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Symmetrical rectilinear burrs can be made of 3, 6, 12, or 24 sticks - no other sizes are possible. The basic six-piece burr was discussed in the previous chapter. The most obvious form for the 12-piece burr is that shown in Fig. 73 on the left. Notice that the axes of the intersecting sticks are not offset as in the six-piece burr, but instead intersect with each other. In order to understand what problem this creates, consider the simple three-piece burr shown in Fig. 73 on the right.
Fig. 73
A little reflection should convince the reader that with any sort of conventional rectilinear notching, the three-piece burr is impossible to assemble, or even more obvious, impossible to disassemble. When you see a three-piece burr of this type, you can be sure that either one of the notches has been rounded so that a piece rotates, or else the sticks have diagonal or otherwise unconventional notches. The same applies to the 12-piece version.
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