Update Oct 28., 2019:
Some of you have asked for the parts I used:

• Bitholder with Magnet, i.e. TOOLCRAFT No. 1614136, or Wolfcraft Bitholder Magnet 60MM 2426000, or Gedore RED R47110014 3300147
• Adapter ¼" to ¼" to mount standard nuts, i.e. Proxxon No. 23701

Update Mar 3., 2019:
Published a Bar to help for not so strong hands. Allows additional force but be carefully with higher torque values (>3Nm) some support is needed to prevent from breaking. See pictures and comment section. Printed mine in 2.5h with 25% infill and PLA.

Update Nov26., 2017:
Published my Torque Measuring Lever (http://www.thingiverse.com/thing:2670493) to easily test the parts with a simple scale. :)

Update Nov 23., 2017:
Added 4.0Nm, 5.0Nm and 6.0Nm cases and rotors. More than about 3Nm is not possible without the bit holder is glued into the rotor, otherwise it would slip. 3Nm means about 100 kg on the edge of the plastic hexagon. I used 2 component epoxy because PETG does not work well with all adhesives. A ¼" magnet bit holder costs about $4, the easy holder with a square less than $2. See pictures. By the way: 6Nm then slowly becomes uncomfortable for the hand. :)

Update Nov 7th, 2017:
Added 0.5Nm case and rotor, added new 1.0Nm and renamed existing to 0.8Nm (was a bit to low in PETG).

Low torques are necessary for many applications, such as carbon parts on mountain bikes or screw connections on electronic and robotic devices. But the usual torque wrenches works reliably only at 4 or 5 Newton meters and above.

Fixed torque handles are available, but always for a high price tag ($100-200). This is why a set of 1/4 "drives with low, fixed torque for self-printing becomes handy. Inspired by a NozzleTorqueDrive from Anders Olsson (I like the design), my parts have a larger diameter with more and thicker fins and the housing is screwed. Through a continuous 1/4 "slot all common inserts can be used and even left-hand crews can be used by placing the insert on the bottom.

The case dimensions are the same for all drives and differ only in the engraved torque values. The screwed cap is also the same for all drives. There is a version with slightly more play if print quality affects the thread. The nut value is 19mm or ¾", very common in most countries. Please do not tighten too much; otherwise, the housing may break!

The rotors receive the different tripping torques by different thicknesses of the fins (main feature) and by different lengths in the vertical axis (fine calibration). In the rest state, there is no pressure on the plastic fins, as a result of which the drives should produce approximately the same resistance over time.

The actuators are designed for normal PETG with 100% infill. If ABS is used, the one number smaller rotor can be used to achieve the same trigger value.

I have printed them in different colors, so I always choose the right part.

Note: I tested the wrench handles with a high-quality 'Stahlwille' torque wrench with calibration certificate, but keep in mind they are self-printed plastic parts: they will never reach the uniformity of professional tools. However, it is usually a question of not exceeding the permitted torque value by several Nm, since we simply have too little feeling in our hands. So often no absolute precision is needed.
A higher value of up to 4Nm (or even 5Nm) would also be conceivable, but then the drive piece would have to be glued in, as otherwise the hexagon in the plastic material would end up with time. We will see...

Enjoy!

Doesn't Matter

No

0.3mm

100% (Rotor), 75-100% Case & Cap

Print in original orientation for best results. I printed all parts in 0.3mm layer height, but of course lower layers will work as well.

Fill the engravings with color if you like. I've filled mine with Crayola pencils, not perfect, but good enough.

More than about 3Nm is not possible without the bit holder is glued into the rotor, otherwise it would slip. I used 2 component epoxy because PETG does not work well with all adhesives. A ¼" magnet bit holder costs about $4, the easy holder with a square less than $2. I bought mine at Conrad and the brand is Wolfcraft.

Fusion360. The thread in the case is an M45 thread with 2mm pitch, but expanded to 46mm. Thread in the cap is cutted by an scale to 98% of the thread in the case.

PS: the last picture shows the countless attempts that were necessary to come to the result... :)