Rectangular Coordinate dimensioning

[dtmbiz]

ASME y14.5m 1994

How does comply with the following?

Fundamental rules; 1.4

(d) Dimensions shall be selected and arranged to
suit the function and mating relationship of a part and
shall not be subject to more than one interpretation.

Considering that coordinate dimensioning is often related to a non-interface corner of a part, I dont see how rectangular coordinate dimensioning complies with the above rule.

Comments ?

 

[KENAT]

If the rectangular coordinate dimensioning is related to a functionally significant feature, then it's fine.

Any dimensioning scheme, not just rectangular coordinate dimensioning, that is not based on function/mating relationship doesn't comply.

Care to better explain you question?

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[dtmbiz]

The part is a simple plate which mounts a variety of electrical components. The drawing was dimensioned from the lower left corner of the part.

The part has a pattern of 6 mounting holes; patterns of holes relating to the respective electrical component. I believe these patterns should be identified with the appropriate relating dimensions. I also believe the mounting holes (pattern) should be a datum.

The lower corner of the part as a dimension origin with coordinate dims shows little regarding the part except that is has a number of holes.

I see this as just laziness. It is the fastest way to throw dims on a drawing. IMO it would not take much time to dimension according to part function and mating relationships.

Additionally I see the use of coordinate dimensioning a contradiction of Rule 1 (d).

 

[powerhound]

Well if a feature is dimensioned 1 inch from one edge of a plate and 1 inch from the other edge, this will result in a square shaped tolerance zone in the absence of any GD&T. Sometimes this is just fine. What other interpretation could there be except put the feature 1 inch in from each edge?

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[KENAT]

Trouble is, setting hole patterns as datums gets confusing fast for anything more than a 2 hole pattern. We've had multiple threads here about it and none really resolved it to my satisfaction.

So unless you have critical fit issues requiring excessively tight tolerances, it may be that an approximation of dimensioning for function is actually better than taking it to the extreme.

Is it worth the extra effort of using a relatively complex datum/tolerancing scheme that many may have trouble understanding?

Maybe you can find a middle ground where you use one of the six holes as datum B, and one of the others as datum C (for clocking) and go from there?

It arguably doesn't reflect function as well as using all 6 holes as the datum pattern, but it's easier to understand and still approximates function better than using edges that mate to nothing.

How many of this part are you making, what is the cost impact...

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[Belanger]

I agree that the square shape is sometimes fine. But the OP also brings up the point about arranging the dimensions. So even if a cylindrical tolerance is created with GD&T, does the fact that the basic dimensions originate from a nonfunctional corner violate the rule?

I suppose if the two edges are truly nonfunctional the this does violate the letter of the law. But a good lawyer could just say, "Well, the function of the part necessitates that the part be manufactured somehow, so the edges are functional in that sense."

As dtmbiz says, I also think that the pattern should be the datum and then have the edges be profiled from that datum.

General rule: coordinate dims shouldn't be used for location. Only for size, chamfer, and radius numbers.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[dtmbiz]

I was hoping to get view points relative to the concept of coordinate dimensioning practice in lieu of Fundmental rule 1(d); not specifically about this simple part.

That is a good point Powerhound, you loose the benefits of a cylindrical tolerance zone on this particular drawing.

If a plate has holes that match hole patterns of a mounted component, then it is advantageous to show pattern dims on the corresponding plate. E.g. 110mm Sq mounting hole distance on electrical contactor flanges would correlate with 110mm Sq mounting hole location dims on the plate. In my experience this is just basic 101 drafting. It is a heck of lot easier to check I might add.

Kenat,
The pattern has 6 holes equidistant from a common center making it an easy to interpret pattern center, IMO. I understand your point regarding irregular hole patterns; however in this case and with a 4 hole pattern of all holes being equidistant from a common center I find them to be a good pick as a datum.

The subject can be theorized forever I suspect; however it seems straight forward to me that coordinate dimensioning defeats the purpose of clearly conveying part function.

It is also another example of designers / engineers looking at Y14 as only needed for "complicated" part dwgs and loosing sight of the Y14 benefits via fundamentals and concepts.

Thanks for the moral support John-Paul.

Thank you to all for your comments !

 

[KENAT]

dtmbiz, a six hole pattern is not simple when you come to inspection and the implications of datum shift or not...

Even a 'square' 4 hole pattern can be tricky.

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[axym]

dtmbiz,

Here's the opinion I think you're looking for. ;^)

One could easily argue (and I often do) that Fundamental Rule 1.4 cannot be satisfied if rectangular coordinate dimensioning is used. That is, if there are directly toleranced dimensions that are not confined size dimensions governed by Rule #1.

When one tries to apply directly toleranced dimensions to the imperfectly formed features of a real part, they are subject to more than one interpretation. If the tolerances are tight, the different interpretations might yield very close to the same result. But the different interpretations are there at some level.

As far as suiting the function and mating relationship of a part, directly toleranced dimensions don't cut it there either. I have yet to see a function or mating relationship that can be conveyed correctly using directly toleranced dimensions. Approximated, perhaps, if certain assumptions are made, but not conveyed correctly. The directly toleranced dimensions might result in controls that satisfy function, but they will also generally over-control other chararacteristics. The fit between parts just can't be conveyed using a disconnected series of straight-line measurements.

I'm not saying that directly toleranced dimensions should never be used. They're often a workable approximation that can be the lowest cost option in many situations. But they're ambiguous and don't match function.

Evan Janeshewski

Axymetrix Quality Engineering Inc.

http://www.axymetrix.ca

 

[drawoh]

dtmbiz,

The functional requirement is that your hole are in the right place. Rectangular coordinates can do this.

We need to make a distinction between rectangular coordinate dimensioning, and the tolerancing method. I would still use the GD&T positional tolerance.

I use hole patterns as datums, as noted above, when I have an inaccurate fabrication process. On a sheet metal part or a weldment, it usually is still easy to create an accurate hole pattern, even if you cannot locate it accurately. At some point, a fabricator can make a drilling or punching template to create accurate patterns, even if, again, he can not locate them accurately.

I would not knock myself out preparing drawings for machine shops. A simple, easily interpreted presentation of the dimensions and tolerances will save you more money than a well thought out set of allowances.

JHG