I've tried 3D printed adapters to make servos into linear actuators before, and, in my experience, they are cumbersome, strip easily, and unreliable. This little kit is not perfect, but it's super easy to assemble and it works well. I used it to make an easy blink mechanism for a robot. Just experiment a bit with the screws so they are not too tight and not too loose.
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BENJAMIN
A Good Design, Experiment, and Educational Product
If you are learning about the capabilities of servos then this product is a good way to explore one of the ways servos can be used. The rack and pinion application was a pleasant surprise to discover for this device type. It does not provide precise motion but is more like a strong motion. The Kitronik Linear Actuator Kit was very useful to me in my design work.