From BlenderWiki

Normal Axis Locking

Description

Movements (translation/scale/rotation, and extrusion in Edit mode) can be constrained to an axis that is aligned with one of the axis of one of the transform orientations, centered on the object’s original world location.

In (Global axes), the cube “B”’s original world location is labeled “C”. The center of the global coordinate system is labeled “W” (Z axis through W not visible).

By constraining a transformation to a global axis you are, in effect, restricting this transformation to one dimension.

All axes in Blender are color-coded, using the analogy XYZ <-> RGB:

• X axis is Red (labeled “X axis”).
• Y axis is Green (labeled “W-Y axis”, for “world-Y axis”).
• Z axis is Blue (labeled “Z axis”).

Their shade is lighter or darker, depending whether they are “locked” or not…

For example, the Y axis is drawn in light green if movement is constrained to the Y axis; labeled “Y axis”.

Now, let’s detail the various types and options of this tool.

Axis locking types

Axis locking

Locks the transformation on only one axis (or forbids transformations along two axes).

An object (or vertex, control point, whatever) will only be able to move, scale or rotate in one direction, along or around a straight line.

Plane locking

Locks two axes (or forbids transformations along only one axis), thus creating a plane in which the element can be moved or scaled freely.

The plane locking only concerns translation and scaling.

Note that for rotations, both axis and plane locking have same effect because a rotation is always constrained around one axis.

Trackball rotations Note also that "trackball" rotations (RR) cannot be locked in anyway!

Axis locking modes

You can lock a transformation on axes of several spaces. In fact, by default you will only lock on World (global) axes. However, if you use the keyboard axis selection method (see below), you will have as second option to choose between the axes of the current transform orientation (unless it is set to Global, in which case a second choice will always be Local).

How to lock axis

There are two ways to constrain movement: using the mouse or using the keyboard.

Using the keyboard

No Constraint.

Global Constraint.

Local Constraint.

You can constrain movement to a given axis by pressing either X, Y or Z, and ⇧ ShiftX, ⇧ ShiftY or ⇧ ShiftZ for plane locking.

A single key press constrains movement to the corresponding global axis (Global Constraint). A second keypress of the same key constrains movement to the current transform orientation selection (except if it is set to Global, in which case Local axis are used). Finally, a third keypress of the same key removes constraints, (No Constraint). The locked axis is (are) drawn in a lighter color to better visualize the constraint. (Local Constraint) and (Global Constraint) are all examples of X axis locking using the X key.

Note that using locked axis do not prevent you from using the keyboard to enter precise numeric transformation values.

Using the mouse

To lock or constrain movement using the mouse, enter Grab/Scale/Rotate mode and move/scale/rotate the object while pressing MMB . While in a transform mode you can use the gesture system to pre-select an axis by moving the mouse in a direction roughly inline with a world axis and then clicking and releasing MMB . For example, if you move the mouse along what visually appears to be the X axis and then click and release MMB the object’s movement will be restricted to the world X axis.

Alternately, you can interactively choose the constraining axis by dragging with the MMB while in Grab mode. All three axes become visible with a guide line that emanates from the object’s original location; labeled “C”. This guide is drawn in white dotted line labeled “S”. As the guide line nears an axis that axis becomes highlighted in a lighter shade and the object snaps to that axis. In this example the guide line is near the Y axis and the cube, labeled “B”, snaps to it.

This is not easy to explain, so try it, you’ll see it’s quite intuitive! Note that with the mouse, you can only choose an axis, not switch between Global and current transform orientation. Nor can you switch between “normal” and “plane” axis locking… You have to use the keyboard for this.