Industry Information

Do non-standard designs need to consider tolerances? How to choose the geometric tolerance of parts?

Of course, it needs to be considered.
Tolerance refers to the allowable deviation range of geometric parameters (dimensions, geometric errors, surface roughness, etc.) of a part during the manufacturing process. The factory reports dimensional manufacturing accuracy, and the smaller the tolerance value, the higher the accuracy and the more difficult the manufacturing.
It plays an important role in the manufacturing industry, where appropriate tolerances can improve product performance and reliability, reduce failure rates, and ensure component compatibility and performance; A larger tolerance range means lower manufacturing accuracy requirements, thereby reducing manufacturing difficulty and cost. Choosing an appropriate tolerance range while ensuring product quality and performance can help reduce manufacturing costs.
The tolerances for geometric parameters of parts include dimensional tolerances, shape tolerances, positional tolerances, etc.
01 Dimensional tolerance
The allowable variation in size, equal to the absolute value of the algebraic difference between the maximum limit size and the minimum limit size.
02 Shape tolerance
Refers to the total amount of variation allowed by the shape of a single actual element, including six items: straightness, flatness, roundness, cylindricity, line profile, and surface profile.
03 Position tolerance
Refers to the total amount of variation allowed by the position of the actual element in relation to the benchmark, which limits the mutual positional relationship between two or more points, lines, and surfaces of a part, including parallelism, verticality, inclination, coaxiality, symmetry, positional tolerance, circular runout, and total runout. Tolerance represents the manufacturing accuracy requirements of a part and reflects its degree of difficulty in processing.
Shape tolerance and positional tolerance are collectively referred to as shape and positional tolerance. These types of errors can affect the functionality of mechanical products. When designing, their selection needs to follow the following principles:
01
Select the core tolerance by part type. There are several pairs of bearing support holes with high requirements, mainly for the flatness of the reference plane and the supporting surface. The main surfaces of box type parts include the reference plane and the supporting surface. Planar parts can choose flatness, narrow and long planes can choose straightness, slot type parts can choose symmetry, and shaft type parts can install transmission gears and rolling bearings. The positioning end face relative to the supporting journal axis line is selected for end face circular runout (or verticality) The shape tolerance items of sleeve parts mainly include roundness and cylindricity of cylindrical surfaces. Different geometric features of the parts can result in different form and position errors.
02
The selection of geometric tolerances should meet the functional requirements of the parts, including their mating properties, working accuracy, assembly interchangeability, and motion balance. To ensure the smooth assembly of bolt holes on components such as the box and end cover, the positional tolerance of the hole group should be specified.
03
Shape tolerance is the total amount of variation allowed by the shape of a single actual feature, while positional tolerance is the total amount of variation allowed by the direction or position of the associated actual feature to the reference. The ideal boundary of the associated feature controls the actual position and direction of the feature, and positional error is the combined effect of the actual position and actual shape, that is, the measured positional error includes shape error. In the process of determining shape tolerance and positional tolerance, once the positional tolerance is given, there is no need to specify the requirements for shape tolerance. In many cases, positional tolerance can control shape error. As long as the corresponding shape tolerance can be controlled and can meet the usage requirements. If it is necessary to mark the shape tolerance, usually the shape tolerance value should be less than