Flatness in the absence of a GD&T callout.

[tcogswell]

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?

 

[dingy2]

J-P

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.

http://www.qmsi.ca

 

[caseynick]

The 1994 standard has basically the same fundamental rule that Dingy quoted from the 2009 standard. 1994 1.4(l) "All dimensions and tolerances apply in the freestate condition......." The exceptions to the freestate rule in the the 1994 standard leave a little more room for interpretation than the 2009 standard. Whereas 4.20 of the 2009 standard reiterates the freestate rule and goes on to talk about restraint and 5.5 is about the freestate symbol the excpetions in 1994 are 2.7.1.3(a)stock..... and "(b) parts subject to freestate variation in the unrestrained condition". So according to the 1994 standard "All dimensions and tolerances apply in the freestate condition" except for "parts subject to freestate variation".?

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.

 

[Belanger]

Dave, that is exactly what I said. Free state is the default, unless there is a need to restrain it (such as a floppy part, etc.). But the discussion here is whether the lack of a restraint note automatically means that the part is rigid. I'm saying that I don't think we can read that into the standard.

John-Paul Belanger
Certified Sr. GD&T Professional
Geometric Learning Systems

 

[DeanD3W]

Hi John-Paul,
- 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

http://www.d3w-engineering.com

 

[Belanger]

Dean, I do see what you're saying, but I still hesitate to superimpose such an assumption onto the Y14.5 standard. I'm not trying to be difficult, but I will noodle about it some more.

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