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ComputerAided_Design_Engineering_amp_Manufactur.pdf

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FIGURE 4.3 Measurement point distribution for basic features.<br />

For holes and circles, the prime number distribution is performed for each circle, in order to detect<br />

the possible lobes on the circle. A pseudo random number generator can be used. Figure 4.3 shows<br />

a typical measurement points distribution on the line, plane, and circle, respectively.<br />

4.4 Alignment of Coordinate Systems, DMIS Code Generation,<br />

and Measurement Operation<br />

In order to perform the CAD-directed inspection successfully, the coordinate alignment has to be<br />

performed between the CAD coordinate system and the CMM coordinate system. Two phases of coordinate<br />

alignment procedures, the rough phase and the fine phase, are explained below.<br />

Rough Phase Alignment Procedure<br />

Rough phase alignment uses the conventional six points method, which is based on six points probing<br />

around the clear cut surfaces of a reference square block. Consider (X, Y, Z) as the CMM coordinate system,<br />

and (X�, Y �, Z�) as the CAD coordinate system as shown in Figure 4.4. In this figure, let A 1(x 1,y 1,z 1), A 2(x 2,<br />

y 2, z 2), A 3(x 3, y 3, z 3) be the probing points data on the plane 1; A 4(x 4, y 4, z 4), A 5(x 5, y 5, z 5) the probing<br />

points data on the plane 2, and A 6(x 6, y 6, z 6) the probing point data on the plane 3. Thus the probing<br />

points data, A 1 to A 6, are obtained as the location of probe ball center in the CMM coordinate system<br />

when the probe ball contacts the respective planes. When X�(a x, b x, c x), Y�(a y, b y, c y), and Z�(a z, b z, c z) are<br />

the base vectors along the CMM coordinate axis, the base vectors can be calculated as<br />

Z� � ( A2 � A1) �<br />

( az, bz, cz) � ( A3 � A1)�( A2 � A1) � ( A3 � A1) Y� � ( A5 � A4) � Z��<br />

( A5 � A4) � Z�<br />

�<br />

( ay, by, cy) X� � ( Y� � Z�)�Y��Z�<br />

�<br />

( ax, bx, cx) (4.12)<br />

(4.13)<br />

(4.14)

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