【转帖】assumed straigtness or flatness

yang686526

assumed straigtness or flatness
hello, i'm having a debate with others to interpret the assumed flatness or straightness on a thickness or "bow" callout. the spec is ansi 14.5m-1982. the thickness callout is .2787-.2745. do i get rfs and the measured "bow" or do i have to place the part down and the entire form has to fit within the maximum of the tolerance. thanks, rich
i'm not clear what you mean "bow" callout.
is it a flat part, or curved into a "bow" shape?
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
yes it has a .002 curve along length of part
if all you have to go on is a thickness dimension, i don't think you can assume anything about flatness or straightness without a specific callout, especially if the measured thickness including "bow" meets the dimension.
people are telling me that my flatness is assumed to be the total tolerance zone eg. .2787-.2745= .0042. that would be fine, even though i don't know if that is even correct, but they take it a little farther and state that if i run at the high end of the tolerance i have .0000 allowable flatness tolerance and on the other hand if i run at the low end of the tolerance i get the full .0042.
without a specific callout, i'd have to agree with them.  all you have to go on is your dimension and its tolerance.
people are telling me that the assumed flatness callout would br the total tolerance of the thickness callout eg. .2787-.2745=.0042. even if i agreed with them about that, which i don't, they take it another step and say that if i run to the max of my tolerance (.2787) i will have .0000 tolerance for flatness. and on the other hand if i run at the low end i will have .0042 tolerance for flatness. i have interpretation of geometrical dimensioning and tolerancing by daniel e puncochar and i don't see anything about it in there.
creech,
   i brought that specific question up when i took my gd&t course.  your assumption makes a lot of sense, but it is wrong as per asme y14.5m-1994.  i am not 100% certain about the 1982 version of the specification.
   according to asme y14.5m-1994, the thickness specification is the thickness at the point measured.  it has nothing to do with flatness.  if you want flatness, you must specify it.
                      jhg
i agree, but from a practical standpoint it may not be measured at two opposing points, but from a flat surface upon which the part lies.
i agree with the others.
i suggest upgrading to asme y14.5m-1994.
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
i think i found it.
  for anyone who is interested. in modern geometrical dimensioning and tolerancing by lowell w foster along with the national tooling and machining association or ntma, the following was stated:
  rule 1 - limits of size rule. where only a tolerance of size is specified, the limits of size of the individual feature prescribe the extent to which variations in its geometric form as well as size are allowed.
   it continues later in the same rule: the surface, or surfaces, of a feature shall not extend beyond a boudary (envelope) of perfect form at mmc. this boundary is the true geometric for represented by the drawing. no variation is permitted if the feature is produced at its mmc limit of size.

chris we're a machining job shop and the prints states the specification. we cannot change it unfortunatly
thank you all