Router shaves edge for gap-free joint

You're troubled by gaps along the mating line between the top and bottom parts of a small box you've just completed. Is there some way to true up the mating edges without resorting to a lot of sanding or planing?

This is a job for your table-mounted router fitted with a straight bit as shown above. For each part, cut spacer blocks about ¹⁄₈ " taller than the inside depth. Fasten them inside the part with double-faced tape. Then, place the part upside down on the router table. Adjust the router to take a light cut—about ¹⁄₃₂ "—off the edge all around. Do the same for the other part. You'll end up with flat, true mating surfaces and a lid that fits.
—George E. Casey, Tacoma, Wash.