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?
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
I think there needs to be something to define and control the arc lengths of the R.75 bits.
RE: Confusing/Odd looking datums
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
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)
RE: Confusing/Odd looking datums
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
Yes, but we don't know what things are missing, and what things just aren't there.
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
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
RE: Confusing/Odd looking datums
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
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 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
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
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)
RE: Confusing/Odd looking datums
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
I'll bring those points up with my colleague.
KENAT, probably the least qualified checker you'll ever meet...
RE: Confusing/Odd looking datums
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 issue
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
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
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
RE: Confusing/Odd looking datums
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
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
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...
RE: Confusing/Odd looking datums
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
I'll let you tell Gary he's using the wrong control though
KENAT, probably the least qualified checker you'll ever meet...
RE: Confusing/Odd looking datums
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
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
RE: Confusing/Odd looking datums
Based upon the figures in '94 spec., or the text?
;P
RE: Confusing/Odd looking datums
(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
"...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
KENAT, probably the least qualified checker you'll ever meet...
RE: Confusing/Odd looking datums
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
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
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
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
I miss typed in 31 Jul 08 13:26 and confused myself, sorry, should have been:
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.
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
Marcelino Vigil
GDTP T-0377
CSWP
RE: Confusing/Odd looking datums
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
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
RE: Confusing/Odd looking datums
RE: Confusing/Odd looking datums
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
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
You can blame ISO for that one.
RE: Confusing/Odd looking datums
Chris
SolidWorks/PDMWorks 08 3.1
AutoCAD 06/08
ctopher's home (updated Jul 13, 2008)
RE: Confusing/Odd looking datums
RE: Confusing/Odd looking datums
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
RE: Confusing/Odd looking datums
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
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
KENAT, probably the least qualified checker you'll ever meet...
RE: Confusing/Odd looking datums
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