In mechanical design and manufacturing, dimensional tolerances are not only the basis for controlling part interchangeability and assembly reliability but also the core parameters guiding the selection of processing methods and the development of processing routes. Different processing methods can achieve varying levels of dimensional accuracy and surface quality, and selecting the appropriate processing method is crucial for achieving the target tolerance. What is 'economic tolerance'? In mechanical manufacturing, higher precision is not always better. Increasing processing accuracy significantly raises costs and time, so it is essential to choose the most economical and achievable processing tolerance while meeting assembly and functional requirements.

The impact of turning on tolerances: Suitable for rotating bodies such as shafts, holes, steps, and threads. It features low equipment costs, flexible operation, and the ability to achieve coaxiality between external and internal holes. Factors affecting tolerances: Tool wear and tool tip radius, fixture accuracy (tailstock, chuck), workpiece material and rigidity (long shafts are prone to deformation).

The impact of milling on tolerances: Suitable for machining surfaces, keyways, cavities, and hole systems. It is ideal for large and irregular parts, with flexible machining paths. Factors affecting tolerance: The clamping method of the workpiece affects the repeatability of the datum. A larger tool overhang length can cause a tool vibration effect. Multi-axis machining centers offer higher precision (up to IT8).

The impact of grinding on tolerances: It is primarily used in precision machining to achieve high accuracy and mirror-like quality. It is commonly applied to hard materials and parts with high fit requirements, such as bearing housings and mating holes. Achievable accuracy includes dimensional tolerances up to IT5~IT6, cylindricality and flatness controlled within 0.002~0.01mm, and surface roughness Ra ≤ 0.4μm. Factors affecting the process include the grit size, hardness, and dressing method of the grinding wheel. Controlling grinding heat is crucial to prevent burn and residual stress, while ensuring uniform coolant supply is also important

Process Selection and Tolerance Allocation Recommendations: Tolerance grades, machining suggestions, and implementation methods. For IT11 to IT13, rough machining is recommended using milling and rough turning. For IT8 to IT10, semi-finishing involves turning and milling, drilling holes, and reaming. For IT6 to IT7, finish machining includes precision turning, grinding, honing, and boring. For IT5 and below, ultra-finish machining involves grinding, matching lapping, and CNC five-axis machining.
The design tolerances of parts must align with the actual processing capabilities to avoid unrealistic precision levels. The selection of machining methods should consider the material, structural characteristics, batch requirements, and assembly accuracy. In the early stages of design, it is recommended to collaborate with the manufacturing process team to develop a 'key dimension machining path' and an 'economic tolerance table.'