【转帖】profile of surface vs profile of line

yang686526

profile of surface vs profile of line
hello, i'm a cad guy learning gd&t so you may see me on here quite frequently as i run into roadblocks.  and yes, my company does have its eyes open for training.
i've searched through past threads and found some information, but some of it seems to contradict what i'm reading in a book (dimensioning & tolerancing handbook by paul drake jr.) under the profile of a line tolerance section, the book says "as the entire feature surface is swept by the tolerance zone plane, its intersection with the plane shall everywhere be contained within the tolerance zone."
so if the profile of a line tolerance is swept over the surface, doesn't that make it the same as profile of a surface?  
profile of a line applies to the whole surface at each individual cross section of the surface. the highest and lowest points within each cross section determine if the part is in tolerance. whereas profile of a surface applies to the surface as a whole. the highest and lowest points at any given spot of the whole surface determine if the part is in tolerance.
i'm sure someone else can explain it better.
david
the basic concept of profile of a surface and profile of a line are similar. the primary difference is that the tolerance zone for profile of a surface is 3-dimensional, and the tolerance zone for profile of a line is 2-dimensional, the tolerance zone for a profile of a line control is a set of lines a uniform distance apart
since the profile of a line tolerance zone applies to each line element individually, so profile of a line provides a limited coverage of a surface, it is usually applied in addition to other dimensions and tolerances that control the surface.
yes, someone else will give more in details
seasonlee
thanks, that makes sense.
how would you control the warping/bowing of a surface? from what i understand the straightness, flatness and profile tolerances adjust to the surface as they sweep across, so the tolerance zone controls the surface undulations, but not the warping of the surface as a whole. would referencing a profile of surface to a drf take care of this? since then the tolerance zone would be locked in place, so the surface couldn't warp out of it?
a profile control can be used to control size, location, orientation and form, warping and bowing is a form control.
one unique aspect of profile control is they can be used with datum feature references as a related feature control. profile controls can also be used without datum feature reference as a form control. when no datum features are referenced, the tolerance zone applies where the part surface actually exists.
if the form is critical to you, then you may use multiple single segment profile control to refine the size and form.
seasonlee

straightness, flatness, and profile do not adjust to the surface as they sweep across. flatness is the 3d version of straightness. if you have a flatness tolerance of .010 then your tolerance zone is 2 perfectly parallel planes spaced .010 apart. every element of the specified surface must lie within that zone and if it doesn't, it is out of tolerance. flatness absolutely controls warping of a surface.   
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the way profile of a line is typically used in industry, it often does equate to profile of a surface and so your interpretation of paul drake's explanationn is correct. and, of course, so is paul!
let me give you an example of how you might use profile of a line as a refinement of profile of a surface.
consider a wing. a big wing.  for the entire surface of a wing related a drf there may be a some reasonable value that establishes a 3d total width zone from wing root to wing tip. however, for cross section of the airfoil itself at any cross section along the wing, the design may require, and often does, a tighter form tolerance which can be accomplished with the profile of a line. thus, a two single segment profile control.
hope this helps.
norm crawford
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applied geometrics, inc.