Go to a hardware store and get the size bit you need that is made for metal. Most cast iron isn't really hard unless it is white cast iron. If the hole is over 3/16, pre-drill it with a small bit first. It might be a good idea to use some cutting oil to.
For more info, see the section on calcium absorption. High calcium foods include dark leafy greens, cheese, low-fat milk and yogurt, bok choy, fortified tofu, okra, broccoli, green beans, almonds, and fish canned with their bones. The daily value (DV) for calcium is 1000mg.
The Health Benefits of Calcium. Calcium is the most abundant mineral in the human body, according to the National Institutes of Health (NIH), and the recommended intake is 1,000 mg a day for adults. Ninety-nine percent of the body's calcium is stored in bones and teeth for structural support.