Drill a hole through cast iron using a drill bit for metal. Cast iron is a high carbon product, so most machinists advise against using lubricant when drilling. For a 1/2 inch hole, keep the drill speed below 1,100 revolutions per minute if the object is at room temperature and below 300 rpm if drilling outside and temperatures are cold.
Cast iron is a relatively soft metal, so any metal drilling bit works. However, it is important to avoid overheating the bit, which causes it to become dull. When drilling larger holes, the drill speed should be slower. Avoid trying to use wood drilling bits or masonry bits. Wood drilling bits are too soft to cut through the metal, and masonry bits are designed for use with a hammer drill.
If your plans include just a few holes, any all-purpose steel drill bit works for the job. For easier drilling with a portable drill, look for a cobalt bit that offers a 135-degree point angle. This sharper angle makes drilling faster and more precise. Gold-colored titanium nitrate bits are a good substitute for cobalt. For a drill press, use a 118-degree point angle bit. The cuts are smoother and produce fewer shards. Thread the hole with a tap for cast iron use.