Reporting Requirements for Geometric Tolerances 1

[axym]

Hi All,

Here's a question (more of a survey, really):

When a first article inspection report is produced for a part with geometric tolerances (position, surface profile, flatness, perpendicularity, etc.), what information do you feel is necessary?

1. Measured values for each geometric characteristic

2. Measured values for each basic dimension

3. Other



Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[KENAT]

2. is of little to no interest.

3. Deviation from basic location, or more specifically if it falls inside the required tolerance.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[Belanger]

Evan, you know what my answer would be

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

 

[axym]

John-Paul,

Yes, I think I know what you would say.

The reason I'm asking is that QA folks get asked for all sorts of things (particularly in a customer/supplier situation), and I'm wondering what people's expectations are.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[greenimi]

Evan,
Is y14.45 relesead? I guess might not be yet.

Are you working on it? I have seen some articles from you on:

http://www.mbccmm.com/cmmq/index.ph...ata-reporting-for-geometric-tolerances-part-1

Thank you

 

[JNieman]

1. Only those on the print.
2. Sometimes (see below)
3. RARELY a customer will require us to document some of the actual values of the "unless otherwise specified" items. Such as "CAD IS BASIC, UOS PROFILE OF SURFACE .060 APPLIES" and after a long conversation, they decided they would be happy with a "color map" showing relative deviation values of the part from the CAD nominal size. Someone in their QA dept apparently just likes those heat/color maps showing high/low spots.

We have had customers -demand- that our FAI report include the measured values of all BASIC dimensions. We protest/explain/suggest all we can within the bounds of 'the customer gets what they pay for' but it really is absurd when your order is for 1,000 widgets, and you only want such a detailed report for the first.

Most people "know better" (biased opinion)

 

[axym]

greenimi,

Y14.45 is not released yet. We're still working on it.

One of the reasons that I'm asking the question about reporting basic dimensions is to assess whether this is a practice that should (or even can) be standardized. As GD&T "purists", many of us have our preconceived opinion that basic dimensions are theoretically exact and therefore do not have measured values. At the same time, measured values of basic dimensions are often asked for and reported in industry (see JNieman's post above, for example). I've heard of similar things going on at a lot of other companies as well.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[CheckerHater]

I am genuinely curious what "measured value" of BASIC dimension 50 would be?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[KENAT]

Deviation (Displacement i.e. size & direction) of feature center from basic location can be useful in dispositioning failed parts - as I spent a few hours yesterday trying to do.

However, you don't really measure the Basic dimension, you measure the distance of the actual feature to the same 'datum' that the Basic dimension goes to - or something like that.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​

 

[CheckerHater]

Exactly. You are measuring everything else BUT the basic dimension.

So, should ASME educate "industries" or bend over backwards for them?

And where to draw the line? May be we just shut down Eng-tips because "this is how we do it in MY industry is universal answer for any discussion?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[JNieman]

I don't think anyone is suggesting that ASME bend over and change to suit less-educated people in industry.

After all, this general topic is meant to establish a dictionary of ideas that we can all reference to improve clear and efficient communication. Given the nature of /what/ it's communicating, I do not think it will modify or add revisions which will accommodate common misconceptions.

Besides, the poor practice I spewed was, what I can only assume, someone requesting extra CYA in an effort to tell a layman-boss/supervisor they "did their part" to ensure quality, somehow. Not that they thought it was the right thing to do /per/ an ASME standard of any kind. My anecdotes were certainly not intended to say it's ok or should be approved of.

I expected the OP was facing similar situations/questions (why else would such a topic arise?) and thought maybe shared-suffering would be appreciated. Misery loves company, right?

 

[CheckerHater]

Well, I don't think the people in the industry are uneducated, maybe they just are educated by wrong sources:

http://files.engineering.com/getfil...e19-ab06-40e554df7bd2&file=Basic_Dim_Wiki.JPG

Somebody has to take a stand, and if this somebody is not ASME, then who?

Either way, I will be waiting for new Y14.45 release. I won't hold my fingers though... :-(



"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Nescius]

Whenever I see a "measured" Basic dimension on a FAI, I ask:

Measured how? From where, to where?

In my mind, attempting to measure a Basic dimension completely defeats the purpose of geometric tolerancing. A common understanding of the question "Measured how?" or "From where, to where?" is arguably the fundamental purpose of GD&T.

 

[Belanger]

I can think of numerous prints where one basic dimension sets the stage for multiple geometric tolerances. So right there it's not even possible to match up a basic dim with a specific FCF.

Also consider profile tolerances, such as Fig. 8-5 in the standard... what is the tolerance on the 45 dim at the top of that drawing? Ain't possible.

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

 

[drawoh]

axym,

Standards aside, if I were inspecting something, my initial report would show the measurements I did on the part, regardless of the dimensions, basic or otherwise. If I interpret the datums correctly and I measure from them, a trained GD&T person, preferably me, can review each tolerance and FCF and determine whether or not the requirement has been met.

In an inspection, the actual measurements are your raw data. There may be some question of how to interpret the GD&T. The raw data describes what you inspected.

--
JHG

 

[axym]

CH,

One of the major challenges that the ASME subcommittees face is to decide which industry practices are worthy of standardization and which are not. Sometimes we examine a common industry practice and find that it contains misconceptions, doesn't stand up to rigorous analysis, and can't be traced back to anything in the standards. Or, perhaps, the practice might be useful in a relatively narrow set of applications and extremely problematic on others. This might be the case with measuring basic dimensions. What we can do is analyze what is being done, and see if it makes sense or not. I don't think that Y14.45 is going to standardize on a flawed practice just because it might be commonly done in induatry - that's definitely not the intent.

Nescius,

Your question relates to what I am generally trying to find out with this inquiry. When measured values for basic dimensions are reported, exactly what do these numbers represent? You are definitely not the only person who thinks that measuring basic dimensions defeats the purpose of GD&T or is a step backward.

John-Paul,

I agree that there are a lot of prints in which a basic dimension applies to multiple features, and/or multiple geometric tolerances. CH's example is a good one, and there are plenty more in Y14.5 itself. There are even examples in which a basic dimension would apply to multiple DRF's. So there would generally not be a one-to-one correspondence between a basic dimension on the drawing and a measurable characteristic on the actual part.

Your example of the 45 mm dimension in Fig. 8-5 is a good one. Two of my favorites are the R82 and R80 dimensions in Fig. 8-6 - imagine trying to measure those.

drawoh,

Part of the problem is that there is often some question on how to interpret the GD&T. Specifically, the meaning of the geometric tolerance, applied to actual part geometry, is often very complex and difficult to measure directly. As you describe, the measured value of the GD&T characteristic may have to be calculated from what measurements were actually taken.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

axym,

I have done some very nasty geometry, and I have worked how to inspect them. On figure[ ]8.6, I can mark stations from one end, perhaps the quadrant of R12, and then measure the height at each. My worksheet shows this. I need to work out whether or not each as-measured dimension is located within the specified profile tolerances. If the profiles are very much sloppier than on figure[ ]8.6, I would print the drawing off 1:1, and sit the part on top of it.

Inspection is one of the things you need to think through at design and drafting.

--
JHG

 

[CheckerHater]

Inspection is one of the things you need to think through at design and drafting.

Finally. And who is going to do that, given generations of "professionals" raised with "functional" mentality?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[drawoh]

Finally. And who is going to do that, given generations of "professionals" raised with "functional" mentality?

I am reading through my copy of Product Design for Manufacture and Assembly. In the first chapter, they have a cartoon chart from Monroe Associates from 1989, which claims that design makes up around 5% of the cost of a product, and affects about 70% of the cost. Anything design renders near impossible is going to be expensive later on, whether this be fabrication, inspection, or shipping and installation.

I didn't say they do it. I said they need to do it.

--
JHG

 

[KENAT]

Well, that chart as is typical for most DFMA material probably applies mostly to relatively high volume production.

For low volume Engineering/Design can be a lot more than 5%.

During design though you should be making sure that the item can be fundamentally manufactured and for acceptable cost, but generally without getting any more detailed than necessary to ensure that. Of course for fundamentally difficult to make parts and/or where the part cost target is aggressive then that can still mean quite a bit of detail.

As to Inspection, maybe I don't spend enough time thinking about that as I could, and will admit it's not an area I know much about.

Posting guidelines faq731-376

http://eng-tips.com/market.cfm?

(probably not aimed specifically at you)
What is Engineering anyway: faq1088-1484​