Confusing/Odd looking datums 1

[KENAT]

Take a look at the attached sketch, it's a MUCH SIMPLIFIED & INCOMPLETE (so invoke 1.1.4 of ASME Y14.5M-1994 etc.) sketch of an actual case our contract checker came across on one of our parts today.

As far as we can tell looking at the standard B & D are valid, D is basically just used to locate the top radius centrally on the width of the item.

However, it just don't quite look right.

Anyone care to opine?


KENAT, probably the least qualified checker you'll ever meet...

 

[ringman]

I believe that if you were to add a tolerance to the .500 dia, mark thru, and correct the positional tolerance for the hole, then remove the positional tol from the R 2.000 and add a atolerance for it, and add a basic location from datum feature C then add a profile of surface all around you would have a total definition.

And by the way that looks a lot like the piece of toast that I had this morning with a mouse hole in it.

 

[KENAT]

How would I add a tolerance to the R 2.000 and then add profile of surface to it as part of the overall profile?

Anyway some of your post it starting to get off the topic of even my extended question into what I thought was covered by my "SIMPLIFIED & INCOMPLETE" qualifier. Barring anything on why the radius isn't a feature of size or related I think I have what I need for now.

Thanks all.

I had cereal this morning, looked nothing like my sketch;-).

KENAT, probably the least qualified checker you'll ever meet...

 

[CheckerRon]

Most everything sounded good to me except Ringman's last post. Why would you want to change the .500 hole to ± verses basic and position? I don't buy that.

The sketch is OK as is, but I too think profile is generally a better callout for this than position, especially if a large quanitiy of them were being made and fully inspected.
From what I know, that would not be the case here.
I do wonder, however, how the height of the part, from datum C to the 2" radius is dimensioned and controlled.
It would be helpful to see that to fully understand the dimensioning scheme.

 

[KENAT]

You caught me Ron, I couldn't remember how the drawing did it and then decided it wasn't critical to the point of my post anyway so left it off the sketch along with most tolerance info etc. My gut would be overal height, but I'm not sure how well that then works with position, another reason I'd prefer surface profile.

I'll let you tell Gary he's using the wrong control though;-).

KENAT, probably the least qualified checker you'll ever meet...

 

[powerhound]

KENAT,
See 5.2(a) for positional and 1.3.17 for the definition of a FOS. There are no opposed elements on your radius that would give you a 2.000 dimension. Determining a feature of size is as simple as applying the "caliper rule". If you can grab it between the jaws or measure it using the ID fingers then it is likely a feature of size. If you have to use the tail end of the caliper or measure it using a depth mic then it's not a FOS.

See

http://www.tec-ease.com/tips/jul-07.htm

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[powerhound]

KENAT,

Also see

http://www.tec-ease.com/tips/august-99.htm

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[btrueblood]

"...but seemed like as shown in my sketch was the preffered way of doing it."

Based upon the figures in '94 spec., or the text?

;P

 

[KENAT]

OK, I already looked at 1.3.17, just looked at Tec Ease and just read 5.2 (but not the sub paragraphs).

1.3.17 Feature of Size One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension.

(I added the bold 'or')

The tec ease site even mentions that the opposing point 'caliper test' isn't in the standard.

I’m just concerned that it explicitly says “OR” opposed points not “AND”. As such I’m not comfortable saying the pos tol on the rad is definitely wrong, and hence I’m not about to tell someone with a lot more experience then I, including lead checker at some notable aerospace places, that he’s definitely wrong.

However, I’m pretty much playing devils advocate as I’d be more inclined to put a surface profile if it were my drawing.


KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

Btrue, I'm not sure I fully understand the standards explicit meaning on this one. What I think it means is shown in this additional sketch. It talks about on an extension of the dimension line, not on it, which is why I think the one indicated in my second sketch isn't OK.

"...but seemed like as shown in my sketch was the preffered way of doing it."

The only version I see in the standards figures matches my original sketch, so yeah I'm kind of breaking the rules and relying partly on the figures.

However, I learnt long ago that just because the CAD lets you do it don't mean it's right! (The opposite is also true.)

KENAT, probably the least qualified checker you'll ever meet...

 

[KENAT]

Oops, I mean this second sketch.

KENAT, probably the least qualified checker you'll ever meet...

 

[btrueblood]

Fair 'nuff, Kenat, thanks, and a star for the lookup effort.

Original sketch is confusing to my eye (again sullied by continued exposure to older std), but your #2 sketch "OK" makes more sense to me. Yes, the "NOT OK" on #2 sketch is how my CAD allows it, and I have used in past to make the definition more clear (i.e. more like the '82 and prior methods). Another habit to break.

 

[CheckerRon]

Neither of the second sketch examples is OK, but even more confusing.
I like the examples and argument re: FOS on the tec-ease posts.

Regarding positioning the 2" R, I envision the center point of the 2" R basic dimenioned off datum C, either on the part or maybe off in space (if the part is < 2"long), but an overall dimension wouldn't work as I see it.

 

[ctopher]

In the second sketch, you could actually get away with either "ok" or "not ok". I prefer to locate it under the dim number or off of the dim extension line for clarity.

Either way you look at it, there is a 50/50 chance someone will question what is the symbol and what does it mean.

Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)

 

[powerhound]

KENAT,

Either way, the radius is not a feature of size. As I said in my previous post, there are not opposing elements that will give you a 2.000 dimension so it is not a feature of size. Being a non-feature of size also means that it should not be located using position but rather profile.
Regarding the second sketch, in the standard there are defined ways of identifying datums and neither of these examples are one of them.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

OK looks like we're at the point where we can't agree on anything.

I miss typed in 31 Jul 08 13:26 and confused myself, sorry, should have been:

placed on an extension of the dimension line of a feature of size when the datum is the axis or center plane.

Regards the datums, I don't see how the one I label "OK?" doesn't comply with the wording in the standard - it is on an extension of the dimension line, but it does not match any of the examples I'm aware of and looks odd to me.

I don't think the one I labelled "NOT OK?" is correct because it isn't on an Extension of the dimension line, but on the dimension line itself.

Powerhound, I don't mean to be a pain and I can see that the R2.00 probably isn't a good feature of size but I don't see in the standard where it say it explicitly says it must have 2 opposing elements, that's just one option.

1.3.17 Feature of Size One cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces, associated with a size dimension.

Don't get me wrong, I want to be convinced so I can tell my contract checker to sort it out but I still have reasonable doubt.

Anyway, some of this is too pedantic even for me, so I'll go finish upsetting people here.

KENAT, probably the least qualified checker you'll ever meet...

 

[vigildesign]

I fully agree with powerhound's 31 Jul 08 18:00 post.

Marcelino Vigil
GDTP T-0377
CSWP

 

[ewh]

I have to agree with powerhound, but only because it just doesn't look right. I don't know why, as it is correct to anchor a datum to the leader stub of hole dimension; so strictly speaking, the "OK?" example should be correct, as it is also anchored on a "stub" of sorts. If the dimension had a control on it, you could anchor the datum symbol to that, but that is another issue.
Sometimes I miss the old ways, where you would just plop the datum symbol below the dimension.

When the people fear their government, there is tyranny; when the government fears the people, there is liberty. - [small]Thomas Jefferson [/small]

 

[CheckerRon]

Have any of you experienced this?

I have marked up drawings to use a symmetry datum IAW Figure 5-4, only to have the Pro/E designer hang the plunger on the extension line, not aligned with the arrowhead and then wonder why I his drawing got rejected.

As ugly and personally confusing the "OK" datum looks to me in KENAT's second figure, perhaps it makes one think a second time about what is being communicated.

I still am not going to use it, however, but stick to the Figure 5-4 method and show this figure to the people that mess it up.

 

[ringman]

Do we no longer differentiate between datums and datum features when communicating online?

 

[CheckerRon]

Point well taken, ringman.