Datums
A datum is a theoretically exact plane, point or axis from which a dimensional measurement is made. A datum feature (actual part surface) is a part of feature that contacts a datum.
The datum system is a very important part of the geometric tolerancing. The datum system is set of symbols and rules that communicate to the drawing user how dimensional measurements are to be made. The datum system is used for mainly two reasons.
1) The datum system allows the designer to specify in which sequence of part is to contact the inspection instrument for the measurement of a dimension.
2) The datum system allows designer to specify which part surfaces are to be contact the inspection instrument for the measurement of a dimension.
The important benefits of datum system are as following.
2) It helps in communicating part functional relationships.
3) It helps in making the dimensional measurement as intended by the designer.
Implied Datums:
An implied datum is an assumed plane, axis, or a point from which dimensional measurement is made. Implied datums are an old concept from coordinating tolerancing.
Planar Datums:
A datum is a theoretically exact plane, point or axis from which a dimensional measurement is made. A datum feature (actual part surface) is a part of feature that contacts a datum.
A planar datum is the true geometric counterpart of planar datum feature. A true geometric counterpart is the theoretical perfect boundary or best fit tangent plane of a specified datum feature.
Since a true geometric counterpart is theoretical, the datum is assumed to exist in, and be simulated by, the associated inspection instrument. The inspection instrument (or gage surfaces) used to establish a datum is called the datum feature simulator. For example, surface plates and gage surfaces- though not perfect planes- are of such quality that they are assumed to be perfect and are used as simulated datums. A simulated datum is the plane or axis established by the datum feature simulator. For practical purposes, in industry a simulated datum is used as a datum. Below figure shows a datum, true geometric counterpart, datum feature simulator and simulated datum plane.
Datum Feature Identification Symbol:
The symbol used to specify a datum feature on a drawing is called the datum identification symbol. The method of attaching this symbol to a part feature determines if it designates a planar datum or a FOS datum. Below fig shows the way of displaying the datum feature identification symbol to denote a planar datum.
