Quote (3DDave)

...shall of it's former self

When you are on this Forum, where you find a simultaneous requirement of some humor, you shall constrain yourself from drawing upon puns.

"Profile tolerances apply normal (perpendicular) to the true profile at all points along the profile."

This is identified only in the Uniform Tolerance Zone section. I would think that specifying the direction of the allowed variation would apply to all profile tolerance cases.

Quote:

The symbolic means of indicating a datum feature consists of an uppercase letter enclosed in a square or rectangular frame and a leader line extending from the frame to the feature, terminating with a triangle.

So, just one letter and it must be in a square or rectangular frame. OK. All squares are rectangles and there is no tolerance given for how much a possible square might not be an exact square; it's not clear why there is any point in mentioning using a square - a rectangle that happens to be a square should be acceptable as a rectangle.

Then comes the contradiction - or with nothing initially saying that there is an alternative means of creating a datum feature symbol:

Quote:

When datum features requiring identification on a drawing are so numerous as to exhaust the single-alpha series, the double-alpha series (AA through AZ, BA through BZ, etc.) shall be used and enclosed in a rectangular frame.

It's not "The means".

A single uppercase letter is one of two means of creating that symbol.

It's "A means".

6.3.2.1 &
6.3.2.2
" placed on an indicated partial datum feature. "

Is it a datum feature or not a datum feature? It doesn't seem as if a datum feature can be "partial."

At least the standard agrees with me -

Quote:

7.12.5 Partial Surfaces as Datum Features
(a) Orthographic Views. A chain line drawn parallel to the surface profile and dimensioned to define the area and
location as in Figure 7-28 indicates a partial feature as a datum feature.

Neglecting that a surface cannot be partial; they can be partitioned or one can limit the scope to a portion of a surface - a surface is a surface is a surface - sure, one can partially clean or paint a surface or partially machine one surface to create another one, but the surface itself is still itself, identifiable as a surface unless it is removed.

Seems like a good way to ruin test scores, in 5-10 years when the test is updated, to ask about partial datum features and, no matter the answer, score it wrong, as there is a contradiction built into the text.

Pack in that "a partial surface in note form or by a datum target" and ask if "chain lines are used to indicate....?" and fail those who say "Yes" to that as well, as datum targets are also partial datum features. Or not.

To test takers - if you miss passing by one point and this is on the test - this may be why.

This seems so stilted.

Quote:

The symbolic means of indicating a datum target shall be a circle divided horizontally into halves.

Doesn't "A datum target symbol is a circle divided horizontally into halves." cover this more concisely?

It's in the Datum Target Symbol section - There is no need to refer to "symbolic means of indicating." If the reader doesn't know what symbols are for then I think that this won't help them. Maybe lead off with what a datum target is so the reader knows why they might use a symbol?? More thinking - this isn't a datum target symbol, this is a datum target identifier symbol - the point, line, or area is the actual datum target; this just has a unique identifier for the symbol itself with maybe some auxiliary info. Too bad. That ship sailed long ago.

Maybe do it in one shot:

"A datum target symbol is a circle divided by a horizontal line* into halves where (when?) the lower half contains an uppercase** letter identifying the associated datum, followed by the target number assigned sequentially starting with 1 for each datum and the upper half is used, as required, for the size and shape of the area (true geometric counterpart) or is otherwise left blank."

I'm not even going to deal much with Fig 7-50. The note says the part is to be restrained but the feature control frames say the only ones that make sense to restrain are not restrained. Or does it mean that the A targets are restrained (because (F) isn't next to the "A") and if the part isn't restrained at the D targets then maybe the D targets are defined in the model so that restraint on the A targets will force the part into contact with the D targets and that fact is invisible in this view? Doesn't a restraint requirement specify how much restraining force is required/allowed? I think a general note about that makes more sense. If that isn't part of the general note then maybe that should not be the general note.

*Not halved using dashes, not by removing segments of the perimeter of the circle, not by a squiggle, not with a yin-yang like division.
**It was important when identifying datums in datum feature symbols to mention uppercase; should be consistent.

Quote (3D)

I'm not even going to deal much with Fig 7-50. The note says the part is to be restrained but the feature control frames say the only ones that make sense to restrain are not restrained. Or does it mean that the A targets are restrained (because (F) isn't next to the "A") and if the part isn't restrained at the D targets then maybe the D targets are defined in the model so that restraint on the A targets will force the part into contact with the D targets and that fact is invisible in this view?

The paragraph about "common datum features" says the MMB/LMB modifier is applied following each reference letter. It wouldn't be needed if the modifier was meant to apply to both parts of the common reference when specified after the second one. So by an extension of a principle the same goes for the "F" modifier. The definition in 7.12 should have been more general. Each letter is treated separately, then the "A" targets are restrained. The D target areas are referenced in free state, and they don't need to be forced into contact with their target simulators. All "A" and "D" target areas act to constrain the same degrees of freedom, so it is an "oversconstrain" by intent. In that case the unrestrained (free state) datum targets are not required to be in contact with the part, and may not be, depending on the particular as-produced part geometry.

The point is that the example doesn't include any useful information if you think that other explanations are enough, unless it is intended for the audience of vehicle hood makers who aren't smart enough to figure out how to fixture sheet metal. Compound datum features? Covered elsewhere. Free state? Covered elsewhere. Restrained condition? Covered elsewhere.

Glad that you agree exactly with what I thought the illustrator was trying to express even though it would have cost literally nothing for the committee to write that out explicitly. It's a puzzle though that sometimes they are explicit and you think they are wrong.

The magic words - "extension of a principle" cover a multitude of omissions and errors don't they?

I didn't agree with the part where you said: "if the part isn't restrained at the D targets then maybe the D targets are defined in the model so that restraint on the A targets will force the part into contact with the D targets". It is acknowledged that for a restrained part, the datum targets which are in free state and take part in an overconstraint may not contact the datum target simulator.

What I find unclear is how restraints are supposed to be applied to the part against MMB datum feature simulator pins, as shown in figures 7-51 and 7-52, datum reference "B(M)".

Forest for trees - why does 7-51 use datum target point symbols in an edge view when that is given as the way to define datum target lines?

Anyway 7.20.4.5

"Anyway 7.20.4.5"
True, but the note fig. 7-52 explicitly states that that restraints are applied on both datum feature simulators "A" and "B". It is not even the barely going through the motions "UNLESS OTHERWISE SPECIFIED" in fig. 7-51 where the "otherwise specified" is only in 7.20.4.5 and not in the figure representing the drawing.

It is not defined how datum target areas need to be depicted on an edge view anywhere outside of fig. 7-51, and it is apparently the same as a datum target point and datum target line. While it is intuitive why a point and a line would look the same in an edge view, a chain line would make more sense for an area.

"The datum target area is indicated by section lines inside a phantom outline of the desired shape, with controlling dimensions added." That's it; all that is needed and no allowance for anything else. There is no need to depict an area in an edge view. I've never felt the need, for example, to show where part marking is to be placed in an edge view. Figure 7-64 shows how one is able to utilize a datum feature symbol that references the datum targets in a way to avoid confusion when clarifying where a datum is to be placed.

Perhaps I'm just tired but I don't see a large problem with 7-52 that isn't swept under the "figures are incomplete" rug. In the spirit of things it's a surprise it's not "THE PART SHALL BE RESTRAINED ..." for inspection of noted features.

It is interesting that Figure 7-27 is not referenced as a restrained condition example when it goes into such detail. That seems like a clear oversight.

The problem with 7-52 can't be rationalized by the figure being incomplete, because the note "PART IS RESTRAINED ON DATUM FEATURE SIMULATORS A AND B" contradicts how datum feature simulators for datum features of size work when the datum features of size are referenced at MMB, as also clarified in 7.20.4.5. Unless someone thinks it is reasonable that a datum feature simulator can both restrain the part and provide datum shift at the same time.

Simply restrain the part, check the toleranced feature, but then if it doesn't comply you could unclamp the part, shift it around that datum feature simulator, and reclamp it for another try.

Belanger,
I agree, but datum feature B is not restrained in any stage of the process, is it?
Can we justify the note in fig. 7-52?

I admit that I didn't look at the figure when I posted that -- I was just responding to your written comment.
It's true that the part isn't muscled against those 4 holes. When I teach about restraint notes, I mention that the note should spell out the exact parameters, such as specific location of restraint, the amount of force/pressure, and even the sequence of clamping (if multiple clamp locations). So I agree that the note in Fig. 7-52 isn't very good.