How do you handle people who belittle GD&T and point to their success without it? 1

[Tunalover]

My boss today discredited my use of the fixed fastener formula of Y14.5. Since he had never heard of it (he's an EE) and probably because I'm a new guy at the company, he essentially discredited my use of the formula and said "we've made hundreds of thousands of parts here over the years I've been here and we've never experienced fit problems between our PCBs and their housings."

I tried to tell him that any process change, tooling change, or supplier change can cause holes to drift away from their true positions. If they do, and the parts still pass inspection, then the parts are still accepted because the formula guarantees that the parts will fit together as long as all the holes come in within their allowable size and positional tolerances. He summarily dismissed the entire subject before I had a chance to show him how the shape of the tolerance zone alone provides a 57% bigger area for the hole centers to land. He stated that GD&T should only be used for fit-critical situations because the symbols drive up the cost of the parts.

How can you talk sense into someone with this lack of knowledge and appreciation for GD&T?

Tunalover

 

[MadMango]

All of the PCB layout guys I have ever worked with seem to "eyeball" placement of holes, components, and board edges. It is only when I complain that "these mounting holes should be in-line with the others" will they be fixed.

"Art without engineering is dreaming; Engineering without art is calculating."

Have you read faq731-376 to make the best use of these Forums?

 

[Belanger]

I guess that's true, Kenat.

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

 

[SwinnyGG]

I would agree with KENAT's assertion that low or no GD&T knowledge won't affect an EE's job prospects. In a past life I spent a fair amount of time in meetings with EE staff who were oh-so-proud of their new PCB layout, asking questions like "What tolerances do you expect on this hole, or this edge?" and getting blank stares in return.

In my opinion, someone who thinks GD&T adds cost either doesn't understand what GD&T does, or is not evaluating things in a big enough frame of reference.

My typical solidworks model of a part takes maybe 8 or 10 hours. Creating a drawing (or set of drawings) for that part takes another 2 or 3 lets say. If I take that basic drawing and add GD&T features where appropriate, it might add another hour or so versus using basic (as in simple, not GD&T BASIC) dimensioning.

So sure, GD&T adds $100 to the cost of the drawing.

Then the guy at the machine shop gets it. If he's competent it maybe takes him a little longer, let's say another hour, to interpret the drawing and quote accordingly. He builds that extra time into his quote- so if it's one part, you add another $100 or whatever.

The real point is that you've now added $200 to part cost- but if you've done your job, everyone now understands what the part needs to do. You've spent $200 so that you don't have to spend $50,000 on part rework after the production line is running, or parts are at the customer, or whatever terrible scenario could arise that gives us nightmares.

I think, regardless of what type of engineering we practice, everyone on this board has at some point worked for a boss or program manager or customer who did not understand why we do things a certain way. My approach in these situations has always been to approach things from the financial side- PMs and bosses care about profit and margin more than anything else- and used correctly, GD&T is a tool which can increase both. If you frame it that way, he will get it.

 

[Tunalover]

Now guys although my new boss is an EE, somebody put him in charge of a group of purely mechanical MCAD designers and MEs who deal with GD&T almost daily in their design of die cast and injection molded parts (I'm fairly new but so far it doesn't looks like any of the designers understand the method; they just give the MEs what they ask for).

After my meeting with him, it dawned on me that he doesn't care to learn anything about something his guys spend a lot of time with. The truth is he will rely on perception rather than facts when it comes to an issue involving GD&T and go with the guy who has the most convincing speech or the guy that he once decided is the one that is usually right, not the guy who puts the facts on the table.

Management is chock full of people who believe that if they don't understand something then it's not worth knowing hence it has no value. The boss I had before this one never strung the three words "I don't know" together. Ever know someone like that?

In any event, I have carried the GD&T torch faithfully for many years and will continue doing so. You know IPC promotes GD&T and requires that all dimensioning and tolerancing conform to ASME Y14.5-2009 on PCB drawings. But it is still true that the standard is entitled "Dimensioning and Tolerancing" not "Geometric Dimensioning and Tolerancing" so the ECAD designers and fabricators fall back on bilateral (±) tolerancing which is approved even though it is vague and subject to interpretation. I have yet to meet an EE or ECAD designer that understands or uses the method or had any interest in learning even the bare essentials.

Tunalover

 

[jflongwell]

I worked in engineering for 29 years and have been an educator at a community college for 16 years teaching the Y14.5 standard. Many of my faculty colleagues think the standard is an overkill and adds cost by tightening tolerances and making product verification more difficult.
I am ASME Senior Technologist for the Y14.5 standard. I frequently conduct training at companies. Last Spring I worked for five companies.
The motivation for the training and potential change is driven internally (frequent problems)or external (customer demands - similar to the ISO 9000 registration need).
Typical domestic business model - if it ain't broke, don't fix it.

John

 

[Belgiancadengineer]

I don't think anyone can deny GD&T is not very intuitive, unlike the concept of +- dimensioning which anyone can understand. I have never been in a precision manufacturing environment but I have never seen GD&T used except for a flatness callout here and there

 

[Tmoose]

Hi Tunalover,

Could you explain a little more how a bilateral tolerance is "vague and subject to interpretation."

thanks

Dan T

 

[ljones]

I agree with Tmoose.
Please explain how a bilateral tolerance is vague and subject to interpretation. I often hear very lengthy discussions in the product design group on how to apply and interpret GD&T. This does not seem to happen when using +/- tolerances.

L. Jones

 

[drawoh]

Tmoose,

--
JHG

 

[drawoh]

Yes, the standard is ASME Y14.5-2009.

--
JHG

 

[JNieman]

I don't see the problem with inspecting that exaggerated depiction. The 'implied 90' rule and youe stated angular tolerance covers form and location (2.1.1.3), and size/location is covered by the linear dimensions. Trying to split hairs beyond that is making mountains out of molehills.

 

[drawoh]

JNieman,

We have had this discussion. thread1103-261904

Do you measure to the holes from the nearest edge?

How do you take into account the angle error of the bottom edge, or is it the side edge?

--
JHG

 

[JNieman]

I understand the complications.

I also recognize that despite the arguments and near-chicken-littling, people have been making and using parts like that successfully since.. the industrial revolution, I suppose.

I guarantee that to this day, gobs and gobs of structural base plates are made from drawings just like yours, or made from drawings with nothing but tail/ordinate dimensions from some point.

The descriptive language defined in Y14.5-xxxx is awesome and clarifies many things, leaving fewer issues open to interpretation. I simply refuse the supposition that it is the -only- way to get things done successfully.

Whether using 'legacy' dimensions or ASME Y14.5 form/size/location controls from basic dimensions, designers must account for the most likely result of their drawings. Risk assessment comes into play. A necessity for knowing how the parts will be made is very helpful.

Yes, one way is inherently less risky than the other. I simply refuse to think the 'sky is falling' (hyperbolic, I know) because people still make drawings using plus/minus dimensions. I think it's too easy to forget that there are entire worlds of manufacturing outside of some of the niches we work in. I've worked in many different types of manufacturing and engineering environments which I'm grateful for. Mostly I've gone to the more-precision-oriented end of the spectrum from start-to-present.

 

[3DDave]

JNeiman,

Eventually parts will be simply made with smaller levels of variation than drawings are likely to call for, eliminating the need for specifying tolerances.

Conventional hole clearances were chosen based on the most likely hole location errors, but since their adoption precision of location has significantly improved.

Many companies put off full part inspection in favor of 'key' characteristics and expecting to catch and fix errors on the factory floor, a strategy that works as long as those variations remain small.

I fully expect concerns about dimensioning and tolerance analysis to disappear in the same way that people no longer care about the C scale or the D scale on slide rules. Calculators carry unneeded precision, but the calculations are so cheap it doesn't matter.

 

[CWB1]

I too have apparently worked for tuna's boss and several variations of him that had particularly odd/wrong/other ways of doing things. Personally I just tend to go with it and do the work in the requested fashion, sooner or later it either blows up on the supervisor or I head elsewhere. Just my opinion, but given that GD&T's been in pretty common use for ~30 years now its rather embarrassing for anyone in industry unable to use it whether they be an EE making board/harness/enclosure/other critical prints or a supplier running parts and definitely career limiting.

 

[drawoh]

3DDave,

I keep claiming that machining drawings are easy due to the accuracy of the process.

Let's design an investment casting, and let's do some DFMA, and ditch the subsequent machining. The bolt holes must be cast in place. Casting tolerances are around .005"/in last time I checked. Probably, I want to use a larger bolt than I would have on a machined assembly, simply because they cannot cast a small hole in place. I need to do quite a bit of analysis to work out the positional tolerance, the tolerance of the hole diameter, and the size of the hole that will make the process reliable.

How accurate is 3D[ ]printing? How accurate is a urethane mold made from a rapid prototype?

--
JHG

 

[dgallup]

I fully expect concerns about dimensioning and tolerance analysis to disappear in the same way that people no longer care about the C scale or the D scale on slide rules. Calculators carry unneeded precision, but the calculations are so cheap it doesn't matter.

Ha Ha Ha Ha Ha. You crack me up. You obviously don't work in a high precision, high production rate industry. Take a look at a modern 8 to 10 speed automatic transmission or thousands of other complex mechanisms and tell me tolerance analysis is going to disappear any time soon.


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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[3DDave]

Ha Ha Ha Ha Ha. You crack me up, thinking that I was referring to every possible application*.

The transmission example, much like that which I've done for multi-stage gear reductions that included temperature variations with milliradian accuracy requirements, isn't all that complex. At most it is tedious. Complex ones include deformation due to loads, temperature offsets, and transient temperatures.

*On other occasions I've mentioned the importance of understanding how floating point calculations are made, but for most people it isn't important. These calculations are done on a lot more complicated machines than any automobile transmission will ever be. Even the cheapest calculator requires manufacturing precision far beyond what goes into a drive train.

 

[SwinnyGG]

Even the cheapest calculator requires manufacturing precision far beyond what goes into a drive train.

Have you ever designed a modern powertrain system, including controls?

Complex ones include deformation due to loads, temperature offsets, and transient temperatures.

 

[3DDave]

Apparently I know what is required.