Wire-EDM or grinding?

Finishing carbide or PCD inserts using EDM or grinding?

 

For every cutting tool insert manufacturer, chosing between grinding machines and wire EDM units, the answer is not that simple. These two different finishing operations generate the question: when should EDM be used and when should grinding be used as a finishing process?

To meet tight tolerances and surface finish requirements, tool manufacturers usually choose for grinding as a finishing operation because it’s fast and efficient. However, wire EDM can be “the” better choice and sometimes the only choice.

Form tool systems, consisting of a toolholder and an insert blank, are designed to machine contours that cannot be produced as quickly while using a single-point turning process. Form tools use a plunge cut method to cut contours in a workpiece blank in a single axis stroke versus traversing the blank, often multiple times, to create the programmed geometry. The insert blanks are made of coated or uncoated hard metals, ceramics, boron nitride or polycrystalline diamond (PCD).  Most tools are manufactured to create part-specific contours of its insert blanks by either grinding or wire cutting (WEDM). The cutting-edge geometries and final dimensions of PCD cutting tool inserts are typically produced on CNC tool grinders. However, since diamond, the hardest known material, is contained in both the workpiece and the grinding wheel, the machining time and tool wear are enormous when grinding.

But PCD tool usage has been gaining market share very fast because titanium-based alloys, along with various composite materials, continue to see increased usage in the aerospace, automotive, medical and energy industries. These materials have created new finishing challenges for tool makers.

PCD tools have a significantly higher service life than hard metal tools when it comes to working on nonferrous metals or plastic with glass or carbon fiber, for instance. However, because of the qualities of the diamond and CBN crystals in these materials, cutting into segments is difficult and requires WEDM. 

There are two types of erosion machines used in producing PCD tools depending on the application; rotary erosion (EDM- grinding) and wire EDM. Wire EDM is today mostly used to remove PCD when manufacturing or regrinding PCD tools.

Since EDM uses non-contact electrical charge to remove material, there is no tool wear, which consequently reduces costs. The quality and finish of most applications is comparable with ground cutting edges, since electrical erosion spark generators employ solid-state circuitry that controls spark duration and frequency within nanoseconds. A finish of Ra 0.1 µm is possible with specific coated brass wire and Ra 0.7µm when using Molybdenum wire.

However, there are some PCD tools that require a ground surface finish, as there is a small degree of thermal damage using EDM. Even when running the EDM unit at a low speed—the faster the more thermal damage and the rougher the tool finish—with the right spark generator settings, surface finishes of ±1.5 microns, for instance, cannot be achieved.

PCD tools are not the only tools that are manufactured with wire EDM units. There are certain tool profiles that cannot be machined any other way. Wire EDM can create some complex tool features that a grinding wheel or disc can’t—at least not as economically—because of the required space of the grinding wheel inside the work area. Also, grinding wheels wear and must be dressed, which generates higher costs compared with wire EDM.

While the EDM process poses many advantages, most tools made of hard metals as well as rotary tools are today still ground. Speed is one distinctive advantage. Unless there is a high amount of material removal necessary, grinding is a faster machining process than EDM. On the other hand, if there is a large amount of material to be removed to achieve the desired shape, using WEDM to machine the part before the grinding process is more efficient.