【转帖】model vs drawing tolerances 9y14.41 0 notes

yang686526

model vs drawing tolerances (y14.41 ?) & notes.
we are using models as the "master data", yet have drawings to clarify what is not on the model. my issue is with tolerances. typically untoleranced dimesions on the drawing default to the title block tolerances (.xx, .xxx). however, when using the model as the data, how are the tolerances handled. i had placed a note saying that any model measurement shall be considered a two place decimal per the drawing title block. i also have a note that states that the (hard) dimensions on the drawing superceed the model data, so that there is no mis-interpretation between the model and the drawing. is this the correct way to pursue this? since i have up to now provided complete details on the drawing i did not have to worry about this. where i'm at now, they have been mostly involved with prototyping, and have been able to get by without this kind of control. but they are now trying to apply the same method to high production, which makes me nervous.
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i think you need to invest in asme y14.41-2003, digital product definition data practices.
this defines how gd&t are applied to 3d models.
art without engineering is dreaming; engineering without art is calculating.
steven k. roberts, technomad
i'm planning on ordering it. however, the customer is reluctant in even using gd&t (the barbarian factor) or i'd use a surface profile tolerance on all untoleranced values. but i'm still going to give it a try...
i use the tolerance within the model the same as what is on the dwg. the problem is when someone else changes the tol on the dwg and not the model.
i do my best to make all dim's 3 place decimal nom, then add tol where needed.
chris
systems analyst, i.s.
solidworks 06 4.1/pdmworks 06
autocad 06
if your customer is providing the models and the drawings, they should determine which is law, not you.  you would hope that the models are created at the mid-range of all tolerances, but we all know that hardly ever happens.
art without engineering is dreaming; engineering without art is calculating.
steven k. roberts, technomad
you have to think differently about dimensioning a model to 2d drawings. say you have a surface made up of 300 parts how can you possibly dimension them all in 2d, especially if most and not rads or planes?
you have to use surface and trimline tolerance and just add positional tolerance to key features like hole positions.
i think that there would be less of a chance of misinterpretation if you made the model the master definition instead of letting on hard drawing dimensions supercede the model.
ptmalm,
   this is the same problem you get when you have fabrication drawings with both metric and english dimensions.  you have two sources of information, one of which is correct, and one of which you provide entirely out of the goodness of your heart, for reference only.  there is no other way, other than not providing the second set of information.  
   your master data must include a complete set of dimensions and tolerances, somehow.  if you cannot implement asme y14.41-2003, then you must prepare complete 2d drawings, and provide the 3d model for reference.
   i used to work as an inspector in the automotic parts industry.  i never saw a drawing.  as far as i can recall, the customers provided inspection fixtures.  maybe this is your solution.
                    jhg
ewh, the issue with the drawing superceeding the model would be (for example) in the case where the part must have a tighter tolerance than standard, or must be a plus zero, minus something for example. since i am not allowed to go "all model" and utilize the y14.41 capabilities of sw, i needed some way to "over-ride" the model.
ptmalm that is exactly how all the major automotive manufacturers work for panels. the model will have a general trim and surface tolerance on it and certain areas will be shown as say a unilateral tolerance and most holes and slots unless for weight reduction or assembly access will have a positional and size tolerance unique to them.
it is also common to show the areas where the part should be clamped for inspection and the datums.
as ajack1 says this is how automotive does it and has done it for a long time.  we were creating minimum content drawings with the math data as the master back in 1998 when i last worked for the gm truck group.  doing details for body panels on maybe a dozen 11 x 17 sheets.
i can't wait for the day that the business i am in now, tool and die, can fully utilize minimum contest drawings.  we have all the equipment to get'er done that way.  only thing standing in the way is the mindset of the people who only know one way of getting things done.
regards,
anna wood
a former automotive sheet metal and interior trim senior designer, cgs jockey, philpot and desmet grad.