A plumbing problem I had to deal with this week was removal of an old bathtub drain flange. What sometimes happens with these drains, and had happened in this case, is that corrosion weakens the metal crosshairs located at the bottom of the flange inside the drain that are normally used to trap a drain wrench such that the flange can be unscrewed. I tried a few standard tricks, including using a cutting wheel to notch the drain collar such that a screwdriver could be hammered against it to cause it to rotate. None of those tricks were successful. Fortunately there is a good custom tool on the market for just this job.
The essential tool I eventually acquired and used was a Tub Drain Extractor by SuperiorTool. In my case, I ordered that tool from Amazon using the link below; few hardware stores have them.
That extractor has a tapered, helical-pattern knurled finish around its perimeter at the bottom, such that after a few hard taps on the Extractor with a hammer into the drain, the tool grips the drain. The tool accepts a 1/2″ size ratchet or extension bar, and using a such a ratchet plus a breaker bar to get more torque (actually the breaker was a carbon tube from an old windsurfing boom), I was eventually able to apply enough torque to rotate the old drain out. After some post-removal cleanup of the leftover areas with a stiff plastic brush, I could easily rotate a new drain into place.