This adapter links 1/8" square shafts used for many hobby robotics gears to circular shafts. The circular side is sized to a typical 1/4"-20 UNC tap drill diameter. You'll have to tap it yourself.
Quasi-legal stuff: This design meets my needs for my own projects. I can't promise it will meet your needs. You will have to examine the design and decide for yourself if it will work for your projects.