Bell System/Western Electric P-12A892/811218924 screw detail

thingiverse

This is a Western Electric P-12A892 (also known under the part number 811218924) "screw detail", a special screw used in the 636A key set (that is, a component with pushbuttons used to select one of five telephone lines) in the (among others) model 564 key telephone (a common fixture in small to medium businesses with telephone service provided by the Bell System). The screw screws into a plastic component of each key. Its narrow, unthreaded end projects into the assembly that allows that key to latch and releases other keys when that key is pressed. Its wide head forms part of a mechanism that prevents two keys from being pressed at the same time. (Contemporary Bell System documentation says that bridging two lines by connecting them together results in inferior transmission characteristics, but I've heard of other Bell System key telephones allowing users to press multiple keys at once to connect two or more lines together, and I've read stories of people bypassing the mechanism that prevents multiple keys from being pressed at the same time with no ill effects.) By default, all keys on a 636A key assembly come with this screw inserted; removing this screw converts the key into a nonlatching, nonexcluding button, and could be done to use the button for signaling purposes instead of selecting a line. My 564HLM telephone came without one of these screws in the rightmost key, but didn't have the wiring changes required to make that key operate as a signal button.A key telephone is a telephone that can access one of several lines by the means of pushbuttons located on the phone. Each line is associated with a pushbutton (called a key), and that line is connected to the phone when the key is down. Pushing down a key releases other buttons, then causes that key to latch; this picks up the desired line and hangs up the previously-selected line (if any). Another key can be pressed to put the current line on hold; this releases its key. Since each line appeared on the key strips of multiple phones, calls could be put on hold and transferred between phones easily. Multiple internal phones could participate on a call to a single external phone number, but multiple lines could not be connected together (usually). Key telephones were a common fixture in small offices and businesses before the late 1980s; they were versatile and included many features needed in business telephone systems, while they were cheaper and smaller than a contemporary private telephone system (PBX). They also obviated the need to give each telephone its own line. Modern telephony systems have in some cases inherited the features of old key systems, but those features have largely been replaced by more straightforward and advanced features.PrintingPlease note that I have not printed this, nor have I tested this for its intended use. Thus, this may not work. The instructions below are speculative.This part has tight tolerances and a small size that make it unsuitable for a FDM printer. I would recommend printing it with an SLA printer that has had its exposure time properly adjusted for the resin in use. Any resin will do. I would recommend printing the part upside-down at an angle.