Basic GD&T question
Basic GD&T question
(OP)
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





RE: Basic GD&T question
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
RE: Basic GD&T question
It's also a great question!
Chris, CSWA
SolidWorks 14
SolidWorks Legion
RE: Basic GD&T question
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Basic GD&T question
--
JHG
RE: Basic GD&T question
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Basic GD&T question
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
RE: Basic GD&T question
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/getfile.aspx?folder=4...
RE: Basic GD&T question
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.
www.axymetrix.ca
RE: Basic GD&T question
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
RE: Basic GD&T question
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
RE: Basic GD&T question
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.
RE: Basic GD&T question
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
RE: Basic GD&T question
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
RE: Basic GD&T question
Depends on what you need to define!
"Know the rules well, so you can break them effectively."
-Dalai Lama XIV
RE: Basic GD&T question
But there is one (or many) between datum and feature controleed wrt said datum: http://files.engineering.com/getfile.aspx?folder=7...
"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future
RE: Basic GD&T question
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
RE: Basic GD&T question
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
RE: Basic GD&T question
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Basic GD&T question
RE: Basic GD&T question
Could you please clarify:
Which attached example is this statement about? What do you mean by: "Not all of the basic dimensions originate from a datum, but all of the datums use basic dimensions"?
RE: Basic GD&T question
Sorry for the missing link. Take a look at this attachment.