Why and how basic dimensions?

[fiambre]

thread1103-295930

Sorry for the long text, but I had this discussion several times before and never reached a conclusion.

I´ve seen the answers from the aforementioned thread, and I´m still not convinced they´re always necessary. Also, sometimes it is impossible to be written in a drawing.

Let´s me explain why. Most of the time (can´t think of an exception to this now) position of the tolerance zone can be unambiguously recovered from 3D model. With tolerance zone orientation this is the most usual case also. Yes, sometimes, it´s really necessary to inform the right orientation of the tolerance zone.

Sure, most of the time they´re desirable to make the work of control people easier. That said it leaves me to another question. How to put basic dimensions?

I´ve seen two schools.
a) Just put enough basic dimensions to position and orient the tolerance zone in reference to the datums (This is what I use to do). Imaging a simple rectangular plate with 8 holes whose location related to the lateral faces are being specified I will just put a basic dimensions of one hole to the datums and then put basic dimensions between holes, seems to be easy to read and no ambiguity.
b) In the example above, put a basic dimension between each hole and the datums, do not put basic dimensions between holes as there are no tolerance specified between them. For polar coordinates, put the basic dimensions in polar coordinates, do not use rectangular notation. This is for sure correct, but usually results in a difficult to read drawing. It can maybe please control guys as no additions and/or subtractions are needed.

I almost forgot, for complex profiles there´s no way to put "dimensions" to inform his form. Have never see a problem to not inform the basic dimension in this case.

I can be completely wrong, but maybe basic dimensions are just something required by the standard.

Note: I have no idea of ASME standard, was always using ISO ones, and not the latest releases. And also, NEVER put title block +/- tolerances.

 

[fsincox]

A basic dimension really just represents the concept of what dimensions always were before the time someone invented the system of implied tolerances, just the ideal size or location on the drawing. The ISO system of fits still refers to them that way as the "basic size", before you assign the “fit” to it (like, say H7/g6). If you have no title block tolerances and do not reference some standard that ‘implies” tolerances to your numbers your dimensions are basic, they do not need a box around them.
Frank

 

[drawoh]

fiambre,

A basic dimension shows the exact theoretical size, profile, location and/or orientation of your component. The assumption is that you will add a feature control frame to specify allowable variation of the feature.

A more fundamental assumption is that you are specifying allowable variation of every feature of your part. A dimension either is basic, or it has a ±[ ]tolerance.

JHG

 

[KENAT]

fiambre, sounds like you're talking about Model Based Dimensioning with a minimal drawing to ensure all tolerances are communicated.

Am I correct in this assumption?

If so then I'd say putting basic dims on the drawing to clarify how the position/profile tolerance is applied is appropriate. Arguably you could use reference dimensions but so long as the drawing is model driven (so will update with the model) in this case you get effectively the same result.

A. or B. is correct, it really makes no difference, so long as any 'chained' basic dimensions eventually lead back to datums. If checking mating hole patterns then dimensioning between features can be useful. However for inspection or machining the other approach can save some math.

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