DIAMETERS SHOWN IN LINE NOT NECESSARILY COAXIAL 2

[KENAT]

Can someone please remind me, in ASME Y14.5 is it basically just 2.7.3 that says that just because diameters are drawn coaxial there is no coaxiality requirement implied. To get coaxiality you need to add controls such as position, runout or concentric (I know, concentric is rarely if ever the right one depending which 'expert' you're talking too).

I get a lot of drawings of parts with 'coaxial' diameters without any controls on the coaxiality.

Do other checkers/peer reviewers etc find the same thing?

Has anyone seen any really good articles or explanations on this I could pass around.

Thanks, Ken


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

 

[KENAT]

MechEng2005 For say a c'bore hole or similar that is dimensioned off a datum structure etc then I think what you put is correct.

However for something like a shaft with multiple diameters or maybe a 'cotton reel' shape there is no dimension to the CL to apply relevant block tolerance too. This is the kind of situation I'm more refering to.

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

 

[powerhound]

Dave,
When you say you would use the number of the digits on the feature to determine how far off center another coaxially shown feature could be, I hope you realize that this would be a 100% fabrication of a tolerance.

The print is incomplete with no controls, so whether or not the part is passed or rejected for a coaxiality issue, it is passed or rejected based on a tolerance that doesn't exist.

vcastro66 nailed it. Whenever production workers and inspectors let things like this slide, it doesn't do anyone any good. It just allows bad drawings to stay in the system and the designers that made them just keep on making them.

I refer all to this link:

http://www.tec-ease.com/tips/sep-07.htm

This is almost what we're talking about...ambiguous prints and what happens when you don't specify EXACTLY what you want. Be sure to watch the little video clip, it has a little more information than the written tip.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

Powerhound, awesome.

Possibly not quite close enough to convince some of the skeptics of GD&T/standards at my employer about the coaxial diameter issue being discussed here but very useful. I had a drawing just last week the the centerline used as a datum.

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

 

[dingy2]

Fit and function rather than theoretical applications of GD&T - Yes, at times that concept has prevailed.

We shouldn't have to figure out the fit and function of features since the drawing should cover the needs but many times it does not happen. Should we look at the fit and function value if a a coaxial diameter is out that far (cam) and does not have a FCF controlling its location?? Absolutely!

If I feel that there is something that could hurt its function, I would reject it, probably, on some sort of stretched tolerance application. We work as a team attempting to please the Customer and saying that Design messed up doesn't help the company. Sure, Design should be aware of the Design deficiency but what if the part was measured on the afternoon shift attempting to achieve approval so the line can start up. What would you do?

These situations happen all the time.




Dave D.

http://www.qmsi.ca

 

[ctopher]

I don't know if this is much help, but here is some more info.

http://www.etinews.com/gdt_glossary.html

http://books.google.com/books?id=EN...ts=DJYgCX4XEf&sig=0slldO8nFDJzcZTVuVVFQo4rhvA

Chris
SolidWorks 07 4.0/PDMWorks 07
AutoCAD 06
ctopher's home (updated 10-07-07)
ctopher's blog

 

[fcsuper]

"You can't assume that just because features are on the same axis and on the same side of the part, that they will be done at the same time."

Says the standard. I was speaking of real world. Find me a vendor that refixtures a part on a lathe to turn one op into 2 or 3 or 4 op, and I will show a business that will not be in business for very long.

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group

 

[KENAT]

At my previous employer we had components that due to size limitations on the machines this was sometimes done.

Specifically some diamters were milled rather than turned.

They've been in business something like 80 years and last I heard were expanding.

Pretty sure I've also seen parts that had to be turned 180 in the lathe to maching the features. Again though this may have been a limitation of the machine/part size.

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

 

[powerhound]

Dave,

What the OP was asking was whether or not features shown coaxial with no controls should be considered relative to each other. The answer is a resounding NO. Are we going to comply with the standard or aren't we?
Applying ANY tolerance to such a feature is pure guesswork, and in the absence of knowing the exact function of said feature, dangerous. Would you really pass a first article on an ambiguous dimension just to get the line going? You also have to work as a team with production, not just design.
As far as rejecting a part on a "stretched tolerance application"; I can guarantee you that if you rejected a part that my guys built based on a tolerance that you effectively made up based on how you "felt" about a part, it would be "on like Donkey Kong" (do they say that in Canada?). I would take that situation straight up the chain and that print would get changed and the cost of the part adjusted accordingly. Oh, and production would NOT take the ding.
I'm not discounting the importance of providing the customer with a part that will work, in spite of their jacked up print, but I am not in favor of allowing bad drawings to proliferate and arbitrarily applying whatever tolerance I saw fit. I'm trying to reconcile your statement about how you would reject a part if you felt it would hurt function. If you had no clue as to what the function was, then how would you even begin to feel that the part's function was impaired? How then would you determine a valid tolerance where none was given?

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

Dave,

Essentially if you try and impose any tolerance or similar requirement onto a drawing that isn't explicitly stated you are accepting a lot of risk.

You have to weigh the problems/costs/time etc of going back to the design authority to request clarification against the possible problems/costs/time etc of imposing incorrect assumptions.

Take a look at the link Powerhound gave earlier about the multimillion dollar costs. Sobering! Normally the talk of a drawing 'being a legal document' seems pretty theoretical but this would appear to be a real world example. Should be a wake up call to everyone creating/accepting sloppy drawings.

Do we sometimes have to try and make 'a silk purse from a sows ear' sure. However, the assumptions we make to do so, and any implications there of, should be made clear in the order/standard terms etc.

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

 

[powerhound]

Matt,
Take a look at the link I provided. All those diameters are shown coaxial but because of openings in interpretation left by the person that created the drawing it cost them along the lines of 8 million dollars. Axes need to be controlled, no matter what. Even though the customer put a geometric tolerance on the print, it was still ambiguous and allowed the vendor to basically pick and choose what best served their purpose.
I understand what you're saying about real world applications and how a part will likely be set up and run, but to leave features uncontrolled just because you think they will be manufactured in a certain way thus somehow guaranteeing coaxiality by definition is a dangerous practice.
What is the resistance to controlling these features all about? There's a way to do it, the standard clearly specifies how to do it, the fundamental rules say you have to do it, so why not just do it?

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[dingy2]

Getting back to the original question about no coaxial requirement if there is no control, is not quite correct.

2.7.3 states "Feature shown perpendicular, coaxial, or symmetrical to each other must be controlled for location or orientation to avoid incomplete drawing requirements".

So, no controls - incomplete drawing requirement.

I gave you a hypothetical situation which caused a bit of a stir but it happens all the time and I acknowledge that I did stretch the tolerance application in this situation.

Would you believe that each and every manufacturing company have Quality Alerts floating around the shop floor? These are requirements that are not on the drawing but should have been.

Over 50% of the drawings with the application of GD&T are flawed in some manner with some of the worst ones coming from one of the big three automotive companies.

How many Designers are applying GD&T without training and don't know what the symbol means, could it be confirmed and is it the most appropriate application?

I have been training in this subject for 20 years and have seen some pretty ugly stuff out there.

Dave D.

http://www.qmsi.ca

 

[KENAT]

So getting back to the point of the OP which was creating unambiguous drawings in the first place, not dealing with bad ones that have been forced upon you...

I believe the standard is pretty clear. If you don't explicitly state some kind of coaxial requirement, there isn't one and later users of the drawing trying to impose one/tolerance of coaxiality is essentially guess work and meaningless in court.

So anyone preparing drawings that has nominally coaxial diameters should put some kind of control, even if it's loose.

If there's no control there's no requirement so strictly speaking anything is acceptable, to the point of the cam mentioned above.

I've got to admit I was a bit surprised by the level of debate on this from regular posters. I thought the standard was clear I was just looking for more information so I can better educate the designers around here.

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

 

[fcsuper]

"Axes need to be controlled, no matter what"...

I think even with your example, I don't by the "no matter what" portion of your statement. It's not about being sloppy or not applying the standard. It's about taking into account the level of detail needed to document the part desired, without getting into specifying processes. Yes, there is more risk to not fully defining coaxial requirements because of the wording of the standard. If 8 million dollars is on the line, then define that sucker like no tomorrow. If it's a prototype and you are having only 1 made in a quick turn around and the part is obviously going to be done in one op on a lathe and criticallity of the diameters coxial nature is understood, then there is no value added by defining it that much, even loosely at that point. Essentially, the part is going to be what it is and adding additional specifications bring no added value. At times, one simply needs to find a balance. The standard is in place to give us all a common understanding of drawings. It isn't there to tell use how to design.

Matt
CAD Engineer/ECN Analyst
Silicon Valley, CA
sw.fcsuper.com
Co-moderator of Solidworks Yahoo! Group

 

[powerhound]

Specifying how to control an axis is no more design than specifying how to locate a hole. You're right that the standard is to promote common understanding but I never implied that it should be used for design.

Powerhound, GDTP T-0419
Production Supervisor
Inventor 2008
Mastercam X2
Smartcam 11.1
SSG, U.S. Army
Taji, Iraq OIF II

 

[KENAT]

Fcsuper, I disagree and doubt I’ll persuade you but for what it’s worth:

If you/your employer are willing to accept the risk associated with incomplete/ambiguous drawings that's your prerogative.

I have major concerns about relying on assumptions of how parts will be made, for many reasons parts aren’t always made how the designer originally anticipated. I’m sure the people creating the drawing that cost $8 million either didn’t know better or were happy to accept the risk or thought that it would be turned on a lathe so would be adequately concentric, I wonder if they’d reconsider that decision now.

Do you leave dimensions and/or tolerances off of drawings for one offs etc., is it not worth the effort? That’s essentially what you’re doing by not giving any coaxiality control.

Part of my remit as checker is to minimize ambiguity on and avoid omissions from drawings. As such I will be requiring coaxiality controls.

“The standard is in place to give us all a common understanding of drawings. It isn't there to tell use how to design.”

And the common understanding in this case would be that showing diameters coaxial does not imply any coaxiality requirement. As such if you show them coaxial but don’t have an explicit requirement stated, you have an incomplete drawing and no coaxiality requirement is stated or implied. They can be way off without failing drawing requirements as there aren’t any.

I don’t see how it’s telling us how to design or what I or Powerhound said that makes you think we use it to design.


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

 

[vc66]

I agree that it's not there to tell us how to design; however, it is there to guide the proper and correct physical representation of our design. The drawing must be COMPLETELY defined. With no coaxial constraints, it is incomplete. Simple as that.

Whether this lack of completeness affects anything is a different question altogether.

V

Mechanical Engineer
"When I am working on a problem, I do not think of beauty, but when I've finished, if the solution is not beautiful, I know it is wrong."

- R. Buckminster Fuller