Is Use of a Center plane for Tertiary Datum Legit? 3

[ringman]

This may be a bit difficult to describe in words alone, but here goes.

We have a rectangular plate, on which we have designated the surface as datum feature A, the bottom edge as datum feature B. So far no problem. The width has been designated as datum feature C which establishes the mid plane of the width. Now the tricky part. We/they have located 2 tooling holes with respect to primary datum feature A, secondary datum feature B, and tertiary datum feature C,(mid plane).

Any thoughts or comments on the legitimacy of this callout,as relates to the tertiary of datum C specifically?

 

[powerhound]

KENAT,

You nailed it.

ringman,
Datum C in not independent of the other datum. A datum reference frame is comprised of 3 mutually perpendicular planes in this case. If you want to control datum feature C perpendicular to datum B tighter than the default implications then use a perpendicularity callout, otherwise, if the part isn't below LMC at any 2 point measurement AND it fits into KENAT's gauge, the part is good. Datum C is not the centerplane of the part itself, it is the centerplane of 2 planes oriented at a 90 degree basic angle to datums A and B and spaced apart just enough so that the part would just slip between them. If there was an MMC modifer on datum C in the FCF then the planes would be spaced out to the MMC dimension of datum C thus allowing C to shift to accomodate the holes if need be.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
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SSG, U.S. Army
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[KENAT]

I must be learning then;-).

KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

I believe our 'sticking point' is whether or not datum feature C is dependent or independent of B.


Drawoh made a point that I belieive is quite significant with regards to locating a point on datum feature B. In theory would work, but not OUR CASE. B HAS TO BE secondary.

 

[drawoh]

ringman,

Datum[ ]B is secondary. Datum[ ]C is tertiary. The bottom, the lower edge and the width are real features, and your centre line is a figment of your imagination. Look at KENAT's drawing and observe how his "centre" is not parallel to the sides.

The whole point of datum specification is that you are telling everyhone how to immobilize your part for fabrication and inspection. If you want datum[ ]C to be equivalent to a centre line, you need to call it up as the secondary datum.

JHG

 

[KENAT]

ringman, my 3rd sketch -22 Jul 08 21:48 - illustrates the 'width' as C/tertiary datum, which you say it has to be.

There is no sticking point, 14.5M-1994 4.2 talks about the part being immobiilized by 3 perpendicular frames. As such the tertiary datum is related to the other datums in that it is perpendicular to both. So it is related to your secondary 'B' datum in so much as it is perpendicular to it.

KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

Kenat

Your sketch does not depict my interpretation, as in fig 4-13 for determination of the center plane, (figment of my imagination). Based on that figure I consider it to be independent of the other features.

Let me make it clear that I do not AGREE with the case that I have stated, but rather am looking to find the proper solution.

I do not believe that a center plane can serve as a tertiary feature.

 

[KENAT]

I'm confused then.

You've posted a question but seem to have already made up your mind and nothing that several posters, some far more qualified/experienced than I, say/explain/illustrate seems able to sway you.

Perhaps if you spent the time and effort to provide an illustration of your specific case and what makes it so different from what other posters and I have said you'd get an answer you agree with.

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

Figure 4-13 clearly states it is referring to 'Primary External Datum Width' this is part of why I say my illustration is "based loosely on fig 4-13 of the standard", since you are talking tertiary and that's what I try to illustrate. If for no other reason than what paragraph 1.1.4 says you should know that we cannot just rely on figures but must also look at the text, though figures are often easier to understand! Look at section 4.5.3 (d), as you state in your 22 Jul 08 20:51 post that that your tertiary datum is RFS.

(d)Tertiary Datum Feature – Diameter or Width RFS For both external and internal features, the tertiary datum (axis or center plane_) is established in the same manner as indicated in c above with an additional requirement: The contacting cylinder or parallel planes must be oriented in relation to both the primary and secondary datum – that is, the actual mating envelope relative to the primary and secondary datum...

Based on this I would hope you see that your tertiary datum is linked to the primary and secondary datums and is apparently legitimate per 14.5.

"Let me make it clear that I do not AGREE with the case that I have stated, but rather am looking to find the proper solution."

I suppose however, based on function maybe your concern is that this scheme doesn't achieve what you think is required.

Perhaps we're coming at it from the wrong angle, what is it that you/they are trying to achieve with this datum scheme?

(Really must stop working on your question and go answer some of my own...)


KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

Kenat,

I NOW see, accept your explanation, and apologize for 'stubborness' on my part.

My thinking in this area has/had been influenced by numerous factors.

Nonetheless, I have concerns in 'our definition of the part'
as it exists.

I sincerely thank you for the enlightenment.

 

[axym]

KENAT,

Powerhound is right, you nailed it.

The diagram you made was exactly what was needed here, and every nuance was correct. Showing how the datum reference frame would be constructed on a part with imperfect geometry is always a great way to cut to the chase.

Well done!

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

Kenat & Powerhound, you each get a purple star for the graphic & explanation.

There is nothing wrong with using a FOS as a datum feature, whether primary, secondary or tertiary. The difference is in the design intent, and from what Ringman describes, it's a perfectly correct callout. As for verification, even for big parts you can use parallels & such to simulate the planes perpendicular to A&B which represent the sides of the feature, then probe off of those rails to find the center.



Jim Sykes, P.Eng, GDTP-S
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