how can i drill holes through a cast-iron tub and avoid sending cracks though the enamel?

Answer 1

use a carbide drill bit, drill from the inside ,enamel side
and keep it cool ,example use a batt powered drill run a small amount of water over drill bit while drilling and keep the speed on low

Answer 2

They make a drill bit especially for drilling into ceramic tile. You can find it at Lowes or Home Depot.
You can use that at a low speed while keeping the area spayed with water to keep it cool.
Use a wide piece of masking tape and cover the entire area you're drilling to help avoid the enamel from cracking.

Just remember SLOW AND EASY.

Answer 3

vari-bit