Naww - it's really constrained by the limits of size. If the bottom surface is perfectly flat then the entire size tolerance can go into variation in the top surface, the worst of which will see 0.6 units of out-of-plane behavior. Depending on the fitting algorithm it might come out as a bit more if the software picks a plane that is not parallel to the bottom surface as the reference plane.
The trick to the question is noting that the individual straightness tolerances do not limit the flatness; their inclusion is misleading with respect to flatness control.
Dave,
The bottom surface does not have to be perfectly flat to allow the top surface to be 0.6 units out-of-plane. It may even have 0.6 of flatness error.
Why 30mm dimension is subject to rule 1? Does not have opposing and parallel elements for its entire width, right? Might be governed by rule 1 in some portions/ area, but does it make it regular feature of size or not?
So, flatness of the upper surface is controled by rule 1 only to a certain degree. Am I right or I am entirelly walking in the weeds?
This question qualifies to be in the GDTP certification exam, because from 3 qualified professionals you get 3 different answers
Not totally up to speed on the 2009 std. Did something change relative to “flatness”?
You can now measure flatness (straightness of line elements) from an opposing surface?
Not ready to buy in on that.
Also straightness of a line element is a refinement of the limits of size. (not applied to an axis or median plane in this example).
A way to measure flatness (straightness) is to lay the surface in consideration on a surface table with a dial indicator that penetrates the surface table. NOT from laying the opposing surface on the surface table. Flatness (straightness of line elements) is relative to the surface itself, NOT another surface. This eliminates the “size tolerance.” (In this case)
IMO the flatness is limited to the max 0.2 straightness callout. The 0.05 straightness will fall within that not added to it.
1. The surface is inspected to be within limits of size
2. The surface is inspected for straight line elements across the length to a 2D tolerance zone of parallel line
elements 0.20 apart.
3. The surface is rotated 90 degrees and is inspected to 2D tolerance zone of parallel line elements 0.50 apart.
(how does the 0.05 zone not fall within the 0.20 zone geometrically ?)
How is any surface element greater than a 0.2 tolerance?
I'm seeking for the correct answer, pls see what I posted at 17 Jul 15 22:34
SeasonLee said:
The quiz originated when I'm reading a book says: a feature can have different straightness values specified in different views. Then I’m thinking what the flatness will be in this case, so I’m seeking the answer, thanks for all of your valuable replies and comments.
5.3 Form tolerances critical to function and interchangeability
are specified where the tolerances of size do not
provide sufficient control. A tolerance of form may be
specified where no tolerance of size is given (e.g., in the
control of flatness after assembly of the parts). A form
tolerance specifies a zone within which the considered
feature, its line elements, its derived median line, or its
derived median plane must be contained.
Straightness could be used for example on a corrugated surface.
If it where along the corrugations, then I could see the profile of the
corrugation limited by its size tolerance; the longitudinal elements would
be controlled by straightness. (90deg to the profile of the corrugation -
"one" straightness control).
If the peaks of the corrugation (tangencies) were laid on a surface plate,
then the straightness is controlling the "flatness" to a degree.
That is not the example from the OP
In this example we have a planar surface with two straightness controls.
Straightness must be less than the size limits or what use is it for a planar surface.
The lesser value (0.05) would need to lie within the (0.2) tolerance.
At any line element within the 0.2 tolerance there is a crossing tolerance zone
90 degrees which is 0.05. I can not see how any element in the 0.05 zone could be
"added" to the 0.2 tolerance. Therefore it must lie within the 0.2 straightness geometrically.
( cant see it any other way from where I am sitting)
To me the straightness control for a planar surface would be limiting the curvature, waviness, or taper of the planar surface,
in a direction. Cant think of why I would use 2 staightness controls in reality. Just would use a flatness control.
Seems like a thought provoking question to consider multiple straightness controls as effecting flatness within
the limits of size.
I agree with what you said, but we have to understand what the standard requires and does not require. After we understand the theory, we can apply such of said theory in some sort of practical way.
I can see that the straightness of each element is controlled to 0.2 in one direction and in 0.05 in the other. However, I don’t see why the 30mm size dimension is a regular feature of size (at least not the entire surface) hence why is subject to rule#1. Each longitudinal element of the surface must lie between two parallel lines 0.05 apart in one direction and 0.2 apart in the other direction. That is my understanding of the theory.
Therefore, my opinion is that each elements could be misaligned to infinity since it is not subject to rule#1. And therefore, the relative "locations" of the "filaments" to each other are not limited by that envelope (Rule #1) requirement.
What am I missing?
As I stated before: if I am wrong won’t be the first time.
What may have been throwing me off track in my last post was that “the question” asks about a “flatness” control while the "FCS are straightness controls". Mixing 2D and 3D tolerance zones.
Limits of size and orientation controls, etc do control flatness to a degree, however actual flatness is not measured from opposing surfaces. The question is more or less a “gotcha” question IMO. The question is asking to derive information from 2D tolerance zones to determine a 3D tolerance zone value. Flatness is not measured from an opposing surface, where straightness can be. What is the question actually “trying to get at”?
What we do know is that both straightness controls are each less than the size limit tolerance and they should be.
We do know that straightness controls are used to refine form more than the size limit.
Knowing those facts, I can not believe that the “flatness question” answer can be equal to the size tolerance of 0.6.
If a grid of parallel line element zones at 0.2mm apart are laid length ways and are separated by ‘some distance’ (in theory there are an infinite number of line elements) from the front face to the back face; and the same is done at 90 degrees with the 0.05mm tolerance; and there is a simultaneous requirement in the controls that I see; now we have "a 3D grid" (topography map) that no element of the surface can violate and still be an acceptable feature.
That grid has a max tolerance of 0.2mm. How than can the flatness possibly be the size limit tolerance of 0.6mm??
I am actually leaning more toward 0.05 max flatness if as the ASME Y14.5 definition the "flatness" is measured relative to the surface itself and not the opposing surface as in the picture of the example question.
The distance from a minimum point of one "line element" to a max point of another line element spaced longitudinally could be 0.6 from one “line element” to another; the size limit. That is for just 1 of the 2 straightness callouts. However you have 2 callouts that the “line elements” must meet simultaneously.
Now we have a “question” regarding “flatness” a 3D tolerance zone relating to straightness controls witch are 2D tolerance zones, AND they are measured differently according to ASME Y14.5.
The only way that I can see to even try to equate the two is to look at the 3D tolerance of the accumulative straightness grid.
Rule #1 only applies at MMC. The straightness control applies relative to high and low points within the limits of size.
Quote:"attempting to show my point regarding simultaneous requirement with diagram"
I don't know how the simultaneous requirements comes to play in this conversation. What the form error straightness versus flatness has to do with the simultaneous requirement?
Quote: "The distance from a minimum point of one "line element" to a max point of another line element spaced longitudinally could be 0.6 from one “line element” to another; the size limit"
I agree here, but only if rule#1 is in effect, but does it? That would be my main question!
Is the 30mm dimension subject to rule#1?
If yes, why yes?
If no, why no?
If yes and no, why yes and no?
Quote: "The distance from a minimum point of one "line element" to a max point of another line element spaced longitudinally could be 0.6 from one “line element” to another; the size limit"
true if only 1 control : straightness 0.2 NOT true with 2 controls : straightness 0.2 + straightness 0.05
The longitudinal line elements must also be included as points in a given
plane with the width line element 0.05 straightness.
Consider for example:
A logitudinal element close to the farside of the surface has a high point at the high limit and then it runs within a 0.2 tolerance zone oriented parallel from the surface dimensioned from.
Another logitudinal element close to the nearside of the surface has a low point at the low limit and then it runs within a 0.2 tolerance zone oriented parallel from the surface dimensioned from.
The first line element can only be lower at any point 0.2 from its high point
The second line element can only rise higher at any point 0.2 from it low point.
That would mean that at the closest points possible with in their respective 0.2 straightness tolerance zones, they would be 0.2 apart. (limit tol 0.6 minus 2X 0.2 tol zones).
For this example those two points (high point of first line element & low point of second line element) are in the same plane across the width.
Now measure the part in the same setup at 90 degrees (across the width) to the 0.05 tolerance zone, oriented parallel to the bottom surface and please, explain to me how those two line elements are contained in the same 0.05 tolerance zone.
They are at best 0.2 apart in this example.
Or else show me "geometricaly with dimensions" how this is possible as others are claiming ?
Take a pack of cards and shift the deck a bit. The straightness along the long sides of the cards remains -0- no matter how they are shifted. The straightness across the depth of the deck can be -0- even if the pile of cards is shifted very far to one side. The size tolerance controls the minimum size of the box the shifted pack might go back into without realigning the cards.
The main flaw in your diagram is that straightness zones are not restricted to being parallel to any datum, so the sloped line should have a zone essentially aligned to the slope.
"The question on flatness with respect to the feature of size 30+/-0.3 indicated on the quiz.
According to Rule #1, this part must fit within an envelope of 30.3 and any two point measurement may not be less than 29.7. If the part passes these size checks, the top and bottom surfaces as well as the derived median plane must be flat within 0.6. Therefore, the stated straightness tolerances would be refinements per the standard."
"Per the standard as I have stated in my response the flatness would be 0.6.
If your question is with respect to the straightness tolerances. Then as I have also stated in my response the flatness must be fall within each of the straightness tolerances for the indicated surfaces, and the size of the feature must fall within the size tolerance. I hope this helps."
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