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Unread 12-05-2018, 03:10 PM
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How to make adjustable hood for ball shooter

Our design subteam's offseason project this year is to CAD a robot that shoots some 6" balls. We were considering making it part of the design challenge to CAD a variable-position adjustable hood for the single-flywheel shooter we were planning on designing. I was looking for some good examples of this to point students at, but I couldn't find any methods that were used that were within our team's means to manufacture.

The only CNC technology we have access to is a router, so waterjetting a custom sector gear is out. From what I could tell, that was the method 254 used in 2016. Other than that, I considered 3D printing a sector gear, but it seems to me that it would have to be printed in parts because of the size it would have to be to accomodate 6" balls.

Other than that, our other option was to CAD a 2-position hood instead of variable-position and use pneumatics to select the hood position. This would be fine, but I imagine there are some games where the variable hood would help.

I was wondering: how do teams without a access to a waterjet cutter build variable adjustable hoods for ball shooters?

And also, a related question: how do teams determine if a adjustable hood is necessary, versus a fixed position hood or 2-position hood while varying flywheel velocity?
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Unread 12-05-2018, 03:22 PM
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Re: How to make adjustable hood for ball shooter

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Originally Posted by Plato2000 View Post
Our design subteam's offseason project this year is to CAD a robot that shoots some 6" balls. We were considering making it part of the design challenge to CAD a variable-position adjustable hood for the single-flywheel shooter we were planning on designing. I was looking for some good examples of this to point students at, but I couldn't find any methods that were used that were within our team's means to manufacture.

The only CNC technology we have access to is a router, so waterjetting a custom sector gear is out. From what I could tell, that was the method 254 used in 2016. Other than that, I considered 3D printing a sector gear, but it seems to me that it would have to be printed in parts because of the size it would have to be to accomodate 6" balls.

Other than that, our other option was to CAD a 2-position hood instead of variable-position and use pneumatics to select the hood position. This would be fine, but I imagine there are some games where the variable hood would help.

I was wondering: how do teams without a access to a waterjet cutter build variable adjustable hoods for ball shooters?

And also, a related question: how do teams determine if a adjustable hood is necessary, versus a fixed position hood or 2-position hood while varying flywheel velocity?
How do you build an adjustable hood without a waterjet?
Usually a hood will be on a shaft. Just bolt a sprocket to the hood and use that to adjust it.

If you want a 'makeshift' setup like 254's, then you can rivet or bolt a length of 25 chain on the back curve of the hood and use the sprocket like a gear to move the hood. This is similar to the way people make a turret

Hood positions?
Determine where you are going to shoot from. If there are 2 general areas, then a 2 position hood will work and you can modify speeds for the small adjustments.

Many positions?
This is really rare. Think hard before making this choice, because it's very hard to be consistent.

Keep in mind as well that calculating the trajectory of the ball so that you have the flattest longest portion of the trajectory at the goal height will give you the biggest sweet spot (we spent a lot of time getting a flat shot in 2014). In other words, a higher release point is generally better because your trajectory is flatter and you have a bigger sweet spot.

Trajectory math is pretty simple, so playing with it to determine general speeds and release angles is pretty easy. It's been pretty rare for you to need more than 2 positions for any game. Usually you'll be varying both hood position and speed, and that is pretty difficult for most teams.

Last edited by Tom Line : 12-05-2018 at 03:29 PM.
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Unread 12-05-2018, 03:25 PM
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Re: How to make adjustable hood for ball shooter

The main ball shooting games I've been a part of were 2017, 2016, and 2012, and in none of those did a team need anything more than a two position hood to be successful. In most teams could get away with a single position. I like to actuate my two position hoods with pneumatics - extend the cylinders to put the extension of the hood in the path of the balls, retract to remove it from the path and decrease the hood angle. Super easy to implement, and if you wanna get fancy you can use some over-center linkages to lock the hood in place.
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Unread 12-05-2018, 04:00 PM
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Re: How to make adjustable hood for ball shooter

If you want just two or four position adjustability - use a pneumatic.

If you want infinite, motorized adjustability - one option that's kind of sketchy but could work for a low load application like this is to tightly affix a chain or belt to the back of the hood, and engage it with a sprocket / pulley.

You can also use a leadscrew to adjust a hood. That's a robust if heavy method. See 1717 in 2009.
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Unread 12-05-2018, 04:16 PM
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Re: How to make adjustable hood for ball shooter

We did 2-position in college with great success.

We just made the shooter backplate in two pieces and hinged them, actuating the top part with pneumatics. We had installed mechanical stops at various points with screw adjustments to get fine tuning.
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Unread 12-05-2018, 08:02 PM
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Re: How to make adjustable hood for ball shooter

I agree with Lawrence that in most cases a 2 position hood is all you need. You can usually fine-tune the shot by varying the flywheel RPM (based off driver presets or vision).


The hood we made in 2014 had an overcenter linkage to hold it in the forward position so it wouldn't open under the ball's compression (see page 21 of 2014 tech binder). This is likely more important in games where the ball is hard to compress (2014 inflated, 2017 plastic).


In 2016, we laser cut the sector gears out of 2 pieces of 1/4" Delrin and then stacked 2 together to match the facewidth of the Servo pinion, increase the stiffness, and help compensate for the laser cutter's kerf & taper. See page 14 of 2016 tech binder. Finding someone to laser something for you (machine shops, tech shops, or just hobbyists/makers in your community) is probably much easier than a waterjet.
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Unread 12-05-2018, 09:50 PM
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Re: How to make adjustable hood for ball shooter

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Originally Posted by Torrance View Post
In 2016, we laser cut the sector gears out of 2 pieces of 1/4" Delrin and then stacked 2 together to match the facewidth of the Servo pinion, increase the stiffness, and help compensate for the laser cutter's kerf & taper. See page 14 of 2016 tech binder. Finding someone to laser something for you (machine shops, tech shops, or just hobbyists/makers in your community) is probably much easier than a waterjet.
Can you speak to the power and speed settings used to cut the Delrin? I ended up setting ours on fire.
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Unread 12-05-2018, 09:52 PM
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Re: How to make adjustable hood for ball shooter

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Originally Posted by sanddrag View Post
Can you speak to the power and speed settings used to cut the Delrin? I ended up setting ours on fire.
Use less power
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Unread 12-05-2018, 10:09 PM
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Re: How to make adjustable hood for ball shooter

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Originally Posted by AdamHeard View Post
Use less power
Are you multi& passing it? I didn't have much luck getting through it at less power.
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Unread 12-06-2018, 12:09 AM
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Re: How to make adjustable hood for ball shooter

We did a multi-position in 2016--used one of the automotive motors (window, I think) and a hub to drive it. I don't recall gearing offhand.

It broke relatively early, and we weren't doing much as far as using the extra positions anyways, so for Champs we ziptied it in place and dialed in from there.
We had a sponsor do the sheet work--it needed some interesting bends as well as cutouts--and riveted it together, both pieces. IIRC, the inner piece was the exit one.
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Unread 12-06-2018, 09:51 AM
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Re: How to make adjustable hood for ball shooter

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Originally Posted by sanddrag View Post
Are you multi& passing it? I didn't have much luck getting through it at less power.
Yes, for most bench-top lasers you'll need multiple passes through delrin. Be sure you have plenty of shop air / ventilation to avoid fires as well. For 1/4" delrin with a 40W (?) laser we would do 3 passes at almost-max power, just enough that there isn't significant melting or charring.
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