【转帖】dual dimensioning standards

yang686526

dual dimensioning standards
ok, here we go.....we are having a major battle here on how to go about a standard of dual dimensioning.  previously we have always dimensioned in english units only but we have decided to add dual (in/mm) dimensions to all of our drawings.  our biggest disagreement is with the conversion of the inches dimension to the "equivalent" millimeters dimension with accuracy.  when we were using only inches we had a standard of the following:  .xxx ± .005, .xx ± .020, and .x ± .030.  is there an "accepted" standard for dual dimensioning while not sacrificing accuracy in the conversion process?  should the metric tolerance hold the same amount of decimal places as the english?  it seems like we are running into the "round up or round down" when it comes to the metric tolerance.  if rounding is done it sacrifices accuracy.  we need to keep the english dimension the "master" dimension and the metric would be the "slave".  thanks for any suggestions.
cygnas
when it comes to protecting yourself or your family it is better to be judged by 12 than carried by 6.
check out our whitepaper library.
the secondary dimension is for reference only.  whoever makes the part is accountable to manufacture to the primary dimension units, regardless of what the secondary says.
why do you feel that the precision (number of decimal places) of a dimension on a drawing has any relation to the acceptable tolerance for that dimension?
mintjulep,
one of the first rules of drafting (using inches) is that the precision of the dimension determines the tolerance as noted on the drawing.  maybe i don't understand your comment.
mintjulep
as far as i know the number of decimal places has inportance and 0.020 is not the same as 0.02. the explanation is that the dimension 0.020 should be checked with a measuring tool of 0.0001 accuracy reading while the 0.02 may be checked with 0.001 tool accuracy reading. meaning, the measuring tool accuracy reading of the 0.020 should be ten times more accurate than the measuring tool for 0.02.
to my opinion this is also the answer to cygnas for the equivalent metric dimension. the number of decimal places on the secondary metric dimension should be such that it will require/force that the measuring tool accuracy will stay the same.
i agree with above answers.
mintjulep, it does matter. would you dim a dwg with .02 or .0200 and the machinist assume it means the same tolerance?
metric dims are more accurate in ref to inches. therefore should not show the same decimal places when on the same dwg. although one is ref to the other as thetick states.
chris
sr. mechanical designer, cad
solidworks 05 sp2.0 / pdmworks 05
if y14.5 is applicable, you need to read par. 2.4.
it should clarify.
as ringman suggested, i read ansi y14.5, paragraph 2.4, which is titled interpretation of limits.  it specifies that tolerance limits are absolute and are assumed to have infinite trailing zeros.  it gives for example, 12.2 is the same as 12.200000, 12.01 is the same as 12.010000000000.
keep in mind though that this is describing the tolerance limits, not the applicable tolerance based on the number of digits (trailing zeros) in the dimension itself.  and i believe the discussion was talking about the dimension itself with respect to trailing zeros as digits, am i right?
a dimension of 12.2 may have different limits than a dimension of 12.200 (and these limits are typically established based on digits after the decimal and defined on the drawing), but a tolerance limit that is stated as 12.2 is assumed to be 12.20000000-->infinity.
and yes my handle says electrical after it so i am asking, not criticizing.  
debodine and all,
   most drawings have a note assigning tolerances to dimensions based on the number of trailing decimals.  if the number of trailing decimals corresponds to an entry in the tolerance note, then it has a tolerance.  this procedure is noted in asme y14.5m-1994.
   obviously, this all assmes the absence of any other tolerance specification, including basic dimensions.  
                   jhg
isrealkk,
agreed on the precision of measurement.  however, precision of measurement is still different from acceptable deveation on a part.  tolerances are not logically related to the number for decimal places in the nominal dimension.  it is simply lazy "standard practice" to relate tolerance to number of decimal places in a drawing title block.
consider a part where the nominal dimension is 4 13/64 (perhaps and old part that hasn't been made in a long time.  dust off the old hand-drawn print and convert it to cad).  4 16/64 gets converted to 4.203125, and depending on the settings of the cad program, and the diligence of the drafter and checker (if one exists) gets rounded of to 4.203, or 4.20, or just 4.2, or maybe 4.20313.  what is the proper tolerance for that dimension?
you cannot cause a non-conforming part to become conforming by measuring it with a more precise instrument.  if i have a part with a feature that needs to be 2 +/- .001 and a part where that feature measures 2.125 i can tell that the part is no good with a simple ruler, even though the precision of that ruler may be no better than 1/8.  i could measure the part with a more precise tool, and learn that the feature is 2.12408, but the part is still no good.
tolerances are linked to number of decimal places only when explicitly stated as such in the title block or a note.  there is no such thing as a default tolerance.