This is a screwdriver/nutdriver for standard 1/4-inch drive hex bits, with a rotating cap.
If you need to scale it to fit a smaller or larger size bit, first print the Test Piece
to get the scaling right.

I had to be careful pushing the rotating cap on, to keep from breaking the pieces that hold it on. When the cap is all the way on, it should spin freely.

UPDATE: 2018-10-14 ==========
I redesigned the clips that hold the cap on the Hex Driver because sometimes
the old clips were breaking off when installing the cap, and included
the new FreeCAD design file.

Also included is a stubby driver.

The hexagonal hole is slightly tapered at the back for a snug fit.

You can also print it maybe 2% bigger, and glue a 6mm diameter magnet
in the back of the hole.

I used PLA at 195C/55C, 0.2mm layer height, (or 0.1mm) 1.2mm walls, 0.8mm top/bottom, 20% infill with a 0.4mm nozzle and cooling fan at 100% after the first layer.

UPDATE: 2018-10-16:
I've also been printing this in PETG, at 235-240 nozzle temp, no bed heat, 0.25 layer height, and 0.5mm line width with a 0.4mm nozzle.
(Just 15% layer cooling fan speed, more cooling made it too brittle with PETG.)

Any infill should work OK with the 1.2mm walls. I even printed a hollow one (0% infill).

Designed in FreeCAD 0.17, sliced in Cura 3.4.1, printed on a Prusa clone using OctoPrint