Drilling a hole through the center of a ball is difficult without a guide, since there are no flat sides for reference to locate the center or keep the drill aimed straight. A ball-drilling jig solves the problem. You can make one from a few scrap pieces of wood and four bolts. The jig holds a ball securely beneath a drill press and indicates where to align the bit to drill through the center of the ball.
Things You'll Need
3/4 x 6 x 12-inch wood
3/4 x 6 x 6-inch wood
Drill bit 1/4 to 1/2 inch smaller than the ball's diameter
4 carriage bolts, 1/4 x 4-inch
4 wingnuts and washers, 1/4-inch
Drill press with bit the size of the center hole
Cut two pieces of 3/4-inch wood, one 6 x 12 inches and the other 6 x 6 inches.
Mark the center of the 6 x 6 wood by drawing an X from corner to corner. This permanent mark will be used to align your drill in the center of the ball.
Drill a hole in the center of each piece of wood. The diameter of the hole depends on the diameter of the ball. If the ball is 1 1/4 inches or less, drill a hole that's approximately 1/4-inch less than the ball. If the ball is larger, drill a hole that's approximately 1/2-inch less than the ball.
Use a rasp to slope the edges of the hole on the underside of the 6 x 6 wood.
Place the two pieces of wood on top of each other with the holes aligned. Drill four holes for 1/4-inch bolts one inch in from each corner. Counter-sink the holes on the bottom board so the bolt heads won't protrude.
Run the bolts up through the bottom board and out the top board. Place the washers on the ends and screw on the wingnuts.
Separate the two pieces of wood by sliding the top one up the bolts. Place the ball between them, resting in the holes. Fasten the wingnuts until the ball is clamped securely.
Lay a straight edge along each line of the "X" and mark each line on the top of the ball, which will show through the hole. The spot where the lines intersect indicates the center of the ball.
Place the sandwich of wood and ball on the table of a drill press. Insert the bit you want to use for the center hole and align it perpendicular to the table. Start at the mark where the two lines of the "X" crossed. When you drill straight down, you'll be drilling a hole through the exact center of the ball.
Make the larger piece of wood the same size as your drill-press table so it's easy to clamp in place.