Notes on Prints

[rbarnes@ontario-plastics.]

Has anyone had the following note appear on part prints?
We have recently seen the following note on a part print for a plastic part, "All dimensions not specified on this drawing are to be taken directly from the model".
The customer wants to use this note as a backup and ammunition if an important dimension is not specified on the print.

[PinkertonD]

Hi welcome and let me see if I can answer this in strong enough terms.

Aaaarrrggggggggggghhhhhhhhhh Run do NOT walk away from that.

The customer is putting the onus on you if anything doesn't match up after machining. Let's say it is a $50,000 mold and the customer "thinks" it doesn't match the model, who is at fault.

You need to give this job to one of your competitors and let them get "rich" off it.

On tiny option that I would not tak,e but will suggest it, is if they gave you an .STL (or whatever) file that can convert directly to GCode to cut the mold (or whatever) then it may remove some risk. But, as I said, I would not do that.

[RWOLFEJR]

I agree this is a concern but I wouldn't get in too big a hurry to walk away from work these days. We get prints for quote in here all the time with errors or unreasonable expectations for the application. What we do is draw the part up as we expect they need it to be and then show them the print with a list of exceptions to their print and an explanation for each exception. We also let them know that these are just recommendations and not etched in stone. Then should things proceed from there we insist that the customer sign off on OUR print and we file it.

Chances are that the dimensions that aren't specified on their print aren't critical. If they were you'd think they'd call them out wouldn't you? The rule is if it doesn't affect form, fit, or function... then it isn't something to worry about. Maybe it isn't that they're wanting to hold a trump card on you but more of a case where the other stuff doesn't matter to them? Or maybe they can't measure that feature with their equipment?

So what I'd suggest is taking their model that they apparently gave you and measure it up. Verify their dimensions while doing your check and add in whatever is missing and slap a tolerance that you're comfortable with on it. Then show them the print and see if they're good with it. Then either have them change their print to match or sign off on yours. Easy Peazy...

Good Luck,
Bob

[jboggs]

Referring back to the 3D model from which the drawing is made might be tempting for the designer but I agree with Dave. They are trying to evade their responsibility. For example, you can refer back to a model but that doesn't tell you dimensional or geometric tolerances.

In my opinion, as one who spends his life creating 3D models and drawings to be used by others, that statement should read like this:
"All dimensions not specified on this drawing are the fault of the designer."

[Belanger]

Hold on ... I don't see this as such an egregious thing. I've worked with LOTS of parts (such as automotive body panels) where it would be a nightmare for the designer to specify EVERY darned dimension! Therefore, it is actually common at many companies to provide many of the dimensions on the print (especially certain key dimensions) but use that general note to provide the full information that y'all could never fit on the print.

The tolerances can still be shown on the print (I'm thinking geometric tolerances, although there are now ways where even these can be embedded in the model).

That said, I certainly agree that the position I'm defending could make the design folks mighty lazy, and therein lies the danger. But in general, the practice that the OP asks about is not all that crazy.

[jboggs]

So what you're saying is it depends on what industry you are working in. What you say makes sense, I guess, but I still see it as a potential disaster. If there are certain fields where this is a successful common practice, then so be it. I'll bet there are also some commonly accepted limitations to that practice.

[PinkertonD]

Belanger, that was then and this is now. In the mid '60s I worked at GM and as you say things were left open top interpretation for the reasons you state. But -- now days 3d drawings are the norm and not the exception and a 3D drawing should be supplied with the model for taking dimensions from. If for no other reason other than for the QC guys to make sure stuff is the size and shape it is supposed to be.

Given that, it is still not up to the machine shop to decide what a note means and how important strict a requirement to just one or two critical unmentioned dimensions might be. We are Machinists and Engineers not artists building clay models.

The ultimate responsibility would fall with the machine shop if the customer queries something that was "taken directly from the model," a little inaccurately.

[Belanger]

Sorry -- I'm not following. You say that "a 3D drawing should be supplied." Is that a paper-based drawing? If so, then how can every dim be shown? If not, and you're referring to the digital package, then that's right back where the OP was: a part print with a legitimate referral back to the model.

[PinkertonD]

Of course I meant digital 3D and that's why I mentioned STL files. A paper 3D drawing would be useless other than a pretty picture.

The "model" referred to by the OP, I assumed to be an actual model made from Papier-mâché, foam, pig-iron or carved from green Mangos. The customer is expecting the OP to use measuring instruments to get the dimensions they have failed to supply and that is a risk not worth taking. Well, to this old fart with 40+ years in business, anyway.

The OP, (Ontario Plastics) sounds to me like they would be stepping into land-mined territory after they have machined the molds and the customer points out an important dimension from the model and it just ain't right.

[Belanger]

It never occurred to me that the OP was speaking of a literal, physical model! I guess I assumed all along that the word "model" written in his note was the "digital 3D model" that is CAD-based (at least since the time when most placed tossed out their drafting boards).

If he means physical model then of course you guys are right. But I have a sneaking suspicion that the word was used in the other way; thus my comments.

Also, good call on the OP - OP pun.

[PinkertonD]

It was an assumption and we all know about the wisdom of those...

...but he said "print" which I assumed to be paper and a carved doodad as the model.

However, he/she does not seem to have returned so maybe it is all moot.

Puns? I try. Or is that -- I am trying?

[Kelly_Bramble]

Has anyone had the following note appear on part prints?
We have recently seen the following note on a part print for a plastic part, "All dimensions not specified on this drawing are to be taken directly from the model".
The customer wants to use this note as a backup and ammunition if an important dimension is not specified on the print.

Ok, here's my take on this subject... Programming and manufacturing to the 3D CAD database or model is common practice in many ********** – especially injection molding plastics and castings. Generally, engineering or design provides a step, ides or similar file to the manufacturing programmers.

Definitions - A dimension is a declaration of length and a tolerance is the capability or variability assigned to the dimensioned feature.
If there is not a tolerance assigned to a feature – then there is no tolerance. The customer cannot hold the manufacturer to the fire for a perceived miss-manufacture. The feature will be manufactured within the manufacturing process capability.
If the manufacturing folks are concerned about what tolerances the customer wants then get in writing what their expectations are in regards to variability of the dimensions.