Error in freeform surfaces: PC-supported clarification - and avoidance of causes
The “Analyze MyWorkpiece /Toolpath” PC tool enables a systematic search for the causes of quality defects in freeform surfaces. This guarantees quality assurance, even before the first machining trial.
What's the problem?
When the surface quality of a free form workpiece fails to meet expectations, virtually all the stations between the CAD system and the tools used could be possible sources of error: the output of the CAD system itself, the output of the CAM system or postprocessor, the parameterization of the CNC and the drive controller, the mechanics of the machine, the tools, the coolant — theoretically, even the workpiece material.
Tracking the “garbage in, garbage out” effect
The quality of each process step between the CAD system and the workpiece depends upon the input data from the previous process step. Thus, in a systematic search for the causes of surface quality defects, the output data of the individual system parts must be scrutinized: the STL file of the CAD system, the mpf file from the CAM system/postprocessor, the logging of position setpoints (IPO trace) generated by the CNC during execution of the parts program and, finally, what the drive controllers have made of this in conjunction with the machine mechanics and the tool (IPO trace of the actual position values). This requires the use of appropriate tools.
The mold-making Quick Viewer in Sinumerik Operate can already visualize the parts program that represents the CAD point cloud as the start and end points of G1, G2 and G3 sets. This can be done online, i.e., directly on the machine. The Analyze MyWorkpiece /Toolpath PC-based tool offers a technologically deeper insight. In addition to STL files, it can also display transcripts of the interpolator output of the CNC (IPO trace), retaining the reference to the parts program: for each data point, the respective set is displayed in the parts program.
Direct comparison in 3D
Analyze MyWorkpiece /Toolpath can display data sets from different origins side by side in 3D for comparison. By directly comparing the output of the CAD and CAM systems or the IPO trace and the CAM data, the causes of error can be identified and isolated. Analyze MyWorkpiece /Toolpath assists with the qualitative data analysis by displaying various selected properties of the data series in color-coded form. Colors represent aspects such as local density of the point cloud, the local curvature of the tool path or the length of the chord section of the tool path for each interpolator cycle (and thus the path velocity). As a result, the data causing the quality defects can be found sooner and indications of the causes can be identified.
Inhomogeneity in the point cloud generated by the CAD system found on the faulty workpiece surface is one example for this: it indicates bad parameterization of the CAD output. Discontinuity in the local tool path curvature may be due to the contour support points being too close together: these “double points” force an atypical and unintended curvature of the tool path, which puts too much strain on the machine tool’s speed control. To discover the cause of the quality defect, it is necessary to check whether these double points in the CAD data coincide with surface transitions or whether they occurred during the implementation via CAM system or the postprocessor.
In the IPO trace’s 3D view of the position setpoints, the chord length is color-coded per IPO cycle and thus the current path velocity is indirectly shown (path velocity = distance per IPO cycle). Anomalies here indicate adverse speed control by the CNC. This is remedied by changing the settings of the Look Ahead function, the COMPCAD data set compressor and/or CYCLE832. However, a comparison with the trace of the actual position values provides an indication of insufficiently optimized drive controllers or machine/tool vibrations.
Quality assurance from the first chip
With Analyze MyWorkpiece /Toolpath, all the critical points of the process chain can be visualized in a practical way. As a result, causes of quality defects in freeform surfaces that are otherwise difficult to detect are systematically resolved. With experience, this results in a considerable increase in expertise regarding the technological relationships and interactions in the process chain.
Analyze MyWorkpiece /Toolpath is implemented as PC-based software so that the machine tool can continue to be used during a search for causes. It is thus possible to perform quality assurance on CAD/CAM data even before the first workpiece is manufactured. This is particularly important when machining large-sized workpieces with high use of material and cutting time. Here, Analyze MyWorkpiece /Toolpath also allows for quality assurance in the position and speed logging for production simulation carried out with the Sinumerik virtual numerical controller kernel (VNCK). VNCK is included in software such as Sinutrain and the NX CAD/CAM system, for example.