In order to resolve this contradiction, a series of data exchange standards have been established internationally, such as Standard for the Exchange of Product Model Data (STEP), and the US Initial Graphics Exchange Specification (IGES). Although the IGES standard has the disadvantages of large data files, long conversion time, and insufficient information, it is undeniable that it is the most widely used interface standard. Most CAD software supports this standard. China also uses IGES as the recommended standard.
The measuring machine software with digital mode detection function currently supports the IGES format. The difference is basically reflected in the loss and damage of individual surfaces after complex digital-analog input, and the speed of import. For a 10M digital model, some may take tens of seconds, and some may take a few minutes. At present, the well-known CMM measurement software on the market has basically solved this problem. Figure 1 shows the ZCRMDT measurement software independently developed by the measuring instrument. When the digital-to-analog software is introduced, the digital-analog size is more than 46M.
For the current mainstream CAD software, some measuring machine software vendors have also developed various direct reading interfaces, such as UG file direct reading, PROE file direct reading, etc., without intermediate file format conversion, to avoid the impact of conversion. However, such interfaces generally require additional purchases.
2. Align
Align is an important part of the CMM software. No matter the number of modes, the machine coordinate system must be aligned with the workpiece coordinate system by alignment. The measured values ​​are comparable.
For box-type parts, the coordinates are basically established in 3-2-1 mode. The surface, line and point features are used to determine the coordinate axis and the origin. The workpiece coordinate system is used to align the workpiece. This is also the most basic and accurate. The alignment method. The well-machined and wide-ranging features should be selected as the coordinate reference to reduce the error caused by the alignment. In general, for the established coordinate system, it is also necessary to perform operations such as panning, rotation, etc. to generate new alignment.
For irregular shapes, the calculation is much more complicated. If there are clear feature points on the workpiece, such as 3 hole cores, the actual value is usually measured, corresponding to the theoretical value, and 3 points are corrected.
We often encounter situations where there are no clear features on the workpiece, ie we cannot accurately correspond the measured values ​​to the theoretical values. For this case, the measurement software is usually a method of iterative alignment. For a single-point trigger acquisition machine, usually the software selects multiple points on the digital-analog surface as the target point, and the selected points should be able to fix the parts in all 6 degrees of freedom to prevent the parts from rotating and moving, and then Move the measuring machine to the position on the workpiece as far as possible to collect the measured point. The software will iterate and correct the measuring point in the vicinity of the target point on the digital model until the alignment error is within the specified accuracy. Some measurement software will guide you to re-measure to a closer point for more accurate calculations when iterating overshoot.
In another case, multiple points are directly measured. The software optimally matches the point group with the theoretical digital model, and aligns the point group with the digital model step by step until the deviation root mean square of the point group and the digital model is the smallest. The more points the method is, the more accurate, but at the same time the more complicated the calculation, the computer
Although each software has different classifications and characteristics for alignment, the above methods are basically used.
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