All the Arduino stuff is hobby grade.
Looking for a reason my circuit wasn't working, I found a video of things he wish he'd known about these drivers. The first was not to power them without a load.
The second was to make sure the current was set correctly. Start by turning the adjust "screw" fully clockwise. Connect a volt meter to your screwdriver, power it up (with stepper connected), and increase the voltage between the top of the screw and ground, then turn counter-clockwise until you have 1/2 of the desired current.