Measurement Path Design in Coordinate Measuring Machine CAI System

Coordinate measuring machines are widely used measuring devices that can measure the geometry, dimensions and relative positions of parts. In recent years, many measuring machines have used computer hardware and software to enhance their functions. However, measurement programs usually still use independent systems to measure different shapes of parts. It is necessary to determine the measurement points and measurement paths in advance, and then prepare corresponding control programs. The process is separate from the part design and processing. Therefore, there is an urgent need for an effective coordinate measuring machine computer-aided measurement (CAI) subsystem integrated with a CAD/CAM system to establish an integrated system for design, processing, and measurement. In this way, for a part model that has been created in a CAD system, the system can automatically generate measurement points on each surface according to its surface features, and then determine the measurement path from the measurement point, generate a measurement program, and control the coordinate measuring machine to complete the measurement task. This system greatly improves its automation performance. 1 Automatic generation of measuring points on the surface of parts The general 3D mechanical parts consist of basic geometric shapes. Planes, cylinders, and cones are the three most typical surfaces. This article mainly introduces the principle of generating measuring points on these three surfaces.

Figure 1 Measuring point on the plane

Fig. 2 N-form solution

Figure 3 Normal solution

The three non-collinear points in a plane determine a plane, but as a general rule, there should be at least 4 measurement points in a plane. This is because there are differences between the actual plane and the ideal plane, and 4 measurement points can reflect the actual situation more accurately. The characteristics of the plane. For a rectangular ideal plane, reasonable measurement points should be evenly distributed over 4 positions, each point away from the boundary is a fraction of the length or width (Figure 1), which takes L1 = L/10, W1=W/10. However, if the rectangle is relatively slender (L/W>4), the determined 4 measurement points will form a long span, which is detrimental to the actual measurement. Therefore, it is necessary to adjust some measurement points appropriately. For example, the length of the rectangle formed by the four measurement points is further divided into eight equal parts, and two points on the diagonal line can be adjusted inward by two grids, see point 2 and point 4 in FIG. 1 . If the selected plane is not an ideal plane (eg non-rectangular or with holes inside), the 4 measurement points determined as described above will fall a little beyond the actual plane. At this point, you must find a point in this plane to replace the original measurement point. If this point is at one corner of the plane, it moves in an "N" shape inward in the diagonal direction (Figure 2). This method is called "N-solving". If this point is close to an edge, it moves inward along the normal to the edge (Figure 3), which is called "normal solution". The solution in Figure 2 is D0, D1, D2, ..., Dn sequentially until the Dn point falls inside the plane. The solution in Figure 3 is D0, D1, D2, ..., Dn sequentially until the Dn point falls inside the plane. Cylindrical and Conical Surfaces For cylindrical surfaces or conical surfaces, the heights of the two measurement circles are h1=r+(H-2r)/5, h2=r+4(H-2r)/5, where H is the total height, r is the probe radius (Figure 4). According to the principle of geometric symmetry, 4 measurement points are generated on each circle, and the central angles of two adjacent points are 90°. In this way, 8 measuring points are evenly distributed on the two measuring circles that are not even in the axial direction.

Figure 4 Measuring points on a conical surface

Figure 5 Measurement points on 3D parts

Figure 6 Measurement path on the plane

Figure 7 Measuring path on a cylindrical surface

The CAD surface model of the combined surface 3D part can be defined from the plan or the 2D projection of the part. After the surface model of the part is established, the system program generates measurement points on all surfaces such as plane, cylindrical surface, and conical surface by the above method. See FIG. 5 . With further research and development, a special measurement path database can be established for the CAD system's geometric database and CAM database. Custom Measurement Points In practical applications, it is also important to determine a sufficient number and an optimum number of measurement points for a given shape surface. In the CAI system, there are two modes of automatic and manual interaction on the computer screen. In the manual interaction mode, any number of measurement points can be determined for the selected surface. Different users can have different choices according to their needs. 2 Part surface measurement path design After the measurement point on the surface of the part is determined, the probe of the coordinate measuring machine approaches the measurement point from the normal direction of the plane where the measurement point is located in the working area, and first moves to the first measurement point at a faster speed. The quasi-measuring point then approaches the first measuring point at a slower speed, which prevents collisions between the probe and the part. The quasi-measuring point is a point that is very close to the measuring point, and it has an accurately measured distance from the normal direction of the measuring point surface to the measuring point. After reaching the first measuring point, the probe retreats to its quasi-measuring point, then quickly moves to the quasi-measuring point of the second measuring point and then to the second measuring point. After reaching all measuring points in turn, the probe is removed from the surface and the measuring path of the probe is generated using the above rules. Referring to FIG. 6, D0 is the starting point, D1, D2, D3, and D4 are four measurement points, and Z1, Z2, Z3, and Z4 are four corresponding quasi-measurement points. When the measurement procedure is performed, the measurement force and the contact condition are strictly controlled, and the measurement data is processed by the computer, thereby eliminating possible human errors. In the part drawing, the cylindrical surface and the conical surface can represent the outer surface, and can also represent the inner surface (hole). The design route must be determined by adjusting the actual shape. Only linear interpolation is considered in this system. Figure 7 shows the outer surface of a cylinder, where D0 is the starting point, D1...D8 are the 8 measuring points, and Z1...Z8 are the corresponding quasi-measuring points. For other conical surfaces or probes on the inner surface of the cylinder can be generated in a similar way. 3 Conclusion The coordinate measuring machine CAI system will be integrated with the CAD system. According to the above method, for the three-dimensional part surface model, the measuring system can automatically generate measurement points on the surface, design the measurement path, and then control the coordinate measuring machine to perform corresponding measurements. , This can greatly improve the automation function of the coordinate measuring machine, shorten the auxiliary time, improve work efficiency, and has low cost and wide applicability.

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