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.

 

[drawoh]

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

 

[moon161]

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.

 

[drawoh]

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

 

[geesamand]

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.

 

[TheTick]

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

 

[FACS]

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-tolerances/

https://www.sigmetrix.com/wp-content/uploads/2016/07/QualityToleranceAnalysis-WP-RyanReid.pdf

https://www.qualitymag.com/articles/92034-corporate-gdt-implementation

 

[3DDave]

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%

 

[FACS]

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

 

[solid7]

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.

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.

 

[1gibson]

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?

 

[solid7]

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.

 

[solid7]

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".

 

[geesamand]

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.

 

[drawoh]

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

 

[drawoh]

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

--
JHG

 

[solid7]

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.

 

[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?

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.

 

[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.

TTFN (ta ta for now)
I can do absolutely anything. I'm an expert!

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[geesamand]

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

 

[drawoh]

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