When you shape the opening in the female part of a box catch you
have to be aware of how much return you’ll get from the spring on
the tongue. If you bend the spring up 1.5 mm and depress it
fully and it only returns .75 mm you can’t make the opening for
this catch to require that the spring returns 1.5 mm in order to
work. The tongue on your basic box catch is two thickness of
sheet. If together they are 1 mm (.6 mm base,.4 mm spring)
thick I wouldn’t make the opening to accept them more than 1.2
mm. If my spring opens .75 mm the catch won’t slip.
I’d suggest making the lower portion of the tongue wider and
thicker (for strength) and soldering a thinner, narrower spring
to that. Make an extra step in the opening in the female portion
to accommodate the wider lower portion of the tongue. This will
prevent someone from raising the lower portion of the tongue
leaving the spring engaged when they’re trying to open the catch.
A thinner spring will have more movement and the catch will
also be easier to operate.
I use 18 kt nickel white gold for springs. I find it works
better than 12 kt or 14 kt and the extra cost is negligible.
I hope that was clear. It would be nice to be able to send
sketch’s in email.