contraption, fetter, machine, instrument
械 centers on mechanical devices and tools, from simple contraptions to complex machines. The fetter sense is a specific application: a mechanical restraint.
械 combines 木 (wood) with 戒, which likely contributes the sound and the idea of caution or restraint. Historically, it referred to wooden fetters or instruments, later extending to machines and contraptions.
A wooden 木 frame with a warning 戒 sign attached: think of a wooden contraption or a restraining device like a fetter. The warning reminds you it's a machine that can trap you.
For カイ, imagine a wooden machine that opens with a key: key -> カイ. You turn the key and the contraption springs to life.
machine; mechanism
apparatus gymnastics; artistic gymnastics
Asia-Pacific Association for Machine Translation; AAMT
Antikythera mechanism (ancient Greek analog computer used to make astronomical predictions)
online machines
session protocol machine; SPM
Turing machine
data processing machine
Protocol Machine; PM
general instrumentation
office automation; OA
artificial machine
virtual machine; VM
optical instrument
packaging machinery; packaging machine
chemical machinery
medical appliances; surgical instruments
simple machine
accepting Association Control Protocol Machine
command-service module (e.g. Apollo spacecraft); CSM