Fully defining a part vs only dimensioning what needs inspection? 3

[GDnTNewb]

Hello, so the last year or so I’ve started designing parts for medium and high volume manufacturing, where the parts go through FAI and have PPAP, whereas before it’s only been low volume stuff.

I’m not entirely sure of what to put on the drawing in these situations. I’m hearing very different things from different people in the company I work for. I want to fully define the drawings with GD&T, qualify all my datums etc, and even if some of my features can have really large variations before they impair function, I want to specify that large tolerance. This is what would make me feel as I’ve done a thorough, good job as Designer. Thinking of each tolerance individually and allocating the largest functional tolerance possible.

But some people say to only put what you want to be measured on the drawing and I get feedback that I should remove tolerances that are wide because it’s not gonna be needed to measure it. Also getting the feedback to not use GD&T on some parts because it implies the part needs to CMM’d and that’s apparently too expensive, or makes people not want to quote. This is confusing me a bit.

Feels like being stuck between a rock and hard place a bit. I want to fully specify my drawing, but I realise if a part has a loose tolerance and uses a very capable manufacturing technology where the I’m most likely gonna have a huge Cpk on that dimension, it does feel like a waste of money to measure that dimension.

I’ve heard of various ways of how to make drawings that are in PPAP. I have heard:
* Only the critical dimensions should be on the drawing, the ones that will be measured continuously or periodically during production, and then you redline on a bunch of other stuff on the drawing that you want to measure during First Article Inspection.
* Drawing has everything that should be measured during FAI, and then you indicate with some symbol which features are critical and to be measured continuously.


But do you then leave out all other things? Even if some dimensions are more critical than others, they all have some limits, do you let those being caught by general tolerances?

Could someone with some more experience on this shine some light how to think regarding these things? Would appreciate it.

 

[Burunduk]

It's a typical problem. The feedback you got saying that you should do partial definition results from seeing the drawing as purely an inspection instructions document rather than a product definition.

For what it's worth, in the ASME Y14.5 standard there are "Fundamental Rules" addressing this issue:

(a) Each feature shall be toleranced. Tolerances may be applied directly to size dimensions. Tolerances shall be applied using feature control frames when feature definition is basic. Tolerances may also be indicated by a note or located in a supplementary block of the drawing format.
See ASME Y14.1 and ASME Y14.1M. Those dimensions specifically identified as reference, maximum, minimum, or stock (commercial stock size) do not require the application of a tolerance.

(b) Dimensioning and tolerancing shall be complete so there is full understanding of the characteristics of each
feature

 

[GDnTNewb]

Would it be a good compromise then to fully define the drawing but when you balloon it up for inspection you don’t put balloons on everything? And then you put that in the Design record for the PPAP?

 

[Burunduk]

I would instead suggest the approach of a "reduced dimensions drawing" AKA "minimally dimensioned drawing". If you release the CAD model along with the drawing for production and inspection purposes, you can refer to the model number in your drawing and write notes that complete the definition for all the features that can be loosely controlled. Such notes typically say that the CAD model geometry is to be considered basic (or "theoretically exact" per ISO terms) and that unless otherwise specified, all part surfaces must conform to a profile of a surface tolerance with a loose value. Preferably, the profile tolerance would reference the functionally appropriate datum features which constrain all 6 degrees of freedom. If the general profile tolerance is to be inspected at all, it can only be measured in a few selected locations that will be agreed upon between design and QA, and documented in an inspection plan.

 

[3DDave]

Is this a case where your company farms out production, so you have no idea what the process capability is?

If so, just do what your co-workers suggest while you look for a new job.

The company culture may be that it is the supplier's job to make parts that work and not your job to make sure the parts are acceptable.

It is normally the task of Quality Assurance to make this negotiation with suppliers. Have you talked to them?

 

[3DDave]

To clarify:

[ul]
[li]" I’m hearing very different things from different people in the company I work for."[/li]
[li]"some people say to only put what you want to be measured on the drawing and I get feedback that I should remove tolerances that are wide because it’s not gonna be needed to measure it. "[/li]
[li]" feedback to not use GD&T on some parts because it implies the part needs to CMM’d and that’s apparently too expensive, or makes people not want to quote."[/li]
[/ul]

You don't have a dimensioning or tolerancing problem.

You have an internal company political problem. One that cannot be solved using references to standards.

If they are following those standards and used what was in them, and you have this situation then those solutions aren't working. More likely they already know about the standards and don't want to follow them because, as mentioned, they know the suppliers don't like to follow the standards.

I expect the by-the-book solution is to have a conversation with your supervisor who will certainly pull the head of engineering and the head of QA and the head of contracts into the conversation, but they already don't want to deal with this or they would have provided a guide book for you to follow and probably some training to go with it to see that you understand the company dimensioning and tolerancing policy.

This mainly leaves the possibility that you will be the scapegoat if any supplier f's up and can say that there is anything wrong with anything you did, even if there isn't. They can replace you more easily than they can replace a supplier and it will make upper management happy that something was done.

Hence the suggestion to find somewhere else to work before that happens.

I see you are logging in from or through Sweden, so maybe they have better job protections than in the USA.

But, who knows? I've worked with a large number of suppliers, worked for a number of companies, and what you run into might see them giving you a big fat raise for pointing out they've been doing it wrong.

 

[GDnTNewb]

Thanks. It’s a young company and I will have the ability to influence the way we do things, hence I want to know whats best, not leave the company. I’m not afraid of being a scapegoat.

 

[Burunduk]

Then consider the "RDD" (or "MDD") option, as I described above.

 

[pmarc]

My recommendation would be to fully define the product on the drawing and then find a way to indicate that certain dimensions should be inspected on more frequent basis than the others.

One way to do that could be to label the characteristics on the drawing as minor, major or critical and define, thru a company document, what those exactly mean from the quality point of view.

To me, RDD or MDD is still an example of complete definition.

 

[Burunduk]

To me, RDD or MDD is still an example of complete definition.

That's the same for me, otherwise I wouldn't recommend it.

 

[3DDave]

Optimism is warranted when you have a larger number of people telling you to fully define the parts than the number telling you not to. I have what you wrote to go by and told you the correct way to proceed. You didn't say you would try that, but appear to have continued on the original appeasement path.

Not sure what a "young" company is when it has enough people set in their ways telling you, not asking or discussing or making policies about the way they want to do business. Per your description, they are more worried about getting bids than doing the job correctly.

The great news is that there are likely a lot of competitors that aren't any better. Some companies can run for a long time like that, but you appear unhappy with the situation. Life is too short to be unhappy.

---

To emphasize what pmarc said - you might first need to develop a company policy document for quality control as to classification of characteristics, not to just leave dimensions and controls off the documentation. Minor, major, and critical are the typical lowest detail which most find sufficient.

 

[drawoh]

My recommendation would be to fully define the product on the drawing and then find a way to indicate that certain dimensions should be inspected on more frequent basis than the others.

I prefer complete documentation too.

My recent experience was with a customer drawing of a casting. It was obvious to us that the attaching holes and the gasket interfaces were dimensionally critical. The machined prototype worked fine. The casting interfered with the top of a screw clamping a printed circuit board. The drawing was of very poor quality, and the cast surface was not inspectable.

My point in thread1103-322065 was that some critical features are easily fabricated to requirements. Others are difficult, and require quite a bit of inspection, even if they are less important.

--
JHG

 

[Eric Gushiken]

I encounter this a lot as well. What those people who don't like too much tolerancing and GD&T on the drawing don't realize is that all the features generally have tolerances either way, in the title block, and they all need to be met. If the parts are being made in house then I think it is more Manufacturing's responsibility to determine what dimensions and tolerances they can ignore based on the typical capability of their machines and workers.

 

[drawoh]

GDnTNewb,

I have one more thought here.

How much time and effort goes into the FAI? They inspect one piece thoroughly. Hopefully, you approve the process. If you are all-knowing and infallible, you will know which features will be difficult to fabricate. On anything complicated, there will be surprises.

From the FAI, you identifiy the marginally compliant features, and your inspectors watch them.

In production, sh*t will happen, and you will react to it. If you have good, complete drawings, you will able to point to the non-compliant features when you complain to the fabricator.

--
JHG