Ancient Chinese Crossbow Trigger - 弩機 (nǔ jī)

Warning:

This is an educational model only. Printed in plastic, it is unlikely to carry any real load.

About this mechanism:

This is a model of a bronze Chinese trigger mechanism for crossbows, primarily from the Warring States to Han Dynasty (c. 5th century BCE – 220 CE). Commonly referred to as 弩機 (nuji), or literally "crossbow mechanism", these triggers were sophisticated, standardized, with three-piece interlocking assemblies (teeth, sear, trigger).

The mechanism consists of a bronze housing that locks the string in place using a rotating hook (often called a "teeth" or "nut") via a sear and a trigger lever. When the trigger lever is pulled, it unlocks the sear, allowing the hook to rotate and releases the string.

About this model:

The mechanism is able to reset after firing by pulling back the vertical spur of the teeth. But because the trigger lever is printed in hollow plastic and very light, you may need to angle the mechanism downwards to have it reset consistently. Flick it back with a sharp pull and it should reset with a satisfying click. It also helps if you sand down all the pinholes to reduce friction.

The "teeth", analogous to the "nut" in western crossbows, is supposed to be casted/welded in one piece, but it is separated into two pieces for ease of printing. If printed flat, the integrated pin of the TeethMain part is not printed in the strongest orientation. It is suggested to increase wall thickness for the pin part specifically, or even set that geometry to 100% infill.

The pinholes are uniformly 10mm across, and the pins themselves have an offset of 0.2mm for clearance. For my printer, I needed to send down the pinholes for the parts to rotate freely, but this also mean that the pins are pretty securely held onto the housing just via friction fit. If you want a smoother working model or an easier fit, it may be easier to make your own undersized pins or scale them down.

All mating surfaces have 0.2mm clearance. The teeth, sear, and trigger are intended to print flat, and have 1mm chamfers to alleviate elephant footing.

Parts:

Housing x 1

TeethMain x 1, TeethAux x 1

Sear x 1

Trigger x 1

Pin x 2