Whuzhee,
I think what you are showing is actually standardized in Y14.8... because of draft angles. Dimensions apply to mold lines.
The definition for the relevant Actual Mating Envelope is also modified from Y14.5 to make sense of it. Don't remember the exact details right now...
A gage with a planar face to simulate the primary datum plane A and a threaded hole perpendicular to it, with a pitch diameter of the MMC size of the external thread's pitch diameter plus the .002 perpendicularity value.
Is such a gage difficult to produce? Of course. Does that make little...
Datum feature B is referenced at MMB, so ideally datum axis B should be simulated by a threaded hole of a corresponding pitch diameter, that is also constrained to be perpendicular to the simulator of datum A (a planar face).
One of the definitions in ASME Y14.5 for a feature of size is a "circular element". Arguably it is one and the position callout can be considered valid.
I agree with your discomfort with this though. It's not clear how an Unrelated Actual Mating Envelope, that is needed to derive the controlled...
The misinterpretation that position can be used as an equivalent way to state perpendicularity probably results from overseeing the difference between applying it to a single feature and to a pattern. When a pattern of holes is positioned relative to planar datum feature to which it is...
SeasonLee,
The standard acknowlesges that composite profile can apply to a single feature which is not any kind of a pattern. That's why instead of P(attern)LTZF and FRTZF the description in the relevant section involves "profile-locating tolerance zone" and the "profile
feature tolerance zone"...
sendithard,
I think I am sort of starting to understand what you are saying, but I would suggest you to post an illustration of the conflict to convince the others there is a problem. Once the problem is graphically clarified the efforts here for solution/conclusion will be more effective.
sendithard,
I think you are missing something.
In this composite tolerance the bottom segment is what really controls the size. It refines size, form, and orientation to A. The top segment doesn't really tell you anything about the ideal size, it just defines the floating range.
There is no fundamental issue with a linear dimension given from a theoretical datum axis. Remember that the theoretical datum axis is the center of a pin located in the measuring fixture. Such a dimension is useful if it is a basic dimension. For example, if it locates the center of a profile...
"I'm guessing that if a hole has no secondary or tertiary datum, it must be because all over dimensions will be measured in reference to the hole, not the hole measured to walls or edges. Sound right?"
That is absolutely right. The position tolerance you are asking about only controls the...
The datum feature selection should usually correspond to how the part is placed in the assembly the part is going to. If it's positioned for inspection similar to the way it connects in the application, then features that are measured good will also be located and function well in the...
Suds,
What you are showing in your sketch looks like 2 different interfaces. The bottom interface seems to be how your part is located - if it's two tapered slots that mate to a kind of rail it could be referenced as a common datum feature, for example A-B (similar idea to how a "between...
A fixture for datum simulation in this case would include:
1. A flat planar surface to simulate datum plane A.
2. A pair of vise jaws perpendicular to the flat surface to simulate datum B (the datum would be their center plane).
3.Another pair of vise jaws perpendicular to the flat surface and...
This may not really answer your question, but the all-around profiled periphery you are referencing as datum feature B in the first version qualifies as a "linear extruded shape" from section 7.3(f) and the chart in figure 7-3 of ASME Y14.5-2018, providing a datum of an axis and a plane. It's...
Regarding the original question:
The idea of the Common Datum Features such as those referenced as A-B or A-B-C is establishment of a single datum from two or more datum features that arrest the same degrees of freedom together.
Whether a third coaxial datum feature cylinder should be part of...
For the D,B datum reference frame the true geometric counterpart for 'D' is an expanding cylinder completely independent from anything else, because it's primary. The true geometric counterpart for secondary 'B' is a pair of two parallel planes collapsing on the edge surfaces of the 100 mm...
