Tag Archives: Build Report

A New Cannon

I’ve said it before and I’ll say it again: I was very pleased with Cannon’s second-place performance at Motorama. Still, its three fights against the eventual champion, Low Blow, highlighted some severe design flaws:

  • Exposed wheels. This is probably the single biggest flaw a robot can have.
  • The ball bearings pushing against the sides on big hits expand the beater holes.
  • There was no firewall; parts could be ejected from inside the robot into the blade.
  • Slow drive motors. 22.2:1s would have been a much better choice.

The scale was hardly Cannon’s friend, so figuring out how to add more material to the robot without adding any weight will be a challenge in and of itself. However, the first (and easiest) place to look is the wheel hubs. Cannon used aluminum “Dave’s Hubs” to hold its wheels on. Much lighter alternatives exist, such as the FingerTech Robotics Lite Hubs. Switching to the Lite Hubs will save over half an ounce off the next version of Cannon. That is more than enough to get started.

The next place I will look for weight will be the back of the robot. The old back weighed about 0.6oz and was made of 3/32″-thick garolite. Although stiff, garolite is about twice as dense as UHMW and is very brittle. The next Cannon will use a piece of (likely 1/16″-thick) UHMW that wraps around the back and protects the wheels. Kitbot creator Pete Smith gave me some UHMW bending tips at Motorama that should mean the clearance between the wheels and the UHMW is better on Cannon than it is on Coercion. Actually, I’m not sure Coercion actually has any clearance!

I put together a quick sketch of my thoughts for the next version of Cannon:

I am not a very talented artist.

I am not a very talented artist.

The new firewall will be bolted to the side rails, which will help to make the entire robot stiffer. I will make a groove in the sides so that the pieces fit together relatively snugly.

I also sketched the side rails (remarkably well). I think I might apply to art school if the whole top-ten school in the world thing doesn’t work out.

Note that the pockets are bigger and I have added holes for the firewall. Also note that the hole that will hold in the weapon bearing has been enlarged. This will allow me to put in a custom bushing, which will increase the surface area inside the hole and keep it from enlarging.

Note that the pockets are bigger and I have added holes for the firewall. Also note that the hole that will hold in the weapon bearing has been enlarged. This will allow me to put in a custom bushing, which will increase the surface area inside the hole and keep it from enlarging.

The next step will be to design the firewall and do more weight calculations. I’ll then need to order parts. Unfortunately (well…fortunately), I will be in Washington, D.C. from March 9th to the 16th, which isn’t conducive to robot design work, and over the next week, I’ll be busy getting ahead on schoolwork to get ready for my week off: not much will get done. Still, it feels good to finally put some of the ideas I have come up with on my blog.

Keep checking back–Bot Blast registration is up on the Builders’ Database. I’ll be taking Cannon, lolcat, maybe Coercion, and maybe Amatol if one of my friends wants to drive it.

Cannon’s Almost Done

I’ve run into quite a few problems with Cannon since I last posted, but I have also made a lot of progress. I’ll get you up-to-date:

  • Cannon’s beater (along with Coercion’s new blade) arrived
  • I realized that Cannon was about an ounce overweight. It had to go on a serious crash-diet.
  • The weapon shaft material has been changed from S7 to titanium. I ordered some Ti, cut it, and tried to thread it. Unfortunately, I had a rethreading die instead of a threading die: although it worked with unhardened S7, grade 5 Ti was a different story. I managed to make enough threads to put on lock nuts, and a good die will be arriving soon.
  • I reduced the weapon reduction from 3:2 to 1:1 by making a new weapon pulley.
  • I wired everything up.
  • I made some pockets in the back to reduce weight.
  • Cannon spun up.

All the weight-saving measures have brought Cannon down to weight without forcing me to resort to using the notoriously brittle FingerTech Lite Hubs instead of the good Dave’s Aluminum Hubs I had planned on using. I will have everything put together by the end of the weekend; I will only have to make a few spare parts and thread the ends of the weapon shaft a little more before Motorama (I only have one weekend left!). My next priority will be Coercion, which ought to be manageable enough.

Here’s a video of Cannon spinning up:

This thing is going to be a lot of fun at Motorama! I have always wanted an antweight beater ‘bot.

Cannon Build Update

Cannon has been coming along very well. I have spent the majority of my free moments–those not dedicated to studying for midterms or doing projects–getting Cannon together. Team Whyachi finally finished Coercion’s new 9.2oz, hardened S7 blade and Cannon’s 3.0oz titanium beater; I expect to have them here by Monday. The goal is to have Cannon in a place where I can plug in the beater and have a finished robot (but this is combat robotics, so that’s a very lofty goal indeed).

Since I last posted, I cut out the back and drilled the twenty holes that will hold Cannon together. I am using .375″ #6-19 button head plastite screws from McMaster-Carr, which are similar to the mind-bendingly strong ones that we used on Tough Nut last year. So far, everything seems to be holding up pretty well. I did the traditional “throw your frame down the stairs” test, and nothing broke. That means it will be super-strong in the arena, right?

The next steps in the build process will be to cut out and thread the ends of the weapon shaft, fashion a pulley for the weapon motor, drill and thread holes in the pulley I already have for the weapon, and then to get wiring. If I have time before Motorama, I will also harden Cannon’s S7 weapon shaft and Lolcat’s S7 weapon shaft. Lolcat’s bent at Bot Blast, so I do not have high hopes for its toughness against all the horizontal spinners that are registered for Motorama. However, the new motors I bought for it should provide a nice speed boost.

Here’s a front view of Cannon (note that I am waiting on shorter plastite screws from McMaster so not all the screws are in it yet):

It would look so much better with a weapon...

It would look so much better with a weapon…

Introducing Cannon

Continuing my alliterating-c tradition of robot names is Cannon, an antweight version of Lolcat. On paper (and in CAD), Cannon looks like it’s going to be a lot of fun.

Cannon CAD Drawing

Very rough, yes, but it still lets me visualize everything and make sure the parts fit.

Here are the specs I have planned out:

  • Weapon motor: Turnigy Aerodrive SK3 2826-1240. It is rated at 150W and will provide more than enough weapon power. The old “Beloved Shardy” undercutter I built simply did not have a powerful enough weapon motor, and this one will be sure to suffice!
  • Weapon ESC: Thunderbird-18. It’s tried-and-true, and the weapon motor is rated at 16A continuous so a fire seems unlikely right now. Unlikely.
  • Battery: Turnigy Nano-tech 3S 460mAh LiPoly. I love the idea of three cells, and in theory, a 460mAh can supply about 14 amps for an entire two-minute fight (throwing out any drops/inefficiencies). I think I will have bigger problems than the battery if the robot requires 14A continuous for two minutes!
  • Drive motors: Fingertech Silver Spark 33.3:1 gearmotors.
  • Drive ESCs: Fingertech tinyESC v2.1, the same kind used in Lolcat. I can’t say I’m wild about these controllers; I killed five or six of them before I learned that I’d discovered a hardware issue. They now seem to work okay, and I do have to say that Fingertech customer service is phenomenal!
  • Weapon: 3.0ish ounce 2.5-inch diameter titanium eggbeater spinning at about 9200rpm. It will be supported by ball bearings and connected to the weapon motor by a urethane belt with a gear ratio of 3:2. The weapon shaft will be hardened S7.

I already ordered some of the parts, and I was actually very impressed by how much higher-quality everything appears. Just look at the packaging.

Turnigy Product Packaging

Look at those beautiful graphics. It’s almost worth the huge premium over normal Turnigy products for the SK3…

Actually, just look at the motor and battery!

Aerodrive SK3 Motor

These motors are actually a little bigger than I expected but feel very solidly built.

460mAh Nano-tech Battery

Speaking of big, these batteries are HUGE for their capacity. Maybe I’m just used to dealing with Lolcat parts.

I do have some small quips with these parts.

  • Both the battery and motor are enormous. It’s a good thing I have minuscule drive ESCs because I may have a hard time getting everything to fit, and the body I designed for Cannon is fairly large.
  • The brushless magnets don’t feel very strong, though the motors feel very precise when you turn them. My feeling is that I am mistaken and when I actually wire the motor up, I’ll be in for a pleasant surprise.
  • The motor wires are small and flimsy.
  • The battery came with a JST connector on it. The battery is rated for 25C (11.5A) continuous, 40C (18.4A) burst. JST connectors are rated for 5A. That’s a waste of plastic.

Still, despite my minor complaints, I am happy with my purchase so far. I can’t wait to see that motor spin.

The motor isn’t the only part of Cannon I am excited about. I learned a lot about building eggbeaters well with Lolcat, and I believe most of it will apply in the antweight class as well. For one, protecting your belt is very important. Lolcat’s belt almost got sliced twice at Bot Blast, so I’ll try to get the side rails to cover up the pully. However, the bottom of the side rails is far more important. Tracked Terror kept getting underneath Lolcat and flipping it over. I worry that wedges could do the same with Cannon, but I have devised a fix of sorts.

Cannon Side View

Side view of the above image.

Notice that the sides are swept back. The beater extends way forward of the front of the supports. I am concerned about what effect this will have on stability. Since the beater, likely the heaviest individual part of the robot, will be centered in front of the support, the entire robot will act even more front-heavy. It could lead to some flipping over on hits; only time will tell. I am also concerned by how small the bottoms of the supports are, but they will be 3/8″-thick UHMW so antweight weapons shouldn’t be able to completely wreck them.

The only way we’ll see how this plays out is in the arena. I will continue to post progress updates on this blog as Cannon comes together.