Marble is relatively easy to drill through providing you use a sharp drill bit. Once I have decided on the location for grab bar placement, I mark the anchor locations with a dry erase marker using the grab bar as a template. Next, I score the marked locations with a carbide punch and soft mallet. This will prevent the drill bit from skating across the surface of the tile.
With marble tile I usually use a sharp or relatively fresh 1/4 inch glass and tile bit to make the initial hole and follow up with a 5/16 inch glass and tile bit. I tend to use 5/16 anchors in combination with toggles in many of my grab bar installations. You can skip the 1/4 inch bit if you desire but doing so does create somewhat more wear and tear on the larger bit. Marble of course can vary in hardness depending upon what variety of marble you are working with. In some cases dipping the glass and tile bit into a container of water or using a wet sponge every 2 - 3 seconds might facilitate drilling through a more difficult marble. When drilling through polished porcelain or granite I would most definitely use water to cool the bit.
When I am drilling and have passed through the layer of marble and have reached the backer board whatever it may be, I ease off the speed and pressure so I can slowly push through with as much control as possible. It is important to push through this layer very gently to avoid introducing an overheated drill bit to an unexpected obstacle which may lay in close proximity. My next step is to probe about in the holes which I have just drilled with a stiff wire probe. This device allows me to feel for structures or obstacles behind the walls which I can often identify by both sound and by feel.
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