×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Profile and Datum Question

Profile and Datum Question

Profile and Datum Question

(OP)
Please read:

thread1103-172988: Surface Profile with One datum reference - Legal?

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.


 

RE: Profile and Datum Question

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.

RE: Profile and Datum Question

...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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

(OP)
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.

RE: Profile and Datum Question

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.

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

(OP)
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?

 

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

(OP)
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.

RE: Profile and Datum Question

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?  

RE: Profile and Datum Question

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." - Robert Hunter
 

RE: Profile and Datum Question

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

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

(OP)
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?

 

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

(OP)
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?

RE: Profile and Datum Question

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

 

RE: Profile and Datum Question

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: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Profile and Datum Question

(OP)
If I call out the pattern as Datum B and I want to locate another feature horizontally to Datum B, I measure it from the vertical centerplane between the two holes? Or from one of the holes itself?

 

RE: Profile and Datum Question

The dimension to the other feature can be dimensioned from one of the holes in the pattern. The centerplane of the pattern involves the inspection setup regarding the datum established by the pattern.

DesignBiz

 

RE: Profile and Datum Question

(OP)
Ok, thanks for all the help!

RE: Profile and Datum Question

(OP)
Instead of creating a new topic, I'll post this question in here. This should be simple.

Picture a cube room. Datum plane A is the floor, datum plane B is a wall and datum plane C is a wall adjacent to B - creating 3 mutually perpendicular planes. I have a basic dimension from C to the wall opposite C, and a profile of surface FCF on that wall. Ideally this wall will be squared up with the rest of the "room". I'm having a hard time picking the datums to reference.

Here are my two ideas:

1- reference A|C. Perpendicular to A and rotated to be parallel with C. Located with the basic dimension.

2- reference only C. Parallel to C would by default make it perpendicular to A. Located with the basic dimension.

Is this correct? In which case referencing A would just be redundant and I'd go with option 2.

 

RE: Profile and Datum Question

Assuming you selected them correctly originally I'd use ABC, maybe ACB.  This gives you a complete datum structure to fully constrain your degrees of freedom and fully define your wall from.  

It really needs to be decided based on function, which you don't really give details on.

Take a look at ASME Y14.5M-1994 sectopm 4.4.  Or maybe someone else here can give a good explanation.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Profile and Datum Question

(OP)
I can't figure out how referencing A and/or B applies to this situation.

Referecing C locks the tol. zone parallel to C, which in turn locks it perpendicular to A and perpendicular to B, and referencing C also gives you the location using the basic dimension.

Now we can translate toward and away from B, and up and down from A, but how is that applicable? All it's doing is moving within itself.

RE: Profile and Datum Question

You can reference only C, but that will increase the tolerances relative to A & B (tolerance relative to C + tolerances of C relative to A & B).

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

RE: Profile and Datum Question

(OP)
Can you explain that a bit more?  

What changes from only referencing C to referencing ABC?

RE: Profile and Datum Question

JLand17,


I would say minimizing the setup for verification.

Reminds me of an old drawing in the 70's of a cubical part dimensions sufficient to provide oveall size and the note ALL SURFACES TO BE PERPENDICULAR AND PERPENDICULAR WITHIN .XXX.  Sounded ridiculous then and still does.

RE: Profile and Datum Question

Correction to last post should have read ...PERPENDICULAR AND PARALLEL......

RE: Profile and Datum Question

JLang17,

To understand the difference between referencing ABC and referencing only C (or any of the other permutations), you have to imagine what would happen with a real (imperfect) room.  A, B and C are all nominally perpendicular to each other, but in reality they will not be perfectly perpendicular - there will be some variation.  You haven't mentioned what the perpendicularity tolerances between A, B, and C are, but there would have to be some tolerance.

When the profile tolerance is applied to the fourth wall, its the orientation of the tolerance zone in the real (imperfect) room depends on the datum referencing.  If ABC is referenced, the profile zone will be oriented exactly perpendicular to the floor (which means that the zone is not exactly parallel to wall C).  If only C is referenced, the profile zone is oriented exactly parallel to C, which means that the zone is not exactly perpendicular to the floor.  There are similar effects with wall B as well.

If the perpendicularity tolerances between the datum features A, B, and C were extremely small compared with the profile tolerance, then the difference between the different datum references would also be very small.  But if the perpendicularity of the datum features was comparable to the profile tolerance, the effects of different datum references would be very significant.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Profile and Datum Question

JLang17,

Per your post of 4-7=09, the only relationship you have rewquired is a parallel relationship to C.  If that is the case B and A would have no bearing, influence or necessity in the FCF.

RE: Profile and Datum Question

JLang17,

Maybe I am over simplifying the control but I do see you are talking about a cube.

For just what you are doing it doesnt sound like you need a full DRF to accomplish it. You could pick the floor to be Datum A. Use a flat control on A. Use perpendicularity to A for "squaring" it up as you say. This would control orientation to A for all 3 walls. You could use combinations of parallesim and perpendicularity referenced to a single Datum. You could use another datum like C to use a basic dim to the opposite wall with a profile callout, accompanied with parallelism and/or  perpendicularity for the other walls. Your post seems to be concerned with orientation.

As mentioned earilier it would depend more on functionality to make a call for a DRF.  

Would any of the "walls"  fit inside of or outside of another part? which if any of the walls intereface with other parts? questions (and others)you could ask yourself to understand functionality and/or interface with other parts. Then you have a better understanding of what DRF(s) are required.

DesignBiz

 

RE: Profile and Datum Question

(OP)
Let me be sure I'm understanding something correctly... When we reference datums in an FCF we are referencing the datum simulators, which in this case are planes derived from the highest points on the datum features (walls) - thus creating the most nominal "wall" possible. Apply this to 3 walls and we have our DRF.

Earlier I was assuming the DRF was perfectly squared up, but I guess it's not huh? It falls somewhere between the datum features and their theoretically perfect planes.

If this is all correct, I think I understand the answers to my question.

RE: Profile and Datum Question

JLang17,

I think you're getting closer.

Datum feature simulators touch the highest ponts on the datum features, but that's not the whole story.  Another key requirement is that the simulators are perfectly oriented relative to one another.  So the simulator for the primary datum feature makes full contact, but the simulators for the secondary and tertiary do not.

If we were to make a simulator set for the ABC datum referencing sequence, it would be three perfectly flat plates that are all exactly perpendicular to each other.  One plate would make full (3-point) contact with the floor, 2-point contact with wall B and one-point contact with wall C.  The DRF is a coordinate system defined in the 3 plates.

If the datum referencing sequence was different (say CAB), the simulator set would be the same but it would contact the floor and walls differently.  There would be 3-point contact with wall C, 2-point contact with the floor and 1-point contact with wall B.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Profile and Datum Question

The question posed assumed that the floor and 2 walls had controls to "square them up".

A DRF is determined by identified theoretical datums on a drawing (or CAD model). In this case "walls" are being used as datums. Datum simulators establish those datums in an inspection setup.

Lets say an inspection setup is made relating 3 datums (gage plates) oriented to each other. They are imperfect yet are made to more accurate gage tolerances (they establish the "theoretical datums"). The part features  are  manufactured at "production tolerances".

The inspection setup for a particular DRF would orient and immobilize the part by contacting the resultant manufactured features to datum feature simulators (surfaces on the manufactured part in contact with the datum simulators of the setup). (E.g. High points on manufactured floor to surface plate for Datum A and so forth).

 If the manufactured part is oriented with "surface A" contacting the "surface plate simulating Datum A" in the inspection setup; then perpendicularity of the walls to the floor could be inspected to a single dautm. A DRF (3 mutually perpendicular planes)wouldnt be needed.

You can turn the part over in order to contact "wall B" to the "surface plate which establishes Datum B".
At this point you could verify perpendicularity of the walls. (wall B,  wall C and the wall opposite wall C to each other).

When you consider a "basic dimension" from wall C to the opposite wall, you will need to check for location. A profile FCF could be used to control location and orientation.

In this case I would probably use datum C as primary (the dimension being verified is from C to the opposite wall); datum B as secondary (a perpendicular wall to B); and A as tertiary. (To me changing B and A around wouldnt have much impact considering the dimension being verified).

When the word "nominal" is used, it makes me think of size. In respect to a DRF you are establishing 3 mutually perpendicular planes with an origin. No size is involved for this.

It would be true that the simulated DRF (inspection setup) is not perfect. However, the drawing DRF does consist of "true geometric counter parts" of features, which would be considered "theorectically exact".


 

DesignBiz

 

RE: Profile and Datum Question

(OP)
Ok, I think I understand this better, but I'm still confused as to what referencing A or B does in this situation. If C is primary, then the tol. zone is located from and locked parallel to datum simulator C. As a result it is perpendicular to simulators B and A. There's no need to locate from B and A since it would just be locating within itself so to speak. Same for rotation, it would just be rotating within itself.

If this were not a wall, but say a cutout on the wall, I can understand referencing the whole DRF - it would locate and constrain the cutout on the wall, but for the wall itself I don't see the need for referencing A or B.
 

RE: Profile and Datum Question

Say datum A is flat within .010 and datum B is perpendicular to datum A within .015, while datum C is perpendicular to datums A and B within .020.
You lock the additional surface to Datum C.  If the location relative to A and B are of no concern, this is fine and you can control it fairly tightly (though not tighter than the flatness of datum C).  If it is a concern, then you have to allow for those accumulated tolerances controlling A and B.

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

RE: Profile and Datum Question

(OP)
Alright, let's say I was also concerned about perpendicularity to B. I reference C|B in the FCF with a profile tolerance of .010. Does this mean that the tolerance zone of .010 is parallel to C, but within the zone, the wall is rotated as perpendicular as possible to datum feature B?  

RE: Profile and Datum Question

JLang17,

Once the functional requirements have been determined, the proper datum features and relationships can be specified.  Otherwise, you can spend a lot of time speculating on what might be an infinite number variations.

RE: Profile and Datum Question

(OP)
ringster,

I understand that, I'm just coming up with some "what if" cases to help me understand the effects of referencing different datums.

 

RE: Profile and Datum Question

JLang17,

For the profile tolerance on the wall opposite C, all that is necessary is the reference to datum C.  In this case, references to A or B would not constrain any meaningful degrees of freedom and should be omitted.  I forgot to mention this in my previous post, but you've got it right.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Profile and Datum Question

(OP)
Thanks axym, that's what I thought.

RE: Profile and Datum Question

I thought I posted this before but couldn't find it.  If you really only care about parallelism to C, and it's geometry as simple as you make it sound so no tol stack issues, why not consider using a +- dim and a parallelism control rather than surface profile?

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources