【转帖】ordinate dimensions to define gdt datum structure

yang686526

ordinate dimensions to define gdt datum structure?
folks,
i am new to this forum, but am hoping someone out there can help answer a debate i am having over gdt datum structures. i claim that the basic dimensions used to define a datum structure must not and cannot be ordinate-style dimensions (ie - the kind that have a 0-0 line predifined).
in our case, we need to define the dimensions from the feature of size to a "centerline" that is to be used as the 0-0 for ordinate dimensions.
the cl goes thru (but perp to) the cl of a bore (datum f). the other defining feature for the cl is an offset from an edge (datum b). i maintain that this offset must be defined by a coordinate style basic (ie- box with two arrows), and not an ordinate style basic (ie - leader line from an edge and a box), since one is defining the offset from the fos to the 0-0 cl  and not specifying the distance from the 0-0 cl to the edge. in addition, this basic is essentially  defining where b is relative to f in space.
i cannot find anything in y14.5m-1994 that covers this. ideas?
thanks
dave
adm44,
   a datum must be a real feature to which you can attach fixtures and measuring instruments.  the correct answer to your question will be whatever allows you to inspect the thing.
   i am trying to visualize your drawing.  you have not mentioned a primary datum.  i am assuming you have one.  
   my guess is that your centre plane (secondary datum) passes through the centre of your bore, and is parallel to your edge, datum_b, your tertiary datum.  if i am right about this, you need a fourth datum somewhere.
   try this datum scheme.
datum a  whatever surface is perpendicular to your bore.  locates three points in a plane.
datum b  your bore.  locates x and y.
datum c  your edge.  locates in rotation.
   the optimal dimensioning scheme probably is to locate everything from the bore.  
   alternately, you can read up on datum targets.  this may be a better solution if your fixturing is weird.  you can even fabricate features in your part to be used for fixturing.  this is done a lot with castings.
                   jhg
hi drawoh,
thanks for the note
i did not mention that yes, we have a primary datum "-a-" that as you suggest, is perp to the bore. so your scheme is in fact what we use. but in doing so, you still need to define basic dimensions between the datums (or datum targets, as the case may be) to set up the datum structure.  due to corporate naming conventions, the structure is
-z - primary (part bottom)
-f - secondary (part bore)
-b - tertiary ( part edge)
the "4rth" datum is the 0-0 cl, but is not called a datum (which isn't allowed). but this cl needs to be defined relative to the above datum structure, and using an ordinate dimension scheme to do that sorta puts the car before the horse in my opinion. problem is that there are very few examples of drawings with gdt and ordinate dimensions
dave
hey dave,
     i would have to agree with you, although i can not find anything in y14.5m-1994 that backs it up. the datum is already established, the edge, you are now defining the "perfect offset" from the edge, -b-, to the fos.
mark
adm44,
   there is no rule that says you must have no more and no less than three datums.  the rule is that you must locate in six dimensions.  you must use enough datums to do this.
   if your bore locates in two dimensions as i described, then the centre plane you are talking about is not important, and you do not need to dimension to it.  dimension your features from the bore.
   if your bore locates only in one dimension parallel to your tertiary datum edge, then you need a fourth datum.  i strongly suggest not doing this.  you could make your edge the secondary datum, and the hole your tertiary datum, but this is still bad practise.
   if you use datum targets, then your dimensions should come off the features defined by the datum targets.  presumably, your bore would not be a datum, and you would show dimensions to it.
   i am not sure i understand your objections to an ordinate dimensioning scheme.  if you have a lot of dimensions coming from one feature, ordinate dimensioning is readable.  if i were doing your drawing, my ordinate zero would be the bore, my datum_b.
                               jhg
jhg
ah- you are correct, the axis of the bore is the zero cl. but, i still need to clock the 0-0 cl. imagine a cam with a hole in its middle. i have one point on one lobe of the cam as my tertiary datum. if i am making the inspection fixture, i need linear-style basics from the center of the bore to the contact point.
now, if i want to use ordinate dims for the rest of the part, i have to create a 0-0 lines. one could make them horizontal and vertical in the view that defines the tertiary datum with no other dimensions, because the basics that define the location of the tertiary will also define the 0-0 cl.
but - if i start with the 0-0 cl and define the location of the tertiary with a basic ordinate dimension, i think that is breaking some standard. that is my question.
it's hard to show without a whiteboard.....
dave
i believe you might want to reconsider and make the bore the primary datum feature rather than the secondary.  the base would then become the secondary.  i believe this would better satisfy your requirements.
the datums should be determined also how they mate with mating part(s).
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05
folks
the question is not what datum structure to use. that is functional and works fine. i am asking a more subtle question regarding dwg standards and how the datum structure is called out on the dwgs. i am asking if its ok to use an ordinate dimension scheme to define the datum structure, or do standards require a linear dimension scheme.
dave
adm44,
   dimensions must come from real features.  your o-o centreline is an imaginary feature.  you cannot fixture to it.  you cannot contact it with a measuring instrument.
   if you draw two sets of dimensions orthogonal to each other, asme y14.5m-1994 states that they _are_ orthogonal to each other.  
   the tertiary datum you are talking about is a datum target.  since it is tertiary, you need only one.  you need to provide a dimension to it.  in asme y14.5m-1994, most of the examples show basic dimensions used to do this, which means that "...established tooling and gauging tolerances apply.".  you may apply a tolerance to the datum target.  you may define the datum target as an area, rather than as a point.
   read the section on datum targets.  try sketching out an inspection fixture.  this might help you visualize your problem.
                            jhg
sorry, i thought drawoh answered your question.
chris
sr. mechanical designer, cad
solidworks 05 sp3.1 / pdmworks 05