RMB for width as tertiary

[Viper555]

I'm having issues with a concept in particular that I was hoping someone could shed some light on (Y14.5-2009).

Part in question: Slender bar with hole in center of larger face (through thickness) and one hole on each side mirrored about the center hole (on same face). Larger face is primary (FS or NS not important), center hole is secondary (perpendicular to primary), and center plane of width is tertiary (position to primary and secondary). All datum reference callouts are RMB.

The example I'm looking at explains how by not using a translation modifier on tertiary, it is possible that the collapsing simulator on tertiary can cause the simulator to not grab on to both sides of the part at the same time when calling out a positional tolerance to primary, secondary, and tertiary of the mirrored holes. I believe the reasoning is that since the secondary datum takes precedence, the converging simulator only does so to the extent that the center plane it derives is coplanar with the secondary, even if only one side makes contact.

It further explains that only by using the translation modifier would you permit the total collapse of the simulator, but would result in a center plane not coplanar with datum axis B.

I can't find anything in the Y14.5-2009 standard that supports the idea that RMB does anything except collapse entirely on the part, but I'm sure it's in there somewhere. Could someone point me in the right direction? Hopefully my example is clear enough.

Thanks!

 

[Burunduk]

The standard requires datum feature simulators for datum features referenced in the same feature control frame to be basically located and oriented to each other, unless the translation modifier or a customized DRF is used *. In the 2009 version It appears in a sub-section of section 4 called "datum feature simulator requirements" or something similar. So yes, that basically means the tertiary datum width simulator must stay centered to the secondary datum hole axis. At RMB this can and probably will cause one-sided contact between the tertiary datum feature and simulator.

*Another exception they forgot to mention in that core definition is when a planar surface used as a secondary or tertiary datum feature has a location relationship to a preceding datum axis, as discussed in your other recent thread.

 

[3DDave]

If all three holes are to be placed such that a plane passing through their nominal axes is nominally coincident with a plane in the center of the bar, then define the holes such that the short direction is controlled to the width of the bar and their spacing along the bar is controlled separately with a separate position tolerance to control the diametral position requirements among them however you like.

Alternatively, since the original hole is centered, just use one face of the bar.

The original case you make is that the location of the bar is to be locked in place by a fastener that expands in the center hole and some overall vice that collapses while centered on the hole to grab the outsides of the bar. In actual practice this tries to shear off the fastener if the vice is strong enough and the material doesn't deform.

If you have an example mechanism that has RFS constraint on both the hole and the width, please post a picture of it.

 

[Viper555]

Thanks for the helpful answers! It's just an example from a workbook.

 

[Burunduk]

Thanks for the helpful answers! It's just an example from a workbook.

A good educational example should start by telling about the use of the part, how it mounts and works. The selection of datum features and geometric controls should follow from that. Too many textbooks include mainly RFC examples to teach the concepts. RFC stands for Regardless Functional Considerations (a term I just made up).

 

[pmarc]

The illustration on the left in the book does not look right to me.

It correctly shows the tertiary datum feature simulator at RMB as a pair of two parallel planes perpendicular to A and centered on B, but the planes are not at the smallest possible separation from each other. The planes should have been contracted further, which would result in a rotation of the workpiece around datum axis B in this view.

 

[Burunduk]

The illustration on the left in the book does not look right to me.

It correctly shows the tertiary datum feature simulator at RMB as a pair of two parallel planes perpendicular to A and centered on B, but the planes are not at the smallest possible separation from each other. The planes should have been contracted further, which would result in a rotation of the workpiece around datum axis B in this view.

pmarc,
I assume the reason for this is the small gap visible between the upper jaw of the datum feature simulator and the left side of the actual part being inspected. Am I correct?

 

[pmarc]

I don't know. To me, the simulator should clearly touch both surfaces of datum feature C.

 

[greenimi]

I don't know. To me, the simulator should clearly touch both surfaces of datum feature C.

Pmarc,
Do you think the "1/3 rule" is applicable here? I mean, the datum should not be established from 1/3 of the applicable surface/datum feature or something like that (if I remember correctly).
Or maybe this 1/3 rule is applicable only to the primary datum (a rocker datum feature primary)?

 

[Burunduk]

I don't know. To me, the simulator should clearly touch both surfaces of datum feature C.

I think I'm not sure I get it. It is because datum feature simulator B doesn't constrain rotation?

Edit: I don't think datum feature simulator C is allowed to rotate about datum axis B while contracting, if that has to do with it.