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Confusing/Odd looking datums

Confusing/Odd looking datums

Confusing/Odd looking datums

(OP)
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...

RE: Confusing/Odd looking datums

Where is A?  It's referenced in the FCFs but not identified that I can see.

I think there needs to be something to define and control the arc lengths of the R.75 bits.

RE: Confusing/Odd looking datums

(OP)
"... it's a MUCH SIMPLIFIED & INCOMPLETE (so invoke 1.1.4 of ASME Y14.5M-1994 etc.) sketch ..."

Just assume A is on a surface in the orientation of the sketch, i.e. perpendicular to B & C.

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

RE: Confusing/Odd looking datums

There is no location for the dim "R 2.000". There is position (FCF) tol, but no dim to locate it.

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

RE: Confusing/Odd looking datums

(OP)
Sorry I didn't feel that was necessary for the point of my post.

Maybe I was unclear.

The issue in question was that having both B, as an edge, and D, as the width looks odd.

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

RE: Confusing/Odd looking datums

Quote:

MUCH SIMPLIFIED & INCOMPLETE

Yes, but we don't know what things are missing, and what things just aren't there.censored

Anyway,  it is "interpretable", although maybe not intelligent.

Any feature located wrt D is Dependant on the actual location of D subject to whatever tolerance is allowed on the 3.000 width dimension.  So it has the effect of making the tolerance zones of such features an oblong, rather than a circle.



 

RE: Confusing/Odd looking datums

Kenat,

How can a datum be confusing or odd?  Firstly they are only theoretical, secondly they are points, lines and planes.  

I think you really meant 'datum features'.

Please accept as it is intended.

 

RE: Confusing/Odd looking datums

As to the sketch,  I do not see a problem other than incompletness.  All that is in place appears to me to be legit per Y14.5.

RE: Confusing/Odd looking datums

It seems legit to me also.  While they may appear odd as used, datums B and D do serve different purposes.
It is more important to locate the Ø.500 hole relative to the edge and the top radius relative to the width, for whatever reason.

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

RE: Confusing/Odd looking datums

Unusual but perfectly legit.

B is fine.  D is fine.  There is nothing that says one can't have both.  Just can't use both at same time.

RE: Confusing/Odd looking datums

I agree that the presence of both B and D is unusual, but I don't see anything wrong with it.

I do have a problem with the position tolerance on the 2.000 radius though.  It's an example of a "partial feature of size" which is a classic pain in the CMM world.  The partial arc (in this case it's around 1/4 of a circle) has no opposed points anywhere, and is technically not a feature of size.  Finding its actual "axis" is highly uncertain if the feature has any form error.  I am also highly suspicious of the MMC reference on the considered feature.  What function would justify letting the arc center drift further away from true position if the arc was a smaller radius?  

I've found that in most cases, partial arc features like this don't function as features of size.  They are just surfaces that happen to be nominally cylindrical.  In my opinion, they should be controlled using Surface Profile.

Sorry for the off-topic rant - in past lives I spent many hours measuring the "size" and "location" of partial arc features and arguing over the results.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Confusing/Odd looking datums

KENAT,

   I agree with the posts above.  Datum D is legitimate.  The positional tolerance referencing datum D is legitimate.

   A positional tolerance referencing datums A, B and C would have been just as legitimate in my thinking, and I would have used a profile tolerance rather than a positional tolerance for the radius.  I find the profile tolerance easier to visualize in a case like this, and I find axym's remarks interesting.

   How about "correct, but poor practise."?

                            JHG

RE: Confusing/Odd looking datums

Datum B is not needed, D takes care of that surface. It's similar to double dimensioning, you have two datums referencing the same surface...or plane.

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

RE: Confusing/Odd looking datums

I agree with Evan on this. The position tolerance on the R2.000 is not good. A radius is not a feature of size and thus not subject to material modifiers nor should it be used with position. Profile of a surface or line is appropriate in this case. The MMC on datum D is legitimate even with profile.  

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

RE: Confusing/Odd looking datums

(OP)
Actually Axym, when I first looked at it my first thought was surface profile for that partial radius.  You may make a good point on the MMC too, it's function is basically as a cover and I think it may be recessed (not sure) if so that may be an issue.

I'll bring those points up with my colleague.

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

RE: Confusing/Odd looking datums

(OP)
Posts while I was typing again!

Ctopher, the hole related to B need to be related to the edge of the part.  The large rad at the top needs to be centered on the width.  Basically ewh got it.

That's why we have both datums, it looked almost like "double dimensioning" to me till I really thought about it and even then still looked odd, hence the post.  However, others here seem to agree that it's fundamentally OK if a bit unusual.

The MMC is functionally OK looking at the function with my checker.  Powerhound/axym, could you clarify why you say it's not a feature of size, you had me convinced but then I looked at 1.3.17 and I don't see that it precludes it, it is a cylindrical surface, albeit not a complete cylinder.  I can see how it may not be a good feature of size to support inspection but, as we barely do inspection that wont be an issuewinky smile.

We talked over the surface profile too, I still lean toward surface profile but it's going to a sheet metal shop and he's concerned it would confuse them, not sure I buy it but still, he has a lot more experience than I.  The current vendor is already making good parts to the current drawing, incomplete/poorly toleranced though it may be.  However, we're tidying the pack to potentially allow outsourcing abroad (as mentioned in other thread of mine) so I'm not sure it's a valid reason for not doing it correctly, if indeed position truly is wrong.
 

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

RE: Confusing/Odd looking datums

Kenat,

Excuse please, this '82-trained engineer who is not completely at ease yet with '94 datum identifiers.

But, Datum D to me at first glance looked like the left edge of the part, i.e. the opposite face to datum B.  If D is intended to be the width (both faces considered together), then it would be more clear to me if the datum "suction cup" was attached to the dimension line for the 3.000 width, and not from a single edge.  Is such a representation legal in '94?  My CAD software allows it, so I've never questioned it.

RE: Confusing/Odd looking datums

(OP)
The examples in -94 show it pretty much as in my sketch.

Quote (3.3.2):

(b) placed on an extension line of a feaure of sie when the datum is the axis or center plane.

My contract checker is more familiar with 82 and asked a similar question.  We talked about moving the D identifier but seemed like as shown in my sketch was the preffered way of doing it.  I suppose I could make an extension of the dimension line and then come off that so the 'plunger' is vertical but I'm not sure if it warrents the effort.

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

RE: Confusing/Odd looking datums

Is the post questioning how to dimension the part or if the selected datum features are correctly defined?

RE: Confusing/Odd looking datums

(OP)
The original question was basically if anyone knew of a reason why having both the width as a datum (feature) for some features and the edge as a datum (feature) for other features was OK, based purely on the fact it looked odd not because I could find anything in the standard that said it was wrong.

I believe I've received adequate responses on this to confirm my belief that it's acceptable, if unusual.

The issue of if positional tolerance is the correct control for the radius came up in axym/powerhounds response.  We're sticking with positional on this drawing but I'm interested in hearing any more arguments in preference of profile for future reference.
 

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

RE: Confusing/Odd looking datums

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.

RE: Confusing/Odd looking datums

(OP)
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 sketchwinky smile.

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

RE: Confusing/Odd looking datums

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.
 

RE: Confusing/Odd looking datums

(OP)
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 thoughwinky smile.

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

RE: Confusing/Odd looking datums

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

RE: Confusing/Odd looking datums

"...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

RE: Confusing/Odd looking datums

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

Quote (ASME Y14.5M-1994):

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...

RE: Confusing/Odd looking datums

(OP)
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...

RE: Confusing/Odd looking datums

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.

RE: Confusing/Odd looking datums

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.

RE: Confusing/Odd looking datums

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)

RE: Confusing/Odd looking datums

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

RE: Confusing/Odd looking datums

(OP)
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:

Quote (3.3.2):

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.  

Quote (ASME Y14.5M-1994):


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...

RE: Confusing/Odd looking datums

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

Marcelino Vigil
GDTP T-0377
CSWP

RE: Confusing/Odd looking datums

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. - Thomas Jefferson
 

RE: Confusing/Odd looking datums

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.

RE: Confusing/Odd looking datums

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

 

RE: Confusing/Odd looking datums

Point well taken, ringman.

RE: Confusing/Odd looking datums

(OP)
CheckerRon, I get that all the time, I'll show the 'plunger' in line with the dimension line on my redline and then the print comes back with it all over the place.

Especially frustrating when it's people that have supposedly had (or had access to) some level of GD&T training.

I'm not saying I'm going to use the "OK?" one but per the text of the standard I think it's possibly legit.

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

RE: Confusing/Odd looking datums

Per my earlier post.
A lot of people don't know what it means anyway.
I have had engineers, purchasing, machinists, checkers and inspectors come to me with a drawing and ask "What does this little triangle with a letter mean?".
With more designs going electronic, paperless, and pushed through quicker...people are not getting the sufficient training for GD&T.
So, IMO...follow Y14.5 to the best of your ability and hope the recipient can read and understand the drawing.

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

RE: Confusing/Odd looking datums

"What does this little triangle with a letter mean?"

You can blame ISO for that one. wink

RE: Confusing/Odd looking datums

thumbsup2

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

RE: Confusing/Odd looking datums

I believe that datums "C" & "D" are irrelevant. The position of the hole is covered by using basic dimensions from 2 edges a and positional tolerance.

RE: Confusing/Odd looking datums

D is used to control the position of R2.000.
C is required if you are going to use a vertical basic dimension to locate the hole (same as B & horizontal).  If the dimension is basic, it has to connected to the related datum in some manner.
Therefore, datums C & D are NOT irrelevant.  It just seems to be an unusual useage, thus the OP.

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

RE: Confusing/Odd looking datums

kanga147: I'm a bit surprised. As ewh just said, basic locating dimensions off datum surfaces is pretty basic stuff. How could C & D not be relavant?

RE: Confusing/Odd looking datums

Yes, I too am curious as to why C and D are not relevant. I'll wait here with ewh and CheckerRon.

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

RE: Confusing/Odd looking datums

kanga147,

   If you do not use datums C and/or D, you have a base and one edge as your reference.  This is inadequate.

                      JHG

RE: Confusing/Odd looking datums

(OP)
Kanga, how much experience do you have with GD&T?  Based on what I know I'd have to side with ewh, ron, powerhound & drawoh.

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

RE: Confusing/Odd looking datums

I hear crickets...:)

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

RE: Confusing/Odd looking datums

sorry I am mistaken, I stand corrected.

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