Grinding allowance of shaft and sleeve parts
As core components in mechanical transmission systems, the precision of shafts and sleeves directly affects the operational stability and service life of the equipment. Proper determination of grinding allowance is a key step in ensuring machining accuracy. Machining allowance refers to the thickness of the metal layer removed from the surface to be machined during the grinding process. It not only needs to compensate for shape errors, position errors, and surface roughness left by the previous process, but also needs to reserve correction space for the final precision inspection. For example, for shaft parts that have undergone pre-turning, if the outer cylindrical surface has a roundness error of 0.05mm, the grinding allowance must at least cover this error value. At the same time, considering the thermal deformation compensation that may occur during the grinding process, a safety margin of 0.02-0.03mm is usually required.
When determining the grinding allowance for shaft and sleeve parts, the machining methods and precision levels of the preceding process must be comprehensively considered. The surface roughness of parts after rough turning is generally between Ra12.5-Ra6.3μm, with form errors of up to 0.1-0.3mm. In this case, the grinding allowance should be set at 0.3-0.5mm. The surface roughness after fine turning can be improved to Ra3.2-Ra1.6μm, with form errors controlled at 0.03-0.05mm. The grinding allowance can be reduced to 0.1-0.2mm. For spindle parts with extremely high precision requirements, such as ball screw shafts for CNC machine tools, the final grinding allowance is often determined through multiple trial cuts. An allowance of 0.05-0.08mm is retained during the initial grinding process. A secondary grinding process is performed after aging treatment to eliminate stress and prevent deformation caused by stress release after processing.
The material properties and heat treatment status of the part also significantly affect the grinding allowance. For bushings made of high-carbon steel or alloy structural steel, a 0.01-0.03mm oxide layer may form on the surface after quenching. This layer must be completely removed during the grinding process to prevent it from affecting the part’s wear resistance and fatigue strength. For ductile iron bushings, due to their brittle material, excessive grinding allowance can lead to microcracks on the surface. Therefore, the single grinding allowance should be controlled within 0.05mm, and continuous cooling should be provided using an emulsion with excellent cooling properties.
Production batch size and processing equipment accuracy are also important factors in determining grinding allowance. In large-scale production, centerless grinding can be used to improve processing efficiency. In this case, the grinding allowance for shaft parts needs to be adjusted based on the positioning accuracy of the feed mechanism, typically 0.15-0.25mm. This ensures a single-shot grinding process while avoiding workpiece runout caused by uneven allowances. In small-batch production of single pieces, for spindles requiring IT5 or higher precision, multiple grinding processes are typically used to gradually reduce the allowance. Rough grinding allows for 0.3-0.5mm, semi-finishing grinding for 0.1-0.15mm, and finishing grinding for 0.03-0.05mm. Multiple clamping adjustments ensure that the final roundness error is within 0.001mm.
In actual production, the determination of grinding allowances requires dynamic optimization based on on-site inspection data. For example, a full-scale inspection of parts from the previous process is performed using a three-dimensional coordinate measuring machine. The maximum and minimum form errors are calculated, and based on this, an initial grinding allowance is increased by 10%-15%. After a trial run of 10-20 parts, the accuracy of the ground parts is re-inspected. If unremoved surface defects are detected, the allowance is appropriately increased. If excessive allowances lead to low machining efficiency, the allowance value is adjusted downward. This closed-loop adjustment mechanism minimizes material waste and reduces production costs while ensuring machining quality.
