To Create Horizontal, Vertical, or Aligned Dimensions (AutoCAD Mechanical Toolset)

Click Home tabDimension panelPower Dimension. Find
Specify the first (1) and second (2) extension line origin.
Move the pointing device to the desired position and orientation of the dimension and click to place the dimension line (3).
Note:
The dimension line turns red when it snaps to a preset distance from the object.
Edit the dimension as required using the settings on the Power Dimensioning ribbon contextual tab, or click Power Dimensioning tabPredefined drop-down ( Find) and select a predefined dimension text template.
Click Power Dimensioning tabClose panel Close Editor. Find
Press ENTER to end dimensioning.