The Knob Latch: A Bored Hole, a Sprung Bolt and a Lug on Each Spindle

"This invention relates to that class of knob-locks in which the knobs are operated independently of each other, and actuate the bolt by means of their spindles, and in which the locking mechanism is controlled by a suitable key or stop inserted through the knobs." "B the main barrel of the lock, which is adapted to be arranged in an opening cut through the door at a proper distance from its edge. C is the bolt-case, which is adapted to be inserted in an opening bored inwardly from the edge of the door, and intersecting the opening for the barrel." "The slot in the barrel, it will be perceived, permits the bolt-case to be centered in doors of different thickness, and obviates the necessity of great precision in forming the opening for said case." Each hollow spindle has a head with "two lugs, h, projecting from its inner flat face, on opposite sides of the spindle. These lugs h project in front of the concave curved faces i of the inner bolt-head, I; and when one of the knobs is turned in either direction, one of these lugs will strike its adjacent one of the curved faces and move the bolt rearward."

Take a length of pine as thick as a door (or glue two boards) and mark a line 60 mm in from its edge, centred on its thickness. Bore the face hole for the latch body with a hole saw, halfway from each side so the exit does not splinter. Bore the edge hole with a spade bit, square to the edge and centred, until it meets the face hole: that is Ludlow's bored bolt case. Chisel a shallow recess for the latch face plate, fit the latch and knobs, and screw them. Measure the bolt's throw and time how far the knob turns to withdraw it. Clamp the block in a vise; wear eye protection while boring.

**Washington I. Ludlow** of Ashtabula, Ohio, filed US 212,956 on 23 December 1878; it is dated 4 March 1879. Besides the latch it covers a key cylinder in the outside knob that can lock the knob or the bolt. **Honest limits.** The patent gives no sizes. Lug radius, throw, spring force, knob size and thread pitch in the notebook are examples; a modern tubular latch's backset and bore sizes come from its own instructions.