Linear punchers are one of the simplest, yet most effective ball launching mechanisms. The name of the mechanism comes from it's linear actuation and the forceful impact on the ball that sends it flying.
A couple of things to note about the design:
The shooting is powered by rubber bands (not shown, but connected from the standoff on the back of the rack c-channel to the front of the assembly)
The puncher is drawn as the rack and pinion set moves the rack away from the ball, and then when the shaved section of the pinion gear reaches the rack, the rack is released and shoots forward
The rack shown in the photo is stopped from exiting the assembly by the horizontal bolts with black spacers.
Linear punchers are used most commonly when needing to launch an object (normally a ball), a far distance. In the past, they were used in Turning Point (2018-19) and in Nothing But Net (2015-16) where teams needed to launch balls (relatively small) across the field. In the case of Turning Point, they were often used because they could be used in different points on the field with angle changing mechanisms.
There is no single best method for creating a puncher. Different game objects require different mechanisms, but there are a number of best practices.
Use a short linear slide length. Longer slides have more power, but suffer from high friction. Shorter lengths have less friction, higher consistency, and will increase the lifespan of the rubber bands.
Make sure the rubber bands are parallel with the linear slide. If the rubber bands pull the slide up or down, it will add friction and reduce exit velocity of the ball significantly.
The slide should have a loose fit in the slide trucks. This can be achieved by filing the slide trucks down. The extra slop is negligible on performance, and the reduction in friction is massive.
Utilize some form of lubrication to reduce friction even further. Common choices for lubricants are white lithium grease or graphite
Some modifications that could be done to the linear puncher include adding a ratchet system that would allow the puncher to stay in place when pulled back. This would be good in scenarios that would require you to quickly launch the object, rather than having to pull back the puncher before firing. Another modification that could be done to the linear puncher is including an angle adjuster to modify the trajectory of the shot. This can be integrated with other sensors such as the vision sensor to create automatic or assisted aiming for the driver. Below are examples of the angle adjust and ratchet system:
BLRS (Purdue SIGBots)