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Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?
5

Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
I would very much be interested to know if anyone can produce any tangible data, to suggest a correlation between GD&T on a drawing, and an increase in vendor procured parts?

I'm not trying to lead a revolt. Clearly, GD&T is the way to go for a great many parts. But for low cost drivers, I believe that many vendors will arbitrarily increase the quoted price, even at the mere mention of the most primitive tolerancing. (or even datums) I cannot prove this, and I can't get anyone to commission a study of the matter. But as a 24+ year engineer, and former business owner, I have firsthand knowledge of this issue. Which does no good, when I can't reliably communicate it to others.

The company that I work for, is currently trying to implement some tolerancing rules, that are broken down by product level. Main product, tooling, electrical, GSE, etc. I believe that we need a very clear delineation in these product lines, as none of them demand the exact level of rigor as any of the other.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

But what do you offer as an alternative? I mean, you have to have some sort of quality control, at least in the sense of being able to specify, and subsequently verify, what it is that you're getting from your suppliers.

Now if you're talking about holding part parameters to too tight of a tolerance, inappropriate for either the intended use, or the expected methods of manufacturing, I might be willing to be concede that often poor application of tolerances, whether they're documented by GD&T or some other scheme, can significantly increase the cost of a component without making it any more suitable for its intended use.

As a young engineer I had to learn the hard way that forcing the machine shop, or a supplier, to meet very close tolerances, just to make sure that parts will always fit together, is often the best way to guarantee the highest possible cost for a part what will work as expected. It took time, and the mentoring by people with years of, in my case, machine design experience, to learn the tricks of the trade where you can specify part designs that will fit together, and perform as expected, even if you use less stringent tolerances by simply thinking smarter about how parts fir together and which dimensions are most critical and which are not, and so you learn to loosen the tolerance where it has little or no effect on fit & function, while holding those that do. Also, you learn to understand the manufacturing processes which are appropriate, again, to assure the best fit & function without wasting time and money on overly complex tooling and work processes. For example, don't specify a tolerance which will require a milling operation when they could have used a drill press to produce a hole to your component, and if the set-up and quantity are such, you might even move it to a punch & die operation, which could reduce the cost even further. As an old timer once advised me, "Don't force the shop the use a 3-axis mill, if cutting it with a torch would have still given you what you wanted."

John R. Baker, P.E. (ret)
EX-Product 'Evangelist'
Irvine, CA
Siemens PLM:
UG/NX Museum:

The secret of life is not finding someone to live with
It's finding someone you can't live without

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
The alternative is a tolerancing scheme that doesn't use GD&T. If you notice my initial post, I suggested that there were multiple product streams, all varying in degrees of sophistication.

This isn't about inter-office revolutions, or radical new ideas. It's about trying to extoll the virtues of allowing simplicity, where simplicity dictates. We currently have a QE mandate that ALL parts must have a 3 datum system, regardless of their complexity. In addition, their policy is that if a part has datums, it SHALL implement tolerancing which relates to said datums. So, we're held hostage to this methodology. I could certainly have it overturned, if I could provide data that would illustrate an unintended negative.

I'm not going to get into a drawn out discussion over the specifics. I'm very confident that I know how to ensure the produceability of parts. The issue here, is whether or not we can prove a cost bias. I have literally hundreds of pieces of very low level tooling that benefit nothing by having anything more than an overall tolerancing scheme. In stark contrast to the hundreds of pieces which deserve more elaborate tolerancing, up to, and including SPC.

My problem is not lack of experience - at least not from my end. I'm 25+ years in the design business, with a machining, R&D, and prototyping background. In addition, I am a former business owner, who was responsible for design, development, prototyping, and delivery of custom products and product lines. In that time, I met more than one vendor who openly admitted that geometric tolerancing was a cost driver. I also established relationships to thwart the price premium. (i.e., work with the vendor in the design process, to reduce the cost, and understand why they build it in, in the first place)

But... surely somebody, or someone that somebody works for, must have conducted an applicable trial to ascertain the pricing effects of product definition... I don't believe anyone who would make the kind of claim that I am making, and can't corroborate it.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

No one will have conducted such a trial because the end result is not verifiable. There won't be a way to ensure that parts will meet some set of requirements that are not there.

However - I think there are a large number of cases where the general precision available produces sufficient accuracy (not the QA "accuracy" vs "precision") that resulting variations are likely to be rejected only in the case of very small tolerances. Case in point - I saw an entire program omit all angle tolerances on 90 degree implied dimensions and not a single complaint seem to have arisen to correct that oversight. If even 1/2 degree out parts would fail to fit.

Likewise I've seen projects where the allowed tolerances would result in 20-50% failure rates of the final product, but the supplier simply made the product not to fail; all tolerances could have been omitted and the parts would still work because the supplier would not want to lose the contract; nor would they complain or ask for clarification.

In contrast I "worked" with a supplier to create an ICD to ensure that parts that would be interchanged (covers to transit cases) would be interchangeable. I made up the drawing, said it was preliminary, and asked if they had different dimension origins to mark those up. Their mark-up was to cross every dimension from the drawing so that their QA would not have to inspect them. Their "solution" was to add a note that the lids for any one order would be interchangeable, but no guarantee that they would fit from the next order.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Well, of course, it's a cost driver; it's got to be cheaper to just crank out crap without regard to any acceptance criteria. The question isn't the cost of THAT piece; it's the cost of trying to get crap to fit together, hunting and pecking for replacement parts, and rework and rejection at the end of the line, when all the mechanical parts have to work together and work smoothly. The Japanese auto industry didn't eat the US auto industry's lunch in the 70s and 80s by building loose tolerance engines and transmissions; they did the exact opposite. They made their parts tight where needed and loose where needed, so their engines ran without problems, were cheaper to build, and required less repair because parts didn't wear because they were fitted and toleranced well. In the meantime, companies like Ford would allow their machining, even with tolerancing to bang against the limits because it was cheaper to machine, but they burned tons of money trying to get one engine and one transmission to play nicely together. And, the cars were ludicriously unreliably.

While I have no direct experience with that type of manufacturing, I was party to second-sourcing a Japanese integrated circuit to be run on a US manufacturer's production line. We had similar "tolerances" and we had test chips on each wafer to ensure compliance to the semiconductor processing requirements, but it usually cost 5 chips in the prime real estate on the wafer. We got Hitachi's mask sets, and no test chips were to be found; we asked and they said, "Don't worry about that." We asked what processing parameters (think surface finish, etc.) were needed, and they said, "Don't worry about, just run it in your standard process." So, we did, and the first batch yielded better than our own products that were specifically designed for our own process, and the second batch yielded even better.

The moral is that the notion that adherence to tolerancing "costs" more is a myth propagated by lazy manufacturers who don't want build quality product. They're willing to do the same thing their grandfathers did and suck up the rejects and poorly fitting parts as the cost of doing crappy business.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
I'm not really looking to turn this into a rant. And for any one individual to just adamantly declare that "no one will have conducted" such an experiment, is - to put it politely - patently uninformed. There are various small scale studies that we regularly refer to in our industry/operation, that are inherently more pedantic than what I'm asking for. And if you knew who I worked for, you'd know that what I speak is true. (no, I won't tell you who I work for in this forum)

Try to step out of your own experiences for a minute. Of course, by asking that, I realize that this immediately disqualifies you from being able to answer the question. But many modern workplaces are data driven, and do not rely on such anecdotal and presumptuous logic. (as what you've put forth, declaring that this exercise has no merit)

I have absolutely no idea where "crap" enters into this discussion. Never did I say that we have any issue producing the product that I'm implicitly referring to. I did not mention tolerancing, fitment, mating conditions, etc. None of that. That's a nice story, but it has no relevance to what I'm asking about. But if that's not sufficient, let me be clear - I'm 100% positive that the requirements that I specify, are sufficient to get a quality product delivered. Without knowing what my product or process is, you have no means to undercut me on my assertion. Nor is there a need to. My issue was that I'm being constrained by a department outside of my own, who is overriding my design intent, with their own internal processes. (i.e., it is MY job to specify criteria, theirs to inspect to what I define - I trust everyone to do their job, and ask the same) In turn, it may potentially mean less money in MY coffers, to work with my entire product line for the year.

So, back to the title. Does anyone have any data to suggest a cost bias for geometrically dimensioned and toleranced parts? I'm not looking for opinions about whether it's the right thing to do, or whether I'm doing it right, or anything else that's not directly associated to this thesis. If the answer's no, so be. We'll close shop, and move on to the next topic.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Now, the current crop of CNC machines probably will build parts to some level of precision, regardless of the spec, since it's built into the lead screws, and tooling, etc. If you can live with that, them that's great, but it's not because you didn't do GD&T, it's just that the machining supported a level of accuracy that was better than you needed. But, there are lots of applications where that could be doomed to failure, if, say, the fit requirements were close to the limit of the unattended manufacturing equipment.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
There are implications to specific tolerancing schemes that have nothing to do with the manufacturing operations, themselves. Consider such things as one vendor's own internal processes, in-process check requirements, final inspection, interpretation of statements of work, or, or, or... There are many facets to this discussion that don't entail the actual part production. Every one of them may get a $ for reasons that don't even appear tangible in the most straightforward parts of the conversation.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

"I believe that many vendors will arbitrarily increase the quoted price, even at the mere mention of the most primitive tolerancing. (or even datums) I cannot prove this,"

You can prove this. Send the drawing out for bid with no FCFs and then send out an update with FCFs.

What you might have difficulty in proving is the drawings are identical in their requirements.

Please share those example studies. We cannot know what is applicable to your products if you don't tell anyone what they are.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
You can't prove this unless you've sent the same basic thing to the same vendor, and asked for two (or more) bids, in some unrecognizable derivative form. Otherwise, you've let them onto the game. One could optimize a part, that has identical volume, identical machine run time, (based on subtractive surface area), and a few other factors. Then, you apply the different tolerancing methods, to each part, see what comes back. Valuable data rarely comes so easily as formulating a sophomoric "just do this" strategy. And what I've described, is a concerted effort. It's actual research.

The problem is, most people don't have the time or scale for this experiment. This is squarely in the realm of data gatherers, not engineers. I do not have a properly established channel for this data.

With all due respect - I'm not under any obligation to provide you with any information about unrelated processes, simply because I've mentioned it. At best, it only serves to take this conversation in another direction. If you feel that I'm flat out lying to you, about a company using intelligent data and metrics to establish pricing strategies, then by all means, please just say so - and I'll ignore you properly. Otherwise, understand that I don't work in the back office at the local foundry. We actually do have some fairly sophisticated practices, that might not be "the way it's always been done"

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
Also... when I say that I can't prove something, it means that I don't currently possess the facts. And unlike the way some things have been presented in this thread, I will not try to pass opinion as fact. What I know from experience, is still just heresay without data. This does not mean that it's absolutely unprovable - only that it is, at this point, unproven. Please don't make the mistake of taking my comments out of context.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Sounds like you have your mind made up to prove the overwhelming majority of the world for the past 40(?) years wrong. While I have no doubt there are studies proving you correct, it will largely be bc there's a "study" proving pretty much every opinion correct including the existence of a flat earth and aliens. As far as I know, most every GD&T and quality text written mentions studies proving GD&T's value simplifying and clarifying prints, as does common sense. I honestly cant think of a part print which GD&T doesn't simplify, unless you don't fully define the part.

I see vendor prints occasionally with overly complex tolerancing that a few simple modifiers could easily define. Much like an unprofessional attitude, they're usually a good indicator that we don't want to do business with that supplier and more often than not those prints are trashed and we move on to other companies. Growing up in a job shop it was much the same, if a customer asked you to copy or repair something without a print, we did it. If a customer provided a lousy print we usually didn't even consider the work.

Quote:

We currently have a QE mandate that ALL parts must have a 3 datum system, regardless of their complexity. In addition, their policy is that if a part has datums, it SHALL implement tolerancing which relates to said datums.

So how does that actually complicate anything? One view defines three datums.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (CWB1)

Sounds like you have your mind made up to prove the overwhelming majority of the world for the past 40(?) years wrong.

And it sounds as if you are taking what you want away from this, rather than what's clearly printed on the page. I'm beginning to see that it's impossible to have discussions here, without endless tangents, unsolicited criticisms, and needless lecturing, or citing of personal tales. My question was simple: do you have data? I didn't ask you to question my premise, to tell me how I should be thinking, to provide me with comedic distraction, or to reiterate the value of a system that isn't even being questioned.

We are trying to establish if there is a particular psychology to the quoting process. Every sample has a use case. I have many uses cases, and a finite budget. I know what I am asking, and why.

As to your experience, please do not presume that I have control over who is selected to do business. Again, not related to the discussion.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

A lot depends on what you need.

A simple prismatic type part, in which standard machine shop practices and default dimensional tolerances per ISO 2768 will yield usable parts, and part qty less than 5 or so, will add cost to add GD&T due to the added time to both interpret and inspect it. Probably not much more. How much more probably depends a lot on the shop.

The tighter and more critical your tolerances become, and the higher your part quantites become, the more benefit you get from GD&T in precisely defining what you need while allowing maximum flexibility elsewhere.

Also, the higher your quantities, the more the up-front print interpretation and inspection setup costs get spread out, making them less of a factor.

I would certainly agree with the assertion that GD&T can always better communicate and clarify the requirements for parts. And if you're quoting thousands or even tens of precision parts, it's essential. But if you're quoting qty 1 welded angle bracket to mount an air cylinder to the side of a machine using clearance holes, a drawing referencing ISO2768 with no explicit tolerancing at all will, in almost all cases, be less expensive overall than one in which the amount that you really don't care about most dimensions is explicitly laid out in GD&T.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
I have hundreds of "qty 1" parts, with relatively zero sophistication, which will NEVER be made twice. In fact, where possible, these parts get 3D printed.

Nobody is getting rid of GD&T for the vast majority of our processes.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

solid7,

Your question depends a lot of who the vendor is, and who is applying the GD&T to the drawings. If everybody understands the GD&T, there ought to be much opportunity to identify non-value-added tolerances, and make the parts cheaper. It is not just the fabricators who do not understand the drafting standards.

There was a good discussion in the Drafting Standards forum on thread1103-322065: Critical Dimension.

--
JHG

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
That's kind of my point, drawoh. If we say that "it depends on the vendor", then we understand that there is some bias. There are many reasons. But the buying process can be completely blind to the underlying factors.

I have constant discussions with some of my colleagues, regarding the necessity of frugality, and the avoidance of "machine shop prices for weld shop parts". We're more or less at bare bones for drawing information - but we have no problem with product definition.

For those who have their boots on the ground - i.e., our purchasing agents - and I mean the ground level, not operational level people, there is an explicit understanding that not all shops are equal. However... the conundrum is, that some of the shops who may be a little less sophisticated, and more prone to charge a premium for GD&T, are also the ones who provide the best price for short lead time. And they deliver the product we need, 100% of the time. We understand how to leverage the strengths of different entities. We just aren't always able to communicate that to our other team members, however. (just as we can see in discussion, people think/say/do as they prefer, not necessarily what's being asked)

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

"We are trying to establish if there is a particular psychology to the quoting process"

Should have led with that.

And it seems like your problem statement is:

"We just aren't always able to communicate that to our other team members"

After all, you state that you can communicate with suppliers to get the best price, best time, best performance, but not your own people. Why are they going to believe random people on the internet when they have you?

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
"After all, you state that you can communicate with suppliers to get the best price, best time, best performance, but not your own people. Why are they going to believe random people on the internet when they have you?"

That is NOT what I said. I said that in a former capacity, that was my firsthand experience - negotiating and developing pricing. Apparently, you don't have a grasp on how disconnected, and full of blind spots, organizations can be in large companies. If you think that you can solve that problem, then you'll be my personal hero, and certainly, you'll be a captain of industry. But for me, I'm working to be part of that solution in my organization.

If it's unclear that I'm looking for a psychological angle, when using terms like "bias", then I'm not sure how much more I can clarify this for you. Let me point out, however, that it was you who suggested sending the same quote out for bid twice. So that certainly implies that you have at least somewhat of a grasp on what's being said.

"We just aren't always able to communicate that to our other team members" means exactly what has been demonstrated in this thread: I can tell you all day long what I want from you, but if you are determined to go your own way, that's exactly what you'll do.

Case in point: I ask for research/data, or some reference to it. What I get, is anything but research/data. Translate that into the office space. As someone in a position to do so, I ask you for something that means something to me, but not you. What I get, is having to put up with a lot of BS - some of it self-righteous, some of it uninformed, all of it anecdotal, and none of it helpful. But somehow, it's thought that I need to hear your official take on the matter, as a whole. If I just tell you my rationale without fact, I get all of it, reloaded, and with ever-gaining forceful momentum. So I like to speak to facts. Everyone's got a story. I'm much less interested in those, unless you're buying me the next round of a nice single malt scotch. Then, we can talk about the other stuff, in a more casual, and less consequential manner.

Back on track - I can suggest to QA why I don't want to add a specific callout. Rightfully, it's well within my boundaries to do that. In no scenario should QA be dictating engineering requirements. It can be negotiated, and it can be pointed out where error occurs, or where improvement is warranted. But in our case, we are transparent, and collaborate amongst groups. Sometimes, that gives a little too much boldness to those to whom it is not entitled. But it helps us all get better at what we do, if approached with the right mindset. But we are fact merchants, first and foremost - not speculators. This is an attempt to prove or disprove, that my past experiences, hold credibility. If I'm correct, there is a direct economic impact. If I'm incorrect, we can state with certainty, that there's no reason not to accommodate an outside request.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
For the record - the QA inspector wants to know the same thing that I want to know. So that you know that it's not just me, on some rogue quest to "undo the last few decades of progress." We are working together to define processes for all who produce product definition. He and I are NOT in disagreement that we can do without the tolerancing, from a pure definition standpoint. Just whether or not the more valuable purpose is to create a homogenous approach for all product lines - or delineate our methods, to achieve value, relative to function. His angle is more to the effect of, "creating a habit".

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

solid7,

I am still thinking about your question. I am a mechanical designer, and I am the guy who prepares the drawings.



What ought to happen is that I prepare drawings with full GD&T. These completely and clearly specify the parts we want, and that we will pay for. The drawings go out to the shop as is, i.e. without anyone providing additional requirements over the phone or through emails. The shops trust the drawings and quote price and delivery in good faith. They manufacture and ship compliant parts. We accept the compliant parts, i.e. if they don't work, we recognize that our design and drawings are wrong.

Metaphorically, the GD&T is a link in a chain. If the GD&T sucks, if procurement does not respect the designers, if the shop does not trust and respect your company and its drawings, if your production does not respect your designers, and the shop, all bets are off. At my current place, I prepare fabrication drawings with GD&T. My boss gets on the phone with the fabricators and tells them how he will use lasers to inspect the parts. What is the shop to do?

A couple of years ago, I was on a contract with a company that was obsessed with GD&T, among other things. They were very good at it. On my last day, I photographed the cash register in the lunch room. It was not always manned (personed?). People were taking coffee and muffins and leaving their change on the counter next to the register. There is a mindset that makes things work.

--
JHG

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Drawoh, I was at a factory on contract and same thing, very picky on drawings, and I think it was like your example in the cafeteria.

On the other hand, I've done contract drafting and been asked to strip GD&T from drawings before sending them to the fabricator, for the stated reason, it was an automatic cost adder.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

moon161,

I have never been asked to strip GD&T from drawings.

As designer, I design stuff and I prepare documentation. I do my fabrication drawings to ASME Y14.5. My tolerances all are based on my design requirements. If sloppy tolerances will work, I specify sloppy tolerances. My drawings specify precisely what I need. If the shop tells me they cannot hit the tolerances I specify, I have a design problem, not a drafting problem.

ASME Y14.5 is a standard that explains what all the stuff on my drawing means. GD&T allows me to do a precise definition of my part. If my fabricator and I do not agree on what the numbers and symbols mean, then the fabrication process is not under control. Anything can happen.

--
JHG

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

In my experience, now approaching 20 years, GD&T can work well or work poorly.

My company had a history of "inspecting" parts on the machining centers while still clamped down. As such, it became common practice to put .001" perp/flatness/true position or everything. It never happened, and so vendors who weren't familiar with our parts would quote to the print and of course, the prices were unrealistic. Nowadays we have our own CMM, inspect everything in the free state, and have updated most tolerances to realistic levels. But some drawings sneak by Engineering and get quoted or made with the old tolerances.

We have also had vendors who see GD&T and add cost for each tolerance. When GD&T is only used for the tight tolerances, that makes complete sense. But when the vendor sees FCFs as a pain, we have agreed to not do business with them any longer.

I've also put GD&T on fabrication drawings and received no-quotes. I have been known to convert GD&T to simple +/- tolerances and notes in order to get a part made, correctly and quickly. (I work in an industry where one-off parts are routine, and being a stuffy engineer who won't flex results in late orders and loss of income. Sometimes the lowest cost option is to alter the drawing for the vendor)

There is plenty of blame for Engineering too. Yes, GD&T is a functional tolerance system, but when the functional tolerance needs are highly incompatible with practical manufacturing and inspection methods, Engineering needs to dig in and find an acceptable solution or get involved with the functional gauges. That process is ongoing, as it should be. Whether and how Engineering participates in this, affects price and speaks to the competence and worldliness of the Engineer.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

GD&T can save a lot of money, especially in a mass production environment.
  • Allows for use of functional gauges
  • Increases tolerance zones
  • Defines fit and function more precisely
It's also useful for sussing out bottom-feeding vendors.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote (Solid7)

o, back to the title. Does anyone have any data to suggest a cost bias for geometrically dimensioned and toleranced parts? I'm not looking for opinions about whether it's the right thing to do, or whether I'm doing it right, or anything else that's not directly associated to this thesis. If the answer's no, so be. We'll close shop, and move on to the next topic.

Move on, man. These guys are not able to answer the question, and will now spend time doing exactly what you told them not to do. A lot of experts in everything you don't want nor care about.

Shut it down... I've yet to see anything more than 10% of the replies in this forum directly related to answering simple questions, they just can't do it.

I'm sure there is a book out there somewhere on this subject, but I cannot find it yet. If I do, I will let you know.

Next topic

https://www.cnccookbook.com/the-high-cost-of-tight...
https://www.sigmetrix.com/wp-content/uploads/2016/...
https://www.qualitymag.com/articles/92034-corporat...

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

FACS - those articles are the opposite of what the OP is demanding. He wants to prove that FCFs are bad and doesn't care about limiting variation.

Welcome to the 90%

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

3Ddave,
I know, that's why I said I couldn't find it yet.
But thanks for reading.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (3DDave)

He wants to prove that FCFs are bad and doesn't care about limiting variation.

This statement alone proves that you have no reasonable claim to: a) being called an engineer, b) answering any question that requires one to have their eyes open while reading. c) technical literacy skills, in general.

Quote (3DDave)

Welcome to the 90%

Makes me wonder if you spend any time self-analyzing. Is your purpose here to contribute meaningfully, or just to be "right"? Cause I have to say, it sounds like you're just here to either be validated, or to lash out strongly against your detractors.

For myself, the purpose of a discussion, is progress, not competition. But in order for that to happen, it requires one (preferably all) to actually adhere to the topic, at hand. Without bouncing around. You know - skills we learned well before engineering school. So FACS is not wrong in his assertion.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

To answer your question directly: Yes, YOU can produce the exact data you are asking for.

Start sending two drawings to every vendor, and get a price for both. One with "full" GD&T, the other with only essential stuff.

It's not some conspiracy, someone has to carry the risk of scrapping parts that aren't within tolerance. Lots of GD&T means the supplier does, too little means the purchaser (or their end user) does. Where do you want the risk to be?

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
I assume you haven't been following this thread.

That's not exactly how "bias" works. But I do thank you for at least sticking to the topic.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (geesamand)

Whether and how Engineering participates in this, affects price and speaks to the competence and worldliness of the Engineer.

Correct. We are the primary cost drivers for everything that happens downstream from us. It behooves us to pay attention to where those costs come from, and how we can mitigate them. Additionally, there is inherent value in "knowing things".

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

I was thinking more about this, and how obviously GD&T boosts price when working with small quantities of low-accuracy parts.

Take for example a bracket sawcut from a short length of angle iron. Drill two holes in one side of it. While GD&T says I should place a true position on the holes, so as to provide bonus tolerance for the hole locations, do I really want to pay for the vendor to build a functional gauge? Or put it on the CMM table? Fine, it's your money, lose business in the price of your product. I just added cost using GD&T here.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?



geesamand,

You have just been asked to quote price and delivery for two pieces as illustrated above, drawn on a napkin or on a piece of scrap paper out of my recycling bin. You need somehow to meet the tolerances I have specified. How thoroughly would you inspect this thing?

--
JHG

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Oops! I missed attaching datum C to the .438/.375 hole.smile

--
JHG

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
Forget that example. Imagine the same part, with no holes, A and B datums, as attached, and datum C as one end. There are no other dimensions, save an overall length. (but for some reason I have 3 datums)

What would you make of that, if you're a vendor? Do you charge extra, based on not knowing exactly what kind of madness you're about to have to deal with? Take it one step farther - if I have a mandate, that if I have datums, I SHALL reference some feature TO those datums, have I just built in a cost? Did I need that "all over" surface profile tolerance - which is one of the simplest "default" specifiers, for the type of parts that we produce?

You are being WAY too specific with parts and tolerances. We were never really headed in that direction. You can debate your example in another thread, because it's not a bias related issue. That's far too technical for the scope of this conversation. We're not talking about what it costs to produce parts with actual requirements.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote (drawoh)

You have just been asked to quote price and delivery for two pieces as illustrated above, drawn on a napkin or on a piece of scrap paper out of my recycling bin. You need somehow to meet the tolerances I have specified. How thoroughly would you inspect this thing?

Well if it didn't have positional tolerances, I'd use a tape measure and/or a dial caliper.

Unless you have a tape measure that consistently evaluates true position tolerances...which I do not.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

That seems to be a different problem than GD&T, per se; you are describing a management problem, doing stuff for the sake of doing stuff, by over specifying. That is not a problem with GD&T, it's a problem of over specification.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote (IRstuff)

That seems to be a different problem than GD&T, per se; you are describing a management problem, doing stuff for the sake of doing stuff, by over specifying. That is not a problem with GD&T, it's a problem of over specification.

Since you didn't quote or reference anything, I'm going to assume it was referencing my post prior.

To whit, how is GD&T not affecting cost, when the inspection operations required to confirm the part in GD&T terms cost more than the part itself?

The point I've been driving at is, GD&T is a great system for maximizing effective acceptability for critical features (cost or function), where volume pays off the overhead of setting up a quality GD&T system. A great deal of manufacturing and engineering design does not fit that context, and costs go up when GD&T is not ideal.

David

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

solid7,

I deliberately went overboard on that thing. The drawing completely specifies what I want.

When I took my GD&T course, the instructor stated that the datums describe the manufacture and inspection tooling. There ought to be a fixture with pins for the 3/8 holes. On the other hand, when the part arrives on my loading dock, I don't know and I don't care how you fabricated and inspected it, or even if you inspected it! If I specify tolerances that are marginally within the capabilities of your process, you need to inspect everything, and possibly factor in a scrap rate. This part is inspectable with a plastic scale from a dollar store.

--
JHG

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Again, it's the misapplication and over specification of GD&T, not GD&T itself that's at issue. When you say that you have datums specified, the question is why? Because if there's no need for the datums, they shouldn't be specified; that's not a fault of GD&T. It's no different than over constraining a design to start with; if you only need ±1/4 inch and you specify ±0.2500, that's not a fault of GD&T, that's a problem with too many significant figures. Having more datums than required is, essentially, putting down too many significant figures.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote (drawoh)

When I took my GD&T course, the instructor stated that the datums describe the manufacture and inspection tooling. There ought to be a fixture with pins for the 3/8 holes. On the other hand, when the part arrives on my loading dock, I don't know and I don't care how you fabricated and inspected it, or even if you inspected it! If I specify tolerances that are marginally within the capabilities of your process, you need to inspect everything, and possibly factor in a scrap rate. This part is inspectable with a plastic scale from a dollar store.

I wouldn't say those things out loud when your ISO 9001 auditors are around.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (IRStuff)


That seems to be a different problem than GD&T, per se; you are describing a management problem, doing stuff for the sake of doing stuff, by over specifying. That is not a problem with GD&T, it's a problem of over specification.

Since we all like tangents so much, I challenge you to point out where I ever suggested that there was a problem with GD&T.

If I challenge you to prove my statements wrong, maybe you'll actually read my posts, with an attention to detail, that will lead you to the point that was always plainly there.

Overspecifying doesn't buy me - or lose me - anything (useful) that isn't already there. So, for the sake of a discussion... If it says the exact same thing, in 2 different ways, does one method induce a bias that influences cost?

At this point, I'm certain that I've asked my question in the wrong forum. Nevertheless, have at it.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote (solid7)

At this point, I'm certain that I've asked my question in the wrong forum. Nevertheless, have at it

I'm just reading now because it's funny and I want to see how deep this goes.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote:

I challenge you to point out where I ever suggested that there was a problem with GD&T.

Your entire thread suggests that, starting with your title. Then, your second post. If there's no problem, then you should be able to use it all time.

Quote (solid7 (Mechanical)(OP)18 Mar 20 23:22)

The alternative is a tolerancing scheme that doesn't use GD&T.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
IRStuff - I'm wondering how you're an engineer, when you make such authoritative statements, without asking any qualitative questions. That's so far removed from my own experience. In fact, in all of your posts, there is only one question, and only posted after much clarification was already offered. It is precisely this:

Quote (IRStuff)

When you say that you have datums specified, the question is why?

That question was answered very early in the thread, and it was information volunteered, not requested. It clarified the premise early on. So I can only conclude that you've stopped by to flex on me. You have made no attempt to understand the issue, before laying down your fully unqualified opinion. I might ask, in the future, before "helping" other, maybe actually pay attention to what's being said/asked, before giving the answer that you've decided they really need.

I judge people in my field more harshly by the questions they ask - and whether they bother to ask them at all - than by the answers they give. I can forgive a person for being wrong, but it's a lot harder when they've put no effort into being right, about the right things, at the right time.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

solid7,

Considering your first and second posts, it seems that you were asking: "Does anyone have data to show that bad GD&T requirements (such as unnecessary datums and tolerances) result in increased costs?" I very much doubt that you're going to find useful data on this, in part because the extra costs would also depend on how bad/severe/unnecessary the GD&T requirements are. I think it's also generally assumed that bad/unnecessary requirements of any kind add more costs.

As far as your more recent question:

Quote (solid7)

If it says the exact same thing, in 2 different ways, does one method induce a bias that influences cost?
The answer likely depends on your suppliers and how comfortable they are with GD&T and how much they (dis)trust you. Suppliers that are comfortable with GD&T and don't expect you to try to screw them probably won't charge you any premium for it. Suppliers that aren't comfortable with GD&T and/or are nervous that you might try to use it to screw them will likely charge a premium or decline to quote.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote:

Forget that example. Imagine the same part, with no holes, A and B datums, as attached, and datum C as one end. There are no other dimensions, save an overall length. (but for some reason I have 3 datums)

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (jmec87)

Considering your first and second posts, it seems that you were asking: "Does anyone have data to show that bad GD&T requirements (such as unnecessary datums and tolerances) result in increased costs?"

I said nothing about good or bad. Again, one of my clarifications:

Quote (Solid7)

His angle is more to the effect of, "creating a habit".

For the vast majority of cases, it's neither good nor bad, right nor wrong. We'll get to the same point, either way.

Re-focus #1337 - Does doing something that's neither good nor bad, right nor wrong, and ends up producing the same product, have an inherently built in COST BIAS depending on which method is used? DOES ANYONE HAVE ANY DATA.

If ya don't know, jest say so. No shame in it.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
Moderator - feel free to close this thread. I really should have known better. The 20% are too busy at work for this.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

There is support for the opinion that there is a costing bias but, without examples of what is supposed to be sufficient and of identical performance, it's impossible to be sure if it applies to this case.

Mostly it's a matter of the correct search terms; I found several.

As far as self-examination is concerned - "You will NEVER get a group of people to adhere to standards and documentation."

The irony and lack of self-awareness carried by the guy who wrote that is staggering.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)
Further proof of intent. Failing to "win" in one thread, one resorts to involving unrelated threads, to make a smaller point.

Have a nice day, 3DDave. Your services are no longer required here.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Quote (solid7)

I said nothing about good or bad.

Quote (solid7)

We currently have a QE mandate that ALL parts must have a 3 datum system, regardless of their complexity. In addition, their policy is that if a part has datums, it SHALL implement tolerancing which relates to said datums. So, we're held hostage to this methodology.

Quote (solid7)

My issue was that I'm being constrained by a department outside of my own, who is overriding my design intent, with their own internal processes.
I assumed that arbitrary GD&T requirements that override the engineer's design intent are bad GD&T requirements. I think this assumption is correct, but please feel free to rant if you disagree.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

Actually I believed that post and take you at your word. Do you recall how you resolved that situation when persuasion failed?

Your premise in this thread is that you want to create some rules (verbal orders, tribal knowledge by oral history?) a document that will always be followed by the other people in your sphere, ones who currently do not take your guidance voluntarily. Yet, at the same time, you railed against exactly what you currently demand; that your employer expects you to adhere to standards and documentation and you don't want to.

It is a conundrum.

To prove my magnanimous nature:

The Politics Of GD&T https://www.eng-tips.com/viewthread.cfm?qid=261325

GD&T part costing more https://www.eng-tips.com/viewthread.cfm?qid=192933

Geometric Tolerancing Can Increase Part Cost and Delay Schedules https://www.linkedin.com/pulse/geometric-toleranci...

These are among the thousands of posts to the contrary.

It's interesting to consider that anyone with the suspicion that there is a quantifiable bias would a make a massive, non-sophomoric study, being somehow able to discern the exact motive from unsuspecting suppliers and assign a dollar figure to it and then just make that difficult and expensively generated result available to just anyone without letting suppliers in on it.

Would it not represent a tremendous competitive advantage to game the system that way? While everyone else is wasting significant sums of money, any company in possession of this information would be either underbidding or taking increased profits. What engineer possessing that information would torpedo his own company by divulging it?

However, some people are prone to seek the attention such a discovery would make, as the LinkedIn post makes clear, albeit without the quantitative, non-sophomoric study expected. Once such an advantage escapes into the wild, I would expect it to spread at rates seen for MBD, CMMI, 6-Sigma, ISO 9000, TQM, Value-Engineering, and Y14.5 itself.

Simple reasoning shows that either no one will divulge such a costly and difficult to perform study at the cost of their livelihood, or that everyone would championing it and be adopting it as fast as possible to cut costs. Since the first is of no use to hope for and the second is not happening, that supports the idea that no such study will be available, even if, I grant, one could could possibly be made; thus the original request is moot.

The most reasonable engineering judgement is that, since no evidence is forthcoming and your company hasn't agreed to any other course, the best action is to find a company that is better at controlling costs; this is unlikely to be the only area in which your current employer in failing to excel. It makes sense to depart. As I'm sure you would say of me - you cannot fix stupid. It also makes for a good line of interview questions to see if any candidate company also has a faulty process for procuring parts; walk out if they are faulty.

One last thing - I have, in fact, suggested that FCFs are not always required when the processing equipment capability is likely to improve by large fraction the necessary variation controls that FCFs are so good at precisely describing. Essentially it is your position. Sadly for you it is sophomoric and not quantitative.

You are hardly the first to have the thought; this is not the first time on this site it's been discussed and there has been no quantitative resolution to the matter.

I am also content that I cannot win in this as the table is rigged. Even so, it's gratifying to analyze the problem in the same way that statisticians analyze gambling without being able to convince the house to be fair.

Edit: Fixed.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (jmec87)

I assumed

Of course you did. You're in good company.

Maybe I should go back and change the title of the thread, to "Assumptions on cost bias... ?" Then we'd have a much more qualified group.

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

(OP)

Quote (the guy who just keeps coming back for "the win")

Your premise in this thread is that you want to create a document

Nope. It's not, and I never said that. Exact quote:

Quote:


The company that I work for, is currently trying to implement some tolerancing rules, that are broken down by product level.

Never said anything about writing documents. The "tolerancing rules" are mandates that will be overseen by responsible design leads, based on existing documentation.

If anything, you've validated my previous (and completely unrelated) comments, by illustrating why the person looking to instill a "habit", is doing so. We have documentation - he simply feels that there is a need to further refine a portion of it, due to past non-compliance, in a much more comprehensive manner. (which is the fundamental nature of my disagreement - we can do better at the other, without implementing a mandatory structure, that isn't needed. If there's a cost component - real or perceived, it's discussion over, and we both agree on that)

You can have the last word. We both know that there is nothing left but a pissing match. So feel free to flay me. I didn't come here looking to take a scalp. Floor is yours...

RE: Data on Cost Bias for Geometrically Dimensioned and Toleranced Parts?

In the metals industry, you pay extra for obtaining ASTM Certs. If you don't ask for the certs, they claim the metal complies regardless.

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