Since the backset is a measurement in inches, it is easy to measure the distance across the face of the door with a tape measure. However, if you check the length of the backset on a large number of doors that are already in place, you might find that the length varies somewhat. Actually there are two different lengths that American door knobs and door knob locks are designed to accommodate. These measurements are 2 3/8 and 2 3/4 inches. Most knob sets are made to be placed at one location or the other, although one company does make a door knob set that has an adjustable part so it can be placed at either distance from the edge of the door.
The length of the latch determines the measurement of the backset that a particular door knob requires. The latch is a built-up piece of metal that connects to the door handle apparatus and runs to the edge of the door and then continues across the very small gap between the door and door jamb and comes to rest inside the striker plate that is placed in the jamb. When not in use the latch penetrates a short way into the jamb and holds the door in place, but when the door knob is turned the mechanism retracts and the door can be opened.
The Circular Opening
The circular opening for the door knob is cut through the entire thickness of the door to form a continuous opening that passes from one side of the door to the other. The placement of this large round opening is determined both by the size of the circle and by the length of the backset. So if you are installing a circular opening in a new door, you need to know the distance from the edge of the door to the center point of the opening, because this point is where you will start the cut.
Circular cutting is done with a round cutting blade, sometimes called a hole saw. To use a hole saw you will need an electric drill that has variable speeds and is reversible. That is because the hole saw is manufactured to fit around a drill bit, and this drill bit has a shaft at one end which fits into your electric drill. When you turn the drill on the bit, the circular cutting blade starts spinning at the same time, because they are made as one unit. The tip of the drill bit penetrates the wood of the door first, but the circular cutting blade follows close behind. As the blade spins, it makes a circular incision that is continued through the entire thickness of the door. However, to finish the cut it is usually best to wait until the tip of the drill penetrates the other side of the door and then finish the cut from the other side using the small hole as a guide.
Door Knob Locks
The door knob apparatus that passes through the door can be of the locking or nonlocking variety. Many door knobs are installed that act as passage handles. With a passage handle the latch, when not in use, goes into the extended position and thus holds the door in place. However, the door can be opened from either side at any time, because there is no locking mechanism. Besides passage handles, there are locking door knobs, which come in many types and varieties.