im trying to find a tool, but i dont know what its called if it exists. Might need to be custom made, not sure. Attached is a pic of the part to be milled, and the tool needed. This is a small delrin link end that will be popped onto a 5mm steel linkage ball. a 4.5mm hole is first drilled through the link, Then the lollipop drops down in the center and interpolates in a circle to form a 5mm sphere socket. So the tool needs a 2.5mm radius, but the diameter squished down to 4.5mm max, preferably 4.25mm or so. Ive found plenty of undercutting tools, but they are a spherical end which wont work for this. Does the tool im looking for exist? Or most likely gonna need custom made? The 5mm socket size is also not required. I will be making both the linkage balls and the delrin link ends so something close to 5mm will work if there is something close in size.