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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[powerhound]

Actually, craftsmanship--as defined by old timers--is way too expensive.



Powerhound, GDTP T-0419
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[MikeHalloran]

Innovation -- as defined by the Harvard Business School -- is way more expensive -- if you account for all the hidden costs -- if you can find them.

The trick is to conceal all those costs until you have cashed out.

The easiest way to conceal costs is to dispose of the people who become aware of them.

Thus endeth the lesson.



Mike Halloran
Pembroke Pines, FL, USA

 

[pmarc]

Because up till this point the discussion was rather based on opinions, I'd like to cite ISO's standpoint about this issue.

The newest edition of ISO 1101 issued in 2011 states in paragraph 5.3 "Definitive drawing principle" that:
"The drawing is definitive. All specifications shall be indicated on the drawing using GPS [GD&T] symbology (with or without specification modifiers), associated default rules or special rules and references to related documentation, e.g. regional, national or company standards. Consequently, requirements not specified on the drawing cannot be enforced."

 

[CheckerHater]

I gave pmarc a star.

Could somebody quote equally short, but powerful statement from ANSI/ASME?

I am preparing a memo for my boss

 

[KENAT]

ASME Y14.5M-1994 1.4 a-c gives this to some extent, though certainly without the last rather appealing sentence.

I wonder if there'd be anything in the inspection standard, but I'm not optimistic.

I didn't see anything in 14.100 with a quick look.

Is this actually a case where even I might think Iso is more appealing than ASME;-).

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[btrueblood]

...but if no spec. is invoked for the print, then how would any paragraph from any spec. apply?

 

[powerhound]

That's a perfect point trueblood, and it's also why the whole Industry Standard discussion came up. When I said there was no specification as to which standard to inspect to, the "Industry Standard" was brought up.

I'm with you Ken; could there actually be something in ISO that I like better than ASME? Whoda thunk it?

Powerhound, GDTP T-0419
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Inventor 2010
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[pmarc]

Just a short clarification. The ISO standard I was citing is 8015 not 1101. Sorry if I misled someone.

 

[juergenwt]

Oh the blessings of having one standard for almost everything manufactured. ISO is not complete but most likely will be the dominant standard. Right now DIN is the most extensive and complete standard and ISO is or most likely will be a 90% copy of DIN. Right now our industry is struggling with at least five - if not more - standards. Here is a list of some:

* American Association of State & Highway Officials (AASHTO)

* American National Standards Institute (ANSI)

* American Society for Testing and Materials (ASTM)

* Institute of Electrical and Electronics Engineers (IEEE)

* National Fire Protection Association (NFPA)

* Society of Automotive Engineers (SAE)

* Underwriter Laboratories (UL)

We transferred the production of machines from Europe to the US, incl. all prints and found everything made to DIN. Bought a few DIN books in English and the rest was easy.

 

[pmarc]

juergenwt,

I do not see a problem in having different standards issued by different organizations as long as the standards are complementary and do not collide one with the other.

ISO is literally one organization but actually divided to different (about 200) Technical Committees (TC) which act like different organizations but behind the facade of ISO. Not sure how DIN looks like but I suppose quite similar.

 

[CheckerHater]

It looks like DIN is very active "inside" of ISO as in "we have some good stuff you guys can use", but at the same time Germany is cancelling some DIN standards and directly adopting ISO as national standards.

The process is happening dynamically "as we speak". If you look thru catalogs of big suppliers like McMaster-Carr, some of their "DIN" fasteners refer to already non-existing standards and are in fact "ISO".

Similar trend can be seen everywhere as EN ("Euro Normen") and BS (British Standard) just copy ISO verbatim and adopt as national standards.

 

[MechNorth]

There are industry standard tolerances for many casting & molding processes; I've seen the docs and used the values for rod and extrusions, rotomolding and thermoforming, and castings. There are also industry standards for "stock" materials. If the rail in the original example was "stock" with only modifications made to it, then those features/aspects of it which remained stock would be controlled by the industry standard. That being said, those tolerances are generally pretty relaxed and most of the decent suppliers whom are trying to stay in business already offer significantly better than their industry standards.

Jim Sykes, P.Eng, GDTP-S
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[KENAT]

However MechNorth - for those 'stock' cases this would effectively be referenced on the drawing by a complete material call out - no?

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[MechNorth]

wish it was always the case, Kenat, but typically all I've ever seen is "2x3 alum angle stock"; sometimes just the dimensions with "STOCK" next to the number ... that's about it. Bad detailing practices prevail. As there are actally industry specs for alum channels, etc, that's technically all that's needed.

Jim Sykes, P.Eng, GDTP-S
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[KENAT]

Pretty much what I figured you'd say.

I'd argue that strictly speaking without somehow referencing the spec that controls the 'stock' sizes, that just saying 'STOCK' alone doesn't cut it as a full definition. That's what I mean by 'complete material call out' - not just the grade but relevant standard that the bar or whatever is to.

That's what we did back in the UK for aerospace/defence stuff and I think I've seen similar for US defense etc.

That said, I'm guilty of that crime here, just saying '6061-T6' or similar and then having 'STOCK' next to a material thickness or similar. I could try to excuse it by saying that I only do it on 'non critical' dimensions or similar but arguably it's still incomplete.

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[drawoh]

powerhound,

Tips on Designing Cost Effective Machined Parts

This has been linked here before. One of the points the author makes is that he has to sell parts. If they look crappy, or you (or your puchaser) are unhappy, he will not be making any more sales. This imposes some discipline on the fabricator.

On the other hand, if you specify that your part is to be fabricated as per drawing 123-456, and drawing 123-456 calls up ASME Y14.5-2009, and you call up fabricatable tolerances, there is no need for the concept of Industry Standard.

JHG

 

[powerhound]

drawoh,

Your link is really good and addresses a lot of issues that I've seen in the past, it doesn't address form at all though. I was sure hoping it would.

I certainly agree that the fabricator should make every effort to produce a part that the customer will gladly buy. A lot of prints I've seen don't even have a surface finish callout but that doesn't necessarily mean it's okay to have a horrible, hacked up surface finish. I know the idea is to make something that the customer will want to buy again or at least provide a service that they will be willing to pay for again. It's the unspecified, yet marginal issues that I'm really asking about. I don't believe having a straightness error of .020" over 36" is unacceptable, especially when there is no indication on the print that any thought was given to tolerances. Every dimension, and I do mean EVERY DIMENSION, is a 3 place decimal which is +/-.005 in the tolerance block. This rail had holes and other features on it. Having Y14.5 specified would have automatically limited the straightness error to .010 but who's to say that would have even been too much since no thought was given to the tolerances at all?

Anyway, I think I'm on the same page as most others here. It's not as easy as saying "too bad, you should have specified it." That might be true, but it won't win me any repeat business. It's such a slippery slope...

Powerhound, GDTP T-0419
Engineering Technician
Inventor 2010
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[drawoh]

powerhound,

My big experience with crappy drafting was a manufactured product. We got to the point where there was one machine shop who knew how to make all the parts. He responded to compaints. He logged the phone calls. He could make the system work.

If he had gone out of business, it would have taken years for another shop to get up to speed.

JHG

 

[btrueblood]

"That might be true, but it won't win me any repeat business."

Probably true, powerhound. I am lucky to have a good number of suppliers who work with me to get us what we want. They call all the time (read to some = "annoy me") to question me about tolerances and "what we really want" vs. what the drawing says. Sometimes we catch errors in the print, sometimes they just need clarification of how the parts work and how we measure the given feature, etc. And when it don't fit, I take the blame when it's my fault. That way, I can slap them silly when THEY screw up and fail to call me right away so we have a shot at salvaging the material.

 

[Toddr8541]

Have any of you also seen the note " must meet quality workmenship standards" I saw that on a print at a company i worked at and asked the engineer what he meant by that. His response "make sure that it's done correctly even if i missed something" I told him to remove it from the print and define the part correctly. Needless to say I let him go a couple of months later.

As a engineer and/or designer or the part its your responsibility to define what you need. Suppliers are not mind readers. In other words do your job and if you screw up admit it dont try and stick the supplier with the bill.

I can tolerate ignorance long before I can laziness. Most of the time its laziness and then its blamed on the supplier to some unknown industry standard.

As for working with the customer if they are always quick to throw the mistake on you they will cost you more money than you will make.