Selecting a DC motor for Servo Motor Use.

One factor in CNC type Servo Motor selection is brush timing.
If the motor being converted was intended by the manufacture
to run fast in one direction the brushes may be advanced.(Advanced
means the brushs are rotated some number of degrees against the
direction of rotation of the armature.)

Advancing brushes improves efficiency and reduces sparking at the
brushes.

For a Servo Motor in CNC application it would be important for the
motor brushes to be set for netural timing. That way the motor would
be equally efficient running CW or CCW (but at somewhat reduced
efficiency).

Some DC motors have adjustable end cases that carry the brush
assembly. This means it may be easy to rotate the brush assembly to
adjust the brushs for netural timming.

This may be a feature to look for in selecting a DC Motor for Servo
Motor useage.