0020// Flatness with Straightness Refinement
For flat surface with demanding " flatness" in certain direction, designer can use straightness as a refinement to tighten the form requirement in certain direction.
Below shown a cuboid with both straightness and flatness requirement,
In this drawing, straighness and flatness tolerance size 1 mm and 2 mm respectively. This means the flatness for top surface need to within 2mm flatness tolerance zone and the line elements along 1500 mm edges need to within 1 mm straightness tolerance zone.
Below are the visualization by using surface graph, (Top: Coordinates of measured points ;Middle: Surface modeling; Bottom: Detail surface contour )
In order to meet flatness spec, surface contour require keep within two parallel planes with 2mm aparts.
In order to meet straightness spec, line elements on all cross sections along 1500mm edges need to stay within two parallel lines with 1mm aparts.
Based on the graphs, section 1 and 5 out of specification because distance between two lines bigger than 1mm.
All in all, the surface graphs above demostarted two important points,
1) Form control along the cross section tighter than form in 3D space.
2) Straightness tolerance zone must smaller than flatness. If no, the straightness control will become redundants.
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