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

 

[Burunduk]

chez311,
I think I now understand your assertion better, and I agree. I think that at the statement "Your measurement equipment and gauging cannot touch theoretical features" the focus was on fixturing and constraints of degrees of freedom, less so on utilizing an origin of measurement, at least not of the type of measurement that allows determination of measured "coordinates" (whatever Mitutoyo mean by those).

My assertion was that the datums directly associated with datum features are meant to be the default dimensioning and measurement origin and their simulators are intended to be used as such in practice. Things become a bit more complex when datums associated indirectly with the datum features such as the "third datum plane" from fig. 4-4 in the 94' standard, or the mentioned tooling ball (in practice), have to be utilized. This is what I meant by "a purely theoretical and virtual entity as far as the drawing is concerned". For the part in fig. 4-4 the probe of the measurement device will actually be touching what is represented in the drawing as a "theoretical feature", or perhaps it would be more correct to say that it is not represented on the drawing at all, but should be concluded from it and set in practice, and only tied to the other datums to complete a 3 normal planes DRF / inspection set up. In an ideal world, this would be no issue at all, as the inspection department would figure it out and invest the time and resources in whatever it takes to work by the drawing.

 

[chez311]

I think that at the statement "Your measurement equipment and gauging cannot touch theoretical features" the focus was on fixturing and constraints of degrees of freedom, less so on utilizing an origin of measurement

Correct.

My assertion was that the datums directly associated with datum features are meant to be the default dimensioning and measurement origin and their simulators are intended to be used as such in practice.

Thats certainly a common practice, and is often the logical conclusion (no need to create more work for either the operators or software when not necessary) however it is by no means a requirement.

Things become a bit more complex when datums associated indirectly with the datum features such as the "third datum plane" from fig. 4-4 in the 94' standard, or the mentioned tooling ball (in practice), have to be utilized.

I agree, though whats that old saying - "if it were easy, everyone would do it." Sometimes complexity is unavoidable and often comes with the territory, however it should be reduced/eliminated wherever possible without sacrificing function or performance (or as little as possible). Thats a key component of good design. Another is taking a stand on the issues that matter and resisting compromise in critical areas.

or perhaps it would be more correct to say that it is not represented on the drawing at all, but should be concluded from it and set in practice, and only tied to the other datums to complete a 3 normal planes DRF / inspection set up.

Yes - this is in the realm of the inspection department, not the drawing definition. As long as the datum features and subsequent relationships have been properly defined its up to the quality department to determine how best to check it and how to design the tooling/probing setup. The drawing does not define this outside of the relationship of the simulators to the tolerance zones. Any decision on how to qualify these simulators (direct probing of simulator, datum feature, tooling ball/hole, or other artifact) is up to inspection.

 

[Burunduk]

chez311, seems there is 100% agreement on all points of discussion, at least from my side.

Now it's time to join Yuyu28 on a ride on his gyrocopter... Or perhaps NOT! Because while we were discussing theory it turned out that the teeter component isn't mounted on any plain surfaces but only hangs on the bolts by the clearance holes as it rotates

I still would like to get a clarification about that part and see what additional advice we can offer.

 

[Yuyu28]

Burunduk, chez311, apologies for my silence; I read you at work so I don't want to take too much time from it. I still like to clarify unanswered questions, so here it goes:

You were right that the beam is not floating; I was missing a part of the assembly. The beam is indeed clamped on plane feature A by the rotor head (not the teeter hub) through the two holes I showed in my sketch. The two swing elements that attach via rolling bearings to the the (not shown) holes at the extremes are lying lightly on the teeter hub. There's also two springs that connects these two swinging parts to the beam, but I don't want to get too deep into the mechanism of the part, both for confidentiality and for my lack of ability to explain things simply

 

[Burunduk]

Yuyu28,
Considering your latest description, chez311's post from 10 Sep 19 13:01 is spot on for your needs.