Highly edited version of the original from Gyrobot and the version from MarcoAlici with independent shaft sizes.(see Remixed from).

Now contains many comments and named parameters.

Features:

• Top and bottom independent hole diameters
• Smooth exterior spirals for coupling
• Hex or square captive nuts
• Recessed bolt heads
• Optimisations for no support printing
• D shafts optional

Notes:

• 100% fill for structural strength
• PLA is less brittle than ABS for this structure.
• Print with no support (unless spiral degrees are very high).
• The longer and thinner the spirals are - the more flexible the coupler will be.
• So increase Spiral degrees for longer arms and Number of Spirals for thinner arms.
• Top and Bottom captive bolts are the same so there is no wobbling at high spin rates.

Can be made flexible by treating a PLA object to a Carburettor Cleaner bath to make it even more flexible.
See http://www.thingiverse.com/thing:98268 for instructions.

Carburettor Cleaner is primarily 30% each of Methanol, Toluene and Acetone. (The Acetone and Methanol do not affect PLA much). In this case its the Toluene which is causing the PLA to expand in size (20% increase in volume) and therefore become more flexible.

However its an organic solvent and dries out. It also weakens the PLA material. Effects are not permanent.

Reference: http://forums.reprap.org/read.php?1,281724