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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    Now you know why they are expensive. – Michael Karas Feb 27 '15 at 8:10

Define "tiny".

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.

hollowing a sphere

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  • Can it bend 0-90 degrees inside stone through 5 mm(diameter) hole? Because I need to hollow out all of inside parts except thin outer layer. It's easy to reach other end inside stone because it doesn't require much bending of shaft. But it's very hard to bend inside through tiny hole to reach your side corner with shaft. Also stone dimension is 120mm x 60mm x 40mm. I'm just wondering if this flexible shaft grinder is long enough for this operation. – user3291766 Feb 27 '15 at 13:51
  • If the hole is 5mm then they are not using a flexible shaft grinder, they are using a straight shank grinder with a half-ball head. Doing it manually requires what is called a reverse or inverse spherical turning setup. Making a setup like this requires a machinist. – Tyler Durden Feb 27 '15 at 14:14
  • It sounds promising. Can you give me link for "straight shank grinder with a half-ball head"? Also do you have video or web link for "reverse or inverse spherical turning setup"? I can ask from local machinist if I can show demo of what I need. – user3291766 Feb 28 '15 at 2:09
  • @user3291766 I added a drawing showing the basic approach – Tyler Durden Feb 28 '15 at 4:13

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