What version do you have?
(Excerpt form the Maya 2009 help)
Mirror copying a mesh
Once you’ve finalized one half of your polygon model (and deleted its construction history) you’ll want to create the opposite half by copying it across the axis of symmetry so you have the complete model. You can produce a mirrored copy of a polygon mesh using Mirror Geometry ( Mesh > Mirror Geometry).
Some things to note:
Before copying one half across the axis of symmetry, you should
check that all of the border edges lie along the axis of symmetry.
If any edges do not lie along this axis, the original half will be
copied based on the vertices that create the widest point and a
gap between the two halves may result.
To ensure that the border vertices lie along the axis of symmetry
Enlarge the front view so you can view the vertices that lie along
the axis of symmetry.
If any vertices on the mesh protrude across the Y axis, you’ll
need to snap these to the Y axis using the Move Tool in
combination with the Snap to grids feature.
Note: Any vertices that protrude across the Y axis likely occurred
as a result of an earlier extrude operation. Because the extrude
feature extrudes components based on the face normal, a few
vertices may have moved across the axis of symmetry.
In the front view, select all of the vertices that should lie
along the axis of symmetry (Y axis) using a bounding box
selection.
On the Status Line, turn on Snap to Grids.
In the Toolbox, double-click the Move Tool to display its tool
settings and ensure that the Retain component spacing setting is
turned off.
in the front view, click-drag the red arrow on the Move Tool
manipulator to the right a small distance.
The vertices immediately snap to the grid line to the right. They
initially will not be positioned at the location you want, but
you’ll notice that all of the vertices you selected are now
aligned to each other.
Drag the manipulator towards the left until the vertices snap to
the Y axis.
On the Status Line, turn off Snap to Grids.
Click off the mesh to unselect the vertices.
Right-click the mesh, change the selection mode to Object, and then select your mesh.
Select Mesh > Mirror Geometry > , set the following options, and then click the Mirror button.
Mirror Direction: -X
Merge with original: On
Merge vertices: On
The mesh is copied along -X to create the opposite half of the model. The two meshes are combined into one and vertices are merged so they become shared.
Save your work.
"When I look at that square... I wish FPSC noobs would stay on their side of the forums and stop polluting these boards." - Benjamin
