Things You'll Need
2-1/8-inch hole saw and mandrel
Heavy-duty 1/2-inch drill
7/8-inch spade drill bit
While most new metal doors come with a hole pre-cut for the doorknob, there may be a need on occasion when you'll have to cut your own. With the proper tools, you can cut through a typical insulated steel door quickly, accurately and safely. When finished, you'll have a standard sized 2-1/8-inch hole suitable for the installation of the doorknob of your choice.
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Lay the door on a pair of sawhorses with the edge of the door where the doorknob latch is set easily accessible.
Tape the doorknob template over the edge of the door positioned where you wish to install the doorknob. The template folds over the edge of the door, presenting a diagram of where the hatch and doorknob holes are located for drilling.
Make a small dimple in the steel of the door edge with a center punch at the center of the latch and doorknob holes indicated by the template. The dimple not only marks the center of the hole for drilling, but also ensures that the drill bit doesn't wander when applied to the door. Remove the template from the door.
Connect the 2-1/8-inch hole saw to a 1/2-inch drill. Connect the mandrel or pilot drill bit of the hole saw to the chuck arbor of the drill securing it tightly in place. Slide the hole saw bit over the pilot drill bit, with the saw tooth end pointed away from the drill body. Secure the pilot saw using the threads in the chuck arbor, screwing them together. Lock the saw into place using the mechanism provided by the saw manufacturer, either the saw will lock by pushing on the head of the saw or it latches into place.
Place the pilot drill bit against the dimpled mark for the doorknob and start the drill. Holding the drill firmly, drill through the metal of the door, using the pilot bit to center the hole for the saw. The pilot bit will begin the hole, guiding the saw as it cuts through the metal covering of the door into the insulation inside. Watch carefully as you drill for the pilot bit to come out of the other side of the door. When that happens, stop drilling and remove the saw from the door.
Repeat the drilling process from the other side of the door, using the pilot hole as a guide, until you've completely sawed through the metal door and the inner insulation.
Use a round file to file the edges of the cut, removing any imperfections from the sawing.
Spray primer onto the cut metal edges to aid in rust prevention.
Switch out the hole saw bit and replace it with a 7/8-inch spade bit. Set the point of the bit against the dimple placed on the door edge for the door latch and then drill the latch hole through the edge of the door into the metal cavity drilled for the doorknob. Make sure that the latch hole is in line with the center of the doorknob hole.