Design And Dimensions Assignment

QUESTION:

During the Detailed design stage, when dimensioning the parts, does it matter which dimensions will be given on the detailed drawing?

This algorithm is applied for axial dimensioning of the shaft from the figure above.

These dimensions are sufficient for the functional dimensioning of the shaft. The dimension Az is given in brackets meaning for information, but can be omitted. As it is seen from this dimension, only the connecting surfaces are not given with their own lengths.

When dimension using the above algorithm, there is a hesitation which dimension to be given, the best criteria would be the OCs theory. It should be given the dimension which has smaller tolerance calculated according to the above described methods.

This method for linear dimensioning of the parts assigns dimensions based on the functionality of the part and it is expected that if two parts have the same geometry but different functions in two different products, they will be dimensioned differently. The method guarantees the biggest possible tolerances for the assigned dimensions, which will reflect in the cost of the manufacturing of those parts.

Dimensioning of the parts according to the above algorithm also guarantees better and cheaper manufacturing not only because of the biggest tolerances assigned to the dimensions but also because it gives much less problems associated with stack up tolerances and process planning which will be demonstrated in the next chapter.

Based on the OCs theory now we can extract rules for classification of the surfaces and dimensioning of the parts.

Analysis of different products and systems show that according to its function, parts surfaces can be classified as follows

1. Main Datums (MD)-number of surfaces which determine the position of the part in the product or system:
2. Auxilary Datums (AD) -number of surfaces which determine the position of other parts according to the regarded part
3. Working surfaces (WS)-number of surfaces through which the regarded part does its function
4. Connecting surfaces (CS)-number of surfaces which connect the first 3 types of surfaces, ensuring the needed distances and rigidity of the part

On the figure below is given an example of the above classification

1. Determine the design datum off the product or until in the selected direction (the 13 14 15 16 17 18 1 Screensh products are 301)
2. Clarified the role of each part in the designed product or system
3. Select the Main Datum of the part in this direction.
4. Give dimensions from the MD to the ADS
5. WS are dimensioned to the closest MD or AD
6. Give dimensions to the own lengths of the MDs
7. Give dimensions to the own lengths of the ADS
8. Give dimensions to the own lengths of the WSS

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