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ComputerAided_Design_Engineering_amp_Manufactur.pdf

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FIGURE 2.15 Formulation of biarc.<br />

Solving Equation (2.4) above yields the radii of the two circular arcs, R 1 and R 2:<br />

© 2001 by CRC Press LLC<br />

R 1<br />

R 2<br />

sin ⎛� �� ��⎞<br />

⎝------------------------- 2 ⎠<br />

2sin �<br />

� �--------------------------------------------<br />

L<br />

⎛⎞ � � �<br />

-- sin⎛--------------⎞ ⎝⎠ 2 ⎝ 2 ⎠<br />

sin⎛ �<br />

� � -- ⎞<br />

⎝ 2⎠<br />

� �<br />

2sin<br />

� � �<br />

� �---------------------------------------------------------------L<br />

⎛---------------------------⎞ � � �<br />

sin⎛--------------⎞ ⎝ 2 ⎠ ⎝ 2 ⎠<br />

(2.5)<br />

It should be noted that R 1 and R 2 are signed quantities; positive radius indicates that the arc center<br />

lies to the left of the tangent and negative radius to the right. For �, positive value is counterclockwise<br />

and negative value clockwise. For the special case of � � 0, it means that the segment from P 1 to Q is in<br />

fact a straight line. Figure 2.16 shows the different types of biarcs formed by various combinations of R 1<br />

and R 2.<br />

It can be observed that the angles of the circular arcs must have the same sign as the radii; in other<br />

words, the relationships R 1� � 0 and R 2(� � � � �) � 0 hold. Furthermore, restricting � to the range<br />

[��, �] for practical purposes leads to the following results:<br />

�2� �� ��(<br />

���), if ��� �(<br />

���) ����2�, if ��� (2.6)<br />

Specifying the variable � determines uniquely the biarc parameters which can be obtained by solving<br />

Equation (2.5). It is possible for the system of Equation (2.4) to produce a degenerate result. This occurs<br />

when � equals � and an infinite number of solutions exists. In this case, it is necessary to specify an<br />

additional variable, e.g., R 1 , to constrain the solution.<br />

Therefore, given the end points P 1(x 1, y 1), P 2 (x 2, y 2), and the tangent angles �, �, after obtaining R 1<br />

and R 2, the arc centers O 1 and O 2 can be obtained.

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