【转帖】flatness at mmc

yang686526

flatness at mmc?
hello everyone,
i am new on the eng-tips forums.  while i am no expert in applying gd&t, i have taken some courses and use y14.5m standard and other handbooks as reference.
i have recently picked up the "dimensioning and tolerancing handbook" by paul drake... highly recommended!  i am struggling to come to terms with an example given in the book.  on page 5-31, in the gd&t chapter, figure 5-27, the example shows a washer with .032+/-.002 thickness and a flatness callout of 0.020 at mmc located below the thickness spec (not attached to any surface).  i understand what the intent is: allow warpage greater than the thickness tol.  this allows deviation from rule #1, but is obviously more restrictive than specifying perfect form at mmc not reqd.
my understanding is that flatness cannot be used with modifiers, such as a tolerance zone, at lmc, at mmc, etc.  is this a mistake in the book?  should straightness have been specified instead? note that the flatness is not called out for a surface; it is called out for the thickness.
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that example is flat wrong. there are only 2 ways to override rule #1 and that way isn't one of them. i would actually be more inclined to believe that you are actually looking at a straightness callout instead of a flatness callout. straightness is a straight horizontal line, flatness is a parallelogram. are you sure you are looking at a flatness symbol?
powerhound, gdtp t-0419
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mmc cannot be used with flatness in the feature control frame. the book is in error.
mmc is only used on feature of size and not on surfaces.
dave d.
powerhound:
yes, i am looking at a flatness callout.  my eyes aren't great, but they're not that bad!
dingy2:
the flatness callout is not attached to a surface.  it is floating below the thickness dimension/tol, similar to a feature tolerance of position control.
on page 5-15 of the book, table 5-1, a chart is shown with all of the geometric characteristics (flatness, profile, runout, etc) and their attributes, such as lmc/mmc allowed, basic dim reqd, etc.  for flatness, two usage circumstances are shown as being allowed:
1. to control a plane, were you have a leader attached to the surface, no datum references, no lmc/mmc condition allowed.
2. to control a "width-derived median plan", where the control frame can be placed below a feature of size dimension, no datum references, lmc/mmc condition is allowed.
this flatness usage in some ways does make sense, but in looking at other reference books, such as alex krulikowski's "fundamentals of gd&t", flatness at lmc or mmc is illegal.  is strightness the proper control of this case?
since you are looking at a flatness callout, the book is wrong. point number 2 that you quoted from the book is also wrong. you should not recommend this book as you did in your op. flatness should always be attached to a surface and never can a modifier be used on it. you can not apply flatness to a feature of size, only to surface elements.
the only other option i can imagine for such a grossly incorrect section on flatness is that the intended explanation of geometric characteristic was straightness as i alluded to in my first post in this thread.
powerhound, gdtp t-0419
production supervisor
inventor 2008
mastercam x2
smartcam 11.1
ssg, u.s. army
taji, iraq oif ii
that is a "flat" out illegal call out no matter how you slice it.  flatness can not have a modifier, it has to be applied to a planer surface and it has to be a refinement of other tolerances (ie it can not be used to increase tolerance).  i would use a note with a functional requirement like "washer must drop under it's own weight between parallel plates .054" apart".
guys, thanks for all the comments.
i agree that the drake book is clearly wrong.  the erroneous flatness callout should have been a straightness control.  the previously mentioned table 5-15 is clearly also wrong.
after digging deeper, i found figure 3-16 in krulikowski's book that clearly shows using straightness at mmc to control the warpage of a washer-like part.  
this paul drake (never heard of him before) either doesn't know straightness from flatness, or has a bad editor and lousy proof reading. get yourself a copy of asme y14.5m-1994, and a teaching book by someone good, like lowell foster, al neumann, george pruitt, or gary whitmire.  
i'm all for handbooks that help in understanding the standard, but having a copy of the standard is essential for reasons such as this post.  a reading of the standard does not insure understanding, and other guides are very helpful in this regard; however when situations such as this appear, the best solution is to study the related section of the standard to best determine what is indeed correct.
that said, i have yet to find this sort of misleading information in anything written by those mentioned by checkerron.
believe it if you need it or leave it if you dare.fff"> - robert hunter
in accordance with the new proposed 2008 standard, the flatness with a mmc modifier is controlling the flatness of the center plane similar to straightness of axis.
thanks gary, so by the sounds of it while it will most likely be correct in the future it's not correct to the current version.
fteixeira, the text you mention isn't in anticipation of the newer version is it?
kenat, probably the least qualified checker you'll ever meet...