Casting discrepancies- what do you do?

Hey all-

I work for a jewelry design studio and we’ve been experiencing problems with casting and I wanted to know what everyone else does. So when we send our projects to casting, they said that they “scale down” ring sizes 1/8-1/4 of a size and then there’s additional shrinkage that takes place during the process. Rings have been coming in 1/2 size too small which makes detailed rings in particular problematic.

I’ve had CAD designers say that they’ll bump up the size to account for additional shrinkage but then this messes with what size the casting partner thinks it is so they’ll try to keep the ring that size as much as possible. One way doesn’t always work. What does everyone else do, we want our rings to be 1/8-14 size smaller for obvious polishing/finishing reasons. Let me know any advice thanks!

It seems that if you want them to be 1/4 size smaller than the nominal finished size, but they’re consistently coming in 1/2 size too small, then you should scale them up 1/4 size, no? Or am I missing something? Is the “casting partner” doing something else to these rings besides casting them?

Hello! Yes our casting partner “scales down” the rings 1/8-1/4 a size. But we’re finding that it shrinks an additional 1/8-1/4 of a size during the actual casting process, so perhaps we should have them discontinue scaling them down and take our chances with whatever the casting comes out to be… Not sure, wanted to see what everyone else does for ring castings, if everyone has them scaled down, or casts true to size, or casts 1/4-1/2 size bumped up.

I’m a little confused? If you’re choosing to send out a cad file or a carved wax model, the casting house should be casting it as received, not modifying it? I make them to the finger size needed. After casting they’re always a hair under, a bit of reaming and finishing brings them up to size. There’s a benefit to having the finished casting a bit undersized. It allows you to remove the hot skin and shape things as you like. It’s never so small that you need to put in a piece. I’m not sure if I’m helping?

Your insight most definitely helps! Asking the right questions. I too was under the impression that we send the file/ring as the size needed and to be a hair under is what’s desired. It’s sounding to me like the casting partner shouldn’t be scaling the ring sizes down any.

Does anyone use Casting House???

I use them sometimes but normally cast in-house. They’re fast and they don’t change my model sizes. They do have occasional casting issues that surprise me, trouble casting pieces that I don’t have trouble with. I’m friends with a guy who has a shop in the same building as CH is in and he says the same thing. But 95% of the time everything is fine. Zero Porosity in Massachusetts pays much more attention to detail but takes longer than CH. As I said I always cast to size. I may make the shank fractional heavier to allow for material removal. If you’re growing your models, you want the casting slightly undersized (as results from the model being the finished size) to allow you to remove grow lines and hot skin. You just need to think about that when determining the model thickness.

Great thanks for the info!

Right off the bat, you can’t vacuum cast platinum. The vertical centrifugal caster for platinum are used because molten platinum is still a “sluggish” metal. The vertical casters reach full RPM in the first few revolutions. I’m not really in the loop anymore but when I was at the factory that was purchased by Johnson Matthey and we started casting platinum, it was a pretty steep learning curve (with no help from them) the burnout to cure the investment was hot enough that when I pulled the flask from the oven I got a “sunburn” where my leather apron didn’t cover my open collar. After it was cast and cold as much investment was hammered off and the castings were placed in
as I recall, hydrofluoric acid. Things may have changed in the last couple of decades - or more. As far as casting rhodium? I think the last time I saw spot on it, it was $4,000/oz. depending on the size of your pieces you’ll still need a large button.
BTW - when I talked about the learning curve, it was a month before we had consistently good castings that completely filled and didn’t crack when we put a ring on the mandrel.
Good luck!