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.
Fun fact: Calcium is the most abundant mineral in the body, and is found naturally in a wide variety of foods and beverages and added to many others! (Hellooo orange juice.) But whether lactose-intolerant or sick of wine and cheese parties, thereâs no need to rely only on dairy products for that daily dose of calcium.