates tool wear.

The material exhibits high viscosity and easily forms continuous tangled chips in cutting. These chips not only bring potential safety hazards but also scratch the machined surface. Thermal deformation induced by high cutting temperature also increases the difficulty of precision dimension control.

35CD4 is generally supplied under normalized tempered and quenched tempered states, both maintaining a high hardness level that further raises precision part machining difficulty.

To guarantee subsequent heat treatment quality, the wall thickness of forged blank after rough precision part machining should be controlled within 30 mm for all cross sections. Standard alignment datum should also be processed to ensure smooth connection between roughing and finishing procedures. The appearance of blank and rough‑machined part are presented in Figure 2 and Figure 3 respectively.

Figure 2 Raw part blank

Figure 3 Profile after rough machining

The cylinder imposes extremely strict requirements on dimensional accuracy, with key precision dimensions illustrated in Figure 4. The dimensional tolerance of end precision outer circle is only 0.022 mm, matched with an inner bore of φ83.37 mm and a wall thickness of merely 2.81 mm. The thin‑walled structure is extremely vulnerable to machining deformation. The deep precision inner bore reaches 287.5 mm in depth, with coaxiality tolerance of φ0.04 mm and cylindricity tolerance of φ0.02 mm, bringing great challenges to straightness and dimensional stability control in precision part machining.

The extended inclined hole on the lug forms a 45° included angle with the part axis as shown in Figure 5. Precision part machining requires adjusting the spindle angle of machine tool, and the cutting tool needs long overhang to avoid fixture interference. The bearing hole located on the lug can only be processed from the left end face. Long tool overhang together with strict aperture tolerance makes precision control difficult.

In addition, the component needs multiple special surface treatments including shot peening, chrome plating, cadmium plating and painting. Each process corresponds to designated processing areas. Shot peening is forbidden on inner bore and specific outer circle surface, and the chrome plating thickness is strictly limited to 20~25 μm.

Based on the comprehensive analysis of material performance and part characteristics, a complete technical process was formulated as shown in Table 3. Multiple rounds of optimization and verification were carried out for key working procedures to overcome technical difficulties one by one.

High viscosity of 35CD4 easily causes deep cutting lines and surface tearing on part end face in initial precision part machining. Parameter optimization for end face turning was implemented as shown in Table 4. After adjustment, surface defects were completely eliminated with smooth and uniform end face quality shown in Figure 6.

In precision outer circle turning, self‑centering chuck combined with steady rest was adopted for clamping support as shown in Figure 7. precision part machining was divided into three stages with sharp inserts of 0.2 mm tool nose radius. Specific turning parameters are listed in Table 5. Early production suffered severe dimensional out‑of‑tolerance, which was confirmed to be caused by roundness error of steady rest supporting surface affecting rotary runout.

Roundness of reference outer circle was strictly controlled within 0.01 mm in revised benchmark procedure with specific improvement requirements shown in Figure 8. After optimization, the fluctuation range of outer circle diameter was controlled within 0.01 mm. A single turning insert can stably process four workpieces, greatly improving dimensional consistency.

Multiple defects appeared in initial inner bore machining. Residual boss existed at the bottom as shown in Figure 9, which was caused by the small oblique angle of flat bottom milling cutter in Figure 10 leading to unsmooth bottom surface. Replaced by small‑diameter boring cutter in Figure 11, the residual boss defect was completely removed.

Inner bore machining also suffered unstable dimension and inner wall scratch after boring. Defect types and proportion are shown in Table 6. Solutions including adopting larger rigid boring cutter in Figure 12, raising cutting speed, increasing finishing depth of cut and optimizing internal cooling system were adopted to solve the above problems effectively, laying a good foundation for subsequent honing process.

The original processing scheme of extended inclined hole adopted extended milling cutter for hole expansion matched with reamer finishing, resulting in large aperture fluctuation. After repeated trials, 7 mm variable diameter milling cutter was applied as shown in Figure 13, achieving 100% qualification rate.

Bearing hole adopted combined processing of milling cutter and boring cutter in early stage, obvious chatter marks appeared on hole wall as shown in Figure 14. After continuous parameter adjustment and process improvement, chatter vibration was eliminated and aperture precision was well guaranteed.

a) Boring cutter

b) Milling cutter

Figure 14 Cutting tools for bearing hole machining

The successful development of 35CD4 high‑strength steel precision cylinder fully proves that the machining difficulties of difficult‑to‑cut materials can be overcome through systematic technical optimization. In precision part machining, targeted optimization of process scheme, tool selection, cutting parameter matching and process sequence arrangement can realize stable mass production under the premise of ensuring precision and surface quality.

The accumulated practical experience provides solid technical support for the manufacturing of similar aerospace precision components. It also offers valuable reference for process innovation and parameter matching in the machining field of difficult‑to‑cut alloy materials.