Machine shop responsibility to comply with "Industry Standard". 7

[powerhound]

I'd like to get everyones take on an issue. If a machine shop gets a print with no GD&T on it as well as no reference to any dimensioning standard, how much responsibility does a machine shop have to catch any unspecified tolerances. For example: a 3 foot long rail with various slots and holes along the length has about .020" of bow in it. Is this something a machinist should have made a special effort to avoid in the absence of any other information or should this part be considered "to print" and called good?

The kickback I'm getting from the customer is that "Industry Standard" dictates that that straightness tolerance should be the same as the tightest tolerance in the default tolerance block. I've heard this before but I thought in modern times, industry standard had fallen by the wayside since it's not documented or enforceable.

Looking forward to the feedback...

Powerhound, GDTP T-0419
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Inventor 2010
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[MikeHalloran]

If it's not on the print, explicitly or by reference, and it's not in any related contract or explicitly accepted workmanship standard, it doesn't exist.

That said, I have worked in a lot of shops where certain expectations of their suppliers had been evolved over many years of working together, and were never recorded, but were understood. I learned of the existence of these unrecorded agreements when some idiot MBA arrived and unilaterally changed the relationship without full knowledge or understanding of the change's impact, and I got nominated to clean up the resulting mess.

This sort of thing almost always happens when said MBA changes suppliers, i.e., sends an old (and technically incomplete) print out for bids, then goes with the lowest bidder. ... who of course does not include money or time to cover unstated requirements, and the first pieces come in 'to print, but wrong'.


Personally, I have never heard of any industry standard relating straightness or camber to any detail tolerance on the print. Industry standards for camber with which I am acquainted typically refer to the part length, stock length, and/or some linear dimension of the section, but not to a tolerance on either.


Mike Halloran
Pembroke Pines, FL, USA

 

[TheTick]

"Industry standard" usually is "industry folklore". Standards are written, codified, and published. Anything else is popular mythology.

 

[MintJulep]

"Industry Standard", "Standard Tolerances" and other such terms are used by vendors to try and get you to accept or pay for whatever piece of crap they have made.

 

[fcsuper]

TheTick and MintJulep are right. There's no such thing as "Industry Standard". It's usually just codespeak for "this is the way we like to do it, so just do it this way and don't question its validity." Most industries do have common practices, but these vary widely between industry types and even regions, or in how badly the vendor wants to get around taking responsibility for something they didn't do right.

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

 

[powerhound]

Thanks for the feedback guys. It seems everyone is on the same page as I am. I just wanted to make sure my opinion wasn't the minority opinion. I still welcome any more feedback so if you're just now reading this please chime in. I realize I posted this over the weekend so I may get a lot more feedback once the workday starts.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
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SSG, U.S. Army
Taji, Iraq OIF II

 

[SDETERS]

Have the customer show you the standards document where this statment comes from. "Industry Standard" dictates that that straightness tolerance should be the same as the tightest tolerance in the default tolerance block" Is specified.

 

[fsincox]

I see the same issue with castings and weldment drawings specified with (2) and (3) place dimensions on drawing title blocks that show +/-.010 & +/-.030 for these. The answer is always "industry standard".
Frank

 

[TheTick]

On a couple occasions, I have felt compelled to let vendors off the hook for not complying with ambiguous drawings (meaning: "We'll pay for the bad parts"). Perhaps I am in the minority, but I believe it's my job to ask for what I need. If I didn't do that right, I deserve what I get.

 

[MikeHalloran]

I like to go easy on vendors when I can, because sometimes I _really_ screw up, and then need their help correcting my mistake.

However, when I draw a part straight, I don't expect it to arrive noticeably cambered.

When it does, I'm thankful for the nitpickers on my team who provided a documented explicit chain of requirements back to, e.g. a specific industry standard for camber of straight bar stock, if not a tolerance or a note on the drawing.

Of course, after the MBAs arrive, the nitpickers are removed as unnecessary, and the survivors are pushed to take up the resulting slack, so stuff gets left off the drawings, and the shop guys start talking about the way things used to be.





Mike Halloran
Pembroke Pines, FL, USA

 

[btrueblood]

fcsuper,

If this is a customer/account you want to keep, can you suggest a method to straighten the rails to comply with their requirementsdesires, and (having fixed their problem) then politely suggest putting a straightness requirement in a note, or (gosh what a great idea) by GD&T.

 

[ctopher]

These days "Industry Standard" means "I don't know, or use, GD&T" or "I don't know how to read drawings".

Chris
SolidWorks 10 SP5.0
ctopher's home
SolidWorks Legion

 

[powerhound]

If I get a problem about being paid for the parts, I will probably just let it go but I won't be re-making them. This is how all the drawing are from this customer and I've brought up the lack of real specification before but as is the case with draftsmen with no GD&T knowledge, the story is always "I've never had a problem before". If a print is sent out and a vendor has to make a few phone calls to ask questions, then the parts get made twice but ultimately the guy gets what he wants, then there isn't a problem with the print. Go figure that logic.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[fsincox]

Don't just blame the little guy, from my experience the engineers, checkers and managers prefer it that way too, because they do not understand GD&T and wish it would just go away, back to the "good old days".
Frank

 

[powerhound]

This is actualy the engineer but I just didn't want to make it sound like it was an engineers problem. Whoever makes the drawing--whether it's the engineer or the draftsman detailing the engineers model--should be held accountable for a drawing with ambiguous requirements.

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
Mastercam X5
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[ewh]

...checkers?

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

 

[SteveMartin]

My take on this pretty much matches what's been said already. If it's not explicitly called out it's not required. Having said that, I would hope my suppliers would question drawings I send that aren't clear rather than assuming they know what I meant.

Technically it's the engineer/designer/drafter's responsibility to provide an unambiguous drawing, but the best results occur when the manufacturer is an active part of the process, not just passively taking what drawings come their way and interpreting (or ignoring - a problem I've also seen) them in a vaccuum.

 

[KENAT]

The situation you indicated is pretty clearly a matter of poor documentation as others mention.

.020, unless the rail is very short, isn't a lot so to even expect the machinist to have asked for the missing info is pushing it.

Not sure exactly what industry but I've never heard that one about the smallest tol in the block tol.

If only they'd invoked 14.5 with it's rule 1, depending on the thickness tol this may have achieved what they thought they were asking for.

As to response, that comes down to how much you value this customers business by the sounds of it.

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[fcsuper]

fsinfox, "Good old days?" GD&T has been around since 1940's. Most engineering projects in existance today happened after that. What good old days? I've described it as an "irrational fear of GD&T".

And to Tick's comment, well, yeah I've paid for my mistakes too.

Matt Lorono, CSWP
Product Definition Specialist, DS SolidWorks Corp
Personal sites:
Lorono's SolidWorks Resources & SolidWorks Legion

 

[CheckerHater]

Just received bunch of parts from new "less expensive" vendor.
Stuff looks like it was intentionally made as bad as our less than perfect prints will allow.
Time to talk to the boss about virtues of standards, tolerances, etc., etc., etc.,... again.
Let me join the Chorus singing "There is no such thing as "Industry Standard"".
Nowadays Craftsmanship is "what you can get away with".
:-(