Understanding datum line requires examining multiple perspectives and considerations. What is a "datum line"? 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?
Datum line that is not a straight axis | Eng-Tips. In this context, the two most recent versions ('09 and '18) of ASME Y14.5 define a 'datum' as: datum: a theoretically exact point, axis, line, plane, or combination thereof derived from the true geometric counterpart. With "line" mentioned as a separate kind. In past discussions such as thread1103-452649 no...
Attaching Datum to leader line or feature control frame?. The '94 standard however is more directive. Moreover, ¶3.3.2 states that "the datum feature symbol is applied to the concerned feature surface outline, extension line, dimension line, or feature control frame as follows:... points to define a datum | Eng-Tips. Building on this, the datum feature is different than datum.
I cannot use three point to define a surface as you mentioned. Because the datum feature, that is the cylindrical surface would not SEAT on this three points as planes do on three point or line on two points. Another key aspect involves, my personal thought is that I need four points to define this cylindrical datum feature. Centerlines as Datums | Eng-Tips.
A datum is always associated with some real world feature. I believe Belanger's comment refers to the fact that you can use the center of a feature as your datum, but that center is the result of clamping the feature (retangular) or pinning it (hole). Either way, it's the engage of the real world surfaces that creates the center. Two holes (or features) used as one datum.
From another angle, it doesn't control the median perpendicularity or anything like that. It would just control each hole to be perpendicular to datum A, but at the same time control their location from each other. Then, when you want to reference back to that datum (of two or more features of size), you'd probably want to modify the datum reference with "M". on the datum | Eng-Tips.
You can find definitions of "datum" in a local dictionary or "online" on internet, such as: {Datum line} (Surv.), the horizontal or base line, from which the heights of points are reckoned or measured, as in the plan of a railway, etc. Thus the "datum" is simply a line used for reference, from which other locations are measured. Datum Feature Symbol placement | Eng-Tips. The surface would be the datum feature and the centerline would be the datum axis (of that feature).
Datum axis' are extensively used but you don't call out the axis (centerline) as a datum.
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