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Minimal distance between engine assembled pipes is an important parameter for evaluating the assembling quality. Feeler gauge is a traditional method, which is time-consuming and low accuracy. In this article, a new approach is proposed which employs an accurate 3D scanner to obtain the surface cloud points of pipes, then extracting the pipe center lines to calculate the minimal distance of two pipes. Firstly, points belonging to the same pipe were picked up. Then equal-interval grids were built along a coordinate direction in which the pipe stretched longest. Point cloud was divided into grid according to its coordinates. Center points of all grids constituted rough tendency points of a pipe. Then, a vertical plane was built on each tendency point passing through it. Then point cloud of the pipe was projected onto the nearest plane. And on each plane, the projected points were fitted into a circle. The center points of all circles constituted center-line points of a pipe. Then traversing method was used to calculate rough minimal-distance (RMD) between two center lines. Then a searching process is conducted around the minimal-point-pair to further calculate minimal distance of two center lines. And minimal-distance of two pipe surfaces was calculated by subtracting radii of two pipes from the minimal-distance of center-line points. Four groups of pipes were examined to verify the proposed strategy. The results showed that the deviations of minimal-distance of two pipe surfaces were within -0.35mm~0.46mm. And the deviations of pipe radius were within-0.1mm~0.29mm.
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