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Position tolerance measurement strategy

Position tolerance measurement strategy

Position tolerance measurement strategy

Could anyone clarify, if the position measurement is a centroid feature or does it apply to all points on the surface?
Attached file explains different target strategy for position measurement in a measurement software. It will be helpful to know which one is the right strategy to be used.
Strategy are:
1. Actual element - Which considers all the fit pints on the surface and position is measured to the maximum deviated point.
2. Fitted element - Which considers only the fitted geometry and the maximum deviated point of the fitted geometry is considered for position measurement
3. Fitted position - Position measurement is done to the centroid of the fitted element without considering other deviation on the related surface.

It will be great if you could support it with a standard document as we have difference of opinion in here.

RE: Position tolerance measurement strategy

Mahesch AC:

ASME Y14.5-2009 paragraph 7.2 allows two interpretations for position tolerance zones - a) axis and b) surface (with MMC and LMC modifiers specified). In the RFS axis case, the axis is derived from the axis of the actual mating envelope - see figures 1-1 and 1-2. In the surface case the virtual condition (inner boundary) concept applies - see a). Evaluation/selection of the feature's position (on your sketches) depends on the (drawing) requirement - axis or surface.

Certified Sr. GD&T Professional

RE: Position tolerance measurement strategy

Have you an example of the drawing that these measurements are representing?

RE: Position tolerance measurement strategy

I would like to understand one or two things here.

Is this question about position tolerance applied to a feature of size (hole, pin) or to a surface?
Based on what I see in the excel file I guess it is rather the latter. If that is true then we have to forget about ASME standard, because only ISO allows to apply position tolerance to a surface.

So the second question would be: What dimensioning and tolerancing standard is specified on the drawing, ISO or ASME?
If that is ISO, then ISO 1101 clearly says that the extracted (actual) surface shall be contained between two parallel planes 0,4 apart, which are symmetrically disposed about the theoretically exact position of the surface, with respect to specified datums. So this would mean that only in Case #1 the surface meets the drawing requirement, because only in this case the surface entirely falls within the specified tolerance zone.

In ASME world this would be equivalent to profile tolerance.

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