My friend's trying to drill fist sized precious stone through tiny hole and remove inside.We want to keep outer side intact. He's going to fill and keep oriental medicine inside. But we can't figure out how to drill and remove inside of stone without making big hole. My friend already smashed one fist sized stone into several pieces while trying to drill it. By the way it's very hard stone. We bought one already done by Chinese craftsman, but it was too expensive. So we decided to do it by ourselves.
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Normally a grinder is used to hollow out a hard rock. Knowing what kind of rock it is may be relevant. For example, if it is subject to fracturing, you definitely want to use a grinder and take special measures.
To hollow out a sphere manually the typical equipment would be a flexible shaft grinder with a right angle attachment. It will be a long process. Professionals can use specialized milling machines to do something like this, but such equipment costs a lot (like $100,000 or more).
You might be able to hire a machinist to do the work. Where I live they would charge maybe $300-$500 to do something like this.
Update based on OP comment: if the hole is 5mm they are almost certainly using a CNC mill to do it with a straight shank hemispheric diamond burr. Programming the mill to do an inverse spherical turn like this requires expertise. If the outside of the sphere is irregular you will need a 6-axis mill, very expensive.
The drawing below shows the basic strategy using a straight shank with a burr. Obviously a complex setup or CNC is required to control the cutter to make a perfectly spherical interior cavity.