Six-point positioning principle and selection of positioning datum (continued)
In Figure 3-33, an example of gear positioning is illustrated. In Figure a, a short pin and a large plane are used for positioning. The large plane restricts three degrees of freedom, while the short pin limits two, resulting in no over-positioning. In Figure b, a long pin and a flat surface are used. The long pin restricts four degrees of freedom, and the flat surface limits one, again avoiding over-positioning. However, in Figure c, a long pin and a large plane are combined. The long pin restricts four degrees of freedom, and the large plane restricts three, which leads to over-positioning due to two elements limiting the same degrees of freedom.
Over-positioning can cause several issues, such as difficulty in loading the workpiece, deformation of the workpiece or fixture, and even damage to the correct positioning. Therefore, when over-positioning occurs, it's crucial to implement measures to reduce its negative effects.
Two common methods to address over-positioning include:
1. **Modifying the Positioning Device Structure**: As shown in Figure 3-34, using a spherical washer can eliminate two degrees of freedom, thus preventing over-positioning.
2. **Improving the Accuracy of Workpiece and Fixture Surfaces**: For instance, in Figures 3-33d and e, improving the verticality between the workpiece's inner hole and end face, as well as the alignment between the positioning pin and plane, can significantly reduce over-positioning issues.
**Choosing the Right Positioning Reference**
When determining how many degrees of freedom need to be restricted based on the machining requirements, there are often multiple options for selecting the reference. This leads to the question of how to choose the best positioning reference.
There are two types of references: rough and finish. A rough reference is used in the initial stage of processing, typically on an un-machined surface of the blank. A finish reference, on the other hand, is a machined surface used for precise positioning.
**(I) Selecting a Rough Reference**
When choosing a rough reference, two main factors should be considered: ensuring the positional accuracy between the machined and non-machined surfaces, and distributing the machining allowance evenly across all machined surfaces.
1. For parts with both machined and non-machined surfaces, the non-machined surface should be selected as the rough reference to ensure the required positional accuracy. If multiple non-machined surfaces exist, the one with the highest relative accuracy to the machined surface should be chosen.
2. When a part has many machined surfaces, the rough reference should be selected to ensure that each surface has enough material to machine. This involves choosing the roughest surface as the reference.
3. A rough reference should not be reused in the same direction to avoid large positioning errors.
4. The selected rough reference should be flat and free from defects like risers or flash to ensure reliable positioning.
**(II) Selecting a Finish Reference**
The selection of a finish reference should ensure the precision of the part during machining, while also making clamping and fixture design easier.
Key principles include:
1. **Standard Coincidence Principle**: The design reference of the machined surface should be used as the positioning reference to minimize positioning errors.
2. **Benchmark Unification Principle**: Using the same finish reference throughout most processes simplifies tooling and reduces errors caused by changing references.
3. **Self-Reference Principle**: For finishing operations, the surface being machined is often used as the reference to ensure uniform machining.
4. **Inter-Reference Principle**: Sometimes, surfaces are used as references for each other to achieve better accuracy and uniformity.
5. The finish reference must allow for accurate positioning, secure clamping, and ease of operation.
**(III) Use of Auxiliary References**
Sometimes, auxiliary references are introduced to improve positioning accuracy. These are not functional surfaces but are specifically designed for the process. For example, a center hole on a shaft or a boss may serve as a temporary reference to simplify machining.
In some cases, minor or non-matching surfaces are used as references to enhance processing accuracy and surface quality. These are also known as auxiliary references.
In summary, the selection of appropriate positioning references is essential for achieving accurate and efficient machining. While ideal conditions are often not fully met, careful consideration and prioritization help resolve conflicts effectively.
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