The Politics Of GD&T 2

[fsincox]

I am starting this thread to remove this discussion from kasabis's "TP & FCF Basics".
dingy2: I do not know if you are or are not the person I am referencing. I really did not mean to single you out, I disagree with the whole politics of “make it easier for them and they will come”.
The old system does not work. The new will not be followed until that fact is generally understood. The 2009 standard finally says it in the forward: ” …stronger admonition than in the past that the fully defined drawing should be dimensioned using GD&T with limit dimensioning reserved primarily for the size dimensions for features of size”.
Anyone who understands zero tolerancing at MMC or LMC should know we are not making the part harder to make. We are trying to provide manufacturing options. The fact the educators have not gotten this across and seem to pussy foot around management is what keeps us in the current state. It may not be popular, it may not be what management wants to here, sorry. I had hoped here I was speaking mostly to true believers and people who want to learn about GD&T not how to avoid it.
"Do not use perpendicularity on small land pilots", (because we can’t check it and it probabally made it anyway?) why shouldn’t I say it then, I don't care if they check it or not, we do not reall check every point on flatness either guys (and girls, I hope?). "Don’t specify finishes on 125 on greater" (because we will get it anyway?) How about in the future when almost everything is rapid manufactured except for fine machining. I think it is our job to state known requirements on the drawing so people don’t need to guess.
I believe, other than the validity of the design itself, it’s proper documentation is engineering’s second most important function, I mean no offence to anyone in particular but I do want to battle this whole culture of lets avoid GD&T.

 

[dingy2]

fsincox:

Circular or total runout should only be applied on small diametrical features relative to a centre line created by another cylindrical feature. If one cannot place the part in a divider head (chuck), then it is too big for either runout. It is not conducive to confirm runouts using a CMM since they can only estimate the runout rather than give a concrete number.

Many years ago, I was a Quality Consultant in high precision screw machine company producing tiny cylindrical parts for the automotive companies. We had tolerances in microns and the only way we could control the off centre and roundness of a feature was with either circular or total runout. Positional was not finite enough to discern movement while runout 100% contacts the cylindrical surface while it is being rotated.

If the product get a bit larger with corresponding increase in tolerances, then positional could be used and one would have a choice of circular, total runout and positional. I would never use concentricty.

Runouts take skill to confirm while positional at MMC is confirmed by the use of a checking fixture on the shop floor. I have an example of total runout at

http://www.qmsi.ca/runout.html

with checking method.

Dave D.

http://www.qmsi.ca

 

[axym]

It's interesting how there are so many different "philosophies" with regards to drawings and GD&T. I think that this relates to one's philosophy of what particular purpose the drawing should be optimized for. Some think that the dimensions and tolerances should be optimized for easy reading and checking by the fabricator. Others think that inspection and gaging considerations should influence what characteristics are specified. Though nobody seems to want to admit it, many people think that the complexity of dimensioning and tolerancing should be limited to what will be understood on the shop floor.

Back in the day, I used to subscribe to the "give as much tolerance as possible, while maintaining function" philosophy. You know, the ivory tower ideal. I would advocate using any Y14.5 tools, no matter how arcane or obscure, in order to open up tolerances. 3 or 4 FCF's for one feature? No problem. If the people using the drawing are intimidated or are not able to understand it, then too bad for them. It's all in the book, look it up. That was my attitude.

Here's an example. There was a feature that had a Total Runout of .005 to datum A. I determined that the function only required Circularity within .005 and Cylindricity within .010. Position at MMC within .020 was adequate for the coaxiality requirement. Way more tolerance for manufacturing! I'm a hero, right? Of course when I suggested that, everyone went ape. "Oh, the shop will never understand that! They wouldn't be able to just use a dial anymore, we'd have to get an expensive roundness tester! This is going to drive the part cost through the roof! Let's just stick with the Total Runout of .005, thank you". Sigh.

This type of scenario repeated itself so many times that I eventually began to soften my stance. I realized that, in (too) many cases, the costs of the extra drawing complexity outweigh the benefits of the extra tolerance. Sad but true. I have found that it is rare that one can get away with using the most sophisticated Y14.5 tools without severe consequences. Particularly in certain industries.

These days, I try to use the function-based dimensioning and tolerancing as a starting point, as one of several possible options. Then simplify the characteristics and tighten the tolerances (the two always go together) until the result is palatable in the client's situation. Occasionally, the client is fully comfortable with the function-based callouts. Sometimes it gets simplified all the way to doing away with GD&T altogether and just specifying very tight plus/minus tolerances instead (when one of the constraints is that the inspection guy has to be able to verify everything with calipers).

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[mfgenggear]

Before the inception of the GD&T there was a msster plaster
that coordinated every tool required or associated with that Major, & Sub Assy's. that way it was assured it would fit together. GD&T is now that Plaster Master.

The problem is the left hand not talking to the right hand.
some one must make sure that all designs are coordinated.
I belive most Engineers & Designer Drafters do a good job.
the checkers must know what he or she is checking for!!

As a manufacture I like GD&T tolerancing. I also like to know the fit, form, & function. also Is it a very critical componet or not? So that more steps can be taken to insure
no mistakes are made.

 

[mfgenggear]

first line should be "master plaster"

 

[KENAT]

"checkers must know what he or she is checking for!!"

Checker, what's that?

;-)

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[fsincox]

axym,
I don't disagree that our perspective gives us different approach to GD&T. My perspective is this, ever since I became the GD&T, guy quite a few years ago, 95% of my time is geting called to the shop or inspection because the .001 runout, or whatever it is, has not been made. So I see the opposite side, some one cut corners, the part didn't make it and now it needs sorting out, generally the part will work, which really angers managment.
I am realistic, otherwise I would not be a supporter of ISO's general tolerancing principle, from my perspective we just cut it too short is all. If your tolerances are such that 99% (you put in your number) pass that is probabally good enough.

 

[drawoh]

axym,

When I produce a drawing of a sheet metal component or a weldment, I take much more time thinking through the drawing than I do if the part is machined. Your runout case is a perfect example of a waste of time in a machine shop, where the original .005" is easily achieved. It would take more time and effort for the machinist to take advantage of your .020" positional tolerance, which is the exact opposite of what you wanted.

When you start trying to locate things within Ø.005" on a sheet metal part or a weldment, you have to understand your requirements, and the fabrication process. Everyone needs to understand the GD&T you are going to use to make everything fabricatable.

JHG

 

[ctopher]

All the years I worked in aerospace, GD&T was mandatory.
Now in non-aerospace, GD&T is rarely used. Interesting how many I come across either don't understand, or don't want to, or simply refuse to use it.
I'm slowly forgetting a lot of it because I have not used much of it for the past 4 years.

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion

 

[fsincox]

Ctopher,
It was manditiory? wow, was it a big company? We are a relitivly small family owned company now part of a huge corporation, so the thinking is very small. I agree that some areas might not be well served by GD&T (yet??), say sheet metal. In my area the drawingss are asuming way too much.

 

[ctopher]

I used to work on fairly large military/space programs. GD&T was mandatory for these programs per contract.
One company I worked at was medium size, the other was a major large aerospace company.

Chris
SolidWorks 09 SP4.1
ctopher's home
SolidWorks Legion

 

[KENAT]

fsincox, pretty much any company doing defense work in US has to use GD&T to some extent, especially if a 'level 3' drawing pack is required (i.e. it can be shipped to any competant engineering shop and built to). Indeed the origins of GD&T are largely from that environment.

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[fsincox]

KENAT,
Sorry, I know the history, I have a copy of MIL-STD-8. I don't mean use it, I mean believe in it, not because someone dictates. I would not say we use it correctly and we are defiantly in the defense business. We use it at about MIL-8 mentality too. Most of the people around me would prefer it say that way.
I have always worked at places where they had in house manufacturing, I'll bet that is the problem, "we don't need it on the drawing we can just talk to them".

 

[KENAT]

The thing is, for many military contracts the govt effectively owns or at least pays for the drawings and so understandably has requirements for how good they have to be to allow for transfer to other suppliers.

We had this battle, though not specific to GD&T, with our manufacturing people back in the UK defence company I used to work for all the time. The drawings have to be to a standard than any competant supplier can build to them. Just because we designed a certain product, and up to now had always won the manufacturing contracts, didn't mean it would never go to someone else.

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[fsincox]

KENAT and all,
I want to thank you guys for the insights I have gotten in the forums the past few months I have been here. I will be on at home, so sorry, you won't really lose me. I am even taking my copy of 2009 home just in case, but, it is the season we think about these things.

 

[mfgenggear]

Basically designing or making parts that are USA military contracts then GD&T is required.

 

[marshell]

When it comes to Military contracts and most aerospace GD&T will be required. Once you stray from those industries the requirements vary dramtically based on the application\industry the work is being done for.

I have worked in all spectrums of these industries from Military\Aerospace in large companies to "ma and pa" job shops. Here is what I have found to be the pros and cons of GD&T amongst them.

In Military and aerospace it is mostly a liability issue. The drawing must contain all design intent, and leave no gray area, and still be producible. I have sat in conference rooms with high end Engineers of major aerospace companies that would review drawings one dimension at a time. I had a tolerance on one part that was +/- .0005. The question was asked where did that come from? Would +/-.0007 be acceptable. Every dimension, and every tolerance had to be justified.

For the smaller shops it comes down to ecenomics. There is no way they want the time to a do a drawing outweigh the time to make the part. The ecenomics are also prohibitive in the fact they do not have the tools to measure many of the GD&T requirements. That is were the "make it as perfect as possible" comes in. They simply cannot measure with the tools they have. Many have the machinist checking their own parts, or do a fit test with something supplied by the customer. Management of these companies also do not want or cannot afford to invest in the training. They have high turn around, and many of the machinist/fabricators/welders know how to run a machine, but not read GD&T.

I am sure there are exceptions to the above but in general I believe the use comes down to liability verses ecenomics.

Just my 2 cents.

Marshall
Marshell Design

http://sites.google.com/site/marshelldesign/

 

[drawoh]

marshell,

If you are required to produce drawings of Level_III quality, you need to work to a standard. This can be ASME Y14.5. This can be the ISO standard. This be the Billy Bob and Cousin Elmo standard, if it is documented and available. GD&T is only relevant because ASME Y14.5 recommends it. ASME Y14.5M allows you to interpret non-GD&T drawings.

The whole point of Level_III drawings is that the military or government or other customer can hand those drawings over to their favorite manufacturing facility and expect to get working product.

If you are a manufacturer producing proprietary product, you can sub-contract to a small group of fabricators who understand your stuff, or you can manufacture in-house. As I noted above, you can produce horrible drawings, and your fabricator can cope with it. As noted in Joe Osborn's article Tips on Designing Cost Effective Machined Parts, your fabricator is going to try to deliver good looking, working parts, no matter how bad the drawings are. This allows your crappy drawings to produce working parts, but note Joe Osborn's remarks about bad drawings.

The other advantage of ASME Y14.5 is that you can find designers, drafters, checkers and fabricators who have been trained in it. If it takes a couple of years to master your in-house standard, you cannot quickly replace or augment employees and fabricators. Note how none of our Eng-Tips GD&T experts claim to have mastered GD&T quickly. Don't re-invent the wheel unless you have to.

JHG

 

[marshell]

drawoh,

I agree with you completely that all drawings SHOULD follow some standard, be it military, commercial, or internal. I was refering mostly to the use of GD&T symbology, and the implentation of its use into any of those standards, and why many companies do not use them.

My post was mostly an observation, and not an opinion. If you were to ask my opinion, I would push companies that are small, and do not use it, to start the training, and start implementing it. My reasoning would be that the goal of most small companies is to become large, and it is much easier to make the jump now, then to wait until you are required by customers to use it, and not be prepared as you grow your customer base.



Marshall
Marshell Design

http://sites.google.com/site/marshelldesign/

 

[Dtuck]

My experience in doing design reviews on products we are asked to make, military included, is that it is easy to find contradictions on hole sizes and positional tolerances. Standard or no standard, the designer needs to have a practical sense of the related components and, more importantly, the processes involved in making those parts. Tolerancing parts to numbers that aren't possible leads to most of the heartache sourcing parts and trying to make products work. 3-D modeling examples are usually more full of problems than straight drawn parts. Standard or no standard, a good designer makes for a more successful product.

 

[ewh]

Standard or no standard, a good designer makes for a more successful product.

Not to be a kill-joy, but this is kind of like saying "The sky is blue" and while true, doesn't really add anything to the subject. I can just as easily say "Standard or no standard, a good checker makes for a more successful product."
A good designer will know how to define the part so that it can be understood by the fabricator and using a common standard makes this much easier to accomplish. Similar to the way we are using english to communicate on these fora as opposed to multiple languages.

"Good to know you got shoes to wear when you find the floor." - [small]Robert Hunter[/small]