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Whether you are working with wood, plastic, steel or other common metals, the versatility of Computer Numerical Control (CNC) machines allows you to produce an excellent product with tight tolerances. While each material requires its own specifications, one material class poses a unique challenge: machining precious metals. Due to their rarity and high price tag, they require a strong understanding of their properties, a careful approach to minimizing waste, and a well-chosen selection of mill ends. Despite their specific needs, they are a highly sought-after product, and with the right strategies, you can enhance your return on investment when machining precious metals.
Precious metals are known for their softness, ability to withstand tensile stress (ductility), and oxidation resistance. Often termed “noble metals,” they are standouts in resisting chemical and environmental factors. For these reasons, precious metals are sought after in various industries, including medical, dental, electronic, electrical, and more.
While gold and silver are precious metals that can be found residing on their own, most of the precious metals are intertwined with non-precious elements in raw ore. These precious metals include ruthenium, rhodium, palladium, osmium, iridium, and platinum. Many have an affinity for each other, and working with precious alloys is quite common. Since extracting these metals is difficult, they have an extremely high cost. Once you begin machining precious metals, getting it right the first time is a must.
Two factors are the most influential when machining precious metals: hardness and percent elongation. Hardness refers to the metal’s resistance to cutting, while percent elongation measures ductility or how much you can permanently deform the precious metal before it fractures.
While hardness can be overcome with speed, feed rate, and end mill coatings, ductility poses a different challenge when machining precious metals. The greater the percent elongation, the higher the tendency of the metal to produce long, stringy chips that build up on the end mill edge. Termed “gumminess,” the problem impacts surface finish, compromises tolerances, and reduces tool life. Under the high pressure and friction of CNC machining, the built-up edge can grow so large it breaks off, gouging the piece and potentially rendering it useless.
Sharp cutting tools are essential for combating the gummy nature of precious metals. Depending on your overall design needs, selecting a 3 or 4-flute design can help control the movement of material, removing it from the immediate area and preventing built-up edges. As more precise cutting is needed, smaller end mills with even fewer flutes may be necessary to ensure a clean and blemish-free finish.
A variable helix is perfect for working with softer materials, and a square profile end mill will also help with smooth finishes and tight tolerances. Both reduce chatter and harmonics – vibrations that mar the surface as you machine precious metals. These vibrations also damage the CNC machine’s parts, potentially causing premature wear on the components.
Solid-body carbide end mills also improve tool longevity, as they provide the stability needed to work with gummy or hard materials. The rigors of machining harder metals may call for specialty end mill coatings. While Titanium Nitride (TiN and Al-TiN) and Titanium Carbonitride (TiCN) are helpful for some materials, the hardest precious metals can be tackled with diamond-coated end mills, allowing you to work carefully and efficiently.
Machining precious metals is about more than simply matching the specs on a page. It’s about understanding how your particular CNC machine works with its component parts, evaluating the machine’s calibration and feedback, and maximizing productivity and profitability.
Having an experienced CNC Machine repair and maintenance company on your side can help you keep your downtime to a minimum and productivity at its peak. Accurate Machine Tool Services understands that machining precious metals is as much a craft as a process. Contact us today and explore how we can support your precious metal machining goals.