Flatness in the absence of a GD&T callout.
Flatness in the absence of a GD&T callout.
(OP)
I apologize if this has been answered in the past. I did see a post on concentricity but not flatness.
I have a part that has a thickness with tolerance called out but no specific flatness called on on it. The part does have a bow in it but the thickness is good through out. Is there an implied flatness in ANSI due the tolerance called out?
I have a part that has a thickness with tolerance called out but no specific flatness called on on it. The part does have a bow in it but the thickness is good through out. Is there an implied flatness in ANSI due the tolerance called out?





RE: Flatness in the absence of a GD&T callout.
Situation can immediately change if indepedency rule is invoked.
RE: Flatness in the absence of a GD&T callout.
Essentially the entire feature has to fall between the stated dimensional tolerance for it's entire length.
Say you have a plate .500" thick +-.010" then including 'bow', the part has to fit in between parallel planes .510 apart. So you 'sort of' get a flatness of .020.
I can't remember the impact of 'perfect form not required at LMC but I'm sure one of the expert will chime in quick enough.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Flatness in the absence of a GD&T callout.
If the drawing does reflect the ASME Y14.5 standard (I have a feeling it doesn't) and using Kenat's .500 +/- .010 example, we could have a flatness of up to .020.
Here is how to calculate the flatness. Measure the thickness of the part. Let's say it was .497. From the maximum part thickness of .510, subtract the actual thickness. In our case we would get .013 and that is the flatness allowed at that thickness. If the part was made at its largest allowable size, .510, there would be not flatness allowance.
Hope this helps.
Dave D.
www.qmsi.ca
RE: Flatness in the absence of a GD&T callout.
Let ma add some clarification (or maybe confuse you more).
If ANSI/ASME Y14.5 is invoked (or implied), you will have a "russian nesting doll" of tolerances.
I made picture (I like pictures) to illustrate it.
If you already read something about Rule 1, you know that your size tolerance is the largest one.
You can refine it with Parallelism. Your Parallelism tolerance zone will be "floating" inside of yours Size tolerance zone.
Then you can refine Parallelism with Flatness. Your Flatness tolerance zone will be "floating" inside of your Parallelism tolerance zone.
But there is a bright side to it. Because your Flatness is always smaller then Parallelism, you can measure Parallelism (sometimes it's easier) to make sure your flatness is good enough.
I am sure there are folks here who will be happy to discuss the issue further, so I am going back to work. :)
Link:
RE: Flatness in the absence of a GD&T callout.
RE: Flatness in the absence of a GD&T callout.
RE: Flatness in the absence of a GD&T callout.
Straightness is a component of flatness on allows a maximum bow in one direction only. I would still stick with flatness since if covers the complete plane.
Dave D.
www.qmsi.ca
RE: Flatness in the absence of a GD&T callout.
RE: Flatness in the absence of a GD&T callout.
Unfortunately, it looks like you have no case.
If ANSI Y 14.5M is explicitly mentioned and there is no note like "PERFECT FORM IS NOT REQUIRED AT MMC" or some other way to slip Independence principle in, things are exactly like on my picture: tolerance you are given should cover EVERYTHING including the bow.
Good luck!
RE: Flatness in the absence of a GD&T callout.
Our QA guy was going on about how we don't have a flatness so couldnt' reject them.
I pointed out the rule 1 and I think we forced the vendor to replace the parts.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Flatness in the absence of a GD&T callout.
RE: Flatness in the absence of a GD&T callout.
So there are two questions: Is the drawing referencing an ANSI/ASME standard? And should it be considered as non-rigid? (i.e., will it normally bow in its intended function?)
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Flatness in the absence of a GD&T callout.
The drawing calls out ANSI Y14.5M 1982. It is a cover that gets screwed down on all four sides. One would think even with a bow it would flatten being screwed down and all, but they did not stipulate a contrained condition.
RE: Flatness in the absence of a GD&T callout.
I think to declare a part non-rigid, the drawing must have a note stating that "All tolerances apply with datum feature(s) x restrained". Without a note similar to this, the default of measurement in the free state applies and the part is considered rigid.
I agree that you're still correct regarding rule 1 though, since sheet metal parts are "from stock", so 2.7.1.3 of Y14.5M-1994 applies. So even if Y14.5 is referenced on the drawing, I think a sheet metal part needs an explicit flatness spec if that is the functional requirement.
Dean
www.d3w-engineering.com
RE: Flatness in the absence of a GD&T callout.
RE: Flatness in the absence of a GD&T callout.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Flatness in the absence of a GD&T callout.
Yes, I agree that a "floppy" part being considered rigid just because it's not restrained for measurement is odd... I'll run through my thoughts on this & I'm sure you or someone else will straighten me out if I'm off track... I'm relatively flexible about this topic.
If a part is considered flexible relative to the mating assembly, then it should be restrained for measurement, since restraint to an ideal mating assembly (a fixture) is likely the best way to model the part's functional state.
If the part is not restrained for measurement, then the assumption would be that restraint was considered and not selected, so the part is probably relatively stiff compared to the mating assembly, so the free state best models the part's functional state.
So, if a non-rigid part is dealt with by restraining specific datum feature(s) to a fixture (probably only one datum feature, since restraint to more than one often causes repeatability problems) then I would say that it follows that a part is considered rigid, per Y14.5's use of these words, if it is not restrained for measurement. I'm using figure 6-54 in Y14.5M-1994 as my justification for saying this (a student accidentally put my Y14.5-2009 in their backpack last class & I won't get it back until tomorrow, so sorry for a '94 only reference).
Dean
www.d3w-engineering.com
Do you agree?
Dean
www.d3w-engineering.com
RE: Flatness in the absence of a GD&T callout.
RE: Flatness in the absence of a GD&T callout.
But I still hesitate to agree that a non-restrained part is automatically assumed to be rigid.
How would we dimension/tolerance a length of garden hose? Must I put a note on there telling everyone how to restrain it?
This topic also brings up the circled F modifier, but I see that as only a way to temporarily override a general restraint note. I don't see it as declaration of non-rigidity. Maybe I should have started another thread on this aspect...sorry!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Flatness in the absence of a GD&T callout.
Per ASME Y14.5-2009 page 8 1.4 (m) states "Unless otherwise specified, all tolerances apply in a free-state condition. For exceptions to this rule see paras. 4.20 and 5.5."
Dave D.
www.qmsi.ca
RE: Flatness in the absence of a GD&T callout.
1)I agree with Dean that if the part is called out made from stock as tcogswell said up the thread then 2.7.1.3(a) applies and perfect form at mmc does not.
2) tcogswell also said up the thread that the 1982 standard applies which I don't have a copy of and am not as familiar with so my comments above are irrelavent in this particular instance.
3) I would turn the problem around to the person that is rejecting the parts and ask him/her what requirement the parts violate.
4) IMHO the parts should be evaluated with restraint simulating their functional restaint.
RE: Flatness in the absence of a GD&T callout.
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems
RE: Flatness in the absence of a GD&T callout.
- I don't know that this clarification is needed, but just in case...
I don't mean to imply that the lack of a restraint note means that a part is rigid, it just means that has been categorized as rigid... That categorization may not reflect reality, and as we will likely agree is pretty common, it may not have been done with awareness of the implications.
- & now to go on a bit more with hope of being more clear:
If we have two categories of "part flexibility" which we call "rigid" and "non-rigid" then there must (or had better) be a way to tell by looking at a drawing which category a given part falls into... So, if a restraint note, then non-rigid... Leading to what I believe is the only logical categorization of a part with no restraint note on the drawing as "rigid".
So, rigid (category) parts are measured in the free state and non-rigid (category)in a condition that is restrained by a fixture. If restraint notes are implemented optimally then parts will be measured in a condition that comes as close as is reasonably possible to modeling their functional state.
Does this help?
Dean
www.d3w-engineering.com
RE: Flatness in the absence of a GD&T callout.
I'll also stir the pot with you're last paragraph -- I don't think you can say that rigid parts = free state and non-rigid parts = restrained. The first two sentences of paragraph 4.20 are clear that all parts are assumed to be checked in the free state unless there is some need to restrain. Later, at the bottom of p. 96, it says that for a non-rigid part (where the dims/tols are met in the free state), it is "usually not necessary to restrain the part unless the effects of subsequent restraining forces on the concerned feature could cause other features of the part to exceed specified limits."
So I think that you're painting too broad of a brush by saying that a non-rigid part equals restrained condition.
At any rate, this is good fodder for poking around in some of those areas of GD&T that aren't discussed very often!
John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems