Hokay. Be patient, this isn't quick. It can be faster to do it than to describe it.
I haven't made one of these for years but this is my recollection. I start with a regular hardened bearing ball of the desired cavity diameter, and a pair of aluminum blocks around 2" square and 3/4 or 1" thick. If you have a mill or you can turn squares in a lathe you'll be able to finish them later to the size you want.
If you have 1100 aluminum alloy, that's what I'd use. I've always had to use
2024 because that's what I had. 2024 won't forge much unless you semi-anneal it. True annealing of 2024 requires 750 - 800 deg. F for two hours. I've gotten it soft enough in a household oven at 500 deg. F for a few hours. After that treatment you have to work without delay. You have roughly one day before the precipitation hardening starts to catch up with you. If you anneal at 800 F it will harden up to around T3 after three days, just sitting at room temperature.The other tools required are a big f**king hammer (I use a 3-pound maul but bigger is better), an anvil (I use a piece of railroad track; I've also used a piece of steel plate laid on a concrete floor), and a couple of files: a double-cut flat bastard and a 12" mill file.
Make a dent in the middle of one side of each block to hold the bearing ball. Center the ball between the blocks and prop them with some wood or cardboard in between so the top block is level enough to stay put. Whack with the maul. Whack really hard until the ball starts to sink into both blocks. When the blocks get distorted on the inside faces, put them in a vise and file the faces with the double-cut. Put the ball and the blocks together again and whack some more. Whack, file, whack. When the ball starts to stick into the aluminum, coat the ball with some moly spray or some graphite and any grease you have laying around. Keep whacking, filing, and whacking. If the ball sticks in one side more than the other, file that side some more.
You should reach a point where the ball is almost completely sunk into the aluminum and the ball it sticking when you pull them apart. You may have to put one block between some parallel whatevers and whack lightly with a smaller hammer on the back side to get the ball out. You're getting close. File the inside faces flat and grease the ball; whack, file, whack.
It really doesn't take long. When the ball in sticking into both sides and the sides are about touching, work carefully. Then you're done. If you want to make a mold of it, drill a sprue hole, mill or turn the assembly square, drill for a couple of key pins and make the pins out of drill rod, and you're done.
I clamp the blocks together with a C-clamp to pour lead. I hope you have something better than C-clamps. d8-)
To try this out quickly to see if you like it, use a small bearing ball. 1/4 inch is good to try it out.
-- Ed Huntress