Six-point positioning principle and selection of positioning datum (continued)

Due to the influence of over-positioning, it may happen that the workpiece cannot be loaded, the workpiece or fixture is deformed, and the correct positioning of the workpiece may be damaged. Therefore, when over-positioning occurs, effective measures should be taken to eliminate or reduce the adverse effects of over-positioning.

Eliminating or reducing the adverse effects of over-positioning generally involves the following two measures:

1 . Changing the positioning device structure

As shown in Figure 3-34, the use of spherical washers eliminates the two-degree-of-freedom limit and avoids the adverse effects of over-positioning.

2 . Improve the position accuracy of work and fixture related surfaces

As shown in Figure 3-33d and e, if the verticality of the inner hole and the end face of the work piece can be improved and the verticality between the positioning pin and the positioning plane can be improved, the adverse effect of over-positioning can also be reduced.

Third, the choice of positioning benchmark

When the number of degrees of freedom that the workpiece should be limited is determined according to the workpiece processing requirements, there are often several positioning criteria for the limitation of the degree of freedom in one direction. At this time, the problem of how to correctly select the positioning reference is proposed.

The positioning reference has a rough reference and a fine reference. In the processing start process. Only the surface that has not been machined on the blank is used as a reference for positioning, and the surface is called a coarse reference. Using a machined surface as a positioning reference is called a fine reference.

(I) Selection of a rough benchmark

When selecting a coarse reference. Two main issues are considered: First, to ensure the accuracy of the mutual position between the processing surface and the non-processing surface; and secondly, to reasonably allocate the machining allowance of each processing surface. Specific reference to the following principles:

1 . For parts with both machined surfaces and non-machined surfaces, to ensure the positional accuracy between the non-machined surface and the machined surface, the non-machined surface should be selected as the coarse reference. As shown in Figure 3 -35a. If there are multiple non-machined surfaces on the part, the surface where the accuracy of the position relative to the machined surface is high is used as the rough reference. As shown in Fig. 3-35b, the part has three non-machined surfaces. If it is required that the wall thickness of the surface 4 and the surface 2 be uniform, the non-machined surface 2 should be selected as the rough reference to process the stepped hole.

2 . For a workpiece with a large number of machined surfaces, when selecting a coarse reference, allowance should be made for the rational allocation of the machining allowance for each machined surface. The rational allocation of allowances refers to the following two points:

(1) It should be ensured that all major surfaces have sufficient allowances. In order to meet this requirement, the roughest surface should be chosen as the rough reference, as shown in Fig. 3-35c. The φ 55mm outer surface should be selected as the rough reference.

(2) For some important surfaces on the workpiece (such as guide rails and important holes, etc.), in order to make the surface finish as uniform as possible, the important surface should be selected as the rough reference. The bed rail surface as shown in Figure 3-36 is an important surface that requires good wear resistance and generally uniform mechanical properties throughout the entire rail surface. Therefore, when machining the guide rails, the guide rail surface should be selected as the base of the rough reference machining bed (Fig. 3-36a), and then the guide rail plane (Fig. 3-36b) should be machined on the base surface.

3 . Coarse benchmarks should be avoided for repeated use. In the same size direction, the coarse reference can only be used once, in order to avoid a large positioning error. As shown in Fig. 3-37, the machining of the minor axis, such as the repeated use of the B-side machining of the A-plane and the C-plane, then the axis of the A-plane and the C-plane will produce a large concentricity error.

4 . The plane selected as the rough reference should be flat and free of defects such as risers or flashes for reliable positioning.

(II) Selection of Fine Baseline

The selection of the fine reference should be based on the assurance of the precision of the part machining. At the same time, it is convenient to consider the clamping and the fixture is simple in structure. The selection of a fine benchmark should generally consider the following principles:

1 . The principle of "standard coincidence"

In order to more easily obtain the relative positional accuracy requirements of the machined surface on its design basis, the design reference of the machined surface should be selected as its positioning reference. This principle is called the principle of coincidence of standards. If the design reference of the machined surface is not coincident with the positioning reference, the positioning error will be increased. The reason and calculation method are discussed in the next section.

2 . The "benchmark unification" principle

When a workpiece is positioned on a certain set of fine references, it is more convenient to process other surfaces. This set of fine reference positions should be adopted in most processes as far as possible. This is the principle of “benchmark unification”. For example, most of the shaft parts use the center hole as a reference; most of gear tooth blanks and tooth shape machining use gear holes and end faces as positioning references.

Adopting the principle of “benchmark unification” can reduce the cost of tooling design and manufacturing, increase productivity, and avoid the errors caused by the benchmark conversion.

3 . The "self-reference" principle

When the workpiece finishing or finishing process requires a small and uniform margin as much as possible, the surface to be machined should be selected as the positioning reference. This is the “self-reference” principle. For example, when grinding the bed surface of the bed, the bed rail surface is used as a reference. As shown in Figure 3-38. At this point, the bed foot plane only serves as a support plane, and it is not a positioning reference plane. In addition, examples are self-referenced, such as reaming with a floating reamer, pulling a hole with a broach, grinding a circle with a centerless grinder, and the like.

4 . "Inter-reference" principle

In order to obtain a uniform machining allowance or a higher positional accuracy, it is possible to use the principle of repetitive machining on a mutual basis. For example, when machining precision gears, first use the inner hole to position the machined tooth surface. After the tooth surface is hardened, grinding is required. Because the tooth surface hardened layer is thin, the grinding allowance is small and uniform. At this time, the tooth surface can be used as the positioning reference grinding inner hole, and then the inner hole is used as the reference grinding surface, so as to ensure the grinding allowance of the tooth surface is uniform, and the mutual position accuracy with the tooth surface is easier to be guaranteed.

5. The refined reference selection should ensure accurate positioning of the workpiece, reliable clamping, and easy operation. As shown in Fig. 3-39b, when machining the C plane, if the "baseline coincidence" principle is adopted, then the B plane is selected as the positioning reference, and the workpiece clamping is as shown in Fig. 3-40. This not only inconvenient workpiece clamping, fixture structure is also more complex; but if the use of A-plane positioning as shown in Figure 3 -39a, although the fixture structure is simple, easy installation, but the benchmark does not coincide, positioning errors.

It should be pointed out that the above-mentioned rough and fine benchmark selection principles can not always be fully satisfied. In practice, conflicting situations often arise. This requires comprehensive consideration, separation of priorities, and emphasis on solving major conflicts.

(III) Application of Auxiliary Benchmarks

When positioning the workpiece, in order to ensure the accuracy of the position of the machining surface, most of the design criteria or assembly standards are preferred as the main positioning reference. These standards are generally the major surfaces on the part. However, in the processing of some parts, for the convenience of mounting or easy to achieve a unified reference, artificially create a positioning reference. For example, the center hole of the process boss and shaft parts on the blank. These surfaces are not working surfaces on the part, but the specially designed positioning reference on the workpiece to meet the process needs is called an auxiliary reference.

Water Drilling Rig

Water Drilling Rig,Water Well Drilling Rig,Hydraulic Water Drilling Rig,Truck Mounted Water Drilling Rig

Sinotai Petroleum Equipment Co.,Ltd , https://www.ts-sinotai.com