CNC machines
One of the most important uses of CNC machine tools is the cutting of complex continuous part paths, or contours. In continuous-path numerical control systems, there is contact between the cutting tool and workpiece throughout the part path while up to five axes are in motion. Therefore, the final workpiece dimensions are directly related to the positional relationship between the tool and workpiece. The ability to monitor this relationship and predict the final part dimensions is important for today’s manufacturing engineer.
For a new part or production run to be manufactured with the use of CNC machine tools, the current procedure is to write the CNC part program using the engineering drawings, execute the part program to machine a test part or prototype, and then inspect the test part, normally with the aid of a coordinate measuring machine, to check for conformance to design tolerances. This feedback of the actual part dimensions (with an adequate degree of precision) is currently the only way to certify the performance of the CNC program. If the test part does not meet the specified tolerances, as is often the case for a first trial, the CNC program is modified, another test part is machined and the process repeated.
This iterative process may be acceptable, although not efficient, in situations where the material is inexpensive and machining time is short. However, in many cases, such as the aerospace industry, the material is costly (expensive forgings to be machined) and machining time is dramatically increased. In such instances, this iterative process of producing dimensionally correct parts becomes economically unacceptable.
Both time and money could be saved if there were a way to directly measure the machine tool’s dynamic contouring accuracy over an arbitrary 3-D path without the necessity of machining an expensive test part. At the same time, the efficiency of the CNC machining process could be greatly increased. The use of the Laser Ball Bar (LBB), a spatial coordinate measuring device, to take dynamic measurements of a path could reduce or replace the need to machine and inspect a test part. In effect, the spatial coordinates of the dynamic tool path measured by the LBB could function as a virtual test part1.
This particular approach must be differentiated from the conventional approach to machine tool metrology. At this time, the normal procedure is to measure the machine tool error motions including geometric, thermal and perhaps process errors. These errors may then be used as pre-machining compensation in the machine tool controller in an effort to give theoretically perfect motions. The machine’s accuracy and, therefore, the part dimensions are based largely on the success of this time-consuming process. Although this research is certainly not a replacement for this body of work, it is also not the same. The purpose of this work is to try and predict the final part dimensions for the execution of a specific CNC part program on a given machine tool prior to cutting the part, not to measure the geometric or servo errors of the machine tool.
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