Cast iron is brittle, but as far as metals go, it isn't very hard -- so drilling a hole into it isn't as difficult as you might expect. Moreover, you don't have to be overly choosy about your drill bit. Any bit intended for drilling through metal will drill through cast iron. The key is to take your time to avoid overheating the bit and wearing it out.
To Lubricate or Not to Lubricate
Although many machinists advise against lubricating when drilling through cast iron, they don't completely agree on the issue. Cast iron has a high carbon content, and since carbon acts as a lubricant it's often acceptable to drill iron without lubricant. Lubricant makes a mess and can prevent metal debris from falling away from the bit as you drill. On the other hand, a lubricant, such as oil or a water-based coolant, cools the drill bit and the metal and helps to prevent cracking or excessive bit wear. The bo**ttom Line is this:** If you're drilling one or two holes, you can rely on the material itself to lubricate your bit, but if you're drilling many holes, your bit will probably last longer if you apply a few drops of cutting oil or spray lubricant before you drill each one.
You don't have to search far and wide for a bit to drill through cast iron, but not every bit is appropriate, either.
What Not to Use
The only drill bits you should not use to drill cast iron are those intended for wood or masonry only. Cast iron would completely destroy the the brad point and extra-wide flutes of a wood boring bit, and a masonry bit is designed to use with a hammer drill; it would take all day for the dull tip of a masonry bit to penetrate cast iron. Spade bits, Forstner bits and auger bits are similarly unsuitable for cast iron -- and metal in general.
What to Use
The best drill bit to use for cast iron is a cobalt bit with a 135-degree point angle. The angle is sharper than that on a conventional bit, making drilling faster and more accurate. Cobalt bits are brass-colored. As an alternative, you can also use a gold-colored titanium nitride bit. In fact, any high speed steel bit intended for metal -- including all-purpose bits -- is suitable for cast iron. When using a drill press, you may find that a bit with a 118-degree point angle drills more smoothly and creates fewer shards.
Tapping -- any tap appropriate for metal works on cast iron, but you might prefer to choose one made specifically for cast iron.
The recommended maximum drilling speed for soft cast iron is 150 surface feet per minute (SFM). This value is related to drill revolutions per minute (RPM) by the formula:
RPM = SFM x 3.82/drill bit diameter
If you're drilling a 1/2-inch hole, then, the maximum drill speed should be 1,146 rpm, which is about midrange on most portable drills. Reduce this speed by half for hard cast iron -- and if you're drilling outside in the winter, reduce it to 300 rpm.