In terms of design parameters, aluminum alloy die-casting plants generally determine the dimensional tolerance first, and then the geometric tolerance. The rationality of tolerance fit is first of all the functional requirements, then the processing technology, and finally the assembly and maintenance technology.
The traditional way of processing is circular processing, so the selection of hole and shaft matching will have the choice of base hole system or base shaft system. Generally, our hole processing is drilling, reaming, and milling cutter directly forming. In this way, for the convenience of processing, we should choose the size of ready-made tools (drills, reamers, milling cutters). For example, the size of tools with diameter of 14 is relatively rare, and the size of tools with diameter of 12, 15, and 16 is convenient to purchase. In this way, the processing cost is very low. For example, the size of tools with diameter of 30 is not 27, although you need 27 for calculation.

In terms of tolerance, when machining holes (no matter the holes on the flat plate or the holes on the round tube), the tolerance zone should be added (positive) tolerance as much as possible, because the tools with special tolerance are very expensive and difficult to buy, and the jitter when the tool rotates (one of the equipment accuracy) during machining will cause the processed size to be slightly larger than the tool diameter. At this time, the shaft is usually machined by turning, and is not limited by the tool accuracy. Therefore, the base hole system is a low cost option (that is, good technology).
In terms of rotation, if it is unidirectional intermittent rotation, or positive and negative movement, select transition fit. At this time, it also needs to have set screws. If it is to transmit large torque, it usually selects interference fit, which usually requires press fitting or even hot sleeve assembly.
Geometric tolerance is the supplement of dimensional tolerance, so the selection of tolerance zone should be less than the dimensional tolerance, otherwise it is meaningless.
For geometric tolerance, you must understand the meaning of geometric tolerance, then you can make the correct choice and the content and scale to be controlled.
For example, for an aluminum alloy die casting, the width tolerance is 0.2mm. If there is a parallelism requirement for the plane on both sides of the width, it needs to be less than 0.2mm, otherwise it will be meaningless. In many cases, the width tolerance requirement is not high, but the parallelism requirement is very high, which may be 0.05mm. Then the parallelism is very important, usually has an important impact on the assembly.