What is a "datum line"? 6

[Burunduk]

Hello.
I have a theoretical question about datums. According to ASME Y14.5-2009 a datum is:

"a theoretically exact point, axis, line, plane, or combination thereof derived from the theoretical datum feature simulator."

I have encountered all of the types of datums listed, except "line". Didn't find an example in the standard either. Perhaps someone on this forum can help - what is a datum line? What is the corresponding datum feature and datum feature simulator from which it is derived?

Thank you

 

[mkcski]

Burnudu:

A good question. Referring to para 4.3(d), notice the "axis (line)" text. Given this, I assume the terms are interchangeable.



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[greenimi]

mkcski,

By the same token, could you or someone else here on the forum, explain what "median line" means in the context of fig 7-60/ 2009 (where this term is used).
In the "Means this" section are shown two leader lines ended with arrows pointing to:
- a straight line, called median line
- an outside surface

Applicable text is "Median line of this surface"

Should I understand that median line it's a collection of the median points/ midpoints of diametrically opposed feature elements ? If yes, then why it is not shown as an imperfect line?

Is this median line an imperfect line (see derived median line definition) or it's a straight line as shown in the left figure of the "means this" explanation?

 

[mkcski]

greenimi:

The OP asked a datum related question with the term "line". Your "median line" response is related to position control. I agree the definition of some terms - line in this case - anywhere in Y14.5 may add to the confusion of interpreting the Standard. But I have not "seen" the use of median line in describing datum concepts. I am not as deep into Y14.5 as you and others who post a lot and that are into the esoterica of the dimensioning "technology". But is there a case for median lines with datums?

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[CheckerHater]

@mkcski: Every axis is a line but not every line is an axis.
Para 1.3.13 clearly says "axis, line, plane", not "axis (line), plane", so I'd say OP's question still legit: how datum, that is line, but not axis looks like?

@greenimi: On 6-70 straight "median line" represents extreme case of feature having maximum location error, but perfect form and orientation.
Still fishy, but I am sure new 14.5 / 14.5.1 clarify the whole "imperfect line"/"line"/"axis" thing. Right?

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[chez311]

Perhaps its a reference to datum target lines?

 

[lfw618]

I think they put both because while every axis is a line, not every line is an axis. One example I could think of would be a line between two holes on a flat plane. The line is not in the "axial" direction of the part so I would not call it an axis, but I still think that would be a valid datum.

 

[mkcski]

chez311:

Let the answer be known by all...great find. My company does not use targeting very often but I believe this is the "line" with datums.

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[CheckerHater]

@chez: Very good interpretation!
So datum is not "line", but combination of (datum target) lines.
This lies in line (no pun intended) with Para 1.3.13.

"For every expert there is an equal and opposite expert"
Arthur C. Clarke Profiles of the future

 

[Belanger]

Not so fast
Isn't a datum target line merely establishing a plane for controlling certain degrees of freedom? So the datum itself isn't really a line.

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

 

[lfw618]

I guess I could see instances where a line could be a datum and not be a datum target. See sketch attached. The two holes might need to be datums or datum targets to be valid, but my interpretation is something like this is why they included line in there. You could probably use a functional gage if the right modifiers were used. That said I'm not sure that would be the most effective way to tolerance it, I think they're just leaving the option open.

 

[chez311]

JP,

I would tend to agree, but isn't an axis also merely either establishing or orienting 2x orthogonal datum planes? Perhaps the definition is....inexact.

 

[chez311]

Ifw618,

Not 100% sure exactly what you're showing there, but I think that would be a plane not a line.

 

[mkcski]

Yeah. I jumped too quick. JP is correct. Line contact establishes a datum plane.

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[lfw618]

Oh yea, thanks chez you're right that'd technically be a plane. I was just tryna sketch out a general idea of what I thought a datum line would be.

So if a datum target line establishes a datum plane, wouldn't a datum target point establish a datum line? Datum target point is valid, yes?

 

[Burunduk]

Thank you all for all the interesting responses so far that generated so quickly. Judging by the replies, I understand that this is not a trivial matter.

JP,

I would tend to agree, but isn't an axis also merely either establishing or orienting 2x orthogonal datum planes?

I think it is a valid point, but it makes me wonder: can datum targets be considered datums? If a datum target line is mentioned in the definition of the term datum, why "area" is not in the list?

 

[greenimi]

On 6-70 7-60 straight "median line" represents extreme case of feature having maximum location error, but perfect form and orientation

Okay. Thanks. Now I understand the meaning of this "means this" pictures.

But is there a case for median lines with datums?

No, I am not aware of. You are correct, I think.
My median line question as been related with the concept of concentricity.

I think chez311 provided a good answer.

Are B1 and B2 in fig 4-48/2009--page 86 considered lines in the OP question context? I would think so.

 

[lfw618]

Does anyone with access to the 2018 standard know if the definition of a datum was changed, and if "line" was maybe removed or clarified?

 

[mkcski]

ifw618

I have a copy and line is still there. Section 3 Definitions para 3.14

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[Belanger]

The definition was indeed altered in 2018, although it doesn't impact our discussion of plane/axis/point. It now says that a datum is "a theoretically exact point, axis, line, plane, or combination thereof derived from the true geometric counterpart."

In 2009 it was "derived from the theoretical datum feature simulator."

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