This style of adjustable drill chuck (similar to the Dremel chuck) is much simpler than the Jacob's style chuck but it does have one quirk: the jaws move with the tightening ring instead of with the body, and hence, the torque seen by the drill bit makes the chuck self-tightening. But conversely, reversing the rotation (like when backing out screws with a driver bit or when using a left-handed drill bit) will cause the chuck to loosen. So this is really only for one-way drilling/driving/dremelling type applications.

That out of the way, it prints and assembles easily. It adjusts from 1/4" (6.35mm) down to 0.045" (1.2mm) and can also be used with 1/4" hex-shank bits. The base has a 1/4" hex socket for connecting to something else, but I've included the STEP file so you can remix it for your own purposes.

Printing
Use PLA because it is the strongest (yield strength), stiffest, hardest, and wears best of the common filaments. Print with 0.2mm layers. Note that the ring part prints nose-side down, and you may want to print it with a brim for stability. Print 3 of the jaws in the orientation shown. The base is the only part that needs supports. Set the support angle threshold to something like 80 deg. so that supports are only generated under the lip of the top knob.

Assembly
Place the 3 jaws around the top knob of the base as shown in the 3rd photo. Slide the ring over the top so that the jaws fit into the three internal slots. From there, you can start screwing the ring onto the base. Keep screwing (moving back & forth to loosen things up) until the jaws are completely closed.

The jaws are held in place by 3 pins formed from short pieces of 1.75mm filament (as shown in the last photo). Start, though with 20mm long sections of filament. With the jaws closed all the way, insert the lengths of filament into the the holes at the top of the ring, angles as shown in the 4th and 5th photos. The ends the pins should bottom out in the slot of the jaws. Leave the pins long, and test the chuck, making sure the jaws stay in place when the chuck is opened. Disassemble the chuck and apply a little lubricant (light grease, etc.) to all mating surfaces. Reassemble and check that everything is buttery smooth.

Finally, re-close the jaws. Remove each pin, cut to a length of 8 - 9mm and reinsert into the holes, poking them all the way in with another short length of filament. Do not use pins longer than 9mm as they will prevent you from ever disassembling the chuck!

Disassembly
Taking this apart is simple, but a little opaque. You just need to use a length of filament or other small diameter rod to poke the pins out from the opposite side. Note, though that the slots in the jaws don't go all through, except for an access hole at the very top of the jaw. Thus, you first have to open the jaws all the way so that the access holes in the jaws aligns with the transverse holes in the ring, as shown in the last photo. You can now poke your rod into the opposite side of the hole for push the retaining pin back out.

Note that if the adjacent retaining pin is too long, it will block your access to the ejection hole for the target pin, rendering your chunk un-disassemble-able.