Machine engraving of seamless bands is a unique surface finishing treatment achieved by diamond tool cutting to produce bright, reflective facets. The technique can be accomplished on manually operated or CNC type machines. Often referred to as diamond faceting or Swiss cutting, the process has been used for many years to enhance the appearance of gold seamŽless bands. Adjustments to the basic method are required to overcome the inherent difficulties associatŽed with machining platinum.

Literature and Previous Studies:
Documentation on machine engraving of platinum seamless bands is comparatively sparse. In 1978, Roy Rushforth of JML Research described detailed machinŽing conditions to achieve reduced tool wear cutting platinum alloys. He noted the poor performance of conventional tools when applying standard gold machining techniques.
A section of literature contained in the 1990 publicaŽtion, An Introduction of Platinum by JML, described the chip formation process and related machining parameters, tooling, and lubricants to surface finish with limited discussion of engraving methods.

A discussion of platinum machining parameters was provided by T. Volpe and R. Lanam in their article Machining of Platinum Alloys for Jewelry presented at Platinum Day VII. Further work was documented at the Santa Fe Symposium in 1999 & 2000. This work focused predominantly on lathe cutting of platinum with no specifics related to engraving methods.

Normandeau & Ueno presented the results of machinŽing studies on 95Pt HTA® materials at the 1999 Santa Fe Symposium. Specific advantages and superior tool performance utilizing an alternative alloy were noted. Examples of machine engraved bands were provided, but specifics related to tooling were not discussed. Overall, there is not much literature published on platŽinum engraving techniques.

Examples of Equipment for Engraving Platinum:
A variety of manual and CNC type equipment is availŽable within the trade to accomplish engraving of platŽinum bands. These can be single or dual tooling head manual units or 7 axis CNC configurations.

Manual type units are practical for small volumes or numerous setup changes. A broad variety of styles can be made, but tooling and machine changeovers become an issue. A combination type unit deploys a horizontal and vertical tool spindle on the same machine. A single ring blank holder travels on a bed to bridge the distance between heads. Rotation may be manual or servo-motor driven.

CNC type units provide an opportunity for enhanced productivity on volume runs of the same style. Thoroughly tested programming, specialized tooling and enhanced skills are required to maximize output on the units. A large capital and training investment is required.

Engraving Tool Design Issues:
Different tool designs are required for machining platŽinum compared to gold. Diamonds for engraving gold are natural specimens with controlled crystal orientaŽtion. Platinum tools are synthetic single crystal (sumiŽtumo) or polycrystalline materials (PCD). Rake angles are set at 0_ for Sumitumo and -5_ for PCD type tools.

Sumitumo type diamonds are purchased close to final shape. PCD diamonds can be EDM cut to shape from a larger mass. They are designed to experiŽence slow, progressive wear of blunt shapes. Specialty shaped diamond products with small or large cut radii can be prepared as required. A larger shaped tool requires a more expensive large diamond as starting stock. Grooved tools are made through patient grinding with a shaped brass master wheel loaded with diamond paste.

In general, natural diamonds have internal flaws that alter performance from one relapping (sharpening) to the next. Consistency and overall performance are much better with synthetic varieties, especially the predictable wear of a PCD type. A specialized set of tools for engraving only platinum will perform better over an extended period of time provided patience and caution are mixed with proper tool selection. The cost of sharpening is 1/8 to 1/20 of the original cost of basic tooling. Diamond tools dull much more rapidly (20-40X) during platinum machining compared to conŽventional gold cutting. Solid tool holders (like those associated with CNC equipment) reduce the rate of wear.

Practical Engraving Issues
Tooling speed of rotation is a critical variable in the process. It cannot be reduced significantly or discrete, large metal cuts will be visually apparent on the ring surface. Polishing to remove tool chatter lines may blur or eliminate the crisp detail of specific styles. High tool rotation speed minimizes surface chatter lines, but increases wear significantly.

For optimum tool life when cutting platinum, it is imperative that a small depth of cut be used at all times. Patient repeated cutting reduces tool wear. There is an audible difference when an engraving machine cuts Pt as opposed to gold. More energy is required to shear platinum than gold. Careful coarse and fine depth of cut adjustments, for a fixed speed of rotation are the main process variŽable to reduce high tool loads.

Lubrication is difficult to control with conventional high tool rotational speeds. Brushing thick oil of winŽtergreen or petroleum oil between cuts or coating the item with beeswax can assist lubrication. Continuous flood feed presents a major spraying problem on open manual equipment. An enclosed CNC unit affords some possibility for enhanced lubrication.

Examples of Engraved Rings:
A number of different styles can be engraved in platinum with various tools and equipment. For examples of these styles and their primary and secondary engraving equipment, please refer to the original article.
 
Summary:
With the use of specially designed diamond tools, it is possible to engrave a broad range of styles in platinum or 18K-platinum combinations. This can be accomŽplished on either inexpensive manual or high volume CNC machines. Engraving parameters must address the adhesiveness, low thermal diffusivity and high rate of work hardening inherent to platinum. Limitations of the fixed workpiece rotating tooling required by engraving equipment must be recognized. Control of the spindle speed and lubrication are limited. Engraving diamonds for platinum should be dedicated to the task and specifically designed with different rake and relief angles compared to gold tooling. Synthetic single or poly crystalline diamonds are required. Be prepared for more frequent tool sharpenŽings compared to gold engraving. Do not attempt to re-sharpen your tools with in house techniques. Work closely with your tooling supplier to resolve issues and try different designs. Consider alternative 95% Pt alloys to assist metal removal during engraving. Patience is a virtue when engraving platinum. 

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Machine Engraving of Platinum Seamless Bands
Greg Normandeau 
IMPERIAL SMELTING & REFINING CO. OF CANADA LTD

This is an abbreviated version of the original work. For full technical details, please consult the original paper.