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.
Can I take opposite points on the radius (since it will be skewed to CS) or only on straight edges of the slot? Those are very short and midpoints based on sets from straight edges only will still be good. I could just use math to show that if the midpoint of the set 2 mm to the left from the center is so much off in Y than the midpoint 6 mm to the left will be that much off based on ratio but I would like to avoid this.
Can I take opposite points on the radius (since it will be skewed to CS) or only on straight edges of the slot? Those are very short and midpoints based on sets from straight edges only will still be good. I could just use math to show that if the midpoint of the set 2 mm to the left from the center is so much off in Y than the midpoint 6 mm to the left will be that much off based on ratio but I would like to avoid this.
Well, if the slot is skewed so far that taking opposed points would mean that it was measuring into the radius, that would still give you really bad results (the cosine error from the slot being skewed would pretty much cancel out because it would be opposite on each opposed point). I would place the points on CAD, and just let them measure as they are.
You could also do a linear closed scan of the slot and dimension it as a profile with graphical analysis to show how it is deviated.
Or you could measure the radius on each end and dimension them. Their centers would basically be equivalent to the midpoints of the opposed sides at the tangent.
