Basic Dimensions 2

[urkson]

I have questions all the time re basic dimensions. Our Engineers will just drop a Basic dimension on the print without any controls. When we question it we are told a. to use the main tolerance control box b. to use the tolerance on the adjacent feature or we get no answer with the 'look' that we should know better than to ask. I do not know if they know how to use the control properly. The problem is neither do I. I think Y14.5 is pretty vague on the subject. Saying a Basic Dimension is the perfect theorectical dimension that can then be controlled, to me, doesn't answer the ultimate question, why use it at all?

 

[dgallup]

You can't use basic dimensions alone. There has to be some associated geometric control such as position or profile. That is where the tolerance comes from.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[JNieman]

It's my understanding that:

A basic dimension will not appeal to any default tolerances. It's closer to a 'reference' dimension than a real dimension in this regard. It is most appropriately used in conjunction with a FCF such as position. If they want to appeal to the tolerance block in the title block or the tolerances in general notes (whichever is applicable) then they need to make it a "live" / "legacy" dimension. Whatever the right name for it is. A plain dimension with no () or box.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[KENAT]

Dimension, Basic. A numerical value used to describe the theoretically exact size, profile, orientation, or location of a feature or datum target. See Fig. 3-7. It is the basis from which permissible variations are established by tolerances on other dimensions, in notes, or in feature control frames. See Figs. 2-14, 2-15, and 3-25.

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(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[JNieman]

That's a much more helpful description than in the 2009 standard, KENAT.

Dimension, Basic
dimension, basic: a theoretically exact dimension.
NOTE: A basic dimension is indicated by one of the methods
shown in Figs. 3-10 and 7-1.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[greenimi]

And, at least in my opinion, one of the most important reason on why basic dimensions are used: TO LOCATE THE TOLERANCE ZONES
Why is that important?
Because a basic dimension can locate a tolerance zone, then the tolerance zone limits the imperfection of a feature's location and/or orientation.

Like someone said here:"While Y14.5's wording doesn't support this nearly as well as I think it should, all explanations of orientation or location tolerances benefit if basic dimensions are thought of as locating and/or orienting tolerance zones, rather than features"

 

[Belanger]

I really tried to bite my tongue on that other thread, but the plain truth is that you guys need some GD&T training, and badly. It's OK if it's not my company, but you gotta look into something if these questions/prints are continually causing debates at work.

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

 

[vonsteimel]

The problem is answered in your first statement.

"Our Engineers will just drop a Basic dimension on the print".... Why are the engineers putting any dimensions on the print? I'm someone that has both a degree in drafting (AS) and in Mechanical Engineering. I can tell you most engineer know hardly anything about drafting. And they definite like to downplay it like there isn't much to learn... which is why its bad news when they start modifying drawings.

But I guess if the engineers are the bosses, basic dimensions are whatever they say they are.
Thanks,

VS

 

[JNieman]

@vonsteimel

It's 2014. Many design engineers have to know how to use CAD/CAE software in many industries. IMO "drafters" are a dying requirement in many industries. At the least, Designers are required, and in some areas, those will be Engineers. In others they will be wise drafters who know a lot about WHAT they are drafting prints of rather than just seeing 'dumb lines'.

_________________________________________
Engineer, Precision Manufacturing Job Shop
Tool & Die, Aerospace, Defense, Medical, Agricultural, Firearms

NX8.0, Solidworks 2014, AutoCAD LT, Autocad Plant 3D 2013, Enovia DMUv5

 

[ewh]

JNieman,
That is very true. Unfortunately, it does not detract from the reality of the "most engineer know hardly anything about drafting" statement. As has been posted on this site before - CAD is a tool, drafting is a skill. An engineer may know a CAD package inside out and still produce worthless drawings.

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

 

[Tunalover]

A computer, via a CAD program, can make junk faster than gold. Well put ewh, many new engineers can be hot on the tube but dumb on doing drawings. Even worse is the engineer who believes that dimensioning, tolerancing, and drafting in general are beneath him and he has no need to know the material.


Tunalover

 

[powerhound]

Unfortunately urkson, you are asking the right questions but getting the wrong answers from the people you work with. You guys would be better off using no GD&T than using this hacked up version the way you are. There's no way you can reject a part for non-conformance because your tolerancing is non-conforming to the very standard you are trying to invoke.

greenimi gave you the definition of a basic dimension in the same way I understand it. I think it's easiest to envision them as locating tolerance zones instead of locating features. The feature control frame is what tells you the size and shape of the tolerance zone. You never default to the tolerance block or anything else. Otherwise, why make the dimension basic in the first place?

You're doing the right thing by questioning this stuff. You may have to resort to doing things their way just to keep the peace AND your job, but their way is definitely wrong.

John Acosta, GDTP S-0731
Engineering Technician
Inventor 2013
Mastercam X6
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[Tunalover]

A computer, via a CAD program, can make junk faster than it can make gold. Well put ewh, many new engineers can be hot on the tube but dumb on doing drawings. Even worse is the engineer who believes that dimensioning, tolerancing, and drafting in general are beneath him and he has no need to know the material.


Tunalover

 

[KENAT]

" Even worse is the engineer who believes that dimensioning, tolerancing, and drafting in general are beneath him and he has no need to know the material." - there are entire threads dedicated to that premise if you look closely enough.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
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[juergenwt]

Urkson - you do not say what system (metric or imperial) you are working in. In Metric if no
tolerance is specified you use Gen. Tolerance as per ISO 2768.

 

[juergenwt]

ISO 2768:

http://www.marylandmetrics.com/tech/DINISO1302extract.htm

 

[KENAT]

juergenwt - would that apply even if indicated as basic?

Posting guidelines faq731-376

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(probably not aimed specifically at you)
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[dgallup]

No. A basic dimension HAS NO TOLERANCE.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[juergenwt]

Kenat - Every dim. even basic must have a tolerance. Otherwise it would be "scale";
One can not have a basic dim of let's say 3" and consider 4" ok.
On metric drawings the gen. tol. class would be indicates on the info block of the print. Check my link to Maryland Metrics.
Here is from another forum:

Question about a basic dimension without a specific tolerance
Published in February 2nd, 2010

Q – If a basic dimension does not show a specific tolerance what tolerance should be applied?

Elton Hughes

————————————————————————————————————————————————

A – Basic dimensions by nature are a theoretically exact value; however, the feature(s) of a part they define as ideal or exact do need to have tolerances to permit acceptable levels of imperfection during manufacturing.

When a feature is defined with basic dimensions, the tolerance for that feature must be expressed through a geometric tolerance. Most often, the geometric tolerance is indicated directly to the feature or feature of size on the face of the drawing; however, some companies include a general geometrical tolerance (such as a position tolerance or profile of a surface tolerance) in the drawing’s general notes. This can be an effective tool when the note is carefully written.

Drawings based on ISO standards frequently use a class of general geometrical tolerances standardized in ISO 2768-2 :1989. The three classes are identified through the use of the upper case letters H, K, or L after the ISO 2768 indication on the drawing. You would need a copy of the standard to interpret the amount of tolerance available.

If you are absolutely certain that a geometrical tolerance has not been applied to the basic dimension (check all drawing views), then the drawing must be considered as incomplete. Without a tolerance, selecting a manufacturing process for the considered feature is impossible to do with any amount of certainty, and there is no acceptance criteria to compare measurements to during the inspection process.

 

[CheckerHater]

@juergenwt:

According to your own post, If a basic dimension does not show a specific tolerance, then the drawing must be considered as incomplete.

What all of that has to do with ISO 2768-1 or -2?