Couple Flatness Questions
Couple Flatness Questions
(OP)
I searched through the forums, but didn't find all of the information I was looking for.
For the sake of the following questions, assume a cimple rectangular part 1.000" high with a size tolerance of ±.005".
Why would one want to apply a flatness tolerance as a refinement to the size tolerance when you could simply reduce the size tolerance? If I have a flatness requirement of .003" on a surface with a size tolerance of .005", why wouldn't I want to just change that size tolerance to .003"?
When a part is at MMC, the flatness tolerance is .000" (per rule #1)? Is that in both directions? Or would the flatness tolerance now be +.000 / -.003?
The GD&T book I'm reading now says that a FCF with flatness per unit (t.g. 0.003" IN/IN) is a refinement of a non-unit flatness callout. Does this mean, I must include the overall flatness callout in addition to any unit flatness? Or can my FCF simply show the per unit requirement?
Thanks for the help...
For the sake of the following questions, assume a cimple rectangular part 1.000" high with a size tolerance of ±.005".
Why would one want to apply a flatness tolerance as a refinement to the size tolerance when you could simply reduce the size tolerance? If I have a flatness requirement of .003" on a surface with a size tolerance of .005", why wouldn't I want to just change that size tolerance to .003"?
When a part is at MMC, the flatness tolerance is .000" (per rule #1)? Is that in both directions? Or would the flatness tolerance now be +.000 / -.003?
The GD&T book I'm reading now says that a FCF with flatness per unit (t.g. 0.003" IN/IN) is a refinement of a non-unit flatness callout. Does this mean, I must include the overall flatness callout in addition to any unit flatness? Or can my FCF simply show the per unit requirement?
Thanks for the help...





RE: Couple Flatness Questions
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Couple Flatness Questions
We could, in theory have a part that is made to 1.005 and from that surface a .003" flatness tolerance (in one direction only) would apply? Or would it be .0015"?
RE: Couple Flatness Questions
How confused am I?
RE: Couple Flatness Questions
Flatness tolerance is in itself independant of size tolerance. Flatness is essentially comparing the deviation of the surfact to its self. Size tolerance is comparing it to some other point/surface/datum.
Take a look in the relevant standard.
I think maybe you're starting to think about what would more properly be parallelism of one surface to another.
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Couple Flatness Questions
Anyone have any simple way of explaining that?
Thanks Kenat for your help... I'll see what I can glean from ASME Y14.5.
RE: Couple Flatness Questions
RE: Couple Flatness Questions
6.5.2.1 lsat para "Where the considered surface is associated with a size dimension, tha flatness tolerance must be less than the size tolerance."
I'm pretty sure I've done a lousy job trying to help so see if any of the this is clearer.
thread1103-204167: Straightness vs flatness
thread1103-211411: Flatness at MMC?
http://www.tec-ease.com/tips/july-99.htm
http://
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What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Couple Flatness Questions
The worst case with your part is that it can be .995" high at one end and 1.005" high at the opposite end. When you fixture the flat face perpendicular to a gauge, the height variation will not exceed .003". Note how the face is not required to be parallel.
A machinist probably will just machine it carefully, but who knows. There could be a bonus allowance in there somewhere.
RE: Couple Flatness Questions
One more question came up today about flatness... We usually specify our flatness tolerances per unit length. Well, that's what people here think anyway.
I was unable to find anything in Y14.5 specifying any unit other than area when using flatness... Is anyone aware whether or not a previous version of Y14.5 allowed flatness per unit length?
Is a flatness tolerance of .003 / 1 possible? Or must it be .003 / 1 x 1?
Lastly, when specifying that unit area, I'm guessing that any trailing zero's are not needed. So, in the above example, it's .003 / 1 and not .003 / 1.000 x 1.000. Correct?
RE: Couple Flatness Questions
If it was "per inch", then where would be the origin from which the surface can deviate from the stated geometric tolerance? Which direction would deviation be allow?
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Couple Flatness Questions
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Couple Flatness Questions
SeasonLee
RE: Couple Flatness Questions
Matt Lorono
CAD Engineer/ECN Analyst
Silicon Valley, CA
Lorono's SolidWorks Resources
Co-moderator of Solidworks Yahoo! Group
and Mechnical.Engineering Yahoo! Group
RE: Couple Flatness Questions
I haven't seen the '84 spec though, so I can't say for sure.
Thanks everyone for your help... I think I'm a little better prepared to offer a solution than I was before.
RE: Couple Flatness Questions
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