Laser cutting and 3D printing required for this build!!!
This is my Mk3 version of Shrapnel. After so many versions of my previous designs, I've organised my Mk versions by what weapon motor size I've used. Mk1 was an 1806, Mk2 an 1804 (weak!) and finally Mk3 with a 2204.
I have been using this version for 9 months now at our monthly meets and at an interstate competition and Shrapnel has been performing pretty well so far!
This version addresses most of the issues in the original Shrapnel design posted on thingiverse (https://www.thingiverse.com/thing:1692075):
Issue 1: The motor mount part of the 3D print would snap off with a hard enough hit.
Solution: An acetal top and bottom plate makes up most of the chassis now and prevents this from happening (a heavy enough hit from a strong vert spinner cracked it once however).
Issue 2: The wheels and drive motors copped a lot of hits and required frequent replacements.
Solution: Enclosed wheels protects the motors. The rear of the wheels are still exposed and vulnerable to horizontal spinner hits which damage the tread. Printed TPU wheels may solve this issue.
Issue 3: The internals were exposed on the weapon side.
Solution: A bulkhead was added to better hold the internals. Hot glue to hold things inside is no longer required!
Issue 4: Shrapnel would get stuck on his side quite easily.
Solution: Added small forks on the bottom plate assembly, this actually causes it to self right upside down but at least you're still fighting! Also created a diamond like shape on the wheel guard to prevent getting stuck on that side.
- Print all of the STL files with whatever settings suit you best. Aim for the Shrapnel Base part to be approx 13 grams or less. Print 2 motor clamps.
- Laser cut all of the DXF files out of 3mm Acetal plastic. Nylon or HDPE would work well too but I haven't tried this. Top plate should be approx. 19 grams and the bottom approx. 14 grams.
- Heat and fold the top plate using a heat gun. The fold should be easy to figure out and use the photo in this listing for a guide.
- Countersink all of the screw holes in the Acetal plates.
- Use a file or linsher to put a 45 Degree angle where the top and bottom plates meet at the wedge tip.
- Mount the motors (wheels attached) with the motor clamps, using small M2 self tapping screws. (I use spare screws from small servos).
- Solder the RC components and fit into chassis. Use the photo in the listing as a general guide. The BESC sits up top, above the drive motors. The battery squeezes between the drive motor connectors. The receiver sits under the right drive motor and the power switch sits under/behind the left drive motor.
- Insert M3 nylon hex risers into the slots in the shrapnel base assembly and screw on the top plate. Use two steel M3x6mm screws to secure the plate to the nylon risers. Use two M3x3mm screws to tap into the base assembly to secure the rear of the top plate.
- Mount the weapon motor to the chassis using two M3x3mm screws. The motor wires should run through the opening in the bulkhead of the base assembly.
- Flip the bot upside down and insert an M5 flange bearing into the bottom plate hole, at the front end. Attach the weapon bar and secure the shaft nut with Loctite (do it up really tight!)
- Mount the bottom plate to the bot in the same was as you did with the top plate. Two M3x6mm into the nylon risers, two M3x3mm tapped into the base assembly. When swapping a battery out during an event, I always access it by removing the bottom plate rather than the top.
- For added protection, glue a 1mm piece of polycarbonate behind the rear wedge to protect the battery (not shown in photos). All it takes is a bot with a sharp point to pierce the battery!
I used 3mm Grade 5 titanium, which I manufactured myself with a grinder. I drilled a 5mm hole in the center to mount it to the weapon motor. To ensure it was balanced I used a propeller balancer from Hobbyking.
Another idea could be to get a bar waterjet or plasma cut from a sheet of titanium, but this can be costly.
A simple steel bar is also suitable but keep in mind it will be heavier.
Aim for a 20 gram bar approx. for this bot.
When soldering the components, you will need to be clever in how you fit everything inside and only leave as little wire slack as possible.
As for what solders where, you can google "combat robot wiring" and find dozens of wiring diagrams to use.
So here's just a few tips for this bot:
- The DF Robot motors with built in drivers come with a connector and cable. Chop off the black connector for both - this isn't needed!
- The motor connector wires are as follows: Red=positive battery supply (after the switch), Black=negative battery supply, Green=signal from your receiver. You could also hard wire to the driver board to save space too.
- The motor drivers don't have a BEC (5V supply for the receiver) so use the BEC from your Brushless ESC (make sure you buy one with a BEC!).
- The micro receiver I use required me to solder Micro Molex connectors in, but you could also just solder the wires directly into your receiver.
- Modifiy all of your wiring so that you don't have excess length - there's limited space in this bot!
When selecting parts the following list isn't necessarily what you have to use. The important thing is to pick parts that are small and lightweight. The total weight for my internal parts (not including the weapon bar, screws and bearings) was approx. 76 grams. I allowed 6 grams for the screws and bearing.
(I will not post all of the web links for these parts as they can be sourced from many places and often the listings for these parts will expire. You should be able to search for the parts using the descriptions I provide.)
Feel free to ask questions on anything I might have missed in the comments!