Section of a section view & section view as base view - Bad practices? 8

[aroy1984]

That taking a section of a section view and using a section view as a base view to project other views is bad drafting practice (except where unavoidable) I have been told often.

But are there any official standards out there which expressly say that such a view is wrong/not to be encouraged? Any specific reasons (logic wise or engineering wise) why these are bad practices?

 

[TWJR]

CH, you take what I say out of context. I said "if". I'm not advocating doing that, since it's pretty much agreed upon that it's a bad practice. I'm making an absurd example of something that's not to be done.

 

[CheckerHater]

if we project from the view that has "broken-out" section, do we show chunk of material ripped out of the part?

I guess we do:

 

[KENAT]

?

Posting guidelines faq731-376

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

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

 

[JNieman]

I can definitely think of one good reason to take a "section of a section" and in that case, I do believe I should show it minus-the-material in the original section. So I guess if you imagine the section-lines being through the center of the part, each time, you'd end up with a quarter-part.

I guess it /could/ be exceptional, however in many contexts it is as common as lunch.

Imagining a cylindrical part with a lot of interior contouring, grooving, threading, and a keyway or broached-grooves parallel to the centerline of the part. I could see a section view being the best Base View, and then a section of that view showing the keyway or groove geometry.

Like I said - I guess "exception" is dependent on context. In the greater world-view of a standards organization, I guess it might be an exception, but it seems it would be a situation that arises often-enough to warrant consideration.

/2cents

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[CheckerHater]

That's exactly why the standard says "should".

"Should" to me means "it's better not to do it the other way, but if you don't have a choice - do"

TWJR and KENAT, sorry, I missed the <sarcasm> tags when preparing my posts.

 

[DiegoLGraves]

Interesting discussion to read thru. I'm currently checking a drawing with sequential views, section B comes from A, C from B, D from C, then there's detail E from the front view, followed by section F from the right orthographic view. The part is sheet metal with 7 planar surfaces. After 35 of fabrication experience, this one takes the prize. The detailer kindly added a pictorial view to the flat pattern; I'll encourage him to follow up with the forming department in case they have any questions.

Diego

 

[JNieman]

@CheckerHater - I could have sworn I read "shall" but I am wrong. Thanks for the correction - that makes all the difference in the world.

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NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[tbuelna]

C'mon, aren't you all engineers?

Isn't the section view of a part basically a planar cross-section having an infinitely thin thickness? And if you take a planar cross-section of that infinitely thin part section, wouldn't the result simply be the intersection of two planes, or a line? What value would the section view of a line provide?

It's not specifically "bad drawing practice" to take a section view from a section view, instead it results in a drawing view that makes no sense.

 

[JNieman]

A section view is not always a section /slice/.

IME, MOST of the time, a section view is merely a view of the part, and what is beyond, with a certain portion sliced away. You still see the mass 'beyond'. It's usually not an infinitely thin slice. I've very, very, very rarely seen that type of section used, actually.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[3DDave]

I agree - most sections are created as if the item was run through a band-saw. It is rare to create an area section.

 

[tbuelna]

A section view is not always a section /slice/....IME, MOST of the time, a section view is merely a view of the part, and what is beyond, with a certain portion sliced away. You still see the mass 'beyond'. It's usually not an infinitely thin slice. I've very, very, very rarely seen that type of section used, actually.

A section view (or sometimes called a section cut) always shows the component shape or construction at the cutting interface defined in the associated view. Sometimes offset sections are used where the cutting interface is defined by angled or offset lines. Sometimes developed sections are used where the cutting interface is a defined curve. But in every case the section cutting interface is defined by a 2D line/curve shown in a 2D view of the the part/assy. So by definition the cutting interface is a projection of the line/curve normal to the view it is defined in. And the result of the section cut are the boundaries of any solid areas that the cutting interface passes thru, or "slices". The only valid features defined in a section cut/view are those existing at the cutting interface. Sometimes features beyond the cutting interface are shown to provide some perspective or clarity to the view, but they are considered as reference and it is not usually acceptable to use them as a basis for defining features at the cutting interface.

The type of orthogonal projection view sometimes defined by a line passing thru the part/assy in a 2D projection view is called an auxiliary view, and they are given the title "VIEW X-X" rather than "SECTION X-X". These are different than section views/cuts since they are interpreted just like any other orthogonal projection view.

A few years back when I worked at BCAG, their drawing practice was that any section view should only show the profile of the solid parts cut by the section interface, with nothing shown in the background. And no cross-hatching was used. The CAD system (CATIA) even had a specific function called "section cut" for making these types of simplified drawing views.

 

[JNieman]

@tbuelna,

I'm in no mood, this morning, to go search through ASME docs to support my statement, and the detail and explanation you give seems like it's probably rooted in something more than "because I did it this way at one company before" so I'm not going to argue. But I will say that what you describe as an "auxiliary view" is quite commonly referred to as a Section View both in industry standards of my entire work history as well as the software defaults of every application I've ever trained on, from Dassault, Autodesk, Siemens, Rhinocerous, and others. It just seems very unusual for the commonplace industry standard as well as the software manufacturers (particular those who /do/ try to base their software on proper specs/standards) to deviate so commonly and universally from the rule you purport.

_________________________________________
NX8.0, Solidworks 2014, AutoCAD, Enovia V5

 

[ewh]

But in every case the section cutting interface is defined by a 2D line/curve shown in a 2D view of the the part/assy.

Be careful when using absolutes - there are exceptions... Y14.3-2012 ¶8.2 states that "The cutting plane may be omitted when its location is obvious", ¶8.5 Half Sections states that "Cutting-plane lines, arrows, and section letters may be omitted when cutting planes are coincident with the center lines", and ¶8.10 Broken-Out Sections states that "No cutting plane is indicated."
And while orthographic views can be defined by a2D line/curve shown in a 2D view, axonometric views are also allowed. ¶9.2.3 Section Views states "Section views may be created
from orthographic and axonometric views. A resulting section view may be orthographic or axonometric."

Section views are not necessarily auxiliary views and visa-versa.
Y14.3-2012 ¶7.7 "Auxiliary views are used in orthographic projections to show true shape and relationship of features that are not parallel to any of the principal planes of projection."
If the cutting plane goes through the part at any point, the result is a section view. If the cutting plane is oriented so that it is not orthoganal and does not go through the part, it is an auxiliary view. Removed views can also be defined by a cutting plane which does or does not go through the part.

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

 

[ewh]

I have to agree with JNieman regarding part background in section views usually being present. Y14.3-2012 ¶8.1.1 Section Views states "The portion of the object between the cutting
plane and the observer is assumed to be removed", which leads me to think that the portion beyond the cutting plane is to remain, unless there is sufficient reason to remove it. This is not addressed directly in the standard, and sections without backgrounds (or partial background) are often used in industry and may be appropriate when drawing clarity is enhanced.

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

 

[tbuelna]

.... But I will say that what you describe as an "auxiliary view" is quite commonly referred to as a Section View both in industry standards of my entire work history as well as the software defaults of every application I've ever trained on, from Dassault, Autodesk, Siemens, Rhinocerous, and others.....

In response to your comment I simply offer the attached screen captures showing the drawing view menu options provided by Dassault's CATIA V5 software application. As you can clearly see there are specific options for orthogonal projection views, auxiliary views, aligned and offset section views, and aligned and offset section cuts.

 

[3DDave]

It sure would be nice if a standards organization would step up and deal with this 200 year plus old mess.

I do have to mention that Catia V5 is a pretty old product and is originated with the French, who don't really have much respect for English translations.

One does have to give the French some credit in drafting. Without them and the snooty British (kickin' a lot of sand on this beach) there would not have been numbered pencils. The Brits had a gigantic lump of graphite and kept the soft, uniform stuff, exporting the contaminated material. A French company got tired of sand-contaminated graphite and ground it up, and mixed it with varying amounts of clay - more clay = Harder pencil. Eventually this resulted in the F, B, and H scales for pencil hardness which was much better than what the Brits made. For more info, Pretrovski's "The Pencil"

Overall, I would not gift any CAD company with being standards compliant, even in areas where there are standards. Some of them bend over backwards to every whim users have, regardless of standards; others aren't even bothered to keep up. I saw one of them dinged recently for failing to currently include the '2009 symbols in feature control frames.

 

[tbuelna]

3DDave-

Don't be too hard on the French and CATIA V5. CATIA V5 is not an obsolete software, and in fact it is a de-facto standard CAD/CAE application currently used by most of the aerospace and automotive industry. As far as drafting standard compliance goes, CATIA V5 user settings can be configured to comply with any standard currently in use or any legacy standard you can think of.

 

[ewh]

The type of orthogonal projection view sometimes defined by a line passing thru the part/assy in a 2D projection view is called an auxiliary view

The problem I have with this statement is that if the cutting plane is "passing thru the part/assy", it is a section view and not an auxiliary view. The Catia snapshot agrees with this (notice how the auxiliary view icon is not sectioned, while the section views are). If the icons are taken literally then I agree that they infer a cutting line thru the part for auxuiliary views, but the arrows are facing the wrong direction; this is not per the ASME standard. I would not base my understanding of the standards on what any CAD software offers, but on the standards themselves.

You don't need an engineering degree to understand drafting standards.

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

 

[Belanger]

Right, ewh ... that's not an auxiliary view. According to paragraph 7.7 of ASME Y14.3-2012, an auxiliary view is just another orthographic view (from the exterior of a part) but one that is taken from an angled surface. IOW, not part of the traditional top/front/side/etc. set of views.

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