engineering fundamentals GDT: Three-Dimensional
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3D Hole
We will now demonstrate the dimensioning of a hole in GDT. As was seen in the 3D Datums section, the part that we are looking at is very much like the one in the 2D section, only we have added more thickness into the page to make the part three-dimensional. The plus/minus dimensioning in Figure 3.2 and GDT dimensioning in Figure 3.3 of the hole in this 3D part look exactly as they did in the 2D case, only the interpretation is slightly more extensive.

Figure 3.2

Figure 3.3

The tolerance zone has now gained a dimension to become a 3D cylinder, as shown in Figure 3.4. The basic dimensions locate the central axis of the tolerance zone, as with the central point in 2D. In addition, the tolerance zone is "true" with respect to A, B, and C: The central axis is parallel to A and B and perpendicular to C. As shown in Figure 3.5, the central axis of the actual hole must lie completely within the tolerance zone cylinder.
Figure 3.4




             Figure 3.5

The central axis of the cylinder is defined as follows: At any cross-section of the hole perpendicular to the datum axis, the center of the cross-section circle is defined as it was in the 2D case. This is illustrated in Figure 3.6.

Figure 3.6

Bonus tolerances for the 3D case are exactly like those of the 2D case, but now the tolerance zone is a cylinder rather than a circle.See 3D Bonus Tolerance for details.

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