![]() ![]() The primary datum can and must control all degrees of freedom that it may. Meaning: with 3-2-1 rule, primary datum has to constrain 3 degrees of freedom(2 rotational and 1 translational).Īs primary datum is bound to constrain 3 degrees of freedom from the other 3?ĭatum precedence is an order of operation. The intent of a feature control frame is to define what order precedence is and minimize error measurement correlation.Īs i understand is i need to take in consideration 6 degrees of freedom when i see 3 datums callout on the print & i can also use it to clear that the designer has put the datums in a correct places. I have seen parts the when measured incorrect datum precedence used they are acceptable, when correct datum precedence is applied the part location now fails. In a perfect world you will never have an issue but we do not work in perfect, never will. A plane can be primary, secondary or tertiary but order of precedence can have a huge impact on accept/reject. With the above, A circle or a line cannot be a primary datum. A plane was primary and 2 post were labeled as B datum or 2 ID’s. I have worked a feature control frame with and A-B callout that constrained all 6 degrees of freedom. Whatever degree of freedom left a point will now stop the last translation is typical but could possibly stop a rotation also. 1- Can be any point circle, intersecting point, centroid on a plane. This can also be a line on an edge or a line from 1 or more circular features or mid points. But a primary can also be a cylinder, the axis can control up to 2 rotation and 2 translation -2- Can be a plane or a line or set features that create a centerline Plane primary-plane secondary the plane secondary controls a rotation that is not constrained by primary and can constrain a translation. 3-2-1 Rule is rules of precedence of constraint as it applies to the 6 degrees of freedom -3- You need 3 point minimum or 3 controls of the 6 degrees of freed -a minimum # point’s construct plane has 3 points and controls 2 rotations and can control 1 translation. Have you looked at all at the ASME Y 14.5? 2009 look at pg 49-50 figures 4-1 4-2 and 4-3 do a good visual depiction of datum precedence and how degrees of freedom are applied here. PS: There are no stupid questions, only stupid answers If i need to input more information let me know PS: Those holes on the part have position tolerance callout diam. 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?) If not then how do i define primary, secondary and tertiary datums?(how do i know which is which?) So, in my case my primary should be Datum C, secondary datum B and tertiary datum A ? – The tertiary datum feature has at least one point of contact with its datum plane. – The secondary datum feature has at least 2 points of contact with its datum plane. – The primary datum feature has at least 3 points of contact with its datum plane. 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? It only applies when all three plaines are used. 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. The issue i am having is do i understand Primary, Secondary & Tertiary datums correctly. 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. I uploaded a png file i made in paint( i didn't have any other program to use so i hope this is fine )įirstly, i did search about my question regarding that clarification from forum & i didn't find a match so i made a new topic. Hi everybody, i am here again to seek your quidance and knowledge. ![]()
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