DATUM (AXIS OR CENTERPLANE)
FOS DATUM FEATURES
Specifying an Axis or center plane as a Datum.
When there is a datum symbol on a FOS then the FOS is specified as a datum feature.
An axis or center plane becomes a datum when the FOS is specified as a datum feature.
There are 5 ways to specify an axis or center plane as a datum which are as follows:
- The datum identification symbol can be placed on the beginning of the leader line.
- The datum identification symbol can be mentioned on the feature control frame.
- The datum identification symbol can be directly placed on the FOS.
- The datum identification symbol can be placed on the dimension to specify an axis or center plane as a datum.
- The datum identification symbol can replace on one side of the dimension line and the arrowhead.
Specifying an axis or center plane as a Datum
When we state a diameter as a datum feature, the datum axis is identified by placing the part in a datum feature simulator.
Datum feature is basically the part surface.
Datum feature simulator is the gage elements.
The axis of the gage elements is considered as the datum axis.
The datum axis becomes the origin of measurement for dimensioning.
REFERENCING FOS DATUM FEATURES
It is very important to communicate the material conditions for establishing the datum axis or center plane when using FOS datum features on drawings.
This is carried out with the help of feature control frames.
The feature control frame consists of the material condition (MMC, LMC or RFS) and the datum sequence (primary, secondary or tertiary).
When no modifiers are used in the feature control frame then RFS condition is applicable.
FOS DATUM FEATURE APPLICATIONS (RFS)
Functions of part is related to datum selection and referencing.
Datum features are usually the feature which orient and locate the part.
DATUM AXIS RFS PRIMARY
A datum axis is formed, when a diameter is designated as a datum feature and referenced in a feature control frame at RFS.
Datum axis are basically formed when there is a contact between the datum feature and the inspection equipment.
Diameter is surrounded by datum feature simulator.
Adjustable devices such as centering devices, collet or precision chuck are used as datum feature simulator.
Datum feature is basically held by the datum feature simulator.
The axis of datum feature simulator basically becomes the datum axis.
Primary Datum RFS
DATUM CENTER PLANE RFS PRIMARY
A datum center plane is formed when a parallel plane FOS is designated as a datum feature and referenced in a feature control frame at RFS.
Due to physical contact between datum feature and inspection equipment a datum center plane is formed.
FOS is surrounded by the datum feature simulator.
Devices that are adjustable are used as the inspection equipment such as contracting or expanding parallel plates.
Holding of the part firmly is done by the datum feature simulator.
Datum center plane is the center plane of the datum feature simulator.
Primary Datum RFS
DATUM AXIS RFS SECONDARY
When a part contains a surface and diameter, it is common to have the surface and the diameter designated as datum features.
In such cases, following conditions apply:
- There must be a minimum of three point contact of the part with primary datum plane.
- Size adjustable secondary datum feature simulator is used.
- Axis of datum feature simulator is the datum axis.
- Datum axis is always perpendicular to the primary datum plane.
- There will be a second and third datum plane which would be associated with the datum axis.
Datum Axis RFS Secondary
DATUM CENTER PLANE RFS TERTIARY
Surface, diameter and FOS are stated as datum features when a part is oriented by a surface, diameter and has an angular relationship with the FOS.
In such cases, the following conditions are applicable,
- With the primary datum plane, the part must have at least minimum of 3 points of contact.
- Datum axis perpendicular to primary datum plane is present.
- Datum center plane that would pass through the datum axis and would be perpendicular to the primary datum plane.
Datum Center plane RFS Tertiary
DATUM AXIS FROM COAXIAL DIAMETERS RFS PRIMARY
In some parts, a single datum axis from two coaxial diameters is desired.
When the two diameters on a drawing are shown on the same axis then they are known as coaxial diameters.
Coaxial datum features are the coaxial diameters which are used to establish a datum axis.
The above figure shows how to establish a datum axis from coaxial datum features.
Each coaxial diameter becomes a datum feature in the drawing.
The datum reference letters are separated by a dash in one compartment.
The datum axis is basically formed by the two coaxial diameters.
This single datum axis becomes the primary datum.
Datum Axis from Coaxial diameters RFS Primary