Preserving design intents 8

[DustyRhodes]

I’m looking for opinion or experienced guidance on methods used to preserve design intent when tooling is built and results in parts that don’t meet intent but are “good enough” to make the decision to spend $$$ and time should wait until tool rebuild at end of tool life (possibly). If prints are changed to make parts QA acceptable then design intent is compromised. A deviation can be written but it is not advised to write a “permanent” deviation. What is conventional wisdom to save original design criteria and still allow production to manufacture product for some extended period of time? I appreciate any and all input. TIA.

Al

 

[hydtools]

We had in place a procedure which required the product engineers to accept or reject parts deviations. One acceptence choice was 'use as is this time only'. If after several lots of accepting the same deviation, the drawing is changed to match the part.

You fall into the position of defending against 'if it is good enough to continue to use, then it is good.'. You are in a poor defensive position if production, maketing and sales, and product engineering continually accept the deviation.

If you can agree with production that when the tooling is replaced, the new tooling creates the part as originally designed, then you can not accept the deviation in the new tooling.

Document all decisions. Especially if you are ISO9000 certified or want to be.

And you go on to more important things.

Ted

 

[MintJulep]

...parts that don't meet intent but are "good enough" ...

I don't quite get that.

If you have parts that work, then hasn't the design intent been met?

 

[GregLocock]

Yes, that was my first reaction, but as an example, say you have park brake handles that are functionally strong enough and look ok, but there is a part line underneath that is just slightly annoying, and larger than the drawing says. You either accept that deviation for the life of the tool, or you don't ship product.

Suppliers play that sort of blackmail all the time, that's why I don't deal with them any more.

Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[PRStock]

We create a double box showing the dimension that is acceptable for "current tooling only".

 

[TheTick]

You can't always get what you want. But, if you try, sometimes you find, you get what you need.

 

[btrueblood]

I hear music...

 

[drawoh]

DustyRhodes,

If your drawings are prepared perfectly, any non-conforming part will not work.

The idea here is that the part does not conform, but it is good enough. This reflects bad design and drafting, a bad attitude by production, and/or poor manufacturing quality. Someone's crappy work and bad methods have been validated, and you will be seeing more of it.

I do not think there is a standard process here. You need to look at all your practises before production and vendors get to assume that all of your tolerances are negociable. Perhaps you are producing bad drawings. Perhaps some of your tolerances are unfabricatable. Perhaps production is unwilling to upgrade or repair bad equipment, or perhaps someone in production is an idiot.

As a designer, I want all my tolerances met. If there is a problem, I want production to come back and talk to me so that we can sort things out. Maybe I am too accurate for the process, and I have to redesign to allow for looser manufacturing. Perhaps they have to try harder, or use a more expensive process.

There was a article out a couple of years ago stating that the Japanese fabricate cars to sloppy tolerances, and they provide spacers and stuff to production. The article described this as an example of Japanese cleverness. My interpretation was that the designers understood the capabilities of their fabricators, and designed their cars to be assemblable under those conditions.

JHG

 

[TheTick]

If your drawings are prepared perfectly,

Good luck with that.

 

[drawoh]

TheTick,

I used the word "perfectly" deliberately. Obviously, nobody every gets everything right in a complex system. Always there are problems.

This does not mean that there always have to be a lot of problems. It also does not mean that since we cannot achieve perfection, there is no point in trying to improve anything.

JHG

 

[DesignerGuy16]

The best way to conserve design intent is consistency in my opinion.

If you're doing everything in the same way, dimensioned in the same fashion, etc. Your success rates will increase just because it will be simpler to find your errors. This is why we have things like ISO.

Now if the end product is functionaly fit and sound, even though it varies from the print.. revise the print unless it's some kind of super critical assembly part or a huge eyesore in a luxury consumer product.

All costs after the design phase start to become exponential so take that into consideration. If it's acceptable as is don't just change it to change it. If it isn't functional, change it. If the end customers won't accept it, change it. Otherwise it's probably fine.

My first job was at a stamping plant that made these friction clips for side view mirrors. The tool design of the dies ended up being too big, so Ford's manufacturers went to the side view mirror vendor and had their die revised to match the manufacturing capability of the die we were running. Ford never changed the part print. Ford manager was fired, another brought on and demanded everything made to print. Now our parts don't work. I explained to him the history, and they agreed to reverse the changes and allow us to keep running our parts as is. Reason being the tooling for this part was upwards of $250,000 in design and manufacturing. Note most of this occured before I ever started working there, but I had to deal with the result of those decisions.

Something that could've been avoided completely if someone had sent the print to Ford to be revised to actual manufacturing capabilities.

James Spisich
Design Engineer, CSWP

 

[KENAT]

One place back in defense/aerospace where we'd fall foul of this was in stress critical areas.

Maybe a part would have some discrepancy that made it slightly weaker. We/stress guy would do some calcs and find the part still had a positive safety factor.

We'd often pass the specific part/parts but not change the drawings.

This was because the change ate into our design margins, the concern was that subsequent additional changes or deviations from drawing might compound to the point of causing a problem. Especially as many of the overall stress reports for items didnt' get routinely updated (customer wouldn't pay for it) this was a real issue. However the cost/schedule impact of not clearing the parts were prohibitive.

I was never totally comfortable with this but it was the way it was.

Generally I'd say if the part is fit for purpose but doesn't meet the print, then the print should (eventually) be changed.

However, there will be 'gray' areas for one reason or another. This happens a lot during development or initial production where the parts aren't quite right but work well enough (or can be finagled on assembly or similar) to prove the concept etc. but wouldn't be satisfactory for full production etc. Also on slight tolerance deviations, it may be decided to use the parts as is if they fit on assembly, i.e. hope the mating parts aren’t using up all of their ‘tolerance budget’. However you may not want to change the print as you don’t want to deliberately make parts that are likely not to fit.

At that UK place our ‘deviation’ process was almost entirely separate from our ‘request drawing change’ process. A deviation (or as we called it a production permit or concession; depending if it was preemptive or after the fact) could be just for a certain part or batch of parts, for the duration of an order/contract or if I recall even till a certain calendar date, though we rarely approved these.

Not sure this really answers the OP though, sorry.


KENAT, probably the least qualified checker you'll ever meet...

 

[hendersdc]

We are currently dealing with this issue on some injection molded parts of an assembly. Some parts are out of tolerance but because we have inspected the mating parts we know they are not using all of their tolerance and the out of spec parts will therefore work in the assembly. We are not going to change the drawings because we want the parts the way they are currently dimensioned. That was what the tolerance analysis was based on.

We get around writing deviations for every production lot by identifying which dimensions are to be inspected durring incoming inspection, and which are to be inspected durring a FAI on a new tool. This way a deviation is done only for the initial complete inspection.

This method works well enough but care has to be taken when some parts roll to a new tool and others do not. The goal of doing it this way is to not have to rev the drawing every time a new tool is made. Quantities of millions a year mean a lot of new tools.

 

[MotorVib]

form, fit, function

When I have been asked the question, can the parts be used I have used form, fit, function. The problem that I have always seen you can only have two On time, Cost, Accuracies. By the way I work low production world with runs from 3,000 per year to 1 per year.

Chris

"In this house, we obey the laws of thermodynamics." Homer Simpson

 

[DustyRhodes]

One thing I should have pointed out originally, this part is a 10 position widget and with only 10 position the cumulative tolerence will not cause assembly failure. However, this is slated to be a family product where there will be as many as 96 positions. At some point the tolerance stack will begin to cause problems with the assembly, eventually to failure. The tool cavities for the 10 position were made for current customer needs and the remaining cavities will be made over time as we approach the product release. We want everything to be the same and it is documented on tabulated drawings. That's why I'm not able to change the print. Again Thanks for all your inputs, I am using our collective opinions to support my argument to not change the print but to find a way to deviate for the current tools set but get it right for future sets.



Al

 

[shaunlane]

Hi there.
i agree with those above who state that if the part is functional and has been tested then the part drawing could
be changed for good.
Tolerance build ups should be investigated during product
design first of all and the part drawing dimensioned accordingly.
If this was done properly,I think your question would not be
to allow a deviation but rather, was the tool designed properly to allow for such tolerances.
Assuming that the deviation will not occur again due
to tool set up , maintennance etc.
To sum up my opinion----
Good design practices and engineering practices should
allow for engineering change requests only if all members
are experienced enough to come to a conclusive decision, that will not result in non conforming products.
Hope this all helps with your decision.
Slane