Using Grinding Versus EDM to Finish Form Tools

On a visit to German-based cutting tool insert manufacturer, Schwanog, we were given a comprehensive plant tour. It included the company’s insert finishing departments that included a bank of Haas grinding machines and Agie wire EDM units. These two finishing operations begged the question: when should grinding be used and when should EDM be used as a finishing process?

To meet tight tolerances and surface finish requirements, tool manufacturers usually opt 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).

Schwanog tools are manufactured to create part-specific contours of its insert blanks by either grinding or wire cutting (EDM). 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 momentum 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 such as Schwanog.

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 EDM, Schwanog’s Production Manager Franz Hummel explains.

There are two types of erosion machines used in producing PCD tools depending on the application; rotary erosion and wire EDM. Schwanog uses a wire EDM unit to remove PCD when manufacturing or regrinding PCD tools. In these machines, a wire uses electric charges to erode the diamond, cobalt and carbide substrate to create the desired geometry on the diamond edges.

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 0.1 Ra is possible.

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 at Schwanog. 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, Mr. Hummel says. Also, grinding wheels wear and must be dressed, which generates higher costs compared with wire EDM.

According to Mr. Hummel, ground tools achieve longer tool life and better part finishes than their counterparts that are machined using EDM units in steel applications.

My time at Schwanog taught me that while the EDM process poses many advantages, most tools made of hard metals as well as rotary tools are ground at Schwanog. 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 EDM to machine the part before the grinding process is more efficient.