Superhard vs. Superhard: Drilling Performance

Toolmakers combine technologies to meet growing demand for PCD tools.

The cutting edge geometries and final dimensions of cutting tools are typically produced on CNC tool and cutter grinders. Tools with PCD cutting edges, however, pose a challenge. Grinding PCD pits the hardest known substance (diamond) against an equally hard grinding wheel. The resulting virtual standoff consumes excessive time and wheels.

“When you think about it, what you are doing is taking two substances of equal hardness and trying to have one remove the other,” said Scott Ries, PCD division manager for Vollmer of America Corp., Carnegie, Pa. “You end up having an almost one-to-one wear, which becomes very expensive.”

Courtesy of Walter

Electrical discharge grinding, shown here on a Walter Helitronic Diamond machine, is an efficient and accurate way to remove PCD when manufacturing or regrinding PCD tools.

One way to limit expense is removing PCD via electrical erosion, in the form of either electrical discharge grinding with a tungsten-copper disc or wire EDMing. The cobalt binder in PCD acts as a conductor for the electrical energy. The main benefit of using an EDG or a wire EDM to shape PCD tools is cost savings, Ries noted. “You are using a noncontact, electrical charge to remove material instead of wearing down a diamond wheel.”

Ries has been working with electrical erosion of PCD since the early 1980s, when, he said, thermal damage of the diamond material up to 0.2mm deep was a problem. “EDM at that time involved capacitors. The capacitors had natural charge and discharge times; with other electrical components we would try to influence that charge and discharge and either make it slower or faster.”

But the poorly controlled EDM pulses were “frying everything up,” according to Ries. “You had a recast layer or you were pulling the cobalt out,” he said. The thermally damaged PCD cutting edge would break down in use, and it was necessary to grind away the damaged PCD to produce a reliable cutting edge.

Today’s electrical erosion spark generators, however, employ solid-state circuitry that controls spark ontime duration and frequency within nanoseconds. “With optimized erosion, in a lot of applications we are actually duplicating a ground finish,” Ries said. A finish of 0.2µm Ra is possible with electrical erosion, he added.

The main benefit of post-erosion grinding is extended tool life. Ries said the decision to follow erosion with grinding to generate a finer finish is usually driven by economics. “It is based on the amount of time you want to take. If I am going to put grinding time into this tool, how much more tool life am I going to get in the application?”

In 2000, Ries compared the life of a PCD tool with a ground finish to that of one with an eroded finish when machining high-silicon aluminum. He said the eroded finish provided 85 percent of the ground tool’s operational life. Based on spark generator improvements since then, “I’m guessing now we’re in the 90 to 95 percent tool life range,” he said. “In an application, if all things are equal, what does that 5 percent tool life buy me? How much time do I put into it, how much grinding wheel cost, how much extra labor and machine time?”

Dual Capability

PCD tools with eroded finishes are effective in many applications. “We have eroded tools that are doing very well machining composites, titanium and high-silicon aluminum,” Ries said.

However, some tools with PCD cutting edges also require grinding on their carbide shanks. And some tools benefit from a finer finish than possible with electrical erosion. As a result, machine builders have developed units to remove PCD that combine traditional grinding with EDG.

Vollmer, for example, builds machines that combine conventional grinding with rotary EDG as well as multiaxis wire EDMs for shaping PCD tools. Walter Grinders, United Grinding Technologies Inc., Fredericksburg, Va., also offers machines with EDG and conventional grinding capabilities. Ed Sinkora, marketing manager, tool division for United Grinding, said Walter has two basic Helitronic Diamond platforms for these applications. In a 2-spindle machine, one spindle is dedicated to grinding, and the other can either be for grinding or erosion. “Those who are just getting into the PCD business will still have a 2-spindle grinder,” Sinkora said.

The other Helitronic Diamond configuration involves a single-spindle machine with a wheel changer that switches both the grinding or EDG wheel and its cooling manifold. A 12-position carousel can hold a mixture of grinding wheel and/or erosion wheel setups, which can be quickly exchanged to produce different tool features.

Courtesy of Vollmer

PCD cutting edges can be shaped via electrical erosion (EDG) with a tungsten-copper disc. The cobalt binder in the PCD tool conducts the electrical energy, and the electrical charge removes the diamond material. Here, the EDG disc in a Vollmer machine is in position at a tool before the flow of dielectric is started.

Vollmer’s Ries said wire EDM can offer an advantage compared to rotary erosion because it can create small inside radii and other complex tool features that a rotary disc can’t. On the other hand, he said, the rotary electrode is a thick, stiff piece of tungsten copper that, unlike a wire, does not offer any deflection. “It is much more rigid erosion, and all things being equal, you have a faster material-removal rate.” The advantage lessens, he said, when removing more than 0.3mm to 0.4 mm of excess PCD stock and wire EDMing becomes easier.

Ries said Vollmer’s wire EDMs present the wire to the part horizontally rather than in the common vertical alignment. That arrangement enables the wire to travel in a larger range of motion than a vertical wire when shaping a tool clamped in the machine’s C-axis.

Field Applications

Toolmakers are seeking ways to increase speed and quality when producing PCD tools in response to industry’s growing use of lightweight materials, such as aluminum and carbon-fiber composites, which can be productively machined with PCD tools.

Big Sky EDM Inc., Hayden Lake, Idaho, produces and refurbishes PCD and PCBN cutting tools for fabricators, tool producers and end users. “Probably 85 percent of our business is cutting PCD and CBN tips,” said Russ Scoffield, president. In addition to wire, sinker and high-speed CNC drilling with an EDM, the shop employs 6-axis robotic automation.

Automation minimizes labor costs during the lengthy process of cutting tips from PCD and CBN discs. “Cycle times are long, so instead of running three shifts a day to keep the machines running, we run one shift and the robot takes care of the rest,” Scoffield said. Big Sky recently began producing its own line of veined PCD tools for machining carbon-fiber and aluminum aerospace parts.

Courtesy of West Ohio Tool

West Ohio Tool recently added a Vollmer QWD 760 wire EDM to create PCD tool geometries unachievable with EDG. A back view of the machine (top) shows that the wire is presented to the part horizontally rather than in the common vertical alignment, an arrangement that enables the wire to travel in a larger range of motion than a vertical wire when shaping a tool clamped in the machine’s C-axis. A front view (bottom) shows the machine in action with a flood of dielectric fluid.