we are attempting to build a catapult in 3 weeks before states. what’s the best way to get started?
VEX HEX I’d advise you to join this server and check #new-content. They have a CAD and in depth videos that you can use to start your catapult and get it working really well. Good luck!
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Make a really quick CAD. take a look at lots of other successful designs, and leave yourself plenty of room to make changes when you tune. Start with a drive, and add the catapult and test it, making changes if necessary. Then add the intake and tune that well, and then add the roller mech, expansion, etc.
Make sure you use a 6 motor drive, it is invaluable during match play and very easy to do with a catapult.
Good luck, 3 weeks is a pretty short time. 
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As I am actually in the same situation (>3 weeks left before state, never used catapult before), I actually have a bit of knowledge on this topic. There are a few catapult explanation videos on Youtube, the best of which are:
2654P Catapult Explanation
606X Drivetrain and Catapult Explanation
99766E Catapult Explanation
A few things I’ve learned so far:
- 6 motor drive is worth it, even though it leaves only 1 motor for catapult and 1 motor for intake/roller.
- Slip gears don’t actually take that much time to make, just take a file and go back and forth a bunch of times. Masking tape actually works to protect whatever teeth you want to leave on from a file.
- DO NOT anchor your rubber bands on your drivetrain.
- Worth stating twice, DO NOT anchor your rubber bands on your drivetrain.
- An aluminum arm will bend a lot. Make sure you use proper cushioning and a steel c-channel. If you can’t find a steel c-channel, box a pair of aluminum ones.
- Low-strength shafts do work, but can bend. I’ve found that using ball bearings actually keeps them from bending, but it’s still better to use a High-strength shaft.
Oh, and also, if anyone wants to see the remains of an aluminum catapult arm, here it is.
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I agree with everything you say but not the steel arm part… You want the arm to be as light as possible, so you can decrease the stress on the motor, so using steel for it is disadvantageous. If done right an aluminum arm should be all it takes for a working catapult. You can build your tower and Ext with steel, but I would not recommend using Steel for the arm itself.
Best advice, don’t. 3 weeks is far too little time to build an entirely new robot design, especially if you don’t even know where to start. Unless you have an insane amount of time over the next 3 weeks, your going to end up at your first comp (state) with a half baked bot. Spend the next 3 weeks working on the e bot you already have.
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Looks like you used the weaker side of the c channel. An aluminum arm works great- you want to keep it as light as possible.
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Lol I have one too:
(What I will say is that out cata is different from others, we have a short catapult arm, which means that we need a lot more rubber bands to make up for it, which meant more stress on the c-channel. Don’t recommend.)
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We also use a short catapult arm, 20 holes long. Because of this, as you said, I had to compensate with more rubber bands. Right now I have 12 rubber bands, and even with this amount, 3-disk shots only barely make it in. Definitely recommend a longer cata arm, but if you have a short robot like me, it isn’t really possible.
Why did you not just box the c channel and add another one under it?
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I actually had it boxed with a c-channel coupler under the stress point, and it still did that . I don’t think I can add another c-channel underneath, as that will run into shafts and motors and other stuff.
I know it’s a pretty bad solution, but I just switched out the main one for steel, and no issues so far. I had to retune a little, but it worked out alright. I had limited time before a tournament.
instead of orienting your c channel so that the wide face hits your hardstop, make it so that one of the flanges hits your hardstop, and simply box the one point of impact. It’s as easy as that. We’ve never bent or snapped a catapult arm ever.
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Well a coupler wouldn’t work simply because of it’s length you would need to box the whole length or it’ll bend
These are good points, I’ll try these.
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