You can add center lines and symmetry lines to your drawing. These are useful for aligning elements and when adding dimensioning parameters.
To create a center line,
This command allows you to create a center line for a circular element. The center line becomes part of the circular element and is moved or deleted if the associated circular element is moved or deleted.
The figure shows two circles before and after adding center lines. The different appearance of the center lines is caused by different absolute and relative offset values.
1. Click Annotation and then, in the Annotate group, click the arrow next to Cen Line.
2. Click Create.
3. Select the circular element, define a selection box, or use the Select tool.
4. Continue specifying circular elements or click to end the operation.
When you dimension center lines and then make a modification, the dimension is automatically updated. The figure shows an example of changing the position of a dimensioned center line.
To create a symmetry line (or other symmetrically associated object),
This command allows you to create elements that are:
• Symmetric with existing elements.
• Associated (or anchored) to the defining elements.
When you modify the defining elements, the associated element is modified accordingly.
1. Click Annotation and then, in the Annotate group, click the arrow next to Sym Line.
2. Click Create.
3. Select the first element to be associated symmetrically.
4. Select the second element to associate with the first.
Both elements must have the same owner.
5. Continue clicking elements or click to end the operation.
When you dimension symmetry lines and then make a modification, the dimension is automatically updated.
To create single or double slot center lines,
1. Click Annotation and then, in the Annotate group, click the arrow next to Cen Line.
2. Click Slot Single or Slot Double.
3. Select an arc or a circle for which you want to create the center line.
4. Continue creating slot center lines or click to complete the operation.
Example: To draw an associated element
This example creates a circle symmetrically associated with two other circles. The figure shows the arrangements of the original two circles and the resulting three.
1. In Creo Elements/Direct Modeling, create a polygonal part with two holes (diameters 10 mm and 30 mm respectively) to produce (in Creo Elements/Direct Annotation) the polygon with holes 1 and 2 in the figure.
2. In Creo Elements/Direct Annotation, Click Annotation and then, in the Annotate group, click the arrow next to Sym Line.
3. Click Create section.
4. Select circle 1.
5. Select circle 2.
6. Click to complete the operation.
Creo Elements/Direct Annotation draws the associated circle 3, which is:
• Midway between circles 1 and 2.
• 20 mm in diameter (the average of the defining diameters).
The figure shows two other examples of symmetry lines.
In A, the symmetry lines are found by clicking the pairs of lines that define them. In B, the symmetry line is found by clicking lines 1 and 2.
Overlapping symmetry lines
When symmetry lines overlap, they are merged into a single line. The illustration below shows a standard front view with hidden geometry.
The first diagram in the illustration below shows symmetry lines created for each face: BC for FACE1, AB for FACE2, DC for FACE3 and AD for FACE4. Each symmetry line has a segment of overlap with at least one other line. For example, symmetry lines for FACE2 and FACE4 share segment AD. Because the symmetry lines all overlap, they will be merged. The second diagram in the Illustration shows the symmetry line Creo Elements/Direct Annotation produces.
Cen Line.
to end the operation.
Sym Line.
Slot Single or 
Slot Double.
