Hi everybody, i am here again to seek your quidance and knowledge.

I uploaded a png file i made in paint( i didn't have any other program to use so i hope this is fine )

Firstly, i did search about my question regarding that clarification from forum & i didn't find a match so i made a new topic.

Now, the main reason why i am here is that i would like to have a straightforward answer that what i read & understand is correct.

The issue i am having is do i understand Primary, Secondary & Tertiary datums correctly. (So 3-2-1 rule)

The 3-2-1 Rule and Points of Contact

The 3-2-1 rule defines the minimum number of points of contact required for a part datum feature with its primary, secondary, and tertiary datum planes. It only applies when all three plaines are used. The 3-2-1 rule says:

So there is a rule for minimum points of contact, meaning that i may take 4 points for my plane, 2 points for my say y-axis as a line & 2 points for my x-axis as a line, did i understand this correctly?

– The primary datum feature has at least 3 points of contact with its datum plane.

– The secondary datum feature has at least 2 points of contact with its datum plane.

– The tertiary datum feature has at least one point of contact with its datum plane.

The 3-2-1 Rule and Points of Contact

So, in my case my primary should be Datum C, secondary datum B and tertiary datum A ?

If not then how do i define primary, secondary and tertiary datums?(how do i know which is which?)

The other question is that i read from forum something about i could use TP instead of datums, does it affect the results in any way(or is it even correct way?)

PS: Those holes on the part have position tolerance callout diam. 0.4 A B C

If i need to input more information let me know

PS: There are no stupid questions, only stupid answers

As always, thanks in advance!

I uploaded a png file i made in paint( i didn't have any other program to use so i hope this is fine )

Firstly, i did search about my question regarding that clarification from forum & i didn't find a match so i made a new topic.

Now, the main reason why i am here is that i would like to have a straightforward answer that what i read & understand is correct.

The issue i am having is do i understand Primary, Secondary & Tertiary datums correctly. (So 3-2-1 rule)

The 3-2-1 Rule and Points of Contact

The 3-2-1 rule defines the minimum number of points of contact required for a part datum feature with its primary, secondary, and tertiary datum planes. It only applies when all three plaines are used. The 3-2-1 rule says:

So there is a rule for minimum points of contact, meaning that i may take 4 points for my plane, 2 points for my say y-axis as a line & 2 points for my x-axis as a line, did i understand this correctly?

– The primary datum feature has at least 3 points of contact with its datum plane.

– The secondary datum feature has at least 2 points of contact with its datum plane.

– The tertiary datum feature has at least one point of contact with its datum plane.

The 3-2-1 Rule and Points of Contact

So, in my case my primary should be Datum C, secondary datum B and tertiary datum A ?

If not then how do i define primary, secondary and tertiary datums?(how do i know which is which?)

The other question is that i read from forum something about i could use TP instead of datums, does it affect the results in any way(or is it even correct way?)

PS: Those holes on the part have position tolerance callout diam. 0.4 A B C

If i need to input more information let me know

PS: There are no stupid questions, only stupid answers

As always, thanks in advance!

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