【转帖】ordinate dimensioning - when to use

yang686526

ordinate dimensioning - when to use
when is ordinate dimensioning appropriate and when is it inappropriate?
my initial throughts are:
appropriate use:
-flat plate
-single horizontal origin and single vertical origin
-?
innapropriate use:
-hole patterns
-?
please offer your thoughts.
don
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i would think it proper to use when there is a quantity of holes in a plate which would just add confusion if dimensioned otherwise.
some people will design a hole pattern with coordinates. i think it is mostly to help the shop, depending on the equipment they use. my preference is to design the hole pattern as a unit, but let its location on the part float a little. for example, an opening with a cover on the side of a casting could move around a quarter inch or so without any harm, but the bolt locations have to be held tight so that it goes together.
i think that ordinate dimensions are great for hole patterns.  it keeps your drawing from getting cluttered.
gk
as engjw states, the hole pattern as a unit is important, and can be controlled with gd&t while being ordinately dimensioned.
ewh,  
i believe that method of dimensioning is no longer allowed by y14.5.  i believe that it went out many years ago. it is replaced by composite positional tolerancing.
ringman,
i admit that it's been awhile since i've used ordinate dimensioning, but it is still allowed per asme y14.5-1994.
while i could not find any examples of mixing gd&t with ordinate dimensioning, the standard does state (para 1.9)"rectangular coordinate dimensions locate features... from a datum or an origin."  if the dimensions are basic and the datums defined, this should be an allowable method of dimensioning.
i do agree however that composite positional tolerancing is the preferred method for this type of control.
i use ordinate dimensions for custom hydraulic manifolds.  knowing how the manifolds are made in-house, on each view i ordinate dimension from one corner, so the origin moves from corner to corner as the block is rotated as it is machined.
i think there is a world market for maybe five computers.
thomas watson, chairman of ibm, 1943.
ewh,
if you use datums and basic dimensions, the positional allowance has to be specified for the allowable variation.
that is where the composite tolerancing comes into play.  it was formerly called 'pltz' and 'frtz'. pattern locating tolerance zone and feature relating tolerance zone.
ringman,
it is a given that the positional tolerance has to be specified.  and i agree that composite tolerancing would be the preferred way to go.  i was just pointing out that ordinate dimensioning is still a viable alternative, not necessarily better.
i respectfully disagree that it is a viable alternative to mix +/- ordinate dimensions with basic dimensions for locating a pattern of holes and be in compliance with y14.5.