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Thread: Sizing of cast rings?

  1. #1
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    Default Sizing of cast rings?

    Hello everyone!

    I have a question (or two) for those of you more experienced with casting, since I have never had any done before.

    Background is; I am making wedding and engagement rings for me and my girlfriend, so naturally I don't want to mess up too badly.
    Since we are both not too fond of golden things I thought I'd have it cast in Palladium. So naturally I need to get it cast at a professional caster since I don't have the equipment to solder or cast it at home.

    The rings themselves I will design using CAD, then let the caster print and cast it. (I have no experience of working with wax either, plus, it saves shipping costs). Since I know our measurements in mm it is also easy to adjust it in CAD.

    According to a caster I spoke to there would be almost no shrinkage due to wax model and casting in Palladium, so that is good.

    Now for the actual questions:

    1) I will be using a court shaped ring (shallow curved on the inside) for all rings since it is more comfortable. However, my question is how this affects measurements. Since fingers are quite soft I would guess that the measurement would be using the maximum inner diameter more than the minimum inner diameter, since that is what will be pressed against the finger and it will become too loose if only using the minimum diameter.
    So, would I make the rings a little bit smaller to match the maximum inner diameter, use the minimum or somewhere in between?

    2) Finishing. I have seen a bit of reference that you will lose a bit of material to finish up a cast. The question is, how rough do they usually come out, how much do I need to polish away, and will it affect the size to a degree that I need to adjust for it?

    I know I can make it a little bit smaller and then size it up a bit, but I don't want to make it too small or too big to begin with since it is the first time casting, and it will be a fairly expensive one at that.

    Do anyone have any (other) advice?

    /Andreas

  2. #2
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    Default

    As the caster says, although the tendency is for a casting to shrink into itself, so in theory it would end up a tiny amount bigger, in practice there is little in it. Also the inside finish will be pretty smooth, but will enlarge very slightly on polishing. So all in all you would make it no more than a half size smaller.

    The real crux is that you must take the size with a sizer of the same profile and width as the final ring. Also it is wise to leave this test ring with the wearer for a week or two to be absolutely sure that they are happy with it.

    As a result I recommend that you make a plain silver ring first to confirm the sizing, or risk painful resizing later. Dennis.

  3. #3
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    Thank you.

    Pretty much as I thought, the biggest issue is that my fiancee is located in Russia and I am in Sweden, so we don't see each other that often. I guess I will need to spend a bit on postage, both from the caster to me and then to Moscow.
    She has tried on a few rings over there (and they do size in mm, so it is quite easy to make a CAD model out of it).
    Still I would feel better if it was tested (and I can see how much I need to clean up the cast) before making the palladium ones.

    /Andreas

  4. #4
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    I always make the model half a size bigger and it depends on how you have the model made for clean up of its grown allow 0.1 to 0.2 mm for clean up depending on the how big the grown lines are if it's milled you won't need as much extra metal in my opinion on

  5. #5
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    Well we have hit a snag in hat I was sure that the ring would end up a fraction big, but joseph who has more regular experience of cast rings says that it will end up a little smaller.

    We need someone out there to adjudicate. Dennis.

  6. #6
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    Great idea, and romantic too.. top marks.

    I'm by no means an expert on such matters, although I do tend to cast a lot of things from wax models I make.

    In my humble opinion, I reckon it'll shrink a little.. or at least thats my experience. Delft clay casting, and of course the more advanced centrifugal / lost wax casting methods should not leave too much clean up to do I would have thought. Casting into something like cuttlefish which is of course textured, does however require quite a bit of clean up on surfaces that are designed to be smooth.

    Cheers,

    Nick

  7. #7
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    Well, this didn't really clarify much then.

    I would also think that it would have shrunk in to itself, making the ring a bit bigger, not smaller. Though I guess it makes sense for it to shrink a bit in diameter as well since it is a circle. I would think the investment somewhat stopped it from getting even smaller.

    So to name an example: I want to make a ring in size N (17.1 mm), so according to Joseph (and Nick) I would make it as a size N (17.3 mm) with an added 0.1-0.2 mm thickness to compensate for cleaning up?

    My brain sooo screams at me that it is wrong to make it bigger in order to get it smaller.

    It would be professionally cast since there is no way I could work with Palladium at home.

    /Andreas

  8. #8
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    Just to add to the confusion

    I often make a ring in silver (as the model), have a latex mould cut and have it professionally cast in gold. The clean up of the castings is minimal, I think allowing 0.1-0.2mm is about right. The rings definitely shrink and like Joseph & Nick I make the model half a size bigger to compensate.

    (now the confusion!!)...I thought that the shrinkage occurred due to the latex mould, when it is heated and cools, so for instance you get minimal difference if using a silicone mould,.....therefore I am unsure whether you will get shrinkage if you are supplying the wax model....??

  9. #9
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    Nice of you to add a bit of confusion.

    The caster says the shrinkage is minimal with printing/milling to wax and then casting directly from it. I am unsure in what tolerances "minimal" is though.
    I would need to compensate for the cleaning up though.

    The plain rings are quite easy to make, so I will just need to decide on a size to cast them in, make them in silver and then try them out for size.
    That said, I would rather have them a tad small so I can figure out how much I need to enlarge my model than to have to reduce it (since I don't have a rig for that).

    As for the casting firm I talked to they don't have a finishing service. Perhaps I should have a dialogue with one that has it and they can tell me exactly how they would have done it if they were to finish it in house.

    /Andreas

  10. #10
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    I added a couple of test rings today too test the amount if any of shrinkage, the rings were both injected into the same rubber mould they were both size N


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    Our ring is the one on the bottom left to be cast in 18 ct white gold


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    also on the bottom left to be cast in platinum, No palladium this week so this is the closest test I can do


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    The amount of metal in the ring will also effect the outcome the more metal the more shrinkage I have found so there is no definite answer just an informed guess. As stated the bluer wax is injected from rubber / silicon moulds .The whiter wax is grown by a 3d printer, the surface finish of the grown wax is rougher than the blue as these are highly polished before the moulds are taken, this is why you should allow for more metal to be polished away. All been well I will post the cast rings tommorrow and we can see what happens. I will measure the rings up and check them against the cad to see the results

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