The positioning of the workpiece during installation is to make a batch or a workpiece occupy the same position when the machine tool is processed, so as to ensure that the workpiece is in an accurate position relative to the tool and the forming movement, and the machined surface can reach the specified position accuracy. The essence of workpiece positioning is to limit the degrees of freedom that have a negative effect on machining. No matter how different the shape and structure of the workpiece is, their six degrees of freedom can be limited by six support points, but the distribution of the six support points is different.
The law that limits the six degrees of freedom of the workpiece with a reasonable distribution of six-point bearing points is called the six-point rule, also known as the six-point positioning principle. The distribution of support points must be reasonable, otherwise the six support points cannot limit the six degrees of freedom of the workpiece, or cannot effectively limit the six degrees of freedom of the workpiece.
The application of the workpiece positioning principle, the six-point positioning principle has universal significance for the installation and positioning of the workpiece during machining, and the specific application should pay attention to the following situations:
1) Two degrees of freedom. The first degree of freedom is the degree of freedom that directly affects the position of the surface being machined and must be limited. The second type of degree of freedom: the degree of freedom that does not directly affect the position of the surface to be machined, and therefore does not have to be restricted. The specific requirements of the machined surface are different for each process of the workpiece, and the first degree of freedom can be from one to six, and the rest are the second degrees of freedom.
2) Full positioning vs. incomplete positioning. If you limit all six degrees of freedom of the workpiece so that the workpiece occupies a completely certain only position in the fixture, it is called full positioning, and the workpiece positioning shown in Figure 1 is an example of full positioning. When the degree of freedom of the workpiece is less than six, it is incomplete positioning. For the disc workpiece shown in Fig. 2, when drilling hole A in the drill die fixture, as long as the center of hole A is on the circumference with R as the radius, it is not required to be at any position on the circumference. Therefore, there is no need to limit the degree of freedom of rotation of the workpiece around the Z-axis.
3) Under-positioning and over-positioning. When the workpiece is positioned, if there is an unrestricted first degree of freedom is underposition, as shown in Figure 3a. Underpositioning is absolutely not allowed in machining, and it should be necessary to limit the degrees of freedom in the Y direction, as shown in Figure 3b. When the workpiece is positioned, if the number of positioning points exceeds six or the degrees of freedom are repeatedly limited, it is called overpositioning. Overpositioning should be avoided as much as possible during processing.