Datum Reference Frames Flashcards

In a datum reference frame, what is the maximum number of degrees of freedom that can be constrained?

Answer: 6

A complete datum reference frame constrains all 6 degrees of freedom: 3 translational and 3 rotational.

Which datum feature simulator contact rule applies when establishing a primary datum plane from a flat surface?

Answer: True contact with at least 3 points

A primary datum plane is established by physical contact with at least 3 high points on the part surface.

A datum reference frame established by [A|B|C] uses datum A as primary. How many degrees of freedom does datum A constrain?

Answer: 3

A primary planar datum constrains 3 degrees of freedom: 1 translation and 2 rotations perpendicular to the datum plane.

What is a 'basic dimension' in relation to a datum reference frame?

Answer: A theoretically exact dimension used to locate a datum or feature

Basic dimensions are theoretically exact values enclosed in a box, used to define the true position of features relative to a datum reference frame.

When a cylindrical feature is used as a secondary datum after a planar primary datum, how many additional degrees of freedom does it constrain?

Answer: 2

A secondary datum cylinder constrains 2 translational degrees of freedom (the two directions perpendicular to the cylinder's axis).

What does 'datum shift' allow when a datum feature is referenced at MMC?

Answer: Additional tolerance as the datum feature departs from MMC

Datum shift is bonus movement allowed between the part datum feature and the datum feature simulator when the feature departs from its MMC size.

In GD&T, what is the term for the perfect geometric counterpart of a datum feature used during measurement?

Answer: Datum feature simulator

A datum feature simulator is the true geometric counterpart (gauge pin, surface plate, etc.) that makes contact with the datum feature to establish the datum.