The best way to create a deep hole, such as  the cord hole in a lamp base, and keep it true is to forget about the drill.

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HoleIsAHole

The best way to create a deep hole, such as the cord hole in a lamp base, and keep it true is to forget about the drill. Turn instead to your tablesaw or router table. If you're cutting the lamp base from solid stock, cut your base blank to its final dimension, plus about 316 " extra width to account for your blade's kerf. Joint or sand it to clean up the faces. Then rip the base in half at your tablesaw. Otherwise, if you're lamina-ting a blank from multiple layers, build up two half-thicknesses, but don't glue the two halves together, yet. Use a dado blade to cut a centered groove in the inside faces of the lampbase halves, as shown. Alternatively, if you absolutely need a round hole, you can use a core-box bit at your router table. Then simply glue and clamp the two halves together.
Michael Kelly, Summit, N.J.