Machining process development
Machining process development involves planning the entire machining process, from rough cut to finished product, based on part design requirements and production conditions. This includes process scheduling, equipment selection, process parameter determination, and quality control. It is a key component in ensuring product quality, improving production efficiency, and reducing costs. A sound process development must comprehensively consider factors such as part materials, structural characteristics, precision requirements, production batch size, and company resources. Adhering to the principles of “quality, efficiency, economy, and safety,” operational process documentation (such as process specifications and process cards) is generated. The process of machining process development is highly systematic and practical, requiring the integration of multidisciplinary knowledge across mechanical manufacturing technology, materials science, production management, and other disciplines to ensure smooth integration of each process and ultimately achieve the part’s design objectives.
Part process analysis is the first step in developing a machining process. By interpreting the part drawing and technical requirements, machining challenges and key steps can be identified, laying the foundation for subsequent process design. First, the structural characteristics of the part must be analyzed to determine whether it is a shaft, disc, housing, or special-shaped part. Machining methods for parts of different structures vary significantly. For example, shafts are primarily processed by turning and grinding, while housings are primarily processed by boring and milling. Next, various technical requirements must be determined, including dimensional accuracy (e.g., shaft diameter tolerance IT6-IT7), form and position tolerances (e.g., roundness ≤ 0.005mm), surface roughness (e.g., Ra ≤ 1.6μm), material properties (e.g., 45 steel hardness 220-250HB after quenching and tempering), and heat treatment requirements (e.g., carburizing and quenching). For complex parts, key characteristics must be identified, such as the coaxiality of the main crankshaft and connecting rod journals and the cumulative pitch error of gears. These key characteristics will determine whether specialized equipment or processes are required. For example, when processing a slender shaft with a deep hole (length-to-diameter ratio of 10:1), the difficulty lies in deformation control and deep hole processing. It is necessary to plan processes such as center hole processing, rough turning, aging, semi-finishing turning, deep hole drilling, and fine turning, and use a tool holder to reduce bending deformation.
Blank selection must be determined based on part material, structural dimensions, production batch, and performance requirements, directly impacting processing efficiency and cost. Common blank types include castings, forgings, profiles, and weldments. Castings are suitable for complex-shaped parts (such as housings and casings). Gray cast iron HT250 castings can achieve dimensional accuracy of CT10-CT12, with a machining allowance of 3-5mm. Forgings are suitable for parts subject to impact loads (such as gears and crankshafts). 40Cr forgings require a forging ratio ≥2.5 to ensure a dense internal structure, with a machining allowance of 5-8mm. Profiles (such as round steel and steel plates) are suitable for simple structural parts. They can be directly blanked for small batch sizes, with a machining allowance of 1-3mm. Weldments are suitable for joining large or complex parts, such as welded blanks for machine tool beds, and require aging treatment to eliminate welding stress. The blank’s accuracy level must also match the processing requirements. Precision castings (such as investment castings) can achieve dimensional accuracy of CT7-CT9, with machining allowances reduced to 1-2mm, making them suitable for mass production to reduce processing costs. Material utilization rate must also be considered when selecting blanks. For example, forgings for shaft parts save 30%-50% of material compared to profiles, but the forging cost is high and it is only economical when the batch size is large enough.
Process routing is the core of machining technology. The sequence of processes must be arranged according to the principles of “datum first, roughing first, finishing second, primary work first, secondary work second, and surface work first, hole work second” to ensure machining quality and efficiency. Datum selection includes both rough and fine datums. The rough datum should be selected from surfaces on the part that do not require machining or have the smallest allowance. For example, for box-type parts, the bottom surface is used as the rough datum for machining the top surface. Fine datums must adhere to the principles of “datum coincidence” and “datum unification.” For example, for shaft parts, the center holes at both ends are used as the fine datum to ensure the coaxiality of each external diameter. There are two ways to divide processes: centralized process (combining multiple steps on a single piece of equipment) is suitable for large-scale production and the use of CNC machine tools, such as machining centers that complete milling, drilling, and boring in a single clamping. Distributed process (each process performs a small number of steps) is suitable for small-scale production and the use of standard machine tools, such as turning external diameters on a lathe or milling flat surfaces on a milling machine. Taking gear processing as an example, the typical process route is: blanking → forging → annealing → rough turning → tempering → semi-finishing turning → gear hobbing → gear shaving → quenching → gear honing. Tempering is arranged after rough turning to eliminate forging stress and improve cutting performance; quenching is arranged after gear processing to reduce the impact of heat treatment on gear accuracy. For parts with heat treatment deformation, finishing processing should be arranged after heat treatment.
