The Structure of Single Crystal Diamond

Like wood, diamond crystals have a 'grain', but unlike wood the 'grain' runs in many possible directions. Diamond can be ground easily as long as the grinding direction is along the 'grain' or put another way – parallel to a 'soft' direction of the crystal. The PG4 works in a very different way to a conventional grinding machine. It is useful to understand why.

Planetary Motion Wheel Spindle

To achieve the correct grinding direction, the PG4 has a wheel spindle which can be positioned in a planetary manner so that the direction of the grinding wheel's motion at the point where it meets the diamond can be aligned with the soft direction of the facet being ground. This enables rapid material removal rates to be achieved. For example, with the wheel in the '135°' position, the grinding direction is against the grain and little, if any, material will be removed. At the '270°' position, the diamond is ground rapidly as the grinding direction is now running parallel to a 'soft' direction of the crystal.

The Floating Table with Constant Grinding Load

Because diamond's hardness varies with crystal direction, it is not possible to grind diamond using a positive in-feed mechanism alone. Instead a constant force is used to simply apply a steady grinding pressure. The diamond tool is mounted on a linear motion floating table which is held in contact with the wheel by a dead weight until it meets a fixed stop. The floating table also contains the air-bearing pivot motion which enables precise radii to be formed.

Edge Perfection and Vibrations

Controlled and low waviness SCD tools can only be produced if vibrations at the wheel/diamond interface are eliminated. A composite granite base to damp vibrations and the air-bearings in the wheel and pivot spindles are all necessary to ensure that the ultimate perfect edges are achieved. Operators can also introduce vibrations as they press switches or tap the touch screen, to avoid this vibration, all aspects of human interface have been assigned to a separate tower which is connected to the main grinding machine via an umbilical harness. Once the program starts there is no further need for the operator to touch the grinding machine itself.

Grinding Facets – The 'Blocking Out' of a Cutting Tool Radius

Under program control, the machine will generate up to 31 facets around the radius of a tool. For each facet, the control will:

• Rotate the wheel spindle to the required planetary position for soft direction grinding
• Advance the wheel a controlled amount and allow the tool to grind and 'float' back to the fixed stop
• Move the tool away from the wheel
• Index the pivot and grind the next facet

Grinding the Radius

• The program will then swing the pivot over the required arc to smooth the flank in conjunction with the radius
• Hard spots 'knarts' can be identified and a sub-arc of pivot rotation selected for focussed attention
• Once the tool is completed the wheel will move away from the tool for inspection/unloading

Simultaneous, Synchronous Axis – Grinding Vectors

The planetary , pivot rotation, wheel traverse, wheel in-feed and rotary module (RM) motions are all programmable and in the basic machine run independently of each other. Optional software is available for the PG4 such that the motion of two or more of these programmable axes can be electronically geared to run with synchronous motion. A repeating sequence of grinding directions or vectors can thus be established enabling complex profiles to be generated, such as elliptical, parabolic, hyperbolic and multiple blended radii.

Multi-Axis Configuration

The pivot and grinding wheel axes feature high precision air bearings and in normal production only these two axes would be running for the very final polishing of a low waviness radius tool.

The remaining machine axes incorporate high precision linear or rotary bearings.

The camera axis alignment is via micrometers.