There has been a hype and fear of GD&T in Mechanical industry so to make your life easier we’ve covered each and every point of Datum targets in this article.
Without wasting much time let’s dive into datum concepts and understand it in a simplified version.
The symbols that describe the shape, size and location of gage elements that are used to establish datum planes or axes is called as Datum target.
The main intention of Datum target is to stabilize an inherently unstable part which could be produced by casting or forging or a warped part such as a weldment.
They are mainly used on parts such as plastic parts or sheet metal with curved surfaces which are free from any type of form.
For establishing datum planes or axes from which the measurement should be taken is the primary goal of datum targets.
Datum target are not present on the part surface, but they are shown on the drawing.
They typically describe the gage elements.
The gage elements only contact a certain portion of a part surface.
A particular relationship is set between the part surface and the gage element with the help of datum targets.
The part surfaces where it is not possible to use the entire surface as a datum feature there the datum target should be specified.
Castings, forgings, plastic parts and weldments are few of the examples.
These types of parts mostly do not have a planar datum feature.
Most of the times, the part will rock, wobble or not rest in the same position.
DATUM TARGET SYMBOLS
There are 2 types of symbols used:
- A datum target identification symbol.
- Symbols that denote which type of gage elements are to be used.
The symbol is divided into 2 parts with the help of a horizontal line.
The bottom half consists of the datum letter and the target number associated with the datum.
The top half contains gage element size information.
Datum Target symbol
The leader line gives us the information whether the datum targets are on the surface or on the hidden surface side of the part.
If the leader line is solid, the datum target exists on the surface shown (datum targets B1 and B2 as shown in the above figure).
If the leader line is dashed then it denotes the datum target is present on the hidden surface of the part (datum targets A1, A2 and A3 as shown in the above figure).
Datum feature symbol always accompanies the datum target symbol.
Datum feature performs the application of establishing a datum.
A datum target symbol or datum feature symbol is used to identify a datum feature.
DATUM FEATURE SYMBOL
A rectangular frame (box) having a datum letter designation indicates to use that feature to establish a datum.
These symbols are used for relating to real physical features on the part.
Imaginary items such as centerlines, center planes or axes do not have datum feature symbol used on them.
DATUM TARGET POINTS
A datum target point is typically specified by an X shaped symbol.
The symbol is mentioned on the plane view of the surface as shown in the above figure.
Basic dimensions should be used to locate datum target points.
The location of the datum target is described by the basic dimensions.
A datum target point is mostly simulated with the help of spherical tipped gage pin.
Datum Target points
The gage pin is shown in the above figure.
As stated in the below figure there are 3 ways to specify a datum target line:
Symbols for datum target lines
- A line on the plane view.
- X on the edge of the surface.
- A combination of line and X.
Basic dimension should be used to locate the datum target.
A datum target is stated where a gage element that represents an area of contact on a part surface is preferred.
A datum target area is mentioned by drawing the outline of the gage.
This outline is shown with phantom lines and the shape is shown in the section A of the below image.
Datum Target area
Basic dimensions are used to mention the shape, size and location.
If the target area is circular, a diameter symbol is used in the upper half of the datum target symbol as shown in the section B.
In certain cases, it is impossible to show the circular target there the method shown in section C can be used.
DATUM TARGET APPLICATION
There are only 3 datum target symbols which can be used in a variety of ways and on a number of different types of parts.
We would understand 3 different datum target applications.
- Partial reference frame creation from partial surfaces.
- Partial datum reference frame creation from offset surfaces.
- Complete datum reference frame creation from irregular surfaces.
Three requirements should be fulfilled to establish a complete datum reference frame when using datum target.
- Use of basic dimensions to define and locate the datum target.
- Six degree of freedom must be restrained with the help of datum reference frame.
- Part dimensioning must be such that the part will rest in the gage in only 1 orientation and location.
So that’s it from us for now!!! Hope you enjoyed going through the content. For any questions or suggestions please feel free to use the comments section. We would be more than happy to be in touch with you all