Re: Using 3 Basics to find Position
I think I know what you are asking…. You have a hole that is not square to the datum features that… its position control references for location… but the basic dimensions are specified “square” to that datum reference. Is that correct?
The designer has chosen a reference point along the hole’s axis (probably where it intersects another feature surface a one end) to detail its location. Is that correct?
The customer wants you to figure the hole’s position deviation at that point using all three deviations from basic using the formula 2*(Xdev^2+Ydev^2+Zdev^2)^0.5. Is that correct?
If those assumptions are correct and there is a position control applied to the hole’s size tolerance… then the position control applies for the entire hole’s axis, not just one point at its end where it intersects another feature. To inspect the hole for its position to the datum features the inspector or inspection programmer has to orient and displace a measurement reference normal to the hole’s specified axis location from the datum features. To do that he must first translate the basics specified into equivalent basics that are normal to the specified hole’s axis. When that orientation and reference move is completed he can probe or inspect the hole’s surface for its position location along its axis.
Here is the problem!
The designer, who has never inspected a part, arbitrarily chose to specify basics square to the datum reference frame not realizing the additional work that his choice will cause the inspector and he will argue if pressed to change his specification that “Basics describe the theoretical exact size form orientation and location” and may be correctly specified equivalently in many different ways.
The quality reviewer, who must comply with ISO requirements, has circled the basic as quality requirement and wants a measured result compared to its tolerance value to discern whether the measured basic is within its limits and the probability of a defect is sufficiently remote. He cannot excuse the basic from having individual limits and from being transformed by the inspector… it is all too confusing for him so he will just declare a non-conformance.
The inspector who has diligently transformed the basics into equivalent basic displacements and orientation from the datum reference and discovered the size of the cylindrical tolerance zone required to contain the measured axis deviation throughout the hole’s depth is being asked to report the equivalent X, Y, and Z measured displacements of the axis at the intersection of the hole’s axis and the surface so that the inspection report will match what has been circled for quality scrutiny.
In a perfect world the designer would specify the basics normal to the feature being controlled just like the inspector has to check them and the quality reviewer would not circle basics but rather just the position control.