Thursday, December 14, 2017

Margin of Safety 4 upgrades

In the short period of time between Dragoncon and Sparkfun AVC 2017 I had a few upgrades in mind for Margin of Safety. First was a horizontal drum attachment with feeder wedges.
What is this nonsense.
The idea was to have the same wedge defeating power as a vertical with narrow wedges, except with a horizontal weapon behind it. Hitting the face of the wedge would impart a strong sideways force, which is unusual.

The hub motor is a new design, able to be used for both the horizontal drum and a new vertical drum, fixing weaknesses with the cantilevered motor on the previous design.
The new vertical drum. Wider and less vulnerable.
The new design requires CNC turning, which is outside of the tools available to me. So I sent it out for fabrication by professionals.
Cross section of the drum.
The drum is two main pieces, the hub motor, made of magnets and aluminum, and the hardened drum itself. It uses a taper fit to allow the drum to span the full width of the motor. With this new design the drum is fully supported on two bearings, the motor is fully protected and not cantilevered, and the wider drum allows for less accurate driving.

The drum without retaining screws.
Horizontal drum with wedges attached.
Horizontal drum without wedges.
I also experimented with a new color scheme for this robot: meat colored. I bought a stack of adhesive printer paper and cured meats and went to town at the scanner.
Plastic wrap on the scanner bed for easy cleanup.
Prosciutto and pancetta.
What have I done.
Sparkfun AVC showed the importance of keeping the 3D prints under compression. At every place where I broke that rule, there was cracking. First was the vertical attachment.
Cracks everywhere.
It split starting from every hole right along the layers. Second were the wheelguards. The rear end of the guards were kept under compression; those sides did fine. The front end was anchored to the front interface with plastite screws. Those came apart frequently, and I ran out of spares.
Boo cracks.
That failure also ripped one of the wheelguard screws out of the front interface.
Womp womp.
Luckily the robot held itself together to win Sparkfun AVC 2017 with a record of 6-1, and was the last robot standing in the rumble. I lost one match due to experimenting with horizontal drum. Due to it's length and narrow stance it requires exceptional accuracy to wedge under an opponent. The sides of the wedges are easy to get under and vulnerable to impact, both from the opponent and from flying across the arena. It is likely still useful against midcutters, though I have not tested that.

All in all, I feel this is an excellent design once the wheelguard issue is fixed, while the flexibility of different weapon modules allows focusing on the fun parts of design while maintaining a tested reliable drivetrain.
Part explosion.

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