Forcing Dimensions - How you do it?
Forcing Dimensions - How you do it?
(OP)
So recently a plastic part we designed was measured and statistics etc about the manufactured parts were gathered.
They suggested we should change our drawings and in-turn models to reflect they're manufacturing capabilities. We have been discussing weather our models should be updated to reflect the manufacturers inability to generate "propper" parts, or if we should just alter the drawings with specific notes indicating why the demensions have been "forced" to reflect the manufacture capabilities (ie they're tooling was probably created impropperly).
Anyhow, a mistake has been made, thats not what my question relates to. What i am asking is do we reflect the mistake in the model? or just the drawing? If just the drawing, how would you indicate the dimension is "forced" and varies from the design intent?
I was thinking coloring the dimension Blue rather then black with a note. This way printed drawings won't "see" it but when looked at electronically (internally) we see what is ... different.
They suggested we should change our drawings and in-turn models to reflect they're manufacturing capabilities. We have been discussing weather our models should be updated to reflect the manufacturers inability to generate "propper" parts, or if we should just alter the drawings with specific notes indicating why the demensions have been "forced" to reflect the manufacture capabilities (ie they're tooling was probably created impropperly).
Anyhow, a mistake has been made, thats not what my question relates to. What i am asking is do we reflect the mistake in the model? or just the drawing? If just the drawing, how would you indicate the dimension is "forced" and varies from the design intent?
I was thinking coloring the dimension Blue rather then black with a note. This way printed drawings won't "see" it but when looked at electronically (internally) we see what is ... different.





RE: Forcing Dimensions - How you do it?
RE: Forcing Dimensions - How you do it?
If only a few dimensions are off, perhaps add a note to the dimensions to show "as designed" state versus "as manufactured".
Ideally, you should change the model to match what you are manufacturing.
More ideally, you could flog the molder until the parts are completely right. This rarely works.
RE: Forcing Dimensions - How you do it?
Hense the indecision about how to proceed. Having a Design Intent model and a "real world" model seems kind of too much.
RE: Forcing Dimensions - How you do it?
If you do change suppliers down the road, do you want the new supplier to make the same part that the current supplier makes, or do you want what you really want the first supplier to make?
At any rate, NEVER disassociate the dimensions from the model.
RE: Forcing Dimensions - How you do it?
I've seen this done on castings a number of times due to tool wear. This way you don't have to change the model. Bad thing is you may no longer have bilateral tolerances around a nominal which is usually preferred.
The whole point of tolerances is to accommodate acceptable manufacturing variation, I would have thought this was an example of that, it’s a variation from the nominal that has been deemed acceptable. While you're at it look at any other tolerances which are unnecessarily tight and adjust those. Ideally tolerancing should reflect function of the part, not some vague idea that it needs to be tight in one area etc. Unfortunately I certainly don't spend as much time on this as I perhaps should or could so many of my tolerances end up a best guess (not on interferences, positional etc but general tols)
I would not force the drawing. This can lead to all sorts of confusion in the future and even unnoticed problems from a geometry point of view. I almost certainly wouldn’t add a note on the face of the drawing no matter what you do, though a note somewhere in the file properties can be useful. If explanation is needed it should be on the ECO or equivalent.
If you decide to actually change the dimensions rather than simply modify the tolerance I'd change the model.
I support the Tick in his appeal to flog the molder. That is the ideal situation.
RE: Forcing Dimensions - How you do it?
Or you can sign-off on a temp waiver and hope the vendor makes them right the second time, which leads to the questions how these passed first artical inspection (if it exists at your company) in the fist place.
"Art without engineering is dreaming; Engineering without art is calculating."
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Forcing Dimensions - How you do it?
RE: Forcing Dimensions - How you do it?
If the part was made per print, but is wrong ... eat the cost, fix the print, send part back to fix or remake, then go on.
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)
RE: Forcing Dimensions - How you do it?
Altering dimensions on a drawing so that they don't reflect the model is a sure fire way to get your a$$ bit later on.
David
RE: Forcing Dimensions - How you do it?
So what to do?
You can accept the out of spec parts only after assembling and testing to ensure that they function within customer requirements.
If everything fits and functions with all the different out of spec parts, then document the difference. The drawing is not the place for this unless you want to change you verified and validated design intent. Work with purchasing or supplier quality on how to document the difference. In automotive world we do this to approve with a Deviation for the out of spec parts for PPAP and after.
The bottom line is that you found one design solution and manufacturing found another.
How do you prove which is the better solution given the budget and schedule limits?
Did you perform all the design analysis to verify your design intent?
Did you evaluate alternate designs looking for design improvements?
Are you confident that your design is right given the millions of dollars at stake?
The tools are made, and production starts very soon.
You have the choice to change tools or test the manufactured product.
Tom Rhodes, GDTP-S
QMC LLC; www.qmc.com
Senior Dimensional Management Engineer.
CeTOL 6 Sigma
RE: Forcing Dimensions - How you do it?
This is not a case of "going to production" its being sold. Its past "getting to production."
Talking with the head eng. He wants a new Symbol representing dimensions as manufactured rather then design intent.
i'm not sure... whats right and wrong at the moment.. up or down for that matter! HAHA.
We also practice another "no no." Critical dimensions and tolerances. so.. i'm wondering where and what is right anyhow.
RE: Forcing Dimensions - How you do it?
What is the purpose of your drawings, to tell the vendor how to make the part or to document how the vendor created the part?
And
Is there a reason that the parts can't be made to print? If you change the spec on the print, should you expect them to meet the new spec next time?
And
Does your tolerance effect your designed dimensions or your "as manufactured" dimensions.
Personally, if you design a part a certain way, you should expect the part to come in that way. If you are okay with the parts you are recieving change the drawing to match them but don't encourage ambiguity by putting two specs on the drawing.
David
RE: Forcing Dimensions - How you do it?
"Art without engineering is dreaming; Engineering without art is calculating."
Have you read FAQ731-376 to make the best use of Eng-Tips Forums?
RE: Forcing Dimensions - How you do it?
Also, "...new Symbol representing dimensions as manufactured rather then design intent". If I were the vendor/machinist I would question what do you want. You will be asking for more problems on future parts and possibly returns from your customers.
Chris
SolidWorks 06 5.1/PDMWorks 06
AutoCAD 06
ctopher's home (updated 10-27-06)
RE: Forcing Dimensions - How you do it?
I still believe that if they are acceptable and will always be acceptable change the tolerances on the drawing to cover it.
If they're acceptable but you'd really rather have them correct to the current drawing and so don't want to 'relax' the drawing requirements then look into some kind of deviation from drawing paperwork. (We used to call in a production permit if it was preemptive or a concession if it was to clean up a mess like this in the UK.) This usually covers specific units or batches but could maybe cover this vendor for this part for perpetuity.
At the end of the day who signs your pay check? They generally have final say (except perhaps for safety critical items) the most you can do (short of walking off the job) is make your concerns/objections clear and document them. Put them in writing and keep a copy of the email or memo. Also if you do an ECO perhaps put on there “As instructed by Head Engineer….” So it’s not your butt on the line in any future witch hunt.
RE: Forcing Dimensions - How you do it?
But it is nice, to know what i thought was wrong is agreed to be wrong by most.
RE: Forcing Dimensions - How you do it?
the dimensions that match the production parts
if that will make your boss happy. You certainly
want the drawing dimensions to reflect what has
been made and what will function for replacement
parts. Do you track parts with discrepancy reports?