Doubt on application of GD&T callout, Datum shift. 2

[Vimalmechs]

Hi,

I am having doubt on GD&T application. This is concerned with a drawing came for review. PFA. In the figure GD&T frame calls Datum C with MMB modifier.

Case 1: Datum C is a plane. As per my knowledge we should not apply MMB to the datum deriving from Planar surface. So wrong GD&T application.

Case 2: Datum plane is centerplane of dimension "2X .010".
a) C Datum plane is deriving from the center points of two ".010" dimension.am i right ?
b) Confused What is the datum shift value will be ?

Please shed some light on me,
Vimal. V. V

 

[CheckerHater]

In both cases your datum C symbol is applied to center line, which is illegal.

This is where confusion starts. Your datum C isn't plane or center plane, it's just wrong.

If you are reviewing the drawing, return it with the note that datum symbol should be attached to actual physical feature.

 

[pmarc]

First of all, which standard governs the print ASME or ISO? If ASME, which version, 1994 or 2009?

As CH mentioned datum feature C symbol is applied incorrectly in case #1. In case #2 it also won't work, because there is no actual datum feature from which the center plane could be derived.

Knowing almost nothing about this application, I have a feeling that functionally both flats should serve as tertiary datum feature. If that is true, I would switch 2X .010 dimension into basic .010 and apply profile callout wrt A and B(M) to both flats plus attach datum feature symbol C to that profile feature control frame.

As for MMB modifier on C, if the print is according to Y14.5-2009, and profile FCF was used, (M) modifier could be placed after C. I do not have the standard handy, but search in chapter 4 for an example.

And there is another thing, there is no geometric control between datum feature B and A on the print. This makes MMB of datum feature B and its datum feature shift impossible to calculate.

 

[CheckerHater]

Pmarc,
Could you elaborate a little bit on using non-FOS at MMC/MMB.
First, are you comfortable with datum feature B being not FOS? When you get access to Y14.5 please note that Fig.4-29, 4-30, 4-31 all are using FOS as a datum preceding datum B.
Which brings us to the question: when we switch 0.010 dimensions to BASIC, what are they represent?
In standard Fig.4-30 basic 15 represents distance from center derived from FOS.
On the OP drawing 0.010 is measured from point derived from non-FOS. Is using diameter even appropriate?
I guess it’s the whole “arc-as-datum” thing again, but wanted to see little more of your opinion.

 

[pmarc]

CH,
I am not comfortable with the secondary datum non-feature of size being directly toleranced. But if it was dimensioned using basic radius with profile tolerance wrt A applied to it, I would have no objections. The standard indeed shows preceding datum features being regular FOS, but in my opinion the approach can work with profiled non-FOS surfaces too.

 

[CheckerHater]

OK, I am cool with that.

 

[Vimalmechs]

Pmarc,
my company is following ASME-1994. By referring 2009 standard i am accepting that i can apply C DATUM with MMC by datum derived from profile tol, flatness tol or angularity.
Will you please detail more about the bottom part of your explanation "And there is another point i am curious, "there is no geometric control between datum feature B and A on the print. This makes MMB of datum feature B and its datum feature shift impossible to calculate"

CH,
PFA.In case 3, datum C is deriving from the surfaces, those surfaces offseted from datum B. as like the fig 4-30 in the standard, dimesnion is from dia.
is your question is valid by considering case 3 .("On the OP drawing 0.010 is measured from point derived from non-FOS. Is using diameter") ?

Pmarc, CH and all other members, thank you for all for sharing knowledge thru discussions.The learnings i have gained from here is tremendous and as well world class.Thanks for all.

 

[pmarc]

Vimalmechs,
For simplicity assume that your datum feature B is not less than 180 deg arc but full 360 deg hole. The only two things that dia. 25.12 +0.01/-0.00 would control in that case would be a size of the datum feature and its form via Rule #1. Orientation of datum feature B with relation to higher precedence datum plane A would not be controlled at all. And that would be wrong for two reasons: 1) per the standard mutual relationship between datum features shall be controlled (see para. 4.9 in Y14.5-2009), 2) the very same paragraph states that such relationship is needed to calculate datum feature simulators boundaries (like MMB) properly.

That being said, the print, due to lack of geometric relationship between features A and B, would allow axis of the hole B to tilt without any limitation with relation to A, and that would make MMB of datum feature B impossible to calculate. In order to avoid this, for full hole perpendicularity tolerance with relation to A would probably be defined. Then the MMB of the datum feature B would be: MMB = MMC(hole) - perp.tol

The same applies to your arc-like secondary datum feature. But since the datum feature B is not a feature of size, you should switch directly toleranced dia. 25.12 +0.01/-0.00 dimension to basic radius and control the surface of the arc with profile of surface wrt A. Only then you would be able to calculate MMB of datum feature B wrt A properly. And that would be the size of inner boundary of profile of surface wrt A tolerance zone.

Did I make it any clearer?

 

[Vimalmechs]

yes Pmarc,

In the bottom part of explanation, 1) Datum C is from surface not Datum B, 2)Profile of surface with respect to A and B.

Thanks.