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

Is Use of a Center plane for Tertiary Datum Legit?

Is Use of a Center plane for Tertiary Datum Legit?

(OP)
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?

RE: Is Use of a Center plane for Tertiary Datum Legit?

Sounds legit. Envisioning basic dimensions from the tooling hole c/l's to B and c/l of C with the holes perpendicularly normal to A.  

RE: Is Use of a Center plane for Tertiary Datum Legit?

If I'm getting the right visual then what you have sounds okay, as long as the datum feature identifier is shown aligned with the dimension line. If the holes are shown centered about, or on, datum plane C then no dimension is required.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Is Use of a Center plane for Tertiary Datum Legit?

ringman,

   I take it you mean you applied a positional tolerance with respect to your three datums?

   It sounds okay to me.  Your datum C is a feature of size.  If this is not an accurate feature, you are going to have to do some tricky tooling, or you are going to have to call it up at MMC.  Is this acceptable?

                      JHG

RE: Is Use of a Center plane for Tertiary Datum Legit?

The callout is legal.

Just make sure that they are creating the datum reference frame correctly:

A: Flat plate (3 point contact)
B: Flat plate, at 90 degrees to the A plate (2 point contact)
C: Two adjustable parallel plates, at 90 degrees to both the A and B plates (generally 1 point contact on each plate)

Regarding legitimacy, what was the reasoning for the datum feature selection?  Why was it one side for B and both sides for C?  One concern is that contracting the vise-like simulator for C might pull the part off of its proper 2-point contact with the B plate.  The likelihood of this depends on the relative lengths of the B and C sides, and their relative squareness.
 

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Is Use of a Center plane for Tertiary Datum Legit?

Legit, yes.  Likely to be confucing to the underschooled.  Be prepared to do much 'splaining.

RE: Is Use of a Center plane for Tertiary Datum Legit?

ringman,

   I am trying to visualize your drawing.

   Datum C should be applied to the width dimension.  Once you have done that, you can draw a centre line up the middle of your plate, and apply basic dimensions to it.  Forget about ± dimensions.  Consider doing this all some other way if the width dimension is not accurate.

                       JHG

RE: Is Use of a Center plane for Tertiary Datum Legit?

Sounds acceptable to me too, except I have had similar parts where I would be using the plate width as datum B (secondary) and the bottom edge as the tertiary.

 So Datum B is my symmetry datum and tooling holes would be in-line with the center of datum B. That way datum B can be clamped 2 points on each side for stability and the machine and/or CMM can find the center axis.

 If the tooling holes were dead center, they would make a great datum B pattern datum.  

RE: Is Use of a Center plane for Tertiary Datum Legit?

Ringman,
I am not able to visualise Datum B since you have defined it as bottom edge, what this means are you trying to define an edge as Datum ?  

RE: Is Use of a Center plane for Tertiary Datum Legit?

2
I agree that fundamentally it sounds legit per ASME Y14.5M-1994, 5.4 is an example of using pos tol WRT Datum Feature Center Planes.

Is the attatched something like what you have?

I've also shown my understanding of what drawoh says about the datum feature at MMC.

 

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

RE: Is Use of a Center plane for Tertiary Datum Legit?

KENAT,

   Your drawing with the alternate FCF is exactly what I am visualizing.

   Let's see what ringman says.

                         JHG

RE: Is Use of a Center plane for Tertiary Datum Legit?

What Ken has drawn is exactly what I was talking about also in my post above.

The only difference being that I like to see primary, secondary & tertiary datums in alphabetical order (e.g. A,B,C), though it doesn't matter what character is used.

RE: Is Use of a Center plane for Tertiary Datum Legit?

(OP)
Ringman says;

That is close on to what I had attempted to describe.
Perhaps a small point, but my part is a LOT bigger.  

My concern is that the mid-plane of datum feature C is established TOTALLY independent of the other 2.  Consequently, in theory will not be perfectly perpendicular to datum feature B.

So whens it comes time to establish the datum reference frame A, B, C, we are trying to center to a line, rather than a point and is essentially  non-achievable.

I have had discussions with some who believed he mid-plane of C is influenced, or driven by B, and that is a point of contention in the discussion,
or has been.

 

RE: Is Use of a Center plane for Tertiary Datum Legit?

Ron, my print is in alphabetical order, or am I missing something?  Like you I've seen the 'width' datum used as the secondary more than I've seen it as tertiary, however tertiary is what ringman says.

ringman, I think swapping the order of your B & C as Checker Ron mentions will solve the problem.  In this case what is currently B will be single point contact so the orientation issue is taken care of by the 'width' datum.

See amended sketch.

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

RE: Is Use of a Center plane for Tertiary Datum Legit?

Oh yeah, as to your size issue, is it really worth even mentioning?  I suppose it might make a difference to inspection methodology but that should't be a major factor in the dimensioning scheme, should it?

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

RE: Is Use of a Center plane for Tertiary Datum Legit?

ringman,

   I suggest that you be careful about discussing imaginary features.  There is no centre plane on your part.  

   Let's use KENAT's drawing as a reference.

   Your datum A, the bottom face, locates your part in one linear plane, and two rotation planes.  Your datum B, the lower edge, locates the part in one linear plane, and one rotation plane.  

   Datum C, your width, locates your part in one linear plane, only.  It is the tertiary datum.  Consider the possibility that your width is grossly non-perpendicular to the lower edge.  A fixture might pick up the top corner on one side of the width, and the lower corner on the opposite side.  

   If the centre defined by the width matters to you, you must follow CheckerRon's suggestion, and make the width your secondary datum, datum B.  A fixture will pick this up at two points on one side, and one point on the opposite side.  Datum B will be perpendicular to datum A.  Your lower edge, datum C, is picked up at one point, somewhere.  

   If the centre really, really, really matters to you, make it your primary datum.  On a plate, this will be very difficult to inspect.

   As I noted above, your whole model is a mess unless your width is accurate.  Using your two tooling holes as datums B and C is a good idea.  Use profile tolerances to control your outline.

                               JHG

RE: Is Use of a Center plane for Tertiary Datum Legit?

(OP)
The alpha sequencing of the datum features is dependent strictly of the design requirements and not alpha sequence driven.

The sketch that Kenat provided, clearly illustrates C as the width of the part.  Therefore the related datum would be the mid or center plane of that feature independent of the other datum features.

I would agree that if we changed the sequence of the datum features to ACB we would have a legitimate callout, however that is not in compliance with the situation of the drawing I am concerned with.  

In the situation I am concerned with datum feature is the orienting feature and C is used for centering of the holes.
Additionally C is RFS.

RE: Is Use of a Center plane for Tertiary Datum Legit?

(OP)
Drawoh

I believe that datum features are real and not imaginary.
I forgot to add that in the previous posting.

RE: Is Use of a Center plane for Tertiary Datum Legit?

By the way I don't see that C (width tertiary per my first sketch) is totally independant.  I think drawoh explains it well above, I've attatched my own effort based loosely on fig 4-13 of the standard (some annotation left out).

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

RE: Is Use of a Center plane for Tertiary Datum Legit?

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
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

RE: Is Use of a Center plane for Tertiary Datum Legit?

I must be learning thenwinky smile.

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

RE: Is Use of a Center plane for Tertiary Datum Legit?

(OP)
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.
 

RE: Is Use of a Center plane for Tertiary Datum Legit?

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

RE: Is Use of a Center plane for Tertiary Datum Legit?

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...

RE: Is Use of a Center plane for Tertiary Datum Legit?

(OP)
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.   

RE: Is Use of a Center plane for Tertiary Datum Legit?

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...

RE: Is Use of a Center plane for Tertiary Datum Legit?

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.

Quote:

(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...

RE: Is Use of a Center plane for Tertiary Datum Legit?

(OP)
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.

RE: Is Use of a Center plane for Tertiary Datum Legit?

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.
www.axymetrix.ca

RE: Is Use of a Center plane for Tertiary Datum Legit?

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.   

2thumbsup

Jim Sykes, P.Eng, GDTP-S
Profile Services
CAD-Documentation-GD&T-Product Development
www.profileservices.ca

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