How to use CMM for surface inspection?

1Traditional Measurement Methods

In the absence of a CAD model for surface inspection by CMM, it is common to generate the coordinates of the cross-section lines and points on the surface model with the relevant commands in the CAD software as the theoretical values, and then control the measuring machine to the corresponding positions for inspection and comparison of the deviation of the coordinate values. This approach requires the designer to provide additional theoretical data, while the compensation of the probe tip ball diameter is not easy to realize accurately, and for single-point measurement, the compensation of the probe is simply not possible due to the inability to determine the vector direction. Therefore, this approach has certain limitations.

2 Measurement Based on 3D Number Models

The use of surface number models for surface inspection is a need for the development of CMM measurement technology. Due to the complexity of surface construction technology, it is also a high-end technology in the realm of CAD applications, and is generally accomplished by specialized CAD/CAM systems. Within measurement software, it is a matter of utilizing the design number model by importing it. In order to achieve this purpose, it is necessary to solve the four aspects of the technical problems: digital model import interface, alignment, tip compensation, theoretical value capture.

One, the model import interface

The use of digital model for inspection, the first thing to do, of course, is to ensure that the model is correctly imported into the measurement software. In fact, due to technology, interests and other well-known reasons, the world's major CAD manufacturers are developing different software and formats, such as domestic influence of UG, PROE, CATIA, etc., can not directly read each other's files.

In order to solve this contradiction, the international establishment of a series of data exchange standards, such as the international standard data exchange STEP (Standard for the Exchange of Product Model Data), the United States of America's initial graphics exchange standard IGES (Initial Graphics Exchange) Specification), etc. Although the IGES standard exists in the data file is large, long conversion time, the information is not full and other shortcomings, but it is undeniable that it is currently the most widely used interface standards, the vast majority of CAD software support the standard, China will also be IGES as a recommended standard.

At present, the measuring machine software with the function of digital-analog detection, all support IGES format. The difference is basically in the loss of individual surfaces and damage after complex modeling, as well as the speed of import. For a 10M model, some may take tens of seconds, some may take a few minutes. At present, the more famous CMM measurement software on the market have basically solved this problem better. Figure 1 for the measurement instrument in the self-developed ZCRMDT measurement software, import the number of models to the detection software, the number of models more than 46M size