Profile and Datum Question

[JLang17]

Please read:

thread1103-172988

Assuming the said cutout is sized with basic dimensions:

-Would you locate it, with basic dimensions, to the center of the cutout or to the edges of the cutout? (are both ways allowed?) I ask because it seems if it were located to center, then it could spin freely and need a datum reference to lock it in place. But if it's dimensioned to the edges, then you wouldn't need more datum references other than A? I guess I'm looking for clarity as to what referencing a full DRF would accomplish.

-Now my other question (which isn't related to the referenced thread, but possibly applicable). Can you locate a datum feature (with basic dimensions) from non-datum features? I feel like it's an obvious "Yes, you have to in many cases!" but I have yet to find something that covers this. Otherwise it seems you'd have to create a new Datum for the sole purpose of locating another Datum.

 

[ringster]

Datum features are selected as required to provide the necessary framework for part verification. They go together to establish a DRF. The location of these features may be established by non basic dimensions from other features and may also be sized by non basic dimensions.

That is intended to be a NO to your question. Surfaces located by basic dimensions must be referenced to a datum feature.

 

[KENAT]

...But that can be by 'stacked' basic dimensions.

Take a look at figure 6-19 & associated text in ASME Y14.5M-1994, I think it may help you with your first question.

In answer to your second question take a look at section 5.2.1.3 "It is necessary to identify features on a part to establish datums for dimensions locating true positions" also specific to surface profile see 6.5.4.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484

 

[DesignBiz]

JLang17,

In support of Ringster's answer you may want to read pg 51 para 4.2.2.1 states that the DRF consists of 3 mutually perpendicular planes that serve as the "origin" for related dimensions and measurements.

In support of Kenat's answer on pg 81 para 5.2.2 in regard to "stack" dimensions, states that positional tolerancing uses basic dimensions for true position unlike unlike plus/minus locating dimensions which can accumulate tolerance.

DesignBiz

 

[DesignBiz]

JLang17,

This question wasn’t clear to me:

"Can you locate a datum feature (with basic dimensions) from non-datum features?"

I don’t believe you are using the correct terminology which makes the question awkward to me.

A datum feature is a feature of a part used to establish a datum. Datums establish a datum reference frame (DRF).
Positional tolerance dimensions are basic and originate at the referenced datums.

If you are asking if non GD&T dimensioned features of a part can be dimensioned from other features rather than datum's, then I would obviously say yes. However if you are using a positional tolerance callout then that feature must be located with basic dimensions back to the DRF.

Hope I am not confusing the question however as you confidently expressed "yes", leads me to believe you may be
asking a different question that is obscured by using incorrect terminology.

I would be careful of the correctness of a callout just because it has been used repeatedly. I would always advise to reference the standard if something looks doubtful. I have seen countless incorrect GD&T callouts carried over from drawing to drawing largely due to ignorance of the standard.


DesignBiz

 

[JLang17]

Thanks for the replies. I'll try to explain what I'm asking a bit more clearly.

Let's say I have a DRF. Primary A is a surface, secondary B is a hole and tertiary C is a hole. Locating A is easy, it's the surface and would maybe only require a form control. Locating B is where my question comes in. I can only reference A for perpendicularity, but to locate B in terms of X,Y I have no datums to reference. B will need a positional tolerance so I need basic dimensions...but from where? Not C, because C is established based on B. This is where I say Yes, locate B from non-datum features, maybe the edges or centerlines....but I'm just making sure that's ok to do.

 

[ringster]

I think you are trying to make something relatively simple, complicated.

In your case as stated, the location of B does not need to be considered. IT IS THE SECONDARY DATUM FEATURE, where ever it exists. The same goes for C.

Yes, they will have a locating dimension and a tolerance on the part, but they do not need to be located with basic dimensions.

 

[DesignBiz]

JLang17,

I believe I understand and this is a common question. Your previous thread states that datums B and C are holes. I will take that as cylindrical holes. You do not locate B or C from the outside part profile. You can make B perpendicular to A (don’t forget the diameter symbol in the FCF in front of the tolerance). This is all that is required. Datum B is the origin not the outside profile. Datum C (another hole) would be basically located from B with a tolerance of position FCF. You now have established your DRF. You would have basic location dims from B and/or C to position the outside profile along with the profile FCF. You are starting with a plate which does not have an outside a boundary defined; locating hole B perpendicular to surface A; locating datum C with basic dims relative to B; locating other hole positions to DRF A|B|C (just guessing at the datum precedent); and then defining and locating the outside part profile with basic dims for definition; and basic location dims back to the DRF.
It may appear with a locating Datum B from the outside profile with basic dims, as if you are locating Datum B from an edge. You are not. With this callout you are locating the outside edges from the hole.

Hope I got your question right.


DesignBiz

 

[JLang17]

I know I'm over complicating this, I tend to do that...

Ok, so on datum feature B I can forget any locating FCF and just use +/- dimensions?

 

[DesignBiz]

As I mentioned above you normally have a "refinement" callout for a Datum(e.g flatness, perpendicularity, parallelism). The perpendicular callout I suggest to use fulfills this requirement. You would use plus/minus for the hole size limits. No plus/minus location dims for Datum B.

DesignBiz

 

[JLang17]

Ok, sorry I posted that before I read your last post. Thank you very much. Thinking in terms of located the edges from B, rather B from the edges helped me understand.

 

[ringster]

DesignBiz,

Im not sure that I am correctly reading your post with regards to datum B requiring a basic location. If that is what you intended to convey, I respectfully challenge that. It may be likely or more probably to be located by basic dimension, but I do not believe it is mandatory.
Agree/disagree?

 

[ewh]

I think DesignBiz is saying that you do not use a +/- dimension to locate datum B; rather you use a +/- dimension to locate an edge or other locating feature from datum B, approaching it more from an inspectors perspective than a machinists.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]

 

[powerhound]

Here are a couple of links to maybe answer your question. The one with the flange is almost what you are talking about except that it's not square. The second tip is an alternative method if both holes would be located WRT the same DRF.

http://www.tec-ease.com/tips/january-07.htm

http://www.tec-ease.com/tips/dec-07.htm

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[DesignBiz]

Ringster,

My post was based on an outside profile that was defined with a profile FCF for a non feature of size. I apparently misread the original post.
The internal cutout has the profile cutout. If the outside profile is dimensioned with plus/minus dims, then it would be dimensioned from the datums with plus/minus dims as Ewh writes.

The Powerhound posted dwgs show what I was trying to describe, except for the outside profile definition relating back to the datums as this thread is discussing for non featrure of size (width as I interpret this thread's dwg in question). For an outside profile that could not be dimensioned by a tolerance of position, then dimensions locating the outside profile should be from the datums to the outside edge, particularily in the case where that shape is defined via a profile FCF which references the internal datums.

I agree Ringster that the post needed clarification.Good catch.

Thanks Ewh and Powerhound for your posts.

Apologies JLang17 for any added confusion.

DesignBiz

 

[JLang17]

So I've been thinking (overthinking) about how my edges are located to datum B, which is a hole. If datum B is wherever it is (as ringster said), then a positional tolerance does not apply, correct? So my edges would be located with +/- dims. However, if I had a form or orientation control on Datum B, then the edges would be located with basic dimensions because the tolerance exists in the FCF.

Does this sound right?

 

[DesignBiz]

JLang17,
Sounds like you're getting closer, but not exactly that straight forward.

Form controls do not need dimensions because they are in reference to themselves. (E.g. flat, straight etc). Some orientation controls could use a basic dimension (e.g. angularity) some may not (e.g. parallelism).
Profile of a surface which has datum references would control form, orientation and location would use a basic dimension. Profile of a line wouldn’t necessarily need a basic dimension; plus/minus are allowable.

The dimensions that we have primarily been discussing in these last posts are concerning location of the edges. If you were to use a profile of a surface control on the edges, then basic dimensions and datum references (establishing a DRF) would be needed. If no profile control then plus/minus dimensions could locate the edges relative to the datums.

Maybe going back to basics may help you to not over think this. You have a plate with an outside shape, an internal cutout and some holes. The holes and cutout have been located from some internal features designated as datums.
The outside edges now need to be located in relationship to the internal features in this scenario. There needs to be a starting point to get to the edges. An x and y dimension from possibly Datum B hole. If you decide to use a profile of a surface contol then you need to have a basic dimension, if not, use a plus/minus dimension. The other corners could be dimensioned horizontally and vertically (and possibly an angle dimension for the right edge). You can use this outside edge starting point with plus/minus dimensions. If you choose the profile of a surface control then the outside profile requires basic dimensions.


DesignBiz

 

[JLang17]

Ok, I have another question about datums.

I have 2 identical holes that share a horizontal centerline. One of them has it's size called out with 2X in front of it and has a FCF with perpendicular callout. I want this hole to be datum feature B.

If I attach the datum identifier to this FCF, what is the datum feature and datum? Is the datum feature the hole and datum the axis? Or is the datum feature considered to be both holes and the centerline between them the datum?

 

[DesignBiz]

Your last "Or" is most correct. If a datum symbol is attached to a FCF identfying a pattern of features, then the datum is the pattern and not a single feature. The pattern's center is the datum; the shared horizontal centerline (actually centerplane) and a veritcal centerplane at an equal distance between the two.

If you would like one of the features in a pattern to be the a Datum, then you can use a separate leader to that particular feature with the datum symbol attached to it, or the datum symbol itself to the feature.

DesignBiz

 

[KENAT]

There was confusion caused by this on my post about 'implied centerlines'.

The poster put the datum identifier on the leader, so it looked like the pattern was the datum. However, they actually meant for it to be the hole.

Attatched the datum to the hole itself. Though, depending on function there is a line of thought that says using the patter may be correct.

KENAT,

Have you reminded yourself of faq731-376 recently, or taken a look at posting policies:

http://eng-tips.com/market.cfm?

What is Engineering anyway: faq1088-1484