Growing Appeal of PCD: Turning Performance

When not to use PCD

In practical terms, knowing what PCD is and how it is manufactured will help you understand when to use PCD tools. But it’s often more helpful to begin by talking about when not to use PCD, and that requires a discussion about PCD’s limitations.

First, the toughness of PCD is relatively low compared to carbide, thus it is more prone to brittle failure. This means that vibration should be avoided, and highly rigid toolholding and workholding are essential. Toolholders should be clean, stout and as short as possible. Drawbar force should be adequate, the spindle taper should be in good condition, and pull studs (when used) must not be overtorqued or worn badly. Ultimately, this means that PCD cutting tools should not be used without careful consideration of the setup and machine involved. In some cases, the machine or workpiece might not be rigid enough to use PCD tools, despite an operator’s best efforts.

Second, PCD is uniquely sensitive to temperature for several reasons. Alone, diamond will decompose into graphite around 1,700° C, but the diamond in PCD begins to decompose around 700° C because cobalt acts as a catalyst to promote graphitization. Cobalt readily dissolves into diamond, which makes it an ideal binder material that “sticks” to the diamond grains quite well; but this affinity also has negative effects. Cobalt atoms dissolved inside the diamond lattice will weaken atomic carbon bonds and reduce the energy required for decomposition. Graphitization aside, cobalt also has a much higher coefficient of thermal expansion than diamond. If the tool becomes too hot, the cobalt binder can expand so much that individual diamond grains become dislodged from the cutting edge. Furthermore, PCD tools often contain one or more brazed joints, and even the best brazed joints will begin to soften around 600° C to 950° C. All of these concerns are the main reasons PCD is not yet used to cut materials like titanium and nickel, where cutting temperatures are usually high.

Third, if they come into contact with iron, the carbon atoms in diamond grains will readily diffuse into the iron and form iron carbide (Fe3C), especially at elevated temperatures. This will lead to gradual erosion of the diamond as it loses atoms to the iron around it, and this ultimately means that PCD is not effective for cutting ferrous materials in most situations.

Knowing these limitations also helps us see several applications where PCD cutting tools truly shine.

When to use PCD

Assuming they are used correctly, PCD cutting tools offer three primary advantages.

The most significant advantage is usually a reduction in cycle time. PCD can hold sharper edges, has a much smoother rake face that reduces built-up-edge (BUE), and has a higher thermal conductivity than carbide. Because of this, typical surface speeds for PCD in aluminum are three to five times higher than carbide. Assuming you have a spindle that allows you to take advantage of these extreme surface speeds, cycle time could be reduced by that same factor, directly affecting your profitability. This potential reduction in cycle time is what might motivate an automotive manufacturer to spend over $10,000 on a single PCD face mill. The savings from a reduction in cycle very soon outweigh the cost of the tooling. But if you’re already maxing out your spindle speed running carbide, then switching to PCD won’t help you in this area.

The second most common reason to use PCD cutting tools is improved surface finish. PCD’s hardness and ability to hold a sharp cutting edge make it capable of creating low-roughness surfaces. In situations where roughness must be controlled carefully, either for aesthetic reasons or for functional sealing surfaces, this can make the difference between success and scrap. But vibration must be avoided, and the entire machine and toolholder assembly needs to be considered as a system.

Lastly, PCD cutting tools are used because they offer increased tool life (and thus better process reliability) in abrasive materials. Cast aluminum parts, with their elevated silicon content, are known for wearing carbide tools. In many cases, switching to PCD can increase tool life tenfold (or more). As automotive manufacturers are fighting to make their EVs lighter by using more cast aluminum, their suppliers are turning to custom PCD cutting tools to increase tool life. This helps them machine these components profitably, not only by reducing cycle time, but also by giving them more confidence in the process — something necessary for automation. Beyond aluminum, other abrasive non-metallic materials are often machined using PCD. Aerospace manufacturers are turning to PCD tools to trim carbon-fiber panels and drill holes in multi-material laminations containing carbon fiber reinforced polymers, and circuit board manufacturers are using PCD tools to drill and trim fiberglass-re-inforced materials. The commercial woodworking industry is also dominated by PCD cutting tools. The benefit here goes beyond any direct reduction in tooling cost a shop achieves. In many cases manufacturers are seeking to improve process-reliability so that they can trust tools and safely run processes with less oversight and less variation from part-to-part.

Although PCD tools do have more unique limitations than carbide tools, there are companies working to overcome these limitations. Companies like Sumitomo offer “binderless” PCD grades that can be used to machine materials like tungsten carbide and ceramics. Some research also suggests that binderless grades are the future of machining titanium and nickel alloys. Innolite GmbH successfully uses monocrystalline diamond to machine ferrous materials by oscillating the tool at high frequency, where intermittent contact prevents the diffusion of carbon into the iron. Companies like NS Tool offer spherical PCD tools (without cutting edges) for burnishing steel molds and increasing the diameter of holes. Moving slowly to keep temperatures low, these tools can achieve finishes unthinkable with other tools, even in ferrous metals. Other companies are investigating specialized coatings that promise to protect the PCD substrate from elevated temperatures.

Will PCD change the cutting tool industry in the same way that carbide did, and could PCD tooling offer your shop the next leap in productivity? The answers are not simple, but there are cases in which PCD tools can make a big difference. In the future, chances are those cases will become more common.