Datum Translation application

[AMontembeault]

Can a datum translation modifier be used when the datum is a planar surface? I can't see anything in the standard which seems to explicitly prohibit this, but every example out there I can find typically only demonstrates this concept using a datum derived from a feature of size (typically a hole or pin).

My application is, I have a rectangular part, with notches cut on opposite ends (Call them datum A and Datum B). I want to control the depth of the notches relative to their respective end face. So my thought was, control the notches on the A datum face back to A, control B datum face back to A, and control the notches on B back to B with a translation callout (otherwise B is controlled at a basic dim back to A, which would give me the wrong stack-up of tolerances).

 

[chez311]

Sure it can, though it only has an impact if said planar datum feature has location constrain to a higher order datum feature.

A sketch would help but your example does not sound like a proper use of the datum translation modifier. I don't even think it would do what you expect it to do. I don't think you need the modifier to be able to control a notch on A to A, and a notch on B to B, and B relative to A. Depending on your application, calling out B and A as separate datum features, while perhaps convenient, might not reflect function. Seems like you could get the result you desire with composite/multiple single segment profile tolerances.

Again more information about the application and a sketch would help.

 

[AMontembeault]

Here is a simplified sketch:

In this design, DIM 2 and DIM 3 are somewhat critical for fit-up. The part is also too massive to be accurately measured with an arm based CMM. In older days, I would have just used limit dimensioning on DIM 2 AND 3, but I'm trying to be consistent with ASME Y14.5 best practices. If I controlled the profile of the top notch to A, then I'm worried that the stackup between datum surface B and the top notch profile would be too large. Because B is held at a basic dimension to A, I thought maybe I would need a translation modifier.

 

[Belanger]

In that sketch, wherever datum B goes (based on its profile control), the other tolerance that references B will try to follow it. So there's no need for a translation modifier.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[chez311]

If I controlled the profile of the top notch to A, then I'm worried that the stackup between datum surface B and the top notch profile would be too large.

If this is your concern, you probably shouldn't have a tolerance zone for the notch controlled to B which can float in location without constraint. As you have it shown the top notch only has to be oriented to B not located (datum feature simulator for B is free to translate).

Ironically, removing the datum translation symbol would get you exactly what you want - a surface controlled in orientation and location to B instead of A. You could add a parallelism refinement if desired.

FYI you probably want to use the "2X" notation even when using an extension line to connect the two surfaces like Y14.5-2009 fig 4-23.