【转帖】flatness in assy

yang686526

flatness in assy
hi everyone,
could you please give me a quick hand. i am checking the drawings of another engineer, and there is something i don't understand.
the basic problem
we are designing a big box from flat 1/2" polypro panels, which get welded together with but joints basic tolerancing for the assembly is +/- .030. on the assembly drawings, there are fcf's [flatness | .10] comming off of every assembly surface with an extention line.
now i am pretty unfamiliar with the flatness feature and it's usage, but from what i have looked up in y14.5
1. it is applied with a leader from the surface
2. it is usually toleranced l.t. the size tolerance on its applicable surface.
i must assume this is on here to control the warping of these 1.2" polypro panels during the welding process, but isn't there a better way to control this.
q1. am i correct that his application is wrong, or should i just shut up.
wes c.
------------------------------
when they broke open molecules, they found they were only stuffed with atoms. but when they broke open atoms, they found them stuffed with explosions...
eng-tips forums is member supported.
1.  attaching it with an extension line is acceptable.
2.  the size tolerance is not related to the flatness tolerance.  the size tolerance locates the plane of the flatness tolerance, while the flatness tolerance determines hoe accurate that plane is.
how's texas treating you?
thanks ewh!
texas is pretty big country. i however, am not staying! i have gotten a nice job offer in atlanta, and i'll be starting out there right after the thanksgiving holiday...
i'll check it out for a few months, and re-eval then...
it seems that all the jobs in texas are contract jobs... and i need at least dental insurance.  
wes c.
------------------------------
when they broke open molecules, they found they were only stuffed with atoms. but when they broke open atoms, they found them stuffed with explosions...
i think you'll like atlanta.  i worked on a contract there for two years and fell in love with the place.  good luck!
size tolerance will also control flatness.  if the size tolerance value is the same or close to the flatness value, no need to add a redundant flatness callout....the end product will be the same.   in addition to form, size tolerances may also control profile, parallelism and orientation.
  wouldn't the size tolerance control just the size?  if you relied on that, you would also need to specify straightness, at least.
  you could call out a tightly conrolled size dimension on a part that is supposed to be slightly "wavey", and it would not force it to be flat.
i would call out the flatness and any other fcf's on the part dwgs. then call out angle tol between parts at the butt joints. use a weld fixture to align the assy.
chris
systems analyst, i.s.
solidworks/pdmworks 05
autocad 06
size tolerances may control straightness (form).  in a case where you would want it "wavy", a tight size tolerance is not desirable.
flatness is on a single surface (plane) and is a unilateral tolerance. if one has an overall dimension to this surface with a tolerance range, the flatness must be inside the tolerance range.
one cannot have an overall tolerance of +/- .030, as an example, and have a flatness tolerance of .075. it would have to be within the size tolerance range other we would have a conflict.
if we took the whole flatness tolerance of .075 we would be outside the size limits - either on the minus or plus.
i would say that most designers do not really look at this but i could be wrong on this.
hope this helps.
dd
dingy,
you have the concept correct.  the rule of thumb i follow is to apply form, orientation, and profile controls to components or assemblies that would prohibit the use of tight size tolerances such as seperable and non-seperable assemblies.   if you're working on parts with tight size tolerances, you may want to reconsider the use of additional callouts as they may be redundant.
gd & t should never be placed on a drawing unless there is a definite function or relationship that is needed. no function and relationship - no gd & t!!!!
if one has a size tolerance, it is only checked or confirmed in a couple of spots at best using an appropriate measuring instrument. the surface is not scanned. if that surface must contain a flatness tolerance because of its function, then the shop floor people in quality and processing are aware of the  situation and will scan the area for flatness.
dd