Don't know what an ice ball maker is, or how it works? Then check out the last page of this Instructable for a video of it in action.
You'll need access to a machine shop, and specifically a CNC machine. TechShop provides all the equipment you need for a monthy fee. I made it at TechShop, and you can too.
Ice ball makers work by melting a large block of ice into the proper shape. It accomplishes this by having two large blocks of aluminum (aluminum has high heat conductivity and capacity), each with a hemisphere carved into a side, slide together to form a single continuous chamber on the inside. What's left is a perfect sphere.
Step 1: Materials and Equipment
You'll need blocks that are in total at least 10 times the volume of your sphere. This is so the aluminum properly melts away the rest of the block. My ice ball maker produces 70 mm diameter ice balls, and the aluminum blocks are each 125x125x75 mm. That's a 13-to-1 ratio, and works pretty well.
You'll also need stainless steel rod. These serve as the bottom vertical guide shafts. I used 3/8" SS rod.
Last, you'll need plastic rod of a larger diameter. This piece slides over the stainless steel rod. I used 3/4" teflon rod, but teflon turns out to be hard to machine. I hear that Delrin machines well.
The only extra material is whatever you think will make the final product easy to use. I added rubber feet and a cabinet knob for lifting the top.
For equipment, you'll need a CNC mill and a lathe, and basic tooling. The inner hemispheres will be cut with a ball end mill, and you'll want as large a diameter here as possible. I used a 1" ball end, but smaller diameters will work as well. You'll also need a tap and die set.
Step 2: Cutting the blocks down to size
I started with 5" square bar stock, which is 127x127 mm, and machined this down to 125x125 mm with a length of 75 mm.
You can do this on a CNC mill or just a normal one. Get a large diameter end mill or, even better, a fly cutter, and machine down each face of the block. For the final finishing step, you may wish to clamp together the two blocks so that even if there is a small amount of error, they still match together perfectly. Make sure to mark a corner so that you know the right orientation!
Alternatively, you can use round bar stock instead of square, and do most of this step on a lathe. You might like the round model better for aesthetic reasons anyway! It's up to you.
Step 3: Machining the Hemispheres
The basic model is easy: use AutoCAD or the equivalent to model the original block, make a sphere of the appropriate diameter, and subtract one from the other. This leaves a block with a hemisphere cut out.
Then, you need to use CAM software to create the toolpaths. You'll want at least two phases: a roughing pass with a large endmill to take away most of the material, and then a finishing pass with the ball end bill to leave a smooth inner surface. Don't try to do the roughing pass with a ball end mill! They don't like being "plunged" directly into metal.
Finally, use the CNC machine to cut the hemispheres. Hopefully, you've learned by practice how to use edge finders, perform air passes, and so on so that you can perform this step safely and accurately. If not, practice some more and find someone to help you!
Step 4: Stainless steel alignment rods
These rods need to be nearly--but not quite--the full height of the ice ball maker. I have chosen 138 mm out of the total 150 mm. The rods start from the very bottom of the bottom block, but do not poke all the way out of the top block. I have left 10 mm of solid aluminum above the top block, and then gave the rods another 2 mm of "slack".
The bottom part of these rods needs to be threaded. I used a 3/8" die, pitch 24 for the threads. I did the threading in a lathe, but you can also do it in a mill or just freehand. The rods need to be as straight as possible, but you can always put some slack in the system later if you need.
I also used a lathe to bevel the edges a bit so as not to leave sharp edges, and make it easier to thread.
Last, I mounted the rods in a drill press, and sanded them (ultimately using 1000 grit) for a very smooth surface. Remember that the plastic pieces slide on these, so the smoother the better.
Step 5: Hole drilling
Since the plastic has a diameter of 3/4", I chose to drill the slightly smaller diameter of 23/32". This meant that I could machine down the plastic pieces to exactly fit the way I wanted.
It's very crucial that the holes on the top and bottom pieces are exactly aligned. After you choose your hole positions, make sure to cut both of them at the same time per block--don't remove it from the vise in-between. Doing so will add measurement error.
Because the holes are so big, you'll want to drill them progressively. Start with a center drill to make the starter hole. Then, use a smaller diameter drill, and only cut a fraction of an inch at a time before lifting it to remove the chips. Do the same with the final size drill.
The top piece has holes of depth 65 mm (leaving 10 mm above), while the bottom piece has depth 60 mm. I left the larger 15 mm on the bottom because we'll be threading holes, and we want a bit more strength here.
Step 6: Drilling and threading the alignment rod holes
Find the tap you need (3/8"-24 in my case) and the drill you need (make sure to look it up in a table!). This should be pretty straightforward: drill the holes in the same location as you did on the top side. As before, make sure you start with a center drill!
Then, tap the hole all the way through. If you use lubricant, this should be very easy since it is not a blind bottom, and the chips can fall all the way through.
When done, you should be able to thread the rods into the holes as shown in the left block in the picture.
You can see in the other picture that I have added rubber feet on the corner; this protects my countertop.
Step 7: Plastic sliders
I started with 3/4" teflon, but as I said you can use other plastics. Approximately half the rod needs to be machined do to *just fit* in the holes you've created. When you get close to the right diameter, only machine off about 2/1000" at a time, and check to see if they fit snugly via friction.
Next, you'll need to drill a hole all the way through of the same diameter as the metal rods--3/8" in my case. Again, use a center drill to start with. You may need to drill from both ends to get the length you need.
Next, you'll want to use a reamer to smoothen and enlarge the hole. Reamers are made with very slightly larger diameters than common sizes: a 3/8" drill is 0.3750", and a common reamer size is 0.3760". This is perfect for us since it gives us just enough gap for a smooth slide.
The other half of the rod should be machined down to a smaller diameter; one that fits freely in the hole. Mine ended up as 0.68" diameter; you may need more or less depending on how precise you are with the other measurements. Constantly try sliding the rod down the metal rods to make sure they enter the hole freely.
Last, you'll want to bevel the end so that it is guided down if there is any misalignment at all. If your measurements are perfect, you may not need this, but I did.
If you want to install a lifting knob, you can do so now. Find a nice cabinet knob at the hardware store, and figure out what diameter screw it takes (mine was 8-32). Find the appropriate tap, and the drill that you use with that tap. Drill in the center of the upper block (again, use a center drill!), making sure not to go so deep that you plunge into the hemisphere. Now tap the hole, cut off the head of the screw (so that you have a short threaded rod), and tighten it up.