How to mark CAD fit tolerances
In mechanical design and manufacturing, fit tolerances are a key factor in ensuring part assembly accuracy and functionality. CAD software (such as AutoCAD, SolidWorks, etc.) provides a variety of methods for marking tolerances. This article will introduce in detail how to mark fit tolerances in CAD, and attach relevant data examples.
1. Basic concept of fit tolerance
Fitting tolerance refers to the allowable variation range of the dimensions of two parts during assembly, and is usually divided into clearance fit, transition fit and interference fit. The following are common types of fit tolerances and their characteristics:
| Mate type | Features | Application scenarios |
|---|---|---|
| clearance fit | There is a gap between the shaft and the hole for easy assembly | Sliding bearings, gear transmission |
| Transition fit | There may be gaps or interference between the shaft and the hole | Positioning pin, key connection |
| interference fit | There is interference between the shaft and the hole and requires pressure assembly. | Fixing of bearings and shafts |
2. How to mark fit tolerances in CAD
In CAD software, marking fit tolerances is usually achieved through the following steps:
1. Use dimensioning tools
In AutoCAD, you can select "Linear Dimension" or "Diameter Dimension" through the "Dimension" menu, then right-click the dimension value and select the "Properties" or "Tolerance" option to set it.
2. Enter the tolerance value
In the tolerance setting interface, you can enter the upper and lower deviation values or select the ISO standard tolerance code (such as H7/g6). The following are examples of common tolerance levels:
| Tolerance class | Shaft tolerance zone | Hole tolerance zone |
|---|---|---|
| IT6 | g6 | H7 |
| IT7 | h7 | G7 |
| IT8 | k6 | K7 |
3. Annotation examples
The following is a typical example of shaft hole fit tolerance marking:
Hole size: Ø25H7 (+0.021/0)
Shaft size: Ø25g6 (-0.007/-0.020)
3. Precautions
1. Tolerance marking should comply with national standards (such as GB/T 1800) or industry specifications.
2. In the assembly drawing, the matching code (such as H7/g6) needs to be clearly marked.
3. For complex parts, you can use the "Geometry Tolerance" tool to add additional annotations.
4. Summary
Marking fit tolerances in CAD is an important part of mechanical design. Correct marking can ensure the interchangeability and assembly accuracy of parts. Through dimensioning tools and tolerance setting functions, tolerance marking can be completed efficiently. In actual applications, it is necessary to select the appropriate tolerance level based on the design requirements and processing technology.
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