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I would guess that you'll have backlash if you do that. These drive systems (e.g., GT2) are all carefully designed such that the belts and pulley's have extremely low backlash. But part of achieving that is correct angles, pitches, tolerances, etc. If you 3D print a pulley, it'll drive the belt back and forth but even being off a bit will lead to backlash and even perhaps slippage at moderate speeds. You can try it though.