engineering fundamentals GDT: Three-Dimensional
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3D Functional Gauge
We have seen how geometric dimensioning and tolerancing to virtual condition ensures part fit. We can use this fact to design a functional gauge that will ensure 100% part fit with minimum good part rejection. We did not cover functional gauges in the 2D section since 2D is more of a theoretical exercise for ease of understanding. In the following example, the objective is for a block with a hole and a block with a shaft to mate with flush sides, as shown in Figure FG3.

Figure FG1 shows a virtual condition gauge for the 3D block that we have been discussing. The block is shown fit to the gauge, with the appropriate datums contacted. The virtual condition is illustrated in VC1.2 in the 3D Virtual Condition section.

Figure FG1
Figure FG2 shows a virtual condition gauge for a 3D shaft block that mates with the hole block. The shaft block is dimensioned very similarly to the hole block. Its datums contact the functional datums of the gauge. The virtual condition is illustrated in VC1.1 in the 3D Virtual Condition section. In the same section, also refer to Figure VC1.3, which shows the shaft and hole virtual condition boundaries superimposed and thus demonstrates guaranteed fit.

Figure FG2

Figure FG3 shows the final result: Parts that mate perfectly despite rough surfaces, fabrication and inspection at different facilities, and handling-induced changes to dimensions before incoming inspection. This is the true power of GDT: control over the design and manufacturing process which leads to lower costs.

Figure FG3

In summary, we have seen how GDT functional gauges designed according to virtual condition ensure part fit. Virtual condition is the envelope for worst-case part fit. A part which fits on such a functional gauge is guaranteed to fit to all mating parts.

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