High-performance Drill Bits (cont.)

Toolholders provide the link between the machine tool spindle and the cutting tool, and must meet the same rigidity and concentricity specifications as the spindle. For example, consider a coolant-fed, indexable drill with a 4-in. projection in a toolholding system that allows a maximum of 0.0002 in. within one inch of the toolholder face. The tip or cutting edges could runout over 0.001 in. Toolholders are available that can true the tip or cutting edge to within 0.0001 in. Such a toolholder can improve workpiece accuracy and tool life significantly at increased drilling speeds.

For high-speed drilling, toolmakers design products that prevent chips from coming into contact with a tool's cutting edges and flutes. Hot chips can soften and smear against the tool, filling in microscopic crevices in its edge surface. This built-up edge becomes the new cutting surface and may force the tool off center, leading to a failure. High-pressure coolant, delivered through the tool, can flush hot chips away from the tool and out of the hole.

Coolants are critical at high speeds, and the choice of coolant must match the material being drilled. For example, when drilling steel, the principal desired action of a coolant is to provide cooling to the workpiece, but when drilling aluminum the primary aspect desired in a coolant is lubricity.

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