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

[solid7]

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.

 

[IRstuff]

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!

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[geesamand]

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.

 

[solid7]

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.

 

[FACS]

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.

 

[IRstuff]

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.

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

TTFN (ta ta for now)
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[solid7]

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:

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.

 

[jmec87]

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:

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.

 

[IRstuff]

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!

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

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.

 

[solid7]

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

 

[3DDave]

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.

 

[solid7]

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.

 

[jmec87]

I said nothing about good or bad.

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.

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.

 

[3DDave]

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-tolerancing-can-increase-part-cost-delay-schedules-lail

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.

 

[solid7]

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.

 

[solid7]

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

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

 

[BUGGAR]

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.