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Reporting Requirements for Geometric Tolerances

Reporting Requirements for Geometric Tolerances

(OP)
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.
www.axymetrix.ca

RE: Reporting Requirements for Geometric Tolerances

Evan, you know what my answer would be :)

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

RE: Reporting Requirements for Geometric Tolerances

(OP)
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.
www.axymetrix.ca

RE: Reporting Requirements for Geometric Tolerances

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)

RE: Reporting Requirements for Geometric Tolerances

(OP)
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.
www.axymetrix.ca

RE: Reporting Requirements for Geometric Tolerances

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Reporting Requirements for Geometric Tolerances

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

RE: Reporting Requirements for Geometric Tolerances

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? :)

RE: Reporting Requirements for Geometric Tolerances

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

http://files.engineering.com/getfile.aspx?folder=f...

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... sad

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

RE: Reporting Requirements for Geometric Tolerances

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.

RE: Reporting Requirements for Geometric Tolerances

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

RE: Reporting Requirements for Geometric Tolerances

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

RE: Reporting Requirements for Geometric Tolerances

(OP)
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.
www.axymetrix.ca

RE: Reporting Requirements for Geometric Tolerances

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

RE: Reporting Requirements for Geometric Tolerances

Quote (drawoh)

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

RE: Reporting Requirements for Geometric Tolerances

Quote (CheckerHater)


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

RE: Reporting Requirements for Geometric Tolerances

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: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Reporting Requirements for Geometric Tolerances

This forum has several threads starting with question: "please, tell me how to measure that".

Strangely, they are left unanswered the longest. wink

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

RE: Reporting Requirements for Geometric Tolerances

(OP)
drawoh,

The profile inspection methods you describe sound reasonable, and I have done both myself. I've measured XY coordinates of points on the surface, imported them into CAD, and fitted them to the model. I've also inspected profiles of flat sheet metal parts using a 1:1 Mylar plot with the profile zones added in.

One interesting thing is, that I believe is lost on many people, is that at a certain level of "nastiness" the profile zone becomes much easier to inspect than the dimensions would be. Once you're willing to move away from the idea of directly toleranced dimensions, things get much more clear.

The profile requirements in Fig. 8.5 and 8.6 would both be relatively straightforward to inspect if a coordinate-based inspection or template-based inspection method was used. The complexity of the basic geometry does not become an impediment - there is just a basic shape with a zone, and you verify the part (or measured points) to that zone. Each point is either in the zone or not in the zone.

The point that John-Paul and I were making, and that others have alluded to, is that trying to inspect each dimension directly (i.e. not the profile zone) would be very difficult. Many (most) of the dimensions would be difficult to inspect repeatably on an actual part, and some of them would be impossible to even define on an actual part.

Evan Janeshewski

Axymetrix Quality Engineering Inc.
www.axymetrix.ca

RE: Reporting Requirements for Geometric Tolerances

As to the OP regarding information I want to see on an inspection report.

Well, I'd like to see the inspector actually interpret the GD&T vaguely correctly.

This having just dug into a failed 'circular position' on an open ended slot where the actual FCF was a bilateral position only on the width.

Posting guidelines FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm? (probably not aimed specifically at you)
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?

RE: Reporting Requirements for Geometric Tolerances

There's a temptation to want some sort of deviation information, to see how far from nominal features and dimensioned happened to be.

But in many cases this isn't as helpful as one might like because often the frame of reference those deviations are calculated from is not fixed with respect to any other frame of reference on the part from which other measurements are made. This is certainly the case for any tolerance that has an (M) or (L) modification to a datum reference, but any unstable datum feature will cause problems.

There should be two sections; one in which it is reported on whether or not a solution was found to accept each feature. The other part will record where all features are located and oriented from a fixed frame of reference so that engineering can create a model that is representative of the as-inspected part and use that model to check where the part is relative to the original design goals.

All that is required to accept the part is the first portion, which is the same as if hard gages were used to accept the part.

Analyzing what the results mean in terms of meeting current goals is a complicated effort that is difficult to represent on a simple report on gathered data based solely on the dimensioning and tolerancing, because, by design, using FCFs is to allow variations that cannot be explained by single numbers.

The latter part of the report can be used to analyze if the parts are being made consistently and pin-point consistency failures in the production system. High-definition point cloud capture can be handy for cast parts subject to heat treat and straightening or injection molded parts, while a simpler geometry descriptions can serve for simple prismatic items.

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