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DI:13.2.4.5 - Part Symmetry

•Perfectly symmetrical parts need no rotation to orient them, completely asymmetrical parts require at most 360° rotation followed by a second 360° to put them in the same position every time.

•There is also a recognition phase required by humans for every orientation. Therefor parts that are not symmetrical, but look as if they are upon quick inspection, will require additional inspection time.

•consider the cases below,

PROBLEM: The part on the left has a stepped shaft at each end, each being slightly different in length. This part can cause three problems: i) it could be installed backwards, ii) a maximum of 360° of rotation might be required, and iii) the operator would require additional time to recognize the orientation.

SOLUTION: Make the steps equal on both sides.

PROBLEM: The offset bar on the left is somewhat thin, and would require some time to determine left/right orientation. If it were small it would also be difficult to manipulate.

SOLUTION: Make the offset a bit larger to make the part easier to handle.

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PROBLEM: The part on the left would require some time to rotate, as much as 360°, and also time to identify orientation.

SOLUTION: Put the hole on both sides, and increase symmetry.

PROBLEM: The part to the left has a slot cut into it on one side. This slot would be hard to see unless the part was picked up and rotated.

SOLUTION: One possibility is to put a second slot on the other side of the piece.

•Alpha symmetry is the largest angle a part would have to be turned about an axis perpendicular to the insertion axis.

•Beta Symmetry is the largest angle the part would have to be rotated about the insertion axis for mating.

•Alpha and Beta symmetry actually range from 0 to 360° (instead of the intuitive 0 to 180°) because it is assumed that the worst case rotation is used.

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Note: For the triangular wedge above there are three possible cases based on the shape of the triangle.

DI:13.2.4.6 - Part Shape, Size and Thickness

•We must consider the basic shapes of the parts being assembled. Two basic categories are prismatic and rotational.

•Rotational parts tend to roll when placed on a surface, suggesting that they will need some sort of holding fixture. This also means that during assembly, they must be supported by hand if not in a stable position when working.

•Prismatic parts tend to have at least one stable orientation that allows them to be rested on surfaces. Unlike rotational parts. If the prismatic parts are made to be stable when put in their final position, then they are much easier to fasten.