Customized Datum Reference Frames
Datums as referenced in Feature Control Frames are used to constrain the six degrees of freedom of a part. The six degrees of freedom are as shown below. X,Y,&Z are translations. u,v,&w are rotations.
Datums as referenced in Feature Control Frames are used to constrain the six degrees of freedom of a part. The six degrees of freedom are as shown below. X,Y,&Z are translations. u,v,&w are rotations.
The order in which the datums are called out in a Feature Control Frame imply which degrees of freedom are constrained by which datums. However, sometimes it is desirable to explicitly specify which degrees of freedom are to be constrained by which datum.
In the figure below, when measuring the two top holes, the degrees of freedom constrained by datum [B] vs. datum [C] could be ambiguous. Therefore it is desirable to specify which degrees of freedom are to be constrained by datum [B] and which degrees of freedom are to be constrained by datum [C].
In the figure below, when measuring the two top holes, the degrees of freedom constrained by datum [B] vs. datum [C] could be ambiguous. Therefore it is desirable to specify which degrees of freedom are to be constrained by datum [B] and which degrees of freedom are to be constrained by datum [C].
In the figure below, notice that the X and Y axis have been defined. According to the right hand rule, the Z axis points out of the screen. This helps us identify which degree of freedom is which.
The part's requirement is that the two top holes be located with the [A] datum constraining the Z translation and the U and V rotations. The [B] datum must constrain the X, and Y but not the W. The [C] datum must constrain the W but not the X or Y. The Feature Control Frame specifies explicitly which datums must constrain which degrees of freedom. In order for the [C] datum to constrain the W and nothing else, the datum feature simulator for the [C] datum must be moveable in the Y direction.
In order for the [B] to constrain the X and Y but not the W, the datum feature simulator must be able to rotate about the Z axis
The part's requirement is that the two top holes be located with the [A] datum constraining the Z translation and the U and V rotations. The [B] datum must constrain the X, and Y but not the W. The [C] datum must constrain the W but not the X or Y. The Feature Control Frame specifies explicitly which datums must constrain which degrees of freedom. In order for the [C] datum to constrain the W and nothing else, the datum feature simulator for the [C] datum must be moveable in the Y direction.
In order for the [B] to constrain the X and Y but not the W, the datum feature simulator must be able to rotate about the Z axis
