Basic GD&T question 4

[artnmotion]

First of all I am a beginner with GD&T....so forgive me if I am totally off.

Do all BASIC Dim's emanate from Datum Features? Or can they originate from features that are not datum features?

Thanks!

-Art

 

[CheckerHater]

You use basic dimensions to establish relations between datums and features you are trying to control.

You understand that there is infinite number of shapes and sizes out there as well as infinite number of ways to apply dimensions to them.
So, it's impossible to create strict simple rules like "you must always place basic dimension from this corner to this center"

But the general idea is, yes, you originate your basic dimensions from your datums.



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[ctopher]

I agree with CH.

It's also a great question!

Chris, CSWA
SolidWorks 14
SolidWorks Legion

 

[ewh]

Not all basic dimensions need to be directly to/from datums, as long as a chain of them eventually lead back to datums

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[drawoh]

I agree with ewg. At the very least, you will be applying basic dimensions to features of size. Often, you want to show hole patterns, rather than the location of each hole with respect to the datum.

--
JHG

 

[Belanger]

No -- not all basic dims originate from datums, nor are they always required to form a chain back to datums. I'm thinking of basic dims that define a true profile, such as in Figs. 8-12 and 8-26 of the Y14.5 standard.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

Belanger,

Could you show an example of feature being controlled wrt datum without using basic dimensions (including implied) tieing datum to feature?



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[pmarc]

artnmotion,
Like ewh said, everything is fine as long as chain of basic dimensions eventually leads back to datums referenced in the feature control frame.

This means that what you have in your sketch is absolutely correct and legal.

In the attachment you can find two more options (b and c) that are 100% equal with yours (option a). Option d, however, is not OK because basic relationship between true centers of both holes and datum C derived from datum feature C is missing.

http://files.engineering.com/getfil...-b250-4d48-a9dc-4479d30903f8&file=capture.PNG

 

[axym]

CH,

I agree with John-Paul. There can be basic dimensions to show the relationship between different features, possibly without any datums being present. This is not the same as controlling a feature wrt a datum without using basic dimensions.

Unfortunately, there is an example of a feature being controlled wrt a datum without using basic dimensions. Figure 8-27 in Y14.5-2009. I am not in favor of this practice, but the standard shows it. In general, the dimensions defining a true profile (and its relationship to datums) should be basic (IMHO).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

Thank you Evan,

I missed Fig. 8-27. The definition of Profile is really loose nowadays.

I have nothing against the idea that basic dimensions are not tied to datums where THERE IS NO DATUMS.

I understood OP's question as if the datum was present.



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

CH, even with datums present, the basic dims do not have to be tied to them. Again I refer to Fig. 8-12. The basic dimension of 17.8 has nothing to do with datum A. (The perp to A is a product of the implied 90º rule, not the 17.8.)

Evan, I think the topic of Fig. 8-27 has come up before, but I really don't see why it's an issue. Recall that profile need not always be a location control; in this case it is controlling the orientation to the datums (as well as straightness). That's still a legitimate use for profile.

Sorry to sound like a contrarian today, guys.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[tbuelna]

A basic dimension is one that has no tolerance except that defined in an associated GD&T block. Basic dimensions do not need to be tied to any particular explicit datum feature, but basic dimensions are meaningless without an associated GD&T tolerance block.

The only exception being a tooling datum plane reference frame defined by a set of datum target points. With model based definition of parts like castings, there are usually general notes that state something like "positional tolerance of cast features to be within .030" MMC of datums A,B,C", "profile tolerance of cast surfaces to be within .030" of datums A,B,C", etc.

 

[CheckerHater]

Belanger and axym,

You contradict yourselves.

Both Fig. 8-12 and Fig. 8-27 still have basic zero degrees and basic 90 degrees tying features to datums.



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

Both Fig. 8-12 and Fig. 8-27 still have basic zero degrees and basic 90 degrees tying features to datums.

CH... The question was not whether it's possible to have basic dims that tie back to a datum. The question was whether it's possible to have a basic dim NOT tied to a datum.
So why point out the implied basics? I agree that they are tied to a datum.
Instead, look at the 17.8 dim in Fig. 8-12 -- that is enough to answer the OP's question: That there are such things as basic dimensions that have nothing to do with a datum.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[ewh]

I agree... in my post 13 Jan 15 15:42, I did not allow for situations where basic dimensions are not tied to datums, but I have used them in that manner.
Depends on what you need to define! ;-)

"Know the rules well, so you can break them effectively."
-Dalai Lama XIV

 

[CheckerHater]

Nobody was saying that ALL basic dimensions are connected to datum.

But there is one (or many) between datum and feature controleed wrt said datum:

http://files.engineering.com/getfil...46da-a149-1d99d63c26a1&file=Implied_Basic.JPG

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

Nobody was saying that ALL basic dimensions are connected to datum.

Yes they were, which is why I even chose to make a comment at 23:52 of Jan 13th.

Let's take it from the top. The OP asks: "Do all BASIC Dim's emanate from Datum Features?" Four posts said that the general answer is yes. I politely disagreed, saying that not all basic dims emanate from datum features. A simple example would be a block that is entirely composed of basic dims and then given an "all over" profile tolerance.
Thus, not all basic dims are tied back to a datum.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[CheckerHater]

I agree.

But OP also used word "originate" which may imply ordinate dimensions or chain, where at least one dimension "originates" from datum (see figures attached to pmarc's post)

My response was that you have to connect your datum to your feature using basic dimensions, while it's impossible to predict all the possible dimension combinations.
If somebody interpreted my answer as "YES, EVERY SINGLE ONE OF ALL THE BASIC DIMENSIONS ALWAYS ORIGINATES FROM DATUM" then I am sorry.
Must be my poor English.



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

No problem. I guess we always have to be wary of blanket statements. Most basic dims tie back to datums (directly or indirectly) but not necessarily. Conversely, not all datums make use of basic dimensions. And so on.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[tbuelna]

Attached is an example of datum planes defined by basic dimensions where no tolerance is involved. Not all of the basic dimensions originate from a datum, but all of the datums use basic dimensions.