I am new to this whole robotics thing. So, I have no idea about how could I build a crossbow-type mechanism?
That’s really a broad question. What are you going to use it for?
Most of the Vex parts do not have the nice flex needed for a crossbow. Rubber bands come closest.
You would use linear track to pull something back and trigger the release of the bow string. The vex catalog does not have a material that works nicely as the bow - fiberglass or wood. The bend range of steel and aluminum is fairly small in comparison and would permanently deform after much travel. So rubber bands act as the string but against fixed elements.
This past season many teams used a slip gear to make as close as you can in Vex equiv. Rubber bands were the bow and you pulled back a linear track until you hit the point with no teeth on the gear and the slide released forward. The gear keeps turning until the teeth appear again and you start pulling back for your next shot.
Alternatively, instead of punching a foam ball, the slider could have a crossbow like bolt on it. When the slider stops suddenly, the bolt continues forward. Just like a crossbow sends a bolt forward.
I’m guessing the mechanism in question would replace the flex of a bow with the stretch of rubber bands. It’s basically a linear puncher, except instead of punching the star at the end of the stroke, the moving piece propels the star throughout the stroke like a crossbow does with a bolt. Is this what you had in mind, @ama8054?
might i suggest that you take a piece of string from the pulley kit and have a rubberband on either side to use for the tension. that way if you need to adjust the tension or drawback length you can just change the length of the string for quick in-competition adjustments.