Hello, have been reading this forum for a while now, great source of information. Recently question came up about measuring method so I would like to get advise from the experts. Based on the drawing datum A is the surface of the the stamped part, datum B is is a hole without any positional tolerance and than there is a round slot. Length and width of the slot are dimensioned separately. Length of the slot is marked as datum C and has non diametrical TP 0.25 MMC, A, B MMC with the associated basic dimension of let say 4.5 mm in X direction (looking at the drawing length is in X and width is in Y). Width of the slot has also non diametrical TP 0.25 MMC, A, B MMC, C MMC with the associated basic dimension of let say 17 mm in Y direction. Both hole and the slot are on the same plane A . At some point insert stopped fitting so measurements started at both ends (our company and supplier). My results were showing TP out with center of the slot being only 3.7 mm in X and 17.15 in Y. Company which makes inserts admitted their punch was loose but said position is still in spec. I was setting alignment by measuring 2 radiuses of the slot, creating line between their centers and rotating alignment to it, they simply used 4.5 mm basic to create offset alignment which also automatically made 17.15 in Y shorter and almost perfect. They are also reporting TP of the slot's center in only one direction since in the other direction would be perfect based on the offset. I presented position in 2 directions (thanks to all the posts regarding generic features and variables), also tried explaining that TP callout is not only for the center but any point along slot's axis. I need some confirmation or correction to my method.
In CAD part is only squared in Z but slot is skewed in relation to X and Y. I can see your point about two-way locator. I was just simply going by ASME Y14.5M 94 GD&T standard which, for noncircular features at MMC states:
In terms of the boundary for an elongated feature. While maintaining the specified size limits of the elongated feature, no element of its surface shall be inside a theoretical boundary of identical shape located at true position. The size of boundary is equal to the MMC size of the elongated feature minus its positional tolerance.
I guess at the end it still comes down to interpretation of the print. To me what defines direction is slot L axis and 4.5 is only to be used for TP evaluation, specially that there are 2 TP callouts of the slot. Using 4.5 offset and evaluating only center makes perfect TP for one of those callouts.
This isn't a slot with a boundary TP. This is a slot that is Datum C in one axis, and has a separate FCF for the other axis. In fact, you shouldn't even think of it as a slot. It's a feature that, as Rich said below, is a midpoint on one axis, and a midplane on the other axis.
This isn't a slot with a boundary TP. This is a slot that is Datum C in one axis, and has a separate FCF for the other axis. In fact, you shouldn't even think of it as a slot. It's a feature that, as Rich said below, is a midpoint on one axis, and a midplane on the other axis.
