BTW I am publishing all my designs on Thingiverse
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This latest design seems to work fairly well! It has a larger hole that is useful both for discarding excess clay, and removing the stone from the mold, by gently pressing it with the fingertip. Also the mold has a diameter of 22.5mm as the clay can never perfectly fit the mold.
Thanks for suggesting printing a mold!
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Wow, wonderful! The mold is the cut ping pong ball?
Btw my mold shape is the intersection of two spheres.
What are you using in step 2 to press on the stone?
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Yes, it’s still the ping pong ball and that’s what I use in step 2 to press it into shape. I got a little bit of paint on it
Great! Did you manage to get a consistent 22mm size?
Yes, the mold is about 22mm in diameter, so I just press until the clay reaches the edges of it
They could be rounder and the edges could be cleaner, but that should not be an issue with your 3D printed mold
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Wow great work! Yeah the issue is with my mold the size is fairly consistent but the edges suffer a bit as they “hit” the wall of the mold. The advantage of the ping pong ball approach is that there is a nice continuous curvature so the edge comes out very clean. So i think it’s a bit of trade off; but the ping pong approach if you get the correct amount of clay, the result can be awesome!
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Have you considered using polymer clays? These usually come in assorted colors, which would remove the need for painting (and avoid the risk paint chipping or wearing off).
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I printed new molds, without a hole, and I am definitely satisfied. The hole was ruining the top of the stone.
Now I first slightly press the clay inside the bottom half (the one with the border), then i flip the stone and repeat, to get an approximate shape. Then I use the top half (the one with the “slot” for the border) to get to the final shape. Pressing lightly ensures the stone does not stick to the mold. Overall, the process requires half a minute at most. The results are really nice. Depending on the amount of clay, I can get stones of different thickness.
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I don’t know what happened today, but I got inspired to convey my design solution for the first time. Here’s a concept.
Clarification:
The idea is to have access clay being squeezed out to achieve uniform size for all stones.
I don’t remember if you use self hardening clay. You might need to get fresher batch to have clay softer and more malleable as a direct result of it being fresh.
Since you’re doing it by hand to have precise control over 2 molds alignment you’ll have to have 2 control rods or maybe more which would be 3 or 4.
There’re 2 holes marked on the drawing, 2 is required minimum.
Steepness of the tilted surface (brown hatching) might be adjusted to make it more steep or less steep respectively.
Distance A likely should be increased compared to drawn design to make it less cramped when using finished product.
Pink line (look through) is internal hemispherical cavity which forms the stone shape.
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Actually my mold is similar; the inside has a cavity like the one you drew; alignment of the mold is guaranteed by the “border” (mold on the right in my pic) and “hole” (mold on the left). I first created molds with holes to get eccess clay out, but pressing hard caused an uneven shape on the top, and the clay would stick to the mold.
You can see the details here: Go stones making kit by ercasta - Thingiverse
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I just noticed that your stones have inconsistent size judging by the flat strip in the middle. You have no way to compensate for deviation in amount of clay you feed into mold. Sealed border makes in impossible for clay excess to come out.
If you would have perfect conditions and you assume that the incoming clay has always same and consistent volume/weight (which is unlikely and you always should take into account measurement error) then your design is perfect.
But if you need to address inconsistency of volume of incoming clay your design has no mechanism for negating this. As a result stones come out in different size/thickness.
Probably there’s a better solution to mine and I wouldn’t be surprised if “AI will refute my joseki”, so to speak. I’m not trying to sell it 
, but rather to keep up the conversation. 
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Yes, you are right about the inconsistency. I also printed a small cylinder whose volume is exactly the volume of a stone, to measure the clay before using the mold, but I find it hard to use so I might decide to improve on that. Also, after a few stones, you get a natural feeling of the amount of clay you need to get.
If you look at some posts before (I know, I am spamming a lot 
), you see molds with a hole, but as per my previous post, this lead to more issues. So I think the best approach is to measure the clay first, and then use the mold
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I also found out hardened clay can be sanded very quickly, to “fix” main issues like imperfections on the edge, or the stone being “skewed” on one side. This means the mold can be used to get a rough shape, without spending too much time on the details. So I would say 30 seconds for the mold, and another 30 seconds for sanding, if one really wants very nice stones
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I can confirm that squeezing the excess clay out on the edges and cutting them with the edge of the mold yields really good results. I noticed that when I made some more stones. Aligning the two halves isn’t super hard, but the guiding rods would make it more convenient. If I ever make some more stones, I would sharpen the edges of the mold to cut off the excess more easily. There are a couple tricks that I have not mentioned. First, you can clean up the edges of the stones by gently running wet fingers along it. Next is to put the stones down on a soft surface, like a cloth, because some of my stones have an indentation from putting them down on the table. Lastly, you can smoothen the surface using sandpaper for a better finish, or more conveniently putting them in a bowl of fine sand and mixing it about a bit (have not actually tried that.)
Also I would suggest adding a lever to your design, so that you can exert more pressure, because squeezing out the clay cleanly requires quite some force depending how much excess clay there is. If someone wants to recreate my method, I got just about the right amount of clay by making cylinders with 16mm diameter and 7mm height as a reference, so make them slightly larger to get a bit of excess.
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I think wet clay is easier to squeeze than self hardening clay, which is more sticky.
As a note, even “real” stones are tumbled in a barrel (for many hours) to get the final results. I don’t have a self-turning barrel so I sand them by hand . There are small tumblers like this one but I don’t have budget for it https://www.amazon.it/ROMYIX-lucidatrice-Professionale-rotelle-Gioielli/dp/B07YKSF6B3
Please let me know if you want me to add versions with holes to the kit I published: Go stones making kit by ercasta - Thingiverse
I made a new version of the tool.
The protruding edge is slighty higher, and the tool can be used to “cut” a consistent amount of clay.
The hilt is useful because the stone sticks to the mold, and I hit the table with the mold to make the stone fall off the mold.
Results are very nice (the slight excess clay is easily sanded off in a refinement step)
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Very nice. I would love to see some finished stones once you’ve sanded them
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Here they are (comparison) !
To sand them, i just put a piece of sanding paper on my hand, hold the stone with the other hand, and pass a few times changing angle and sanding strength depending on how much I want to sand them. I am experimenting with different sanding paper right now I am using 100, a kind for light materials light plaster.
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