Six-point positioning principle and selection of positioning datum (continued)
An example of gear positioning is illustrated in Figure 3-33. 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 restricts two, resulting in no over-positioning. In Figure b, a long pin and a flat surface are used. The long pin limits four degrees of freedom, and the flat surface restricts one, so there is also no over-positioning. However, in Figure c, a long pin and a large plane are combined. The long pin limits four degrees of freedom, and the large plane limits three, which leads to over-positioning due to the overlap of constraints from two positioning elements.
Due to the effects of over-positioning, issues such as difficulty in loading the workpiece, deformation of the workpiece or fixture, or even damage to the correct positioning can occur. Therefore, when over-positioning happens, it's essential to take effective measures to reduce or eliminate its negative impact.
To address over-positioning, two common approaches are typically used:
1. **Modifying the Positioning Device Structure**
As shown in Figure 3-34, using a spherical washer instead of a standard one can eliminate two degrees of freedom, thereby avoiding the problems caused by over-positioning.
2. **Improving the Accuracy of Workpiece and Fixture Surfaces**
In Figures 3-33d and e, if the perpendicularity between the inner hole and the end face of the workpiece is improved, along with the alignment between the positioning pin and the plane, the adverse effects of over-positioning can be significantly reduced.
**Third, Choosing the Right Positioning Reference**
When determining how many degrees of freedom need to be restricted based on the processing requirements of the workpiece, multiple reference options may exist for a single direction. This raises the question of how to correctly select the positioning reference.
Positioning references can be either rough or fine. A rough reference is used in the initial stages of machining, where an unprocessed surface on the blank serves as the reference. A fine reference, on the other hand, involves a machined surface used for positioning.
**(I) Selecting a Rough Benchmark**
When choosing a rough reference, two main considerations are important: first, ensuring the positional accuracy between the machined and non-machined surfaces; second, distributing the machining allowance evenly across all surfaces. Specific principles include:
1. For parts that have both machined and non-machined surfaces, the non-machined surface should be selected as the rough reference to ensure proper positional accuracy. If there are multiple non-machined surfaces, the one with the highest positional accuracy relative to the machined surface should be chosen.
2. When a workpiece has several machined surfaces, the rough reference should be selected to ensure sufficient machining allowance on each major surface. For instance, the roughest surface should be used as the reference to avoid excessive material removal.
3. Coarse references should not be reused in the same size direction, as this could cause significant positioning errors.
4. The selected rough reference should be flat and free of defects like risers or flash for reliable positioning.
**(II) Selecting a Fine Benchmark**
The selection of a fine reference should focus on achieving precision during machining and simplifying clamping and fixture design. Key principles include:
1. **Standard Coincidence Principle**
The design reference of the machined surface should ideally be the same as the positioning reference to minimize positioning errors.
2. **Benchmark Unification Principle**
Using the same set of fine references throughout the process helps reduce tooling costs and improve productivity.
3. **Self-Reference Principle**
For finishing operations requiring uniform and minimal machining allowances, the surface being machined can serve as the reference.
4. **Inter-Reference Principle**
When high positional accuracy is required, mutual referencing between surfaces can help achieve consistent machining results.
5. The fine reference must allow accurate positioning, secure clamping, and ease of operation.
It’s important to note that in practice, the ideal reference may not always be achievable, and trade-offs often arise. Careful consideration and prioritization are necessary to resolve conflicts effectively.
**(III) Use of Auxiliary Benchmarks**
Sometimes, auxiliary benchmarks are introduced for convenience or to meet specific process requirements. These are not functional surfaces but are specifically designed for positioning. For example, a center hole on a shaft or a boss can serve as an auxiliary reference. Similarly, some minor surfaces may be used for positioning to improve machining accuracy, even if they are not critical to the part's function.
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