Normal Modifier

The Normal modifier allows you to unify or flip the normals of an object without applying an Edit Mesh modifier.

Select an object. > Modify panel > Modifier List > Normal
Standard menu: Select an object. > Modifiers menu > Mesh Editing > Normal Modifier
Enhanced menu: Select an object. > Modifiers menu > Geometry (Convert to Mesh) > Normal Modifier

For example, if you wanted to fly inside of a procedural object, such as a sphere or a cylinder, and wanted to retain control over the radius and number of segments, you couldn't collapse the object to an Editable Mesh and maintain the procedural nature of the primitive.

Flipping the normals of a sphere creates a sky dome over a city.

Tip: If you are animating the creation of a complex object such as a nested Boolean or a loft, and you think the operation might result in inconsistent faces, apply the Normal modifier to the result and turn on Unify normals.

Tip: The Lathe modifier sometimes creates an object with normals pointing inward. Use the Normal modifier with both Unify and Flip turned on to fix "inside-out" lathe objects. The Normal modifier allows whole-object manipulations of normals to be performed quickly without using an Edit Mesh modifier.

Patches

Patch objects coming up the modifier stack are not converted to a mesh by this modifier. A patch object input to the Material modifier retains its patch definition.

Procedures

To use the normal modifier:

Select an object, then on the Modify panel Modifier List, choose Object-Space Modifiers Normal.
The object appears to turn inside-out, since Flip Normals is on by default.
If the object has some faces pointing inward and others outward, turn on Unify Normals to make all the faces point similarly.
Tip: To flip or unify normals on part of an object, convert it to editable mesh and select faces or polygons. On the Surface Properties rollout in the Normals group, use the Flip and Unify buttons.

Interface

Unify Normals: Unifies the normals of an object by flipping the normals so that they all point in the same direction, usually outward. This is useful for restoring an object's faces to their original orientations. Sometimes normals of scenes that have come into 3ds Max as part of a DWG or DXF file are irregular, depending on the methods used to create the scene. Use this option of the modifier to correct them.

Note: Unify Normals does not work on editable poly objects; before applying the Normal modifier, convert the model to editable mesh format or apply a Mesh Selector Turn To Mesh modifier.
Flip Normals: Reverses the direction of all the surface normals of the faces of the selected object or objects. Default=on.