detents

Babbage was not a fan of springs, which at the time were not uniform and were prone to failure. One of his 1837 "Principles of the Engine" was

"to have no springs to do work, only retaining springs".

But he certainly made liberal use of spring for detents, which impose one or more preferred locations for an otherwise continuous movement. It is, as Tim Robinson observes, a "soft" version of the locking that he uses when parts should not be moving at all. Here are examples, some of which include the option of a hard lock.

So how to easily implement detents, with fewer parts -- and in particular fewer vertical shafts? My first attempt was to use a "spring plunger" from McMaster-Carr, which is an acetal plastic nose in a steel 8-32 threaded case, with an embedded spring that provides 1 to 3 pounds of force. I used it screwed into a 3/8" vertical shaft and had it engage with notches of the kind Babbage showed.

It works, but the small 1.7mm tip size requires small notches and increases the areal force, which makes it too hard to move the part off the notch. I have since switched to using 4mm x 5mm  ball plungers available on Amazon (at 1/50th the price!) that have a 3mm ball and similar 6 Newton (1.3 lb) force, but a larger contact radius.

With my new (used) mini-lathe I drill an undersized hole in the end of 1/4-20 bolts that the plungers are force-fit into. The bolts are then screwed into threaded holes drilled in 1/2" shafts. I was able to increase the size of the notches in the parts that need to be be partially immobilized (in this case the carry warning arms), and the action seems both smoother and more positive.