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positional control and datum references GD&T
[FONT=Calibri][SIZE=3][COLOR=#000000]Referring to the picture, the part consists of a hole extruded through a rectangular plate and is located with basic dimensions, with respect to datum B and C (shown in the picture). Datum A, B, and C are primary, secondary, and tertiary datums respectfully.[/COLOR][/SIZE][/FONT]
[FONT=Calibri][SIZE=3][COLOR=#000000]My question is, if a positional control is applied to the circular feature of size, then why is datum A referenced in the FCF? To me, datum A is irrelevant for the reasons I mention in the following:[/COLOR][/SIZE][/FONT]
[LIST=1][*]The focus is to control the position of the feature which is located with respect to datums B and C alone.[*][COLOR=#000000][FONT=Calibri]The positional tolerance zone resides within a region measured about true position relative to datums B and C. [/FONT][/COLOR][/LIST]
[FONT=Calibri][SIZE=3][COLOR=#000000]What I’m trying to point out, in 1-2, is that the main features, true position & tolerance zone, that define the positional control have nothing to do with datum A.[/COLOR][/SIZE][/FONT]
[FONT=Calibri][SIZE=3][COLOR=#000000]The only [I][U]reason[/U][/I] I can think of for controlling the location relative to datum A is because datum B is typically oriented with respect to datum A, and C to A and B. Therefore datum A must be referenced because the orientation of datum B to A and C to B & A will affect the accuracy of locating the FOS to its intended true position. BUT if this is the case, then why do books on tolerance analysis not take this into account? My “[I][U]reason[/U][/I]” for why datum A is consider is no longer making sense, given the statement in the previous sentence.[/COLOR][/SIZE][/FONT]
[FONT=Calibri][SIZE=3][COLOR=#000000]Can someone help me make sense of all this?[/COLOR][/SIZE][/FONT]
[FONT=Calibri][SIZE=3][COLOR=#000000]Thank you.[/COLOR][/SIZE][/FONT]