A ratcheted crank/knob, maybe based on something like [www.thingiverse.com] would be printable (albeit, I'd think you'd want ABS and not PLA), and could serve a similar purpose, especially with a spring to grant additional elasticity to the pulley.
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I suppose it really depends on your goal. A mass manufactured tuning peg is likely to be cheaper, more reliable, and slightly easier to use. Plastic could be made lighter, could possibly be integrated into the printed arm, and isn't a vitamin.
Edit: Or maybe do a ratchet the other way around: put the teeth on the outside (a shaped hole integrated into the design of the arm), and print the flexing part of the ratchet on a peg in ABS that can be stuck into the hole. Once tensioned, the tension should be enough to keep the tuning "peg" from moving too much.
I suppose it really depends on your goal. A mass manufactured tuning peg is likely to be cheaper, more reliable, and slightly easier to use. Plastic could be made lighter, could possibly be integrated into the printed arm, and isn't a vitamin.
Edit: Or maybe do a ratchet the other way around: put the teeth on the outside (a shaped hole integrated into the design of the arm), and print the flexing part of the ratchet on a peg in ABS that can be stuck into the hole. Once tensioned, the tension should be enough to keep the tuning "peg" from moving too much.