Profile of a Surface : Can I use "per unit area"? 2

[Motalu]

Hi All,

First post, so go easy on me!

I have a part with six nominally co-planar mounting bosses over a total area of approximately 450mm x 450mm.
I have received feedback from the supplier that they can hold the "flatness" of these bosses (their terminology, not mine) to ±0.05mm per 100 x 100mm area. Thus they propose an overall "flatness" tolerance of ±0.2mm for the faces of these bosses. This (just about) acceptable to us, so we would like to capture this on our piece part drawing.

I would prefer to use Profile of a Surface in place of a flatness tolerance for a couple of reasons; the ability to control position (using a basic dimension) within the same tolerance and also, most importantly, the implicit inclusion of co-planarity provided by Profile of a Surface (Sec. 8.4.1.1 of ASME Y14.5)

I am proposing setting a datum plane (Z) to be co-planar with all six boss faces and then applying one of the attached geometric tolerances to the surfaces. (please see attached image)

The issue that I am having is that ProEngineer only offers "per unit length" as an option for Profile of a Surface, not "per unit area" (which is offered when creating a flatness tolerance, for example).

Is "per unit area" not an acceptable modifier to a Profile of a Surface tolerance?
Logically I think it should be allowed, but can find no explicit reference to it in ASME Y14.5 (which does mention per unit length in 8.3.2.2.).
I have seen it used in other places (

http://www.tec-ease.com/gdt-tips-view.php?q=112

for example), but wanted to come here and solicit some other expert opinions.

Thanks in advance for any help you are able to provide!

 

[pmarc]

I see no reason why it should not be allowed. Not every torelancing case is specifically shown and explained in the standard.

One comment though to your profile FCF's:
Single common 'Z' for sure is not allowed per Y14.5. I would suggest splitting it into both segments similarly to fig. 7-50 in Y14.5-2009.

I hope you know the difference between having and not having 'Z' in bottom segment of profile FCF

 

[MechNorth]

Nothing wrong with it; it's an extension of principles is all.

If I understand you correctly, you want to make the six coplanar surfaces the common primary datum feature for datum Z. You want (or will accept) 0.4mm coplanarity overall between all 6 pads. You want 0.05mm flatness on each pad individually. Correct? If so, then you don't need per unit area; the first level of the composite control would be 0.4mm, and the second level would be 0.05mm, with INDIVIDUALLY added behind the fcf of the second level. The profile control needs a 6X (for '09) or 6 SURFACES (for '94) with the feature control frame, and preferably with 6 leaders or a phantom line showing the relationship between the surfaces.
If I have misunderstood the intent, pls restate for me.

Pmarc, pls explain why you feel a single common Z datum is not allowed. The use of multiple datum features resulting in a single datum is well understood in the standard.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim, not sure if I understand your question correctly, but I was saying that Z symbol cannot be common for both segments just like position tolerance symbol is. I was not saying that there cannot be Z only in upper and Z only in bottom segment.

 

[MechNorth]

Ah, you're saying that "Z" can't be referenced in the PLTZF & FRTZF of the composite control? If that's the case, that's not true. In that situation, the six bosses being related to some other datum, Z (as opposed to establishing the primary datum), the first level establishes that all 6 pads must be within the 0.4mm zone centered at a basic distance from Datum Z; the second level establishes that the 6 pads together must be within a floating zone of 0.05mm width (floating within the 0.4mm zone)which is parallel (or oriented by basic dimension wrt datum Z) to Datum Z. Without the Z reference in the second level, the 0.05mm zone could incline at something other than 0/180-deg to datum Z. If I've still missed your meaning, pls help me out.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[Motalu]

Thanks for the quick feedback guys.

Taking an extract of Jim's post:
"If I understand you correctly, you want to make the six coplanar surfaces the common primary datum feature for datum Z. You want (or will accept) 0.4mm coplanarity overall between all 6 pads. You want 0.05mm flatness on each pad individually. Correct?"

Not totally correct.

My intention was to keep the 0.1 Profile of a Surface over the 100x100mm area that the supplier states they can hold this within.
The reason I want to retain this is that a couple of the bosses are within 100mm of each other so I want these two boss surfaces to be more tightly controlled with respect to each other than the 0.4 overall tolerance (to avoid any significant "step" over a relatively small distance).

I hope I have explained myself correctly.

Thanks again.

 

[MechNorth]

Ok, then skip the composite control. Use the 6X profile within 0.4, then put a separate control for the surfaces you want controlled more tightly wrt each other. This has the same effect as a composite control for those select features.

The problem is that unless your pads are actually large enough, you don't have a 100x100 surface area and therefore it wouldn't be understood (beyond your single supplier) what it means.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

Jim,
Please look at attached picture - hope this explains what I meant.

http://files.engineering.com/getfil...9-dd9db3652a7d&file=composite_profile_FCF.JPG

 

[Motalu]

Thanks to both of you, a separate control will do the job nicely.

It also has the advantage of avoiding me having to try and achieve something that ProEngineer's geometric tolerancing configuration is preventing me from doing! (Apply a per unit area tolerance to a Profile of a Surface).

 

[MechNorth]

Ah, tks pmarc; I didn't see the trees for the forest.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[fsincox]

pmarc,
I never knew where that symbology came from (not the std) I have allways assumed it was just a CAD package issue and therefore would have to be allowed. I realize most packages now are updated but I run into this all the time when the standard changes, like right now SW does not allow the "all around" symbol on dimension leaders only note leaders and it doesn't even support "all over". I have to ask, where does it say it can't be done that way?
Frank

 

[pmarc]

Frank,
Paragraph 3.4.4 of Y14.5-2009 says: "A composite feature control frame contains single entry of a geometric characteristic symbol (position or profile) followed by each tolerance and datum requirement, one above the other".

Don't know if this is sufficient for you but for me is.

Additionally some figures in the standard somehow supports my point of view (7-42, 7-43, 7-44, 7-50).

The only thing I do not like in 3.4.4 is that it seems to exclude composite flatness or straightness FCF's as shown in fig. 5-4 and para. 5.4.2.2 for instance. Unless these FCF's should not be called composite...

 

[fsincox]

Purely a matter of interest. For example, I see ACad drawings with the tolerance block or datum attached to the center of the rectangular box off of an extention line extending from a surface edge view, where does it show that in the standard?
Frank

 

[fsincox]

Sorry, SW2011 does have the "all around" symbol.
Frank

 

[Belanger]

Right, pmarc. I think that flatness and straightness would not be called composite, but rather "per-unit" control. However, I don't like to be so formal ... I call 'em all double-deckers (except when I'm teaching a class).

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

 

[pmarc]

J-P,
If flatness and/or straightness FCF's with per-unit length or area refinements would not be called composite, then can profile of a surface with per-unit area refinement in the lower segment be called composite? If it should not be called composite too, then maybe 3.4.4 does not apply here? Hmm...

But seriously, coming back to the main topic, I do not think that even if there was a single common datum reference for both segments of profile FCF in OP's question it would create any confusion. I believe the intent would still be clear.

It is only a matter of how legal one wants to be in using this particular concept of GD&T.

 

[Belanger]

I'm just going by what the standard says; they use "composite" for position as well as profile. But never for any other symbol. (Most people would understand what you mean, though.)

I think the difference is because a composite FCF for profile or position implies that the datums in the lower segment have an "orientation-only" role to play. Since flatness and straightness never reference datums, they aren't composite in that sense.

Which brings up your good question: If we are showing a single profile symbol in front, but merely want to use the lower segment for a per-unit refinement, should it really be called composite? Probably not, methinks. But this terminology question is a very small aspect of the discussion.

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

 

[fsincox]

So basically no one here knows where this common datum type of symbolic notation came from, it is not shown in the any versions of the standard as I recall?
Frank

 

[MechNorth]

no, single datum reference for multiple lines is not legal in the standard, and never has been in the standard. It is a CAD thing. The problem with GD&T modules in CAD packages is that the programmer reads a "GD&T for Dummies" set of notes and thinks he understands it. Some packages respond to complaints & correct it; others just ignore it (from personal experience).

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[pmarc]

For sure it is not in Y14.5's 1994 & 2009 editions.
I do not have 1982 version so cannot say.

Also cannot find it in any of existing ISO GD&T standards.