# The Off-Line Simulation on Measuring through Software PC-DMIS CAD++ V4.3
## 4 Discussion
The alignment ensures only the initial position determination of the verified machine part in the working space of the coordinate measuring machine, which in the next steps allows performing all working movements with the control unit in the CNC mode. For rough alignment of coordinate systems, it is necessary to scan on the surface of the measured part (Figure 18):  
Three points PNT1, PNT2, PNT3 to define the plane; subsequently, the \(z\) axis direction from the coordinate system of the part is identified with the direction of the direction vector by three points of the defined plane,  
The circles CIR1 and CIR2, where the order of their selection determines the direction vector \(v\); the coordinate system of the part is then rotated about the \(z\)-axis so that the direction of the \(x\)-axis is identical with the direction of the direction vector \(v\),  
The circle CIR3 as the available drawing documentation determines the position of the origin of the part coordinate system at its prescribed zero point (Figure 18). This requirement is met when the origin of the \(x\)-axis and also the origin of the \(y\)-axis are shifted into the scanned circle CIR3. The last step is the movement of the origin of the \(z\) axis to the plane scanned using points PNT1, PNT2 and PNT3. Fine alignment. Compared to rough alignment, the basic difference between is the controlled movements managed by the control unit of the coordinate measuring machine, which ensures that all points required for alignment of the scanning system to the point of contact of the contact tip with the surface of the part are scanned with constant force and constant approach speed. With the mentioned constant parameters, in addition to the points required for fine alignment, all required elements of the verified machine part are scanned, which is a basic prerequisite for achieving correct measurement results of precise machine parts. For fine alignment of coordinate systems, the same elements are scanned on the surface of the verified machine part as by rough alignment, while their replacement geometric elements are determined from several scanned points. Figure 18: Elements of rough alignment. Sanning of elements necessary for evaluation of verified geometric specifications. The technical documentation of the machine part clearly specifies which geometrical specifications are necessary to be verified. Therefore, all necessary points are scanned on their surface. Based on computational algorithms, they are translated with substitute geometric elements representing the real elements on the part. As the scanning system moves during scanning, it is necessary to avoid its collision with the measured part. In this case, the usage of the so-called envelope plane is the most suitable way for elimination of possible collisions (Figure 19). Definition of evaluation elements and generation of measurement protocol. In the last phase of the measuring program preparation, an evaluation element is assigned to each replacement geometric element, which relates to the nominal value read from the technical documentation and the associated tolerance. Deviations in the size, position and shape of the substitute geometric elements then represent deviations from the nominal value of the dimension, position and shape of the actual element on the part. The output phase of the measuring process becomes a measuring protocol, which gives clear information on whether the verified machine part meets the given requirements or becomes repairable or a not-separable failure. In addition to the nominal value and the associated tolerance, the measurement protocol also shows the measured value, its deviation from the nominal value and from the tolerance band, which is expressed both numerically and graphically.