Question

 

 

A) What is the difference between datum, datum simulator and datum feature? Illustrate by example.

B). Why is surface & plane and line & axis specifically mentioned as datums? Are they not same?

 

C) When is there a need to use datum targets?

 

D) What are the different types of datum targets? 6

 

Answer 1

A) A datum is theoretical exact plane, axis or point location that GD&T or dimensional tolerances are referenced to. All GD&T symbols except for the form tolerances (straightness, flatness, circularity and cylindricity) can use datums to help specify what geometrical control is needed on the part.

A datum feature is usually an important functional surface where all other dimensions need to be in spec with reference to it. However, things get a little tricky when clarifying what the datum actually is. A Datum is a perfect point, line, plane or surface but only exists theoretically. However a Datum Feature is a tangible surface, point or axis on a part where that theoretical datum is located.

A datum feature simulator is the manufacturing or inspection equipment contacting the datum feature of the part. Datum feature simulators are used to establish the simulated datums.

B) Surface and plane, though, seem to mean the same thing but have a major difference.Planes are perfect and only exist theoritically. Whereas, a surface is a tangible feature and has waviness and bumps. Same goes for line and axis, with the former being a tangible aspect.

C)In GD&T, a datum target may be used to specify a point, line, or area on a part to establish a datum. This is sometimes necessary when a part feature is too large or naturally irregular to create a datum. Parts that are cast, forged, or molded often benefit from the use of datum targets.

D) There are three types of datum targets namely, line, point and area.