Although grinding is a widely employed method for hard and brittle materials, drilling microholes requires the use of an ultrasmall-diameter grinding wheel that is difficult to fabricate and breaks easily when grinding force is applied, resulting in a high tool cost. To solve this problem, cemented WC micropins were fabricated by electrical discharge machining, and microholes were drilled using them as micro-grinding tools with the assistance of ultrasonic oscillation. The micropin tools can be employed in grinding because the convex parts of the electrical discharge craters formed on their surfaces serve as cutting edges of abrasive grains in grinding wheels. To clarify the drilling conditions necessary for ultrasmall-diameter tools to drill holes with a diameter less than 5 µm, the relationships between drilling conditions and drilling characteristics were investigated. The drilling conditions included the tool rotation speed, tool feed speed, ultrasonic oscillation amplitude, and use of grinding fluid. The drilling characteristics included the hole diameter and grinding force evolution. The investigation showed that a high tool rotation speed prevented the increase in the grinding force because of the faster grinding speed. A low tool feed speed was favorable to lower grinding force. At a high tool feed speed, most likely, the tool was fed before the workpiece material was sufficiently removed, thereby exhibiting a large grinding force. The ultrasonic oscillation amplitude had no noticeable effect on the grinding force. The hole diameter was not affected by the tool rotation speed, tool feed speed, or ultrasonic oscillation amplitude. Although the use of grinding fluid reduced the grinding force, the hole diameter increased, probably because enhanced lubrication caused the tool to slip on the workpiece surface at the beginning of drilling, resulting in an increased rotational runout. Under the favorable conditions obtained, a hole of 3.2 µm in diameter was successfully drilled in crown glass.

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