Tolerance of Positoin - feature positioned to itself ??? 2

[DesignBiz]

Please review attached drawing for comment on the tolerance of position callouts circled.

1. I do not understand how a FCF can reference the feature positioned to itself in a datum reference block. (e.g. datum A is positioned with reference to itself in A-B)

2. I believe the location tolerance and the basic dimension relavent to it should have the same number of decimal places. (Not shown on the drawing) (e.g. if tolerance in FCF is .XX then its basic dimensions should be XXX.XX NOT XXX) Is this in the standard?

DesignBiz

"Quality is in the details"

 

[DesignBiz]

I don’t think my example was that much of a stretch. Just trying to get minds back to basic engineering fundamentals. Engineering is in large part communication. It is a basic fundamental to document a design. If one is similar in many ways but is different in any way, then minimally the differences are defined by a modification drawing.
It is fairly easy to agree that the standard does not cover everything. In this case, A-B has been explained in detail as to its meaning vs. the standards basic DRF concept. I don’t find it that hard to understand, however my “bottom line” is that this deviation must be defined as an amendment to the standard, as all deviations should be.
Evan, although you have walked me through your thought process on this callout meaning and have pointed out yourself that it doesn’t fit the standard’s current definitions; I don’t see why, that my main point of documenting the deviations is any different than any other documentation of a design requiring definitions for modifications. It’s neither that I don’t understand your presentation nor reject it, I simply say if this type of “extension” of the standard is used then it must be documented as an amendment to the standard.
This particular part has been talked about as being fixed at both ends and therefore both ends are equally important. I have commented that both ends indeed connect to other tubes, however if you look at most any exhaust tube (and I have mentioned before that this is what was posted, a rigid exhaust tube) it is truly held in place by hangers. I have mentioned before in almost any part that certain features can be found that are equally as important. I have contended in this case that the “equally as important” argument seems to ignore other important features of this component. The tubes are normally of considerable length and each segment has minimum clearance at some point. The segments must be manufactured within tolerance as well, or it won't matter if the ends come out in the correct position because it still wouldn’t fit.
MechNorth has also mentioned a few days ago “design intent” is the way a component should be dimensioned. I have personally been a vocal advocate of this principle. However as I have thought more on this I find many times the manufacturing process must be “considered”. If for example I know I have a cast or molded part, it is a part of the design to consider minimal wall, draft, transitions from one thickness to another, etc. If my design does not consider the manufacturing process and its relationships to part function, then it could easily be a poor design.

Thanks for all the comments.


DesignBiz

"Quality is in the details"

 

[axym]

DesignBiz,

I think we're both in agreement on the need for explanations of practices are aren't covered in the standard. I'm still a bit confused about what the situation in your company is. Is it that there is resistance to documenting those explanations, so that if something is not covered in the standard then you can't use it?

Regarding the "equal importance" of features, here are my thoughts on that. I wouldn't use the term "importance" when it comes to choosing datum features and how they are referenced. It all comes down to degrees of freedom and how they are constrained. The tubes are fixed at both ends, therefore the two end cylinders are the features that constrain the tube's degrees of freedom and should be the datum features. Each cylinder has an equal role in constraining the degrees of freedom, so they should be referenced in a way that treats them equally (i.e. A-B). Other features on the part might be equally "important", and must be in tolerance in order for the part to function. But these other features don't constrain the DOF's and are therefore not referenced as datum features.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[MechNorth]

DesignBiz, I've never said to ignore manufacturing and inspection. The PRIMARY intent of GD&T is to convey design intent, not to convey manufacturing and inspection techniques. A common problem is that manufacturing overwhelms design with their demands and the design suffers. It seems that many / most companies want to maintain their US vs THEM department division mentality. As part of my training material, I advocate Design Previews rather than Design Reviews. The difference is that everyone (design, manufacturing & inspection) gets a chance for input on the controls and tolerances before the GD&T annotation is added. After the drawing is done, it's little value to go back and shred the designers's work and self esteem. More advanced companies will even set up design groups to solicit manufacturing and inspection input before designing the workpiece. Some companies have integrated this into DFM processes.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DesignBiz]

Axym,

The company does not want to take the time to document the amendment because it would need to be approved by a committee. Therefore it is the local opinion that it is better for the callout not to be understood, than to go thru the process of documenting it's meaning.


MechNorth,

Never claimed that 'you said' to ignore mfg or inspection, just wrote that you had mentioned that design intent is the way a part should be dimensioned; I agree, but this also got me thinking that mfg needs to be considered more some times than others. "I thought more on this..." (my previous comment) is in regard to my usual response.... "dimension for design intent/functionality"; meaning... maybe I should answer a little differently in the future.


DesignBiz

"Quality is in the details"

 

[MechNorth]

One of the joys of text-based communication is the freedom to read in / read out subtleties. Thus, misunderstandings. No problem.

Jim Sykes, P.Eng, GDTP-S
Profile Services

http://www.profileservices.ca

TecEase, Inc.

http://www.tec-ease.com

 

[DesignBiz]

FYI,
Although I enjoy the comments from all of the site's contributors, I find particularly interesting the detail and perspective from the inspection set-up aspect that yourself and Axym provide.


c u folks on another thread.....

DesignBiz

"Quality is in the details"