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nrostrander (Aerospace)
3 Jun 09 12:15
All,

     I was reviewing a drawing and came across a pattern callout as shown in my attached example.  Is this right?  I have not seen it done to this format prior to today that I can recall.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

KENAT (Mechanical)
3 Jun 09 12:18
No, as far as I'm aware assuming ASME Y14.5M this is not legal.

They are defining both datums twice.

I guess the intent is to indicate that the hole pattern is centered in the part but this isn't how to go about it.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

nrostrander (Aerospace)
3 Jun 09 12:32
Kenat,

     Thanks.  I was sure it wasen't.  What section of the ASME would call this out?  The part being created, must have the holes centered on the plate.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

KENAT (Mechanical)
3 Jun 09 12:42
3.3.2 of the 94 version kind of addresses not having multiple datums with the same letter.

As to 'implied centering' or symmetry.  This is an area of some debate.  Look at figure 5-4 for an example in the standard.  However, this example isn't explicitly demonstrating 'implied centering' so some people doubt it's applicability to the question.

To my mind, if it's obvious the featurs are centered, and the tolerance FCF reference appropriate datums you don't need centering dimensions or similar, they just clutter the drawing.

 

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

nrostrander (Aerospace)
3 Jun 09 12:44
Kenat,

     This is what I beleve it should look like.  Comments?

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

KENAT (Mechanical)
3 Jun 09 12:52
I meant to reference thread1103-240068: Implied Centerlines/Symmetry on the centering issue.

Looking at your re-post.  I don't think it's right.  I'm not familiar enough with correct application of symetry control to go into detail without doing research I don't have time for right now.

I would leave off the symetry FCF and just add centerlines to the part to make it obvious the hole pattern is nominally centered, much like figure 5-4.

To me the reference to B & C in the position FCF, combined with showing nominally centered (aided by Center Lines) is perfectly clear.

KENAT,

Have you reminded yourself of FAQ731-376: Eng-Tips.com Forum Policies recently, or taken a look at posting policies: http://eng-tips.com/market.cfm?
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

dingy2 (Mechanical)
3 Jun 09 13:22
This revision is still incorrect. The symmetry feature control frames just don't make any sense at all.

Take the symmetry FCFs out of the design and then you would have the face as datum A, overall width as datum B (would end up the centre line of the width) and overall length as datum C (again, the centre line of the length). The positional feature control frame is now correct as long as there is a tolerance on the hole size.

Hope this helps.

Dave D.
www.qmsi.ca

JLang17 (Electrical)
3 Jun 09 13:46
Don't datums B and C need a condition modifier in the FCF? Due to datum shift.
KENAT (Mechanical)
3 Jun 09 13:53
JLang17, that would have to be decided based on function.  You don't 'have' to have the modifier.

KENAT,

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Helpful Member!  SeasonLee (Mechanical)
3 Jun 09 13:58
Besides all problems highlighted by Kenat and Dave, here is another mistake on the re-post:
Material modifier is not allowed on Symmetry callout.

SeasonLee
 
dingy2 (Mechanical)
3 Jun 09 14:13
SeasonLee:

You are correct about the modifier not allowed on symmetry but the symmetry FCF is all wrong in this application so it should be deleted.

Dave D.
www.qmsi.ca

SeasonLee (Mechanical)
3 Jun 09 14:29
Dave: Yes, the symmetry callout is definitely wrong here, I meant for all symmetry application, material modifier is not allowed.

SeasonLee
 
nrostrander (Aerospace)
3 Jun 09 19:50
So this is how I now understand it if there were two differnt patterns.  Comments?

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

SeasonLee (Mechanical)
3 Jun 09 21:05
1. Vertical basic dimension missed for the four big holes.
2. Where are the basic dimensions for the four small holes?
3. Datum D is illegal here, datum C is good enough as the tertiary.

SeasonLee
 
nrostrander (Aerospace)
3 Jun 09 22:51
Season,

     I see the missing spacing dims.  I should have stated that I was wondering if perhaps datum D was good or not.  I don't understand as to how the centering pattern of the smaller holes could be made using C.  Unless would it be understood that the 4 smaller holes are centered between C and the edge of the part.  I suppose for this, I would be better off just basic dimming the smaller holes from the center of the larger holes or the edge.  This is why I was wondering if the same concept used for symmetrical of a slot (fig 5-61), could be used for a hole pattern alignment.

P.S. - Spacing dims added.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

SeasonLee (Mechanical)
4 Jun 09 11:50
Try to use the 4 big holes as a pattern and assign as datum D, add the necessary basic dimensions wrt datum D on the 4 small holes, then the 4 small holes' positional tolerance can be called out as |TP|Ø.001|A|D|.

Attached is an example (from Geo-Metric III page 270) for your reference.

SeasonLee
 
looslib (Mechanical)
4 Jun 09 12:10
You cannot assume anything such as holes being cenetered just because they have GD&T to the center of the part. You have to dimension one hole from the edges of the part or the implied part center.  

"Wildfires are dangerous, hard to control, and economically catastrophic."

Ben Loosli

SeasonLee (Mechanical)
4 Jun 09 14:38
Ben

Please ref to the original post, its a symmetrical part.

SeasonLee
nrostrander (Aerospace)
5 Jun 09 22:17
I got this answered by a 40+ year ME with a PHD and a professional GD&T certification.  Having family and friends in the same field as myself is VERY helpfuly.  I thank everyone for this answer, especially him!

Attached is the answer to this ever-growing debate as to if the figure in the ASME is missing dims or not.  See attached document for full details.  I was not supprized as to the outcome personnaly. thumbsup2

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

powerhound (Mechanical)
6 Jun 09 1:49
Actually, the example that you have marked out as wrong and incomplete is the correct example. A pattern absolutely CAN be understood as centered, especially when centerlines are used as you have shown. When a feature or pattern of features is shown centered, it is implied centered unless there is a dimension to define it otherwise. This is a commonly understood principle. It's in Geometrics IIIm by Lowell Foster and in Fundamentals of Geometric Dimensioning and Tolerancing by Alex Krulikowski. Also see this tip and take note of where it says that many people would add a half dimension from center the to edge of a feature that is shown centered and they would be incorrect to do so.

http://www.tec-ease.com/tips/mar-08.htm

The example you are showing as correct and preferred is incorrect because datums B and C are the centerplanes of the part yet you are showing a basic dimension from the edge of the part. This is not good. What if the outside dimension of the part is big to the top of the tolerance? Let's assume .030" is the top. Do you check the holes WRT to the center of the part or WRT to the edge of the part? According to your FCF, you would check it to the center and ignore the .500 basic dimension thus giving it the status of a reference dimension. The example shown as "right but not preferred" would be better labeled as "acceptable but not checkable".

Are you sure your 40+ year ME has a GDTP certification?  

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

nrostrander (Aerospace)
6 Jun 09 3:56
Powerhound,

     I will have to look into this data you have provided me.  I currently do not have a copy of the book to which you have utalized as to your source of referance.  I will order a copy on Monday.  As for your statment as to renaming the view as "acceptable but not checkable", I would agree with you.  That statment is made better, though I was sent the dwg and just added the comments at the bottom.  I did not even read into it for grammer puroses.  As for the ME, I know for fact he has taken the courses and got certified years ago when I was young.  I know this only because he, along with my father, was always quizing me on ISO, ANSI, and ASME documents. (Never tell someone what you may want to be when you grow up, you just may not like the outcome,)  I dispised it, mainly because they only wanted to have me, shall I say, qoute the standards (the basic stuff) and memorize their sections (teaching me would have been prefered).  I will mention though, that he has rarley ever let me down. On the otherhand, he was rather confused as to seeing the datum callouts in the current standard (he's use to the rectangle with the letter between the dashes).  Also, he is retired, so perhapes he is loseing it these days?  I will defenatly look further into this, as far as i can see though, no where in the standards does it state that just because it looks to be cenered, it shall be understood in that mannor, thus the "implied" intent is not finite.  That is the real question in debate.  I am not disregarding your referance, but I can not consider it to be finite without performing an in-depth analysis of your afromentioned referance.  I will say that i have heard a great deal as to that book being very intuitve and explanitory.


     On a side note, I jsut read thrugh the entire GD&T portion (basically 75%) of the Y14.5-1994 (Reaffirmed 2004) verion once again to verify his statments agenst the aapplicable stadard our company is attempting, key word there,to follow.  No where does it stat that his definition may have been misconstruded. But like I said, I will still get a copy of the afromentioned book for utalization and scrutiney.


    Last thing, and I do not mean to be sharp, may I point out, that just because showing a center mark in the middle of a feature pattern whoms centerlines look to be centered within a given set of datum planes, that one can not guarentee it's centering upon the FCF's referanced datums.  take for instance that the designer/draftsman was drawing the part and placed the holes on the part. By the way, dimensing is always done after modeling the part. Then he proceeds to model the rest of the part in question.  Upon completion, he then proceeds to work on the rest of the assy to which the part gets applied to.  After may hours, and/or days of work, the task of dimensioning the part begins.  Just because he drew them in what appers to be centered, he just uses the FCF Positional tolerance callout and leaves the centering lines.  What if the holes were built as "implied" centered yet were actually .010 off center?  This is what GD&T is suppose to prevent.  So the unofically documented "implied" centered, should truly never be allowed.  Now if it were a adopted practice, and prperly documented as such, then I would have no problem with it.  The point of this post was that a part in question was improperly specifying a patterns symmatry along a pair of perpindicular datum planes.

   All-in-all, I do than you for your consideration into this matter, and do take your comments with great consideration.  Perhapes everyone should write the ASME committee about this topic, and have then defin it it the best, and most accurate and majoraly accepted mannor.  I would love to have this debate put to rest by the standards.

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

SeasonLee (Mechanical)
6 Jun 09 4:15
1. Left figure (preferred method) is not correct for the following reasons :
*Datum symbol B and C placed in line with size dimensions, it means a datum center planes established, any basic dimension (if needed) should start from the datum center planes instead of from the edge, so the basic dimension .500 and 1.250 should be deleted. Actually, these two dimensions are not required at all.
*You should have tolerance on the hole size since there is a MMC on the positional tolerance callout.

2. If you really care about the holes location wrt the center planes, I will recommend using the composite positional tolerance to control it, I don't think symmetrical callout is a right way here.

SeasonLee
 
nrostrander (Aerospace)
6 Jun 09 9:16
morning

Ok, I knew this would cause more debate. lol. Though I will admit the I do enjoy constructive critisizum. :D

Season,

As for your comment about basic dims should come off of the CL's, I will contact him as to if he may have ment for them to be that way.  I totaly understand where your comming from.  And in reguards to the symmetry symbol, it is, technically, correct, but how is one to percicely verify that.  Not every part made will be the same, as is the pourpose of tolerance to begain with.

On a side note, perhaps we can cheat the system, as my attached image shows.  There is no dought in my mind that it is understood and fianit.  The only catch here is perhaps, I should show the entire part to ensure clarification. (note: This is, for lack of a better term, a cop-out method.)

 

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

powerhound (Mechanical)
6 Jun 09 11:37
nro,

  Regarding your scenario about holes being .010 off center, if the holes were supposed to be centered but were actually built .010 off, then whatever tolerance you have in your FCF will let them know whether or not it's good. In the case you have presented it would be out of tolerance.

  Regarding your latest submission, this is no good either because the ASME standard specifically prohibits datums indicators from being attached to centerlines. Here's another tip for your review. Be sure to watch the video because there's a little more information on it.

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

  Going over the Fundamental Rules in the standard should alleviate any of your concerns over "implied" dimensions. The standard is FULL of them; implied 90s on features shown as perpendicular, implied positions of features oriented at 90 to each other, implied patterns using the simultaneous requirements rule and the list goes on. Just go back to the previous tip I sent. Don Day shows just a few of the implications you can legally make.

  The new datum indicator has been like that for over 15 years so if your ME friend was surprised to see it, then he has certainly been out of the loop for awhile.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

axym (Industrial)
6 Jun 09 12:03
I'm basically in agreement with powerhound on this one, with a few things to add.

The dwg labeled as wrong and incomplete is laid out exactly like Figure 5-4.  It is implied that the hole pattern is nominally symmetric relative to the centerplanes.  While this "implied basic symmetry" is commonly applied in industry and is in several GD&T textbooks, one could argue that it isn't explicitly stated in the '94 standard.  In the Fundamental Rules section on page 4, there are rules about implied 90 degree angles but no similar rules about implied symmetrical relationships.

The dwg labeled as right and preferred is wrong.  As SeasonLee pointed out, the extra basic dimensions should come from the centerplanes and not the edge of the part.  Figure 4-8 on page 58 of -94 shows an example of this.

I've never seen anything quite like the dwg labeled as right and not preferred.  The Symmetry characteristic has very few applications in the first place, and this is definitely not one of them.  Symmetry is usually applied to features whose surfaces are nominally symmetric relative to a datum plane, such as the slot in Fig 5-61 of '94.  It cannot be applied to entities like basic dimensions or imaginary lines through holes.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

nrostrander (Aerospace)
6 Jun 09 12:48
Ok, I got an email back from my source.  He stats that he ment for the dims to be off the CL's and not the part edges. Also he apologizes for the error.  So it is not implied but defined as off the CL's.  I don't beleve that I need to sketch this, as I for one can agree with everyone else as to how the dims should have been off of the CL's, but I did just for everyones sake.

See attachment for dim correction.

Side note, until I see a admenment to the standards stating implied centered, then I will not use it, I will go by this.  But I will state that I have personally took into consideration everyones comments on this subject matter.  Perhaps this subject should be dead, but untill it is finite in a standard, I can foresee this being an ever-growing debate. sadeyes

P.S. Hole tol can be whatever you wish it to be. Just an example people.  

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

SeasonLee (Mechanical)
6 Jun 09 13:01
The extra basic dimensions .750 and .500 is not required at all, what is the purpose of these two dimensions ?

SeasonLee
nrostrander (Aerospace)
6 Jun 09 13:21
Everyone,

     I just had an ipifany (rare for me), I decided to look outside of the box, we are all missing a very improtant referance, ASME Y14.2 (Line convensions and Lettering).

Quote: "Center Lines are used to represent axes or center planes of symmetrical parts and features, bolts circles, and paths of motion."

Thus the answer to all are problems lies right befor our very eyes. Implied may not be written in black-and-white like it is in Y14.5 for 90deg angles, but it is implied!

Seasonlee is absolutly right! I will have to let the ME know that he is loseing it. lol

 

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

fcsuper (Mechanical)
7 Jun 09 11:10
You cannot apply a datum to a centerline unless you define that centerline from a surface dim, which would be bad practice.  The way you apply to a center of a part is added the datum to the overall dim in that direction.

Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group

ewh (Aerospace)
8 Jun 09 9:21

Quote:

You cannot apply a datum to a centerline unless you define that centerline from a surface dim
Very true... you can however label a centerline as "CL (Datum)A" or "AXIS (Datum)A".

"Good to know you got shoes to wear when you find the floor." - Robert Hunter
 

KENAT (Mechanical)
8 Jun 09 11:35
Something just occured to me, take a look at 2.7.3 of ASME Y14.5M-1994, it talks about features shown symmetrical and the need to add controls for location/orientation.  It doesn't say you need to add dimensions.  So long as you have a control on 'how symmetrical' the affected features are, I would say this allows you to just 'show' the nominal symmetry.

I'm not sure the debate on implied centering/symmetry will go away anytime soon though.  I can't recall if the new version of the standard addresses this, I have a vague recollection of someone saying it does but can't be sure.

I will say this, if in doubt, don't use it.  I make use of it as it simplifies drawings & makes them less cluttered and easier to read.  I believe it to be perfectly legal and so am happy using it.  In any situation where it may not be clear though, I'll add the 'centering' dimension from the CL of the datum or equivalent.

 

KENAT,

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nrostrander (Aerospace)
8 Jun 09 17:33
"In any situation where it may not be clear though, I'll add the 'centering' dimension from the CL of the datum or equivalent."

Kenat,

     I agree with that statment 100%.  Take for instance that you had two datum planes running parrallel to each other used for centering two differnt feature patterns. Now take for instance that the planes both run very close to one-another.  In this case, I would add the dim to the appropriate CL to achieve clearity.

     I suppose we all need to take into consideration what drawings are ment to do; relay part information while allowing only one interpritation. I recon, if it can be misinterprited, it will, thus eliviate the possiblity.

thumbsup2

P.S. - Unfortanatly I agree in that this topic may never be truly resolved.  At least until it is "black-and-white" in the Y14.5. :(

V/R
Nathan
CAD Technician/ISO Director
Compass Systems, Inc.   ( http://www.compass-sys-inc.com/ )

KENAT (Mechanical)
8 Jun 09 17:53
nrostrander, while I'd never say never, I'm having trouble envisioning a situation where having 2 parallel datum's used as centers for 2 different sets of features is the best way to do things.  Certainly your example showing similar seamed over complicated.  However, that's a bit off track.

I'd definitely take a look at 2.7.3 and see if that makes you happier using it.  It's generally accepted that showing diameters coaxial combined with a tolerance on 'how coaxial' is OK.  To my mind 2.7.3 also extends this principle to symmetry.

KENAT,

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powerhound (Mechanical)
8 Jun 09 22:19
nro,

The best way to achieve the clarity you desire, using your example, is to call the features out relative to the appropriate datum. Centerlines are not nearly as critical to detail drawings as you seem to think they are.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

SeasonLee (Mechanical)
9 Jun 09 1:07
What means a center line?

Every circle on a drawing will have its center at the intersection of two center lines, center lines only exist on drawings, not on actual parts. A center line represents the axis of symmetry for a view, so center line is a type of line shown on the drawing.

For a circular part, you can't use the center lines as datum, the datum should be the part axis, As for when you can dimension to center lines, Professor Don Day gives us a good example, hope this will help you.
http://www.tec-ease.com/tips/july-02.htm

SeasonLee
 
KENAT (Mechanical)
9 Jun 09 1:11
Interestingly that example shows the very thing that is under debate, impled center/symmetry.  There is no "12" dimensions centering the CL on C.  Then again some other controls are also missing.

I still think 2.7.3 makes it legal but what would I know.

KENAT,

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powerhound (Mechanical)
9 Jun 09 10:42
KENAT,

  What purpose would having a half dimension from centerline serve in this case? Why would you want to center a centerline on C? Also, what controls are missing?

 Implied symmetry does not apply here. The entire profile of the part is fully defined with nothing to be implied.

Powerhound, GDTP T-0419
Production Manager
Inventor 2009
Mastercam X3
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

KENAT (Mechanical)
9 Jun 09 11:24
Powerhound, it does have "implied symmetry" - the 4 hole pattern is 'shown' as centered on -C-.  It isn't explicitly dimensioned as such.  You might be able to argue the 'basic 90' from -B- covers the hole pattern I suppose.

Also the width that is -C- is itself shown centered on B with no centering dimension.

Also the slot is shown centered on -B- without a centering dimension.  And in the other orientation the 10 dim is to the CL of the slot and the slot width doesn't have a dim to the center line.

Now all of these have controls on them per 2.7.3 so to my mind are perfectly legal but at least -C- & the slot appear to rely on implied symmetry to get their nominal location to which a geometric tolerance is applied.

Maybe use of the term implied is misleading, perhaps 'shown symmetry' more closely matches the wording of 2.7.3.  However, the idea of just showing something 'symmetrical' and then adding a control of 'how symmetrical' it is is used widely in the standard.  Just look at how slotted holes are typically dimensioned, or figures 4-15 thru 4-17.

Although I admit the control I thought was missing isn't, the all around surface profile covers it.
 

KENAT,

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MechNorth (Mechanical)
9 Jun 09 11:50
Good debate.  Not surprised to see that "implied" conditions, particularly symmetry / centering are still a contentious issue.  They are addressed more succinctly in Y14.5-2009, though I'm sure loopholes will be found if we look hard enough.

I would like to remind the OP that the Y14.5 certification process didn't start until after the '94 standard was released.  At that time, the old datum symbol [-A-] was disallowed and replaced with the ISO datum symbol.  If the 40yr ME with a PhD didn't know that, it's unlikely that they were actually certified.  It's still a hot-button topic for many veteran practitioners.

I would agree that Tec-Ease's July '02 Tip (tks to all who reference our tips) is incomplete in details.  As a single example, the outer profile is not controlled as shown on the drawing.  In reality, there would be a general surface profile control and maybe a few extra basic dimensions might be thrown in.  We often leave details off of Tips drawings so that the focus of the poste is not lost.

Now one last item re the July'02 tip.  The width dimension for Datum-C is shown as a basic dimension.  This tends to bother some people because they've been told over time that only a Feature of Size can be used to establish a datum center plane or datum axis; that requirement is not stated in the Y14.5 standard.  To the contrary, if you look at Fig. 4-10 thru 4-13, the width feature (pls note that I didn't say "Width Feature Of Size") doesn't even have a dimensional value, much less a toleranced one, and yet there is a datum feature callout symbol inline with the arrowhead, and it clearly indicates what the meaning is in each case.  So, it is quite valid and supported in the documentation.  The extension issue that I tend to see in cases such as the July'02 Tip is that some people will say that you are making a circular reference because the general surface profile, which controls the surfaces generating datum-C, is also invoking Datum-C.  This confusion seems to come from the problem of mentally separating datums from datum features.   

Jim Sykes, P.Eng, GDTP-S
Profile Services  www.profileservices.ca
TecEase, Inc.  www.tec-ease.com

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