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.
Probably the best source of dietary calcium is fat-free organic Greek yogurt. 1  It gives you 450 mm of calcium per serving, plus vitamin D and protein, and two servings fulfill your calcium needs for a full day. Other sources of calcium include: 2  Leafy green veggies like spinach and kale. 3  Legumes and beans. 4  Sardines.