CB Central Bobbin
The CB or Central Bobbin is strongly identified with the Singer Class 15 machine and is described as having a vertically mounted oscillating hook, employing a stationary bobbin case and round bobbin positioned in the centre of the hook mechanism. Unlike a full Rotary Hook the CB oscillating hook travels only halfway around its race, stops and returns to the starting point to form the next stitch.
The CB hook mechanism can face to the left known as side loading as in the Singer 15 Class or can face to the front of the machine known as front loading. Loading refers to the bobbin case being inserted from the left or from the front.