I want to drill a hole (about 12mm diameter) through the floor of a shipping container. From the inside of the container, the wooden (probably plywood) floor is visible, and from the outside there is the steel hull. I have no idea what is in between those two layers.
How do I get a hole through the whole floor?
Unless this is a powered shipping container, e.g. one with a builtin refrigerator or other such features, there's probably nothing interesting between the plywood floor and the steel skin. So just get a drill bit suitable for steel and have at it.
Important tip: Once you're through the wood and drilling into the steel, stop and apply a few drops of a light oil to the bit to help keep it cool and cutting better through the steel. You'll want to pull the bit out of the hole and apply the drop or two of oil every 10 seconds or so, just to ensure you don't overheat the bit. The wood will help insulate the bit and keep even more heat in, so it's important to keep the bit cooled. If you don't, the heat will cause the bit to dull even faster and will frustrate you, though you'll make it through eventually.
Drill through the wooden flooring first, shine a light into the hole, when you'll be able to see or feel what's directly below. If it's the steel bottom (an magnet would prove) then using a drill bit for steel, carry on through. If there's something else in the way 9doubtful), you can always fill the hole in in the wood, if needed.
An auger may do a better job in the wood, but a tungsten carbide, etc., bit will do both.
