How to grind any diamond in its soft direction
On a scaife bench the position of the diamond can be changed on the plate or the dimaond can be rotated. The soft direction can be found. For a cutting tool with a radius, a grinding machine with a pivot is used. The tool is in a fixed position. The soft direction can be found by moving the wheel relative to the diamond.
These machines are therefore called "planetary" grinding machines as the grinding wheel spins but also moves like a planet around the sun (the diamond).
Wheel position - Hard Direction
Normally the wheel (grey) rotates anti-clockwise. With the wheel centre positioned at the 3 o'clock position, the grinding direction is vertically downwards as shown. The diamond (yellow) has soft directions shown in red.
In this position the wheel is trying to grind the diamond in the 'hard' direction and little if any progress will be made.
Wheel position - Soft Direction
By moving the wheel centre to the 4:30 o'clock position, the direction of grinding is now parallel to one of the soft directions of the crystal and the diamond can be ground easily.
Click this link for more information on planetary grinders.