Datum shift

[Viper555]

Hello,

I'm trying to understand an illustration which uses datum shift to show one possible "means this". I'm referring to Fig 7-58(c) in the 2009 standard. The illustration shows the VC for both cylinders, but it's the location of the three vertical lines relative to each other that I don't fully understand. If you were to apply the datum shift to the max extent possible (at LMC for Datum Feature A), then the part would shift to the rightmost position of the .1 cylindrical tolerance zone and is denoted by the .05 radial distance between the Datum Axis A and the datum feature A. This is clearly shown.

Why then does the distance between the upper cylinder at its leftmost location position mate up with the boundary edge? It seems the .45 distance between the upper cylinder feature axis and Datum Axis A would be reduced to .40 since you would be taking .05 away due to the shift. I'm not saying you lose your total positional tolerance, but rather you would have .40 to the left of the Datum Axis A and the remaining .05 to the right.

Thanks in advance.

 

[3DDave]

Datum shift is applied solely to the datum feature. The measurements are made from the datum feature simulator, not the as-made feature.


The datum feature is shifted in the datum feature simulator. The controlled feature is allowed to move in the Virtual Condition boundary.

 

[Viper555]

I'm still not sure I understand the final measurements in that figure.

Suppose that both cylinder features were produced at LMC and that the upper cylinder was at its maximum positional deviation of .45 from Datum A. Since Datum A is always fixed, wouldn't any radial shift of the part, and by extension the lower cylinder, permitted by datum shift cause the .45 distance to change by a commensurate amount? .05 in the case of Fig 7-58 which would then cause the the axis of the upper cylinder to now be .40 from Datum Axis A?

 

[3DDave]

The measurements are from the datum represented by the axis of the datum feature simulator, not between the features.

The problem is the problematic vocabulary. The correct term should be datum feature shift. The datum itself never shifts. Since the committee is inconsistent in using the English language, you end up with this question.

In this case, because they are allowing the datum feature to shift relative to the datum, the distance from the datum feature axis to the feature axis is the sum of both the shift allowed to the datum feature plus the location variation allowed to the considered feature.

For this example the feature is shifted left 0.45 and the datum feature is shifted right 0.05 for a total distance between them of 0.50.

This generally isn't going to happen with datum feature shift but they have contrived this example to try to illustrate the effect.

 

[Viper555]

Ah. So it's just the feature that's shifting and not the entire part. Thanks!

 

[Burunduk]

Ah. So it's just the feature that's shifting and not the entire part. Thanks!

I consider the feature and the entire part to shift together as long as the part is a solid one-piece object.

The main idea is that the simulated datum and the tolerance zone or virtual condition boundary (as shown in the 2009 7-58 example) are one fixed system, and you make the actual part interact with it by mating the datum feature with the datum feature simulator according to the applicable defaults or modifiers and the related rules.
The standardized term "Datum Feature Shift/Displacement" (4.11.9 in Y14.5-2009) is related to material boundary modifiers, and describes the "play" available in that interaction to optimize the fit of the feature being controlled to the tolerance zone or virtual condition. In 7-58(c) the part is moved all the way to the right as enabled by the difference between the actual datum feature and the datum feature simulator sizes, so that the controlled cylinder is just barely inside the VC boundary. You can see that this allows the maximum total of a 0.5 axis to axis offset. Had datum feature shift not been available (RMB) or less of it available (MMB and larger actual mating size of the datum feature), the big cylinder would end up violating the VC and non-conforming.

 

[3DDave]

Great catch. But.

Right in the '-2009 explanation of datum feature shift:

If the datum feature simulator geometry is
such that it does not fully limit or constrain the feature
such as rotating away from the datum feature simulator
beyond the established boundary limits, as shown in
Fig. 4-31, illustration (c), then the feature must remain
in contact with the datum feature simulator, and datum
shift or displacement is not allowed

The inconsistency in using the standardized term and failing to use it again anywhere else in the standard is typical. Clearly not a concept worth their time to mention in the examples. Instead they use the standardized term "displacement" multiple times. Clearly the "shift" concept is left in the document to avoid having training companies need to rewrite their older materials.

This error was corrected in the next version, but the "shift" concept wasn't referenced anywhere else in the body of the document in 2018 either.

But great job for being on language patrol. Those Tec-Ease guys need a kick in the butt as well. They have several like the following:

"Play" is a term not included in the standard.

The part is also moved all the way to the left. Same thing, as it is contrived to trap the part between extremes of the feature and datum feature.

Had there been thought given to it, it would have been part shift to allow the datum feature(s) and the considered feature(s) to meet their respective surface limitations set in a rigid combined datum reference and feature control coordinate system.

 

[Viper555]

So if you are indeed shifting the part, then I guess I'm still confused.

Before the shift scenario:
The upper cylinder feature is produced exactly at MMC .45 away from the fixed Datum A and the lower cylinder is produced at LMC at true position.

Shift scenario:
The lower cylinder (and entire part as a system) can shift maximally .05 radially away from fixed Datum A due to MMB.

After shift scenario:
Since the entire part has shifted, the axis of the upper cylinder is now closer to Datum Axis A by the shift amount (.05 radially in this case).

How is the upper cylinder still .45 away from Datum A after the shift instead of .45 minus the shift?

EDIT: I just realized, the requirement of the .045 positional tolerance from Datum Axis A applies AFTER the shift and not before. I think that was the source of my confusion.

 

[3DDave]

They are simultaneous motions of the part. If the feature was farther than 0.45 and there was not more than 0.05 available the part would not fit in the gauge/would not be acceptable/ would be rejected.

This example is the case where all of the datum feature shift is used to exactly offset and accept all of the feature position variation.

The Datum Reference Frame exists independently of the part, which is what makes it so useful - one can create a gauge before any part is made and can be used to accept or reject every part that is made. There are some cases where firm contact is required, so-called "regardless of feature size" and others where the part is allowed displacement between the datum features and the datum feature simulators (MMB and LMB using the MMC and LMC symbols on the datum feature references.)