Once again, Ganoksin’s knowledge base provides a nice article on design considerations for creating cast hinges for mass production.
This gives a good starting point, but I’ll add a bit more information below for designing your own hinges in CAD.
Some Useful Advice For Making Hinges in CAD
1.) There are two ways to make the hinge cylinders. The first is that every single hinge cylinder has the same length (much like a door hinge). The second (and the one I prefer using for jewellery) is to have the total length of a hinge divided evenly between the odd and even cylinders on either side. This means the length of all the odd hinge cylinders and the length of all the even cylinders should be equal. For example, in the case of a 3 cylinder hinge, this would mean that the middle cylinder is twice as long as the outer cylinders.
2.) When making hinges, checking your tolerances is key. You need to allow for sufficient thickness in casting for the hinge to survive. This means a minimum wall thickness in silver of 0.75mm for the tube wall, and a hinge pin of at least 1.5mm. Gold and platinum would be smaller– 0.5mm thick tube wall and 1mm hinge pin. When making hinge pins for casting, it’s a good idea to make the hole for the pin smaller than it would ultimately be so it can be drilled out cleanly.
3.) Finally, you want to make sure that the hinge cylinders have recesses in the opposite side in which to sink into, otherwise the hinge parts cannot fit through each other.
For more information, there are specific jewellery books like The Metalsmith’s Book of Boxes and Lockets which can give you a lot more detail on hinge construction.