GD&T Datum Reference Frames 3

Question 1

What is a 'datum target' in GD&T?

Accepted Answer

A specified point, line, or area used to establish a datum on irregular surfaces

Answer

Datum targets are designated points, lines, or areas on a part used to establish datums on castings, forgings, or other irregular surfaces.

Question 2

Which symbol designates a datum target area in GD&T?

Accepted Answer

A circle divided by a horizontal line with the target ID

Answer

A datum target symbol is a circle divided by a horizontal line; the lower half contains the datum letter and target number (e.g., A1).

Question 3

How does a tertiary datum typically constrain degrees of freedom in a 3-2-1 locating scheme?

Accepted Answer

1 rotational DOF

Answer

The tertiary datum in a 3-2-1 scheme constrains the last remaining rotational degree of freedom (clocking the part).

Question 4

When referencing a datum feature of size at RFS (regardless of feature size), datum shift is:

Accepted Answer

Zero — no datum shift is permitted

Answer

At RFS, the datum feature simulator expands or contracts to make contact with the actual feature, so no datum shift is allowed.

Question 5

A part drawing calls out datum reference frame [A|B|C] on a position tolerance. What does the order of datums indicate?

Accepted Answer

The precedence (primary, secondary, tertiary) of datum constraint

Answer

The left-to-right order in the feature control frame compartments defines datum precedence: first = primary, second = secondary, third = tertiary.

(GD&T) Geometric Dimensioning and Tolerancing Certification Practice Test