A carriage drive apparatus positions a carriage along a guide rail. First and second front drive rollers are rotatably mounted on a carriage drive housing on opposite sides of a nonrotating guide rail. The first and second front drive rollers are adapted to frictionally engage the guide rail. Third and fourth rear drive rollers are also mounted on the carriage drive housing on opposite sides of the guide rail. The third and fourth rear drive rollers are adapted to frictionally engage the guide rail. A motor is mounted on the carriage drive housing and is coupled to the front and rear drive rollers. The motor is used to drive the carriage drive housing along the guide rail. A sensor is used to sense the position of the carriage housing along the guide rail. The drive can be disengaged from the drive rail while the sensor remains engaged to permit teaching the controls various stop positions while manually moving the carriage drive housing.