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pattern positioning

pattern positioning

pattern positioning

(OP)
I am trying to determine if it is legal within ASME Y14.5M to locate a pattern of holes with conventional dimensioning (no datums) from an outside profile and then use GD&T to control the spacing of the pattern to itself. We are reviewing an old (80's) drawing package in which this was done extensively and are trying to determine if this is/was legal.  The holes within the pattern are located with basic dimensions.  The feature control frame does not include any datum references.  Thank you in advance.

RE: pattern positioning

I would not consider it strictly legal. What I would interpret the callout to mean would be that there is an implied primary datum (the surface where the holes are located). The holes are located with reference to the implied primary and to each other. There is no requirement that anything in the pattern is locate in reference to anything else, except that the conventional dims must be met. If you are updating the drawings, add the needed datums. If you are working to the drawings, determine what is needed as if you are creating the drawings from scratch.

Peter Stockhausen
Senior Design Analyst (Checker)
Infotech Aerospace Services
www.infotechpr.net

RE: pattern positioning

dnobie,

Was ANSI Y14.5M-1982 referenced on the drawings?

Locating a pattern of holes with plus/minus tolerances and using a geometric tolerance to control the pattern spacing is legal.

I don't think it was legal in '82 to have a Position feature control frame with no datum references though.  Especially with a regular hole pattern - they've only recently brought in the idea for coaxial holes.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: pattern positioning

(OP)
axym,

The drawings only call out ANSI Y14.5 (no year).  Thank you for the feedback.  This was also my understanding, however I am having a hard time finding any reference (in '94 standard).  I will do some more digging.

RE: pattern positioning

I believe the method you describe was allowed prior to 1982.  I also believe that it was 1982 Y14.5 that departed from the rectangular pattern locating tolerance zone to allow only the diametric.  (I have been of the opinion that that was not well thought out.)

Without a date for the Standard on your drawing you are faced with a dilemma. I think that it was only in the 1982 that the requirement to add a note stating specifically the date was added.  FWIW.

Im not sure just how you go about applying portions of 1994 Std to a pre-existing Std.   

RE: pattern positioning

Looks like a highjacked post thread. :) I also iterate that the year of the ASME Y14.5 needs to be specified.  No other ASME standard requires the release year, only Y14.5.  This is because each release changes how one should read a drawing.  Even though 1994 and 2009 are similar, there are fundamental differences.  Both are certainly different from 1982.

Matt Lorono
Lorono's SolidWorks Resources & SolidWorks Legion

&

RE: pattern positioning

Evan,

You said that locating a pattern of holes with plus/minus tolerance and using geometric tolerances to control pattern spacing is legal.
Could you please give me any example from Y14.5 (1994 or 2009) standard that shows this method of specifying position tolerance? Sorry to answer, but I couldn't find such.
 

RE: pattern positioning

Of course there should be "Sorry to ask, but I couldn't find such"smile
 

RE: pattern positioning

Quote (pmarc):


Evan,

You said that locating a pattern of holes with plus/minus tolerance and using geometric tolerances to control pattern spacing is legal.  

   He said it was legal.  He did not say it was recommended or explicitly shown in the standard.

   Even in the 2009 standard, it is legal.  It is strongly recommended that you use positional tolerances, but ± tolerances are defined, and have meaning on a drawing.

               JHG

RE: pattern positioning

some companies have an internal policy the Y14.5 applies on the inception of the drawing. government contracts are normally the date of design unless other wise specified or
updated contract.

nfgenggear

RE: pattern positioning

Even if legal, realize that it is ambiguous because the implied datums will not have clear precedence (one over the other).

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

RE: pattern positioning

Quote (Belanger):

Even if legal, realize that it is ambiguous because the implied datums will not have clear precedence (one over the other).  

   The ± dimensions apply from the edges.  Datums are not even called up.  I do not see ambiguity here.  

   A more serious problem is that the edges you are measuring from are controlled in perpendicularity by the angle tolerance note on the title block.  No fabricator in his right mind is going to submit parts with a 0.5° or 1° error, but he is within his rights to do so.  He has every right to be paid.  

   Like I said, legal, interpretable, but not recommended.

               JHG

RE: pattern positioning

I agree that it's legal, interpretable, and not recommended.  But realize that when you say that the ± dimensions apply from the edges, that automatically makes those edges "implied datums."

The problem is that the inspector won't know if the vertical edge should be primary and the horizontal secondary, or vice versa (or what about the face of the part?).  Even if the 90º angle between those two edges is off by a tiny fraction of a degree, the shop now has a 50/50 chance of making the part as the designer intended!

 

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

RE: pattern positioning

pmarc,

I don't know of any examples from Y14.5 that show it, but I still think that it's legal.  drawoh is right, positional tolerancing is the preferred method for locating features of size.  So the standard doesn't show an example of the non-preferred method.

John-Paul,

I didn't see the problem with implied datums.  I was envisioning the hole pattern as a datum feature (probably secondary) and the edges of the part being inconsequential.  It would be more clear to apply a surface profile tolerance to  the edges, but if the tolerance was large then plus/minus might be just as workable.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: pattern positioning

I agree that locating the pattern using +/- is legal. Does it not depend upon the function and relationship of the pattern? In some cases, the FRTZF is more important that the pattern location and I don't see anything wrong with locating the pattern with +/-. It does reflect the design intent.

As far as and Inspector not knowing the implied datum set up, well, I was a layout Inspector at one time. Believe it or not, we did have a datum structure using a primary, secondary and tertiary datums and any experienced layout Inspector or CMM Operator should be able to figure it out.

Dave D.
www.qmsi.ca

RE: pattern positioning

Quote (Belanger):

I agree that it's legal, interpretable, and not recommended.  But realize that when you say that the ± dimensions apply from the edges, that automatically makes those edges "implied datums."

   This is the question I asked in thread1103-261904: Plus Minus Tolerances.  

   If I specify the bottom of the plate as the primary datum, and the edges as secondary and tertiary, then the inspector picks up two points on the seconary edge, and one point on the tertiary.

   I believe that this all is irrelevant if I do not specify datums, and I use ± dimensions.  The inspector picks up the edges the dimensions come from.  If the edges are not perpendicular, I am in trouble.
 

               JHG

RE: pattern positioning

I'm confused.  EVERY measurement has a datum.  If you're measuring a simple length, then it doesn't matter which side is the datum, but there is still a datum.  Now we are talking about position, and that involves TWO directions of space (ignoring the perpendicularity to the face for right now).

So JHG, what do you mean why you say that "this all is irrelevant if I do not specify datums, and I use ± dimensions"?

I think I agree with your last statement: "If the edges are not perpendicular, I am in trouble."  That's why I maintain that the datums must be specified in an order of precedence; otherwise you are saying that the part is always made at exactly90º -- good luck!

Dave,
What do you mean that your inspection shop had "a datum structure using a primary, secondary and tertiary datums"?  Was it an in-house rule of thumb? I think I would want the print to tell me what the datum precedence is.    Just wondering...

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

RE: pattern positioning

Belanger,

   I suppose that the nominally straight edge that my hole is dimensioned from can be considered a datum.  I think you are assuming that the datums are perpendicular as per ASME Y14.5.  I think that only applies if I call up ASME Y14.5, and explicitly call up the datums.   

               JHG

RE: pattern positioning

JP:

You stated "Dave,
What do you mean that your inspection shop had "a datum structure using a primary, secondary and tertiary datums"?  Was it an in-house rule of thumb? I think I would want the print to tell me what the datum precedence is.    Just wondering."

This was many years ago at GM but we would review how the part was machined and simulated the datum structure from machining. The primary datum was the mounting surface (3 point set up) while the secondary was a side (2 point set up) and the tertiary was also a surface (1 point set up). We did not use holes as secondary or tertiary datums but, at that time, I was in the casting plant.

Today I would use the same datum structure that is already shown on the part. We could have a pattern shown in positional while another, on the same plane, is shown with +/-. Guess what? Same datum structure.

I can see situations where the pattern location is not important while inside the pattern is vital to its function. Let's say we have a cover that is to assemble onto a machined casting. The hole pattern on the cover and the mating part are vital to its function so the mating part could have the FRTZF in positional while the pattern position is shown in +/-. This truly reflects the design intent.

If we have a composite FCF, the shop floor would end up developing and using a checking fixture for the pattern location when it is important to its function. This is a waste of $$ and time. Of course, inside the pattern (FRTZF) certainly is vital and a checking fixture should be used to simulate assembly.
   

Dave D.
www.qmsi.ca

RE: pattern positioning

KENAT,
I didn't realise you were so young. These practices were perfectly legal and shown by the committee in the standard  before the '82 version.
The idea that this stuff is just going to disapear or doesn't still exist makes me wonder where you all work. I too wish these practices would all go away, but on the contrary, I think this is most engineers' concept of good GD&T, from what I see.  

RE: pattern positioning

I see plenty of bad GD&T, though our industry didn't really get started till the 90's.  

I've seen much older drawings at previous employer but wasn't as hot on GD&T back then.

My comments were more to do with the suggestion it was legal to the 94 version of the standard.  I'm not going to catagorically say it isn't but I have my doubts based around the two things I posted.

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RE: pattern positioning

Even ISO standards (ISO 5458:1998) do not recommend using coordinate tolerances for locating pattern of holes. They say the meaning of +/- dimensions is in this case not standardized. But of course they do not give precise explanation why it is not recommended.

I can only imagine the reasons, and I see quite a lot of them...

The other thing is that 'not recommended' does not mean 'forbidden'.
 

RE: pattern positioning

pmarc,

They say that the meaning of +/- dimensions in this case is not standardized.  That's explanation enough for me as to why it is not recommended!

One could also argue that the +/- tolerances locate the edges relative to the holes, as much as they locate the holes relative to the edges.  That's the thing with +/-, it isn't even clear what the considered feature is and what the reference feature is.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: pattern positioning

I believe that the +/- for locating a pattern was replaced by the composite tolerancing scheme, in the 1982 version.  My opinion was that they totally disregarded the benefits of a rectangular locus for the PLTZ.


 

RE: pattern positioning

It is not recommended because it is ambiguous.  I would ask which edge the hole is measured from.  Look at the dimension lines, you say, and follow them back to the two edges.  OK, I say, now tell me which one takes precedence over the other.  You reply that it doesn't matter.

Then you are saying that the angle between the two edges will always be guaranteed to be perfectly 90º, or else you are saying that it's OK for a drawing to be ambiguous.  Either way, it's not good!

Ringman -- using GD&T over ± location does not eliminate a rectangular tolerance zone.  It simply means that you'll need two feature control frames (one for each direction), and they should not have the diameter symbol in front of the number.

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

RE: pattern positioning

Belanger,

I would conclude from your statement that you have used rectangular locus for PLTZ.  If so, was it accepted as legitimate.  I have felt that an example in the standard would be of benefit.

RE: pattern positioning

I guess that's true; rectangular zones with composite might be a little tricky.

Hey -- fodder for a new thread!

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

RE: pattern positioning

Belanger,
The committee admits as much in the new 2009 standard basically any plus and minus "except for features of size" is not well defined. This is the logical extension of your point, too. What are people suposed to do redraw everything? after ever new standard? It is just not going to happen. This is why the divide between us few believers  and the "people" is so great,  Change without outside motivation, like the collapse of manufacturing in this country and the desperate need to be competitive, it will be in fact a VERY slow process.
Frank

RE: pattern positioning

Yes, I certainly was not implying that every old drawing must be changed. But the OP was asking about the legality of doing so, and I pretty much agree with the others who have said that it may be legal, but not recommended.  So those of us who are responsible for adding tolerances or modifying tolerances should be aware of the potential pitfalls.

I have indeed used composite position tolerancing without the diameter symbol for a pattern of holes, with no problems.  However, it was in one direction only. The tricky part might be when another composite tolerance is applied in the other direction -- the way the FRTZFs intersect might be the sticky part, but I'll have to think about that some more.   (KENAT, I did a search and didn't really see that issue addressed specifically. But I'd like to know more.)

RIngman,
I am curious why you are such a big fan of the rectangular zone (or locus). Are you envisioning holes/pins or are you thinking of its use on planar features of size?

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

RE: pattern positioning

John,
I agree, too.
Frank

RE: pattern positioning

Ringman,
After noodling this out a little more, I think I see what you are getting at.  Even if we are considering a regular pattern of holes on a part, we might want to allow them (as a group) to drift north or south only a certain amount -- perhaps because of a concern about wall thickness between the hole and the edge of the part. And to use a diametrical zone would force our tolerance cylinder to stay inside of the square tolerance zone, thus losing some real estate on the corners.  

So I agree that there may be value in keeping a rectangular zone, and that can be done with GD&T.  I can't quite get my mind around composite tolerancing with a rectangular zone, and since I am an opponent of pure ± tolerancing for location, I'm guessing that there's a solution that lies somewhere in the middle.

Perhaps a feature control frame for position to the edges of the part (no dia symbol), and a separate frame for hole-to-hole position showing the dia symbol.  Ugh.  

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

RE: pattern positioning

Belanger, I'm pretty sure Ringman or Ringster has brought up the rectangular tolerance zone before, and I thought at least one in the context of composite tolerance.

thread1103-242095: Composite Positional Tolerancing & Simultaneous Requirement RFS might be it but I thought more it had been discussed in more detail elsewhere, maybe I'm thinking of thread1103-238012: Unequal bilateral positioning or thread1103-242404: Drawing conversion from plus minus to comply with GDT Std but after a search I'm guessing thread1103-217781: Pattern Locating Tolerance Zone shape was what was in my memory.

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RE: pattern positioning

In my opinion, this all starts with the "wink, wink, I don't really care about this dimension" thing I have been here railing against. I keep trying to argue that to say: "I don't care about the perpendicularity of large diameter/small length pilots to their mounting face, because I can't check it" leads to the same road. The people who locate patterns this way are doing the same thing: I don't care about the location of the pattern from the edges just the pattern itself. Once you allow the out of "well it really isn't that important to me" instead of "this describes the actual condition" it is hard to know where to stop, everyone has an opinion. All general tolerances concepts are based on this.
Frank

RE: pattern positioning

Well Frank:

I don't think anyone has said "wink, wink, I don't care about the dimension" but sometimes GD&T trainers profess overuse of GD&T when it is not required for the design intent of the respective feature.

A simple linear +/- is sufficient and legal for the location of a pattern (PLTZF) of holes where the pattern location is not important to its function. Positional tolerances (FRTZF) may be vital inside the pattern if we have a mating part assembling to the features inside the pattern.

So many GD&T trainers profess that all features of size should have positional tolerances and all surfaces should be covered with a profile of a surface without regard to their function. It has been said here in this forum that this is better practice while the reasoning is somewhat weak.

Could this lead to higher costs for the product? Could we end up checking positional tolerances on patterns that have no bearing on the part function? Could we end up not confirming positional tolerances on certain features where there is a need since all other features of size have the same positional requirement?

I do care about each an every dimension but the application of GD&T should be predicated by the feature's function and not a shotgun approach.

Dave D.
www.qmsi.ca

RE: pattern positioning

Dave,

Sorry to drag this thread out, but do you think it's crystal-clear when ± dimensions are used to locate a feature of size?  I keep beating the drum that it's not, because the precedence of the two implied datums is not given.

And we can't say that it doesn't matter, because somebody is going to have to check the X and Y location for that feature, and there are two possible readouts.  Thus we don't know which one is really the intended location.

Plus/minus dimensions are only really clear when showing a size, chamfer, or radius.  It's best to use GD&T for location -- hey, if it ain't critical, then just give it a generous tolerance!
 

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

RE: pattern positioning

JP:

Yes it is clear just as long as one comes from a secondary datum if it is a hole or from the secondary and tertiary datums if they are surfaces. Crystal Clear! I could have positional tolerances on some patterns while linear tolerances on other patterns. Some patterns would then require checking fixtures and included in the Control Plan while others would only be confirmed sporadically.

If one used positional for all features of size, how should it be interpreted? One could have a note that states positional tolerances above a certain value have no bearing upon it function, then fine, I would agree. This just doesn't happen though.

I will remain diametrically opposed on the concept of all features of size must have positional tolerances.


Respectfully,

Dave D.
www.qmsi.ca

RE: pattern positioning

But recall that my beef is that we don't have any datum precedence.

You say that "it is clear just as long as one comes from a secondary datum if it is a hole or from the secondary and tertiary datums if they are surfaces."  But may I ask how you know what the secondary datum is?  I humbly submit that we don't have any idea what the secondary or tertiary datums are if you are using ± tolerancing for the location.

You also ask "If one used positional for all features of size, how should it be interpreted?"   Well, I hope we agree that every feature of size must have a location tolerance.  So I'm not sure what the question is asking.  I simply meant that position should control a feature of size because the datums are clear -- I don't care if the zone is rectangular, cylindrical, or another shape, and I don't care if the tolerance value is 0.0001 mm or 10 mm.

Using GD&T is not a statement about the importance of a feature. It's about defining that feature in a clear way.

Maybe I'm still not seeing something, but I hope my basic idea makes some sense.

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

RE: pattern positioning

Dave,
I must disagree, I am sorry you say you care but your logic says: "I dont care". The magnatude of the tolerance given should say enough to an educated reader. What you are doing is sticking to the old way "because it is not important enough". While you have the right to do that, I stand by my statement that it is one reason the standard has such trouble being really generally accepted, the other issure was it changed too much for people who don't like change, the worst thing they did in '94 was not make the datum change a recommended option.
Frank

RE: pattern positioning

Frank,

Can you elaborate on the 'datum change a recommended option'?


 

RE: pattern positioning

Dave,
I myself would like to see these practices preserved, too. I feel the standard does not adequately provide enough options in handling flexible parts say like sheet metal and plastic.  The only one I know of is the restrained condition; they want me to spend time defining the manufacturing condition on a part I can bend back later, anyway.
My criticisms of the standard are too many restrictions (the whole feature of size thing) too much you can't do this and can't do that. The parts they show are just starting to be more realistic.  I want an option like the one in this thread for parts were I really am saying: "the number from the edge is just that, a number, I don't really care that much and when we assemble this part we can force it back if we have too. There are a lot of parts out there that are like that. Many people see no justification in using too much GD&T on. I agree with that, but, I do want it understood it means I don't care enough to worry about spending lots of time on.
.All of my life, I have seen this done on weldments and or castings, not large mass production mind you.  Onesy Twosey stuff," just make something that looks like this" if it is not exactly are you going to throw it away, not likely. When I first started I made the mistake once of placing +/-.06 on a weldment drawing to avoid rounding a dimension to 2 places. The manufacturing people had a fit, I asked: "well what is the tolerance then?" The truth was they don't know and don't care what the tolerance was, you get what you get.  The ISO is developing standards on this stuff. I don't care may be the reason GM is the number 2 auto maker in the world the last I heard, I know, Toyota just had a fall, big shots need that every once and a while. It is a shame for the people though.
Frank
 

RE: pattern positioning

Ringman,
I meant, I think, the ISO datum symbol adoption should have been a "recommended" option. From my point of view it made the whole adoption of the '94 standard optional.  I am certain it has inhibited its adoption in the companies I have been in due to the resistance to change I have been talking about here. The '94 standard mostly offered clarification on principles, except the feature of size change they snuck in. 2009 is more of the same and thankfully tries to undo some of the damage of feature of size from '94. They brought back symmetry and concentricity that were never REALLY gone. I feel these standards are mostly consistent and build upon each other, not radical revisions. It is good but not enough to make their adoption mandatory, I can look at a later standard and say restatement of a secondary datum "means this" and it does not contradict the '82 standard. Composite profile just more composites, etc.
Frank
 

RE: pattern positioning

Gentlemen:

I did not say that a +/- tolerance is not important (every dimension is important) but I did say that it is legal to use and I would recommend the practice if there is no function or relationship of the feature to any other feature on the part or mating part.

Why would one place a positional tolerance on a pattern of holes used to lighten the product? There is no function or relationship of the location of the holes and will give a false impression that the hole location is important to its function. Could we end up making a checking fixture for this pattern? Absolutely! Could we end up confirming the location of the pattern on a regular basis? Absolutely! Does the positional tolerance on on this pattern reflect the design intent? No!

If our primary intent in applying GD&T is to "define the feature in a clear manner", why have a composite feature control frame when the feature is already covered in a "clean manner" with a geometrical symbol? Maybe it is design intent??

As per ASME Y14.5-2009, it states "it is even more important that the design more precisely state the functional requirements". This is a reasonable statement and I totally agree with it.

JP - I did not find anything that stated "Using GD&T is not a statement about the importance of a feature. It's about defining that feature in a clear way."

I do not come from a design background and maybe that is the difference. I do come from the Quality field and I see the struggle people have interpreting drawings with the overuse of GD&T, inappropriate application or just plain wrong. If we could only reflect the design intent with the proper use of GD&T, it would help manufacturing produce a better product at less cost. This just my opinion.

   

Dave D.
www.qmsi.ca

RE: pattern positioning

This may in fact be another reason to stick with the '82 standard as I said above it lays the groundwork for most of the good stuff that came later while at the same time contains, in the appendix, references to the style of dimensioning (as former practices) desired in this thread.
It seems that some people here prefer to not remember or are to new to know these practices did exist and were in fact sanctioned by ASME. I would suspect there must be millions of drawings like this out there and to sit here and pretend like they don't exist, it takes a lot of gall to say these people were wrong, when ASME itself supported it.  Really you should say that ASME was wrong.
 I believe the ASME can only really recommend standard practices that is why so many have a hard time deciding if this is legal or illegal.  The sad fact that so many companies I see are so far behind the current standards shows the great divide between the standard and the potential users.  Walt says: (in another forum) "we have to remember the bottom line is to make money".
The large corporations' representatives and the GD&T sales people who sit on the committee have apparently lost touch with the small companies and shops that, I suspect, use this stuff the most and find lots of it irrelevant to their daily use. Guys, I am not pleased to have to say these things, but we must have a balance.
I am finding myself in debates with members who claim they care about every single dimension like they are all the same, now I will agree that, "anything can become important when it becomes important" but I can not accept that all things are really equal, this sounds good as a cliché but I believe it is not being honest to the way most of us really work. Do you not have general tolerances on your drawings?? The ISO is brutally honest when it talks about the reason for general tolerances. ASME 14.5 2.1.1 mentions them but never says why most of us, use them. (I would imagine any who don't use them to be in a small minority).
I have never worked at a large mass production company and always wanted to because I though maybe they would actually appreciate this stuff. I suspect, these companies may make more actual parts in quantity but are not the majority of drawing users out in the field.
I submit it is because most companies/people do not believe all features are equal that title block tolerance usage is so common and that there are features on parts that if they are out you will be willing to throw the part away and many more you would just will live with. This is not the mass production world, but the world I am from and I suspect the majority of the users are too.
I suspect the lawsuit argument is not a big winner in the real world either as I don't see many out here so afraid of them that they hurry to adopt the latest standard either. I suspect many here are not in a hurry to give up their beloved '94 version for the 2009 version, that I definately find superior, not perfect.
Frank
 

RE: pattern positioning

Quote (dingy2):

I did not say that a +/- tolerance is not important (every dimension is important) but I did say that it is legal to use and I would recommend the practice if there is no function or relationship of the feature to any other feature on the part or mating part.

Why would one place a positional tolerance on a pattern of holes used to lighten the product? There is no function or relationship of the location of the holes and will give a false impression that the hole location is important to its function...

   Let me come at this from the design point of view.

   I would carefully apply tolerances to lightening holes.  They would be very sloppy tolerances.  The relationship of the lightening holes to the datums might not matter as much to me as their position with respect to the sloppily toleranced outline.  In that case, there would be sloppy ± tolerances to the edges.

   If I told the machinist to somehow keep a hole within a 2mm profile, would you make a fixture to inspect it?

   One of my concerns is that, while the tolerance does not matter this time, it might matter next time.  Next time, the mass, and the weight distribution of the part may be critical, in which case, I will apply accurate tolerances to the lightening holes.  I do not want machinists and inspectors trying to work out whether or not I mean it.

               JHG

RE: pattern positioning

Dave,
Isn't the fact you don't need me to tell you specifically which is primary, secondary or tertiary show that you don't really care that much?
Frank

RE: pattern positioning

Frank & drawoh:

This is a can or worms with no resolution whatsoever but I will quote 1.4D from the ASME Y14.5-2009 standard.

"Dimensions shall be selected and arranged to suit the function and mating relationship of the part and shall not be subject to more than one interpretation."

The standard also states that positional tolerances are recommended (not mandated) on features of size.

1     Would applying positional tolerances at MMC on ALL patterns of holes without regarding their function and relationship of the part comply with 1.4D? Some patterns or single holes with no function or relationship would be shown in the same way so there could be more than 1 interpretation.

2     Would applying positional tolerances at MMC with relatively large tolerances comply with 1.4D? As long as we knew the line of demarcation, it would comply but nothing will ever be shown on the drawing. I think that there could be more than 1 interpretation.

3     Would applying positional tolerances on holes or pattern of holes with no function or relationship in RFS while the holes or pattern of holes with a function and relationship are reflected in MMC? I think that there could be more than 1 interpretation but the chances of this happening are minimal. This may be the best solution????

4     Would applying positional tolerances at MMC on holes or pattern of holes with a function or relationship while holes or patterns without a function or relationship are shown in linear tolerances meet 1.4D? Yes it would.

These were just some final thoughts on this controversial subject. As I stated before, no resolution will be found in this forum but I appreciate the fact that we can discuss it and appreciate other points of view.

Dave D.
www.qmsi.ca

RE: pattern positioning

You're treading a fine line aren't you though?  It says to "suit the function", it doesn't say to "to make the function explicitly clear to every user of the drawing" or similar.  If the GD&T is applied properly then there's only one interpretation of what the pass fail criteria for the part is, isn't that what 1.4d is talking about?

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RE: pattern positioning

Excuse the irony, but there seems to be more than one interpretation of what "shall not be subject to more than one interpretation" means.  Dave's statements (and I'm not sure that I understand them correctly) indicate that it means that a given feature could be toleranced in more than one way by the designer.

To me, dimensions and tolerances should not be subject to more than one interpretation when applied to a real (i.e. imperfect) part.  So that each dimension, and more importantly, each tolerance, can be defined on a real part in a unique and unambiguous way.  Measurement uncertainty will always exist, but that is a separate issue.  We're talking about specification uncertainty here, where the specification itself is open to multiple interpretations.

In order to truly obey the statements in Y14.5 1.4D, plus/minus tolerances must be confined to sizes (diameters, widths, etc.) of proper features of size.  Applying them to anything else either doesn't suit the function and mating relationship, allows more than one interpretation, or both.

Dave, I agree that we're never going to agree on this.  I know that you believe that GD&T should be applied to features that have a function and relationship, and that plus/minus should be used on features that don't.  For me, it's just not that simple.  I don't even agree with how you decide whether a feature has a function or not!  But I agree that it's good that we can discuss various points of view.   

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: pattern positioning

Another thought, people keep mentioning lightening or vent holes etc. as examples of features with little or no real functional requirement, but I'm not sure that's true.

You typically don't want the holes getting too near to each other, or the edges of the part etc, so is the real functional requirement a fairly loose LMC?

The thing I probably have most trouble deciding on appropriate functional tolerances are things like cable lengths.  There is typically a functional minimum, however, except in rare cases where impedence or space concerns etc. are very significant, there isn't a very pressing maximum length requirement.  You could just give a min dimension but in isolation that would imply an infinitely long cable is OK, which is probably isn't.  You can pick a number based on a loose manufacturing capability tolerance, but if a cable came in say 1/8th" over that it would most likely work and you probably wouldn't reject it.  So what is the correct answer?

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RE: pattern positioning

The answer:  long enough to hold the car if the elevator cable breaks from the top floor!  (Just kidding)

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

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