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Author Topic: Sportsman Saw Arm  (Read 65 times)

BOTSHED Robotics

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Sportsman Saw Arm
« on: October 24, 2017, 01:39:23 pm »
Greetings! I'm looking to build a sportsman saw-bot. Think something along the lines of Gloomy or MegatRON. I have built two other bots, a 30lb sumo and an antweight, but I'd still consider myself to be a newbie. I have many questions related to the construction of an articulated saw arm:

- Would a cut-off disc such as this: https://www.homedepot.ca/en/home/p.10-x-332-in--metal-cut-off-disc.1000719776.html be a better option than a circular saw? I see many sawbots get their blade stopped when they press it into the opponent, I was thinking something without teeth wouldn't be as vulnerable to this.

- I am planning on using a brushed motor for my weapon, so that the it would have the torque necessary to power through cutting rather than stalling. I see most builders opt for brushless weapon motors, am I making the right choice?

- What kind of motor+gearbox should I use to move the arm? On my antweight, I had tremendous satisfaction from using a servo in my lifter. The control was responsive and precise, and it would hold its position if I released the controls. Are there larger servos suitable for a sportsman, and if so, would they be practical? I would love to be able to lower the cutter slowly into the opponent rather than clumsily slam it down.

MikeNCR

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Re: Sportsman Saw Arm
« Reply #1 on: October 24, 2017, 03:27:30 pm »
The cut-off disc will be less prone to getting stuck, but will be more vulnerable to side loading and will be easier to break. It may be worth having a few different styles of blades on hand and a way to change them out quickly to adapt for who you're fighting.

Without more detail on your weapon system, it's hard to say if you're making the correct motor choice. With that said, I had been considering the A23-150 from ampflow for the rebuild of Spanky's circular saw attachment.

There aren't many servos at a scale that would suit the arm of a 30lb bot well. The easy route would be gearing a normal motor low enough that the braking torque on the motor and the bit of resistance in the gear train is enough that it stays put. To give an example, with Nyx, the lifter arm is geared 9:1 beyond the DeWut in low gear. The arm generally will stay in position unless I provide control input or a large external force is exerted on it. Another option would be finding a gearbox (or building one) that's got a worm stage or other similar mechanism that resists backdriving. It'd likely be heavier/more complicated, but it would work.

rcjunky

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Re: Sportsman Saw Arm
« Reply #2 on: October 24, 2017, 08:51:57 pm »
abrasive blades as linked can shatter pretty easily. Lenox makes these impressive abrasive disks I think will work well against metal bots, but for cutting plastic a toothed blade will cut better

http://www.lenoxtools.com/pages/lenox-metalmax.aspx

Andrew Burghgraef
Canadian Carnage Robotics

Great Hobbies- Canada's leading retailer of radio controlled models and related hobby supplies

BOTSHED Robotics

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Re: Sportsman Saw Arm
« Reply #3 on: October 29, 2017, 05:37:28 am »
Thanks for the help thus far, all info has been duly noted. At this point, it's pretty clear to me how little I know!

I've got a decent little plan put together so far: Two very grippy 5-inch diameter wheels, giant snow shovel scoop, articulated saw arm. Scoop, push, pin, saw. Hopefully it should be plenty entertaining. The shape and size of a large plastic snowshovel seem perfect for my plans, and I'm hoping it can withstand sportsman combat. I'll bring spares for sure.

- For drive motors, I was thinking of going with these: http://www.banebots.com/product/M7-RS775-18.html. For gearboxes, I am currently undecided, probably either the 16:1 or the 64:1 (going stock to save $). That seems like a big difference, and I know you can't know for sure which one is right without knowing the voltage and how fast the bot intends to be. I'd like something fast, of course, but with two wheels I imagine something too zippy would be difficult to control. For voltage... I can't say for sure because I don't even know what battery I'm going to use. Presumably around 18v.

- I hope to attend Franklin, and I know LiPos aren't allowed there. Oh, how this complicates things! I've heard A123 batteries are the next best option, but everything from purchasing to assembly to charging seems intimidating. Would it be easier just to use NiCads? Also, would a second battery for my saw weapon be necessary?

- Back to the saw arm, I've got a spare 12v drill motor I was lucky enough to find. Running at the roughly 18 volts I hope to have, would that be anywhere close to useful as the weapon motor? As for the lifter, I was hoping some other banebots gearmotor combo would suffice, combined with a chain drive. This arm should be: A strong enough to self-right and press a running saw through an opponent, and B fast enough to self-right quickly but no greater. My other worry with the arm is what will happen when I inevitably accidentally overextend the joint. Do I need to install limit switches, or some kind of friction device that will slip before anything else breaks? What do other builders do? To further add to potential problems, if the arm takes a good hit at its end, couldn't it act sort of like a long wrench and apply way too much torque on the system? So many questions!

I'd always love to go with the simplest options possible, or at least the most realistic ones for my setup. I'm basically building this in a garage,  and the only power tools I can reliably access that can work with metal are a drill press, beefy corded drill and a dremel. I also don't exactly have specific budget in mind... but I'm always open to cheap alternatives.

MikeNCR

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Re: Sportsman Saw Arm
« Reply #4 on: October 29, 2017, 01:54:11 pm »
775's make for a decent drive motor option in the 30lb class. With 5" wheels the 16:1's will probably be a bit too fast, and the 64:1's will likely be far too slow. You can play around with this to get an idea what you'll end up with - http://sparc.tools/TorqueCalc.php (use the nominal battery pack voltage for the voltage number, so 3.3v per cell for LiFe) This gearbox gets to a not bad spot speed wise- http://www.banebots.com/product/P60S-333-7.html (13.5ft/s top speed with 5" wheels, if you want a bit faster 6" wheels would work too)

If you're going A123 or another LiFe chemistry you'll likely want to run a 6s pack, or set of smaller packs in series that total 6s (19.8v nominal)

The drill motor may be enough for the saw, but it's hard to say for sure as it's not a setup I've messed around with. (Spanky used a cordless circular saw motor with a bit of extra gearing)

A gearmotor with some extra reduction is a solid option for the arm. You've got a few options for how to protect the motor/gearbox - Torque limiter, current limiting, limit switches, or hope. On Nyx's lifter I use current limiting to keep from massively overdriving the lifter and that's been a reliable option. The axe setup on Nyx uses a torque limiter and it's worked quite well there and has been plenty good for self righting.

An impact at the end of the arm will apply the torque to the system, so I'd lean against backdrive resistant systems (worm gears or similar) as the system being driven back can absorb a lot of the force without doing as much damage to the mechanism.

If you want a good look at how I dealt with some similar issues, take a look here: http://www.instructables.com/id/Nyx-30lb-Fighting-Robot-Re-Made/