pic: Combat Robot Weapon 2

So I changed my mind and went from a single-tooth drum to an asymmetric horizontal bar spinner. This is the business end of the bar, the other end is going to have a steel counterweight instead of a tooth. Bar is 0.5" 6061 aluminum, tooth is hardened 4041 steel with the option for either carbide cutting inserts or a hardened s7 tooth (still to be made). Bar is 11" long, hoping to be able to put the hole for the shaft/bearing/upper deck mount 9" from the tooth. This would give me 18" of “reach” with my weapon.

The inserts will crack almost immediately. The only successful use of carbide inserts I’ve seen is in the 150 gram class.

I came to say the same thing. A nice piece of tool steel would be cheaper and less prone to shattering. Carbide is too brittle. Tool steel may be too brittle as well.

Yeah I realized this shortly after machining the tooth to fit the inserts. That’s why I’m making a tool steel insert for it :slight_smile:

Any recommendations for actually mounting the tooth? I’m thinking 5x grade 8 1/4-20 volts would do the trick but I am open to any recommendations :smiley:

The big thing is to try to limit any stress risers like sharp inside corners. If your using bolts be careful not to load them in shear. The best designs usually utilize geometry to hold the tooth in place when under load rather than just a faster. It’s also important to be aware of the strong likelihood of damaging the fastener head pottentially making it very difficult to remove.

Could you please explain this a bit more? Sorry if that’s a dumb question, but I don’t quite understand what you’re talking about (not sarcasm, I actually don’t know. My engineering lingo is limited).

Basically, if you only use screws to lock things in place instead of making the metal parts fit into each other via small teeth or something, then when you hit a robot it’ll put a shear load on the screw. If the head of the screw gets hit then you may be unable to remove it too.
Having something like a slot and an extrusion that fits into the slot is very strong, and then the screw just pulls the two pieces together instead of stopping it from moving.

Those carbide inserts are pretty nifty! I never thought about using those on a combat robot, but it’s too bad they’re liable to chip easily. At least with HSS bars you can grind them to custom profiles! :smiley:

No problem, I’ll try my best. A stress riser/stress concentration is the reason you often see fillets on the inside corners of parts. An object is strongest when force is evenly distributed over its area, when you have a sharp corner and force is applied the stress will concentrate over that sharp corner potentially causing it to crack.

A good example of this is when you have a crack in polycarb sheet from an impact. If it gets impacted again the crack will likely grow longer as the stress will concentrate right at the tiny point at end of the crack. That’s why its a good idea to drill a hole at the end of the crack. Even though your removing material it will actually make the sheet stronger as the stress can be distributed evenly around the much larger smoother hole rather than the tiny crack and hopefully prevent the crack from continuing.

This will definitely be taken into consideration when fitting the counterweight. Thank you! :slight_smile: Seeing as to how it’s too late to help the tooth mounting situation, how could I strengthen the connection it in its current state?

Strange as it may sound, taking a small round file and rounding out the inside corner of the aluminum bar will reduce the stress riser there, making it more durable. Counter-intuitive, remove material to increase strength, but it is real.

Don’t take my word for it though: Get 2 smaller pieces of aluminum (scrap is fine) and cut a square corner inside like on the arm, but make the part sticking out (along the steel in your photo) much narrower (like 3/4", using 1/4" plate) - you want to be able to break it. Take one piece and file it as I suggested, a nice 3/4 round hole. Then, clamp it in a vise and, using a calibrated force*, whack it and see which breaks first.

*Get a weight and drop it, controlled, from a (measured) height. Increase the height to increase the force. Control is easy: Attach the weight to a long-ish stick, and hold the stick at one end. Think broom handle with hole drilled in far end, a bolt holding it so the near end (with the weight) drops the same each time.