Baseline or chain dimensioning (Para 5.2.2 from 1994 removed in 2009)

[vnangli]

I am glad to be posting my first thread, I work in medical devices industry. I request your help for my question listed below.

In 1994 standard, according to para 5.2.2 (page 81) if dimensioning in basic, there will be no difference between chain dimensioning or base line as long as the toleranced features had identical position tolerances.
*This para has been removed in 2009. Does it now mean the chain dimensioning can be replaced with base line dimensioning and vice versa irrespective of identical or unidentical positional tolerances between features as long as the features have the same DRF?
As an example, in 2009 standard refer page 64 and fig 4-19a, hole with dia 9.2 is positioned with refernce to datums A,B,C but is nowhere dimensioned directly from either B or C. According to 1994, you cant add 8.3 and 8.7 as the pattern of 4 holes and 9.2 dia hole dont have identical tolerance...
Thanks in anticipation
Vj

[Kelly_Bramble]

A basic dimension is a basic dimension regardless of how or what acceptable dimensioning style one chooses to use, tolerances are contained within the FCF or other approved method and the tolerances are not cumulative.

Figure 4-19a, the feature you are referring to is dimensionally (basic) tried back to datum's B and C via the center/extension lines. Technically there is a basic Zero dimension, however it is not shown where instead the center/extension lines are used to indicate zero distance relative to those datum features.

See: 2.1.1.4, page 24 and 1.4 (k), page 8 of the ASME Y14.5-2009 standard.

I am glad to be posting my first thread, I work in medical devices industry. I request your help for my question listed below.

In 1994 standard, according to para 5.2.2 (page 81) if dimensioning in basic, there will be no difference between chain dimensioning or base line as long as the toleranced features had identical position tolerances.
*This para has been removed in 2009. Does it now mean the chain dimensioning can be replaced with base line dimensioning and vice versa irrespective of identical or unidentical positional tolerances between features as long as the features have the same DRF?
As an example, in 2009 standard refer page 64 and fig 4-19a, hole with dia 9.2 is positioned with refernce to datums A,B,C but is nowhere dimensioned directly from either B or C. According to 1994, you cant add 8.3 and 8.7 as the pattern of 4 holes and 9.2 dia hole dont have identical tolerance...
Thanks in anticipation
Vj

[vnangli]

I sincerely thank you for your response, but.....I have a couple more questions

I agree with you, the tolerances are not cumulative. But the resultant tolerance between any two features in an array is dependent on the scheme of dimensioning (be it basic or +/- tolerancing, in basic the resultant tolerance will be dependent on the geometric tolerance for the respective features).
That is where 1994 standard, para 5.2.2 clearly specifies when dimensioning an array of features there is no difference in dimensioning either by chain (in basic) or baseline (in basic), the resultant tolerance will remain unaffected as long as each feature has same positional tolerance. Now this paragraph is completely removed in 2009 standard, I want to understand the implication...

As far as the feature I have refered in 4-19a (2009 std), the 9.2 dia hole is not dimensioned directly to datum "B", the dimension will be sum of 8.3 and 8.7 (this addition was not allowed in 1994 as the 9.2 hole is dimensioned to B with 4 hole array in between with a different tolerance).

Thanks
Vj

[Kelly_Bramble]

As far as the feature I have refered in 4-19a (2009 std), the 9.2 dia hole is not dimensioned directly to datum "B", the dimension will be sum of 8.3 and 8.7 (this addition was not allowed in 1994 as the 9.2 hole is dimensioned to B with 4 hole array in between with a different tolerance).
Vj

Nothing has changed in regards to the interpretation of distance or orientation as given by Basic Dimensions relative to specified static datum’s. The distance between datum B and the tolerance boundary (dia 0,26 at MMC) within which the as-built axis of the dia. 9.2 hole is 8.7 + 8.3 in ALL GD&T standards in existence.

The tolerance boundaries (zones) are statically located via basic dimensions relative to specified datums.

Now, the translation datum as applied to datum C is a new concept introduced in 2009 standard, however this has no effect on the distance between datum B and the 9.2 hole feature.

[vnangli]

Thank you so much, Yes I had to visit 2009 standard..I am now convinced with your explanation. I have the "basics" straightened up..

I believe I can post my questions about 2009 standard from now on..

Thanks
Vj

[Kelly_Bramble]

vnangli,

Just to followup - see para 2.6, page 27 of the 2009 standard.

see the text following "NOTE"...

[dahsr]

Regarding illustration 4-19(a), the distance from the FCF hole is 8.3 to the location specified, as defined between the dimension arrows, correct? So, if I set my hole feature of the FCF hole as "zero", I would measure the distance to the dimension specified - to the location specified between the dimension arrows of 8.3. Am I missing something? Am I adding anything?