Samurai Design Guide

Introduction

Welcome to the Samurai Design Guide!

Samurai is a new (first iteration in 2019-20 season) experimental 12lb Battlebot. The robot is a fully invertible vertical bar spinner. This guide details and justifies design choices. We will go through the overall design subassembly by subassembly, and provide any useful and important notes, observations, and tips that we learned along the way.

Design Overview

Samurai is a fully invertible 12lb vertical bar spinner. Full invertibility is achieved by our primary design feature: a hinge. This hinge allows the entire weapon assembly to pivot. As such, the weapon assembly will self-right when the robot is flipped over. The weapon and hinge assembly is mounted in a chassis driven by a two wheel geared direct drive system. The design overview will break down the robot into its sub-assemblies.

Chassis

Design

The chassis subassembly, shown in Figure 1, is a trapezoidal structure composed of 1/4in aluminum 7075 plates bolted in a closed loop. The overall frame is reinforced with two 1/8in aluminum top and bottom plates. In order to achieve the trapezoidal shape, the front plates and the inner/outer side plates are joined at an angle. The plates mesh with planar puzzle fits and are secured with angled corner blocks. Note that the top plate is a very important structural component, as without it, the back plate would be entirely responsible for keeping the two halves of the chassis together. Around the chassis, 1/4in HDPE plates serve as armor to protect the chassis.

Manufacture of the chassis was fairly standard. All plates were waterjetted and milled/tapped to create screw holes and pockets. For corner blocks, stock was first reduced to their final outer dimensions. Using a waterjetted fixture, we were able to hold the blocks at an angle in the mill and cut the angled face.

Justifications

• The trapezoid shape ensures that enemy robots must move closer to our weapon in order to land a hit on our chassis. In addition, it also reduces the number of sharp angles opposing weapons can catch on.
• Using corner blocks prevents us from having to machine any angles into the plates' puzzle fits, reducing manufacturing complexity and time, and increasing the strength of the joint as less material needs to be removed.
• HDPE serves as sacrificial material to absorb energy by deformation.

Issues

• Assembly of the chassis was notoriously difficult due to misaligned screw holes and bent plates.
• Using button head screws on the bottom plate made their heads very easy to knock off.
• Holes were not initially made through holes, making tapping more difficult than necessary. They were later redrilled to be thru with a hand drill.

Weapon Carriage

Design

The weapon carriage, shown in Figure 2, is a symmetrical kite shaped structure built of 1/4 and 1/8in aluminum 7075 plates. Two outer walls are held together by a pair of connecting plates. The weapon blade itself is mounted on a 15mm hardened dead shaft at the front of carriage. The shaft is held in the carriage by a pair of retaining rings. To constrain the weapon axially along the shaft, a pair of aluminum spacers are inserted between the bearings' inner races and the outer walls of the weapon carriage. At the back of the weapon carriage, the weapon motor and hinge bushings are mounted coaxially. Protecting the motor is a bent sheet of 1/8in HDPE. Power is transmitted to the weapon from the motor by a polyurethane flat belt.

Justifications

• Kite shape ensures that the weapon carriage is symmetric when inverted.

• Connecting plates act as standoffs separating outer walls. 1/8in aluminum was selected to minimize space usage, allowing a larger weapon. The semicircular cutout at the front of the connecting plates serves a similar purpose.

• Retaining rings were used to minimize weight and space which would otherwise be occupied by the head and nut of a shoulder screw.

• Spacers transmit force between outer walls and provide additional support to the carriage at the front where hits are more likely. They also constrain the weapon and ensure it remains steady. It is important to ensure that spacers do not touch the outer race of the bearings to minimize friction.

• The weapon motor is mounted in the carriage and coaxially with the hinge to save space. The robot would otherwise be too large and heavy with the motor in the chassis.

Issues

• The motor armor was mounted rather weakly. This caused it to easily rip off and deform into the path of the rotating carriage. This caused our hinge to jam and caused us to lose a match.

• Combination of tightly toleranced spacers and retaining rings made assembly of the weapon carriage difficult.

• Cutting retaining ring grooves in a hardened steel shaft is very difficult, resulting in low quality grooves that struggled to hold in retaining rings.

• Weight reduction in the outer walls was excessive, causing a beam to snap when hit by horizontal bar

• Flat belt tracking was not done properly, causing the belt to jump pulley walls. Proper flat belt tracking should be achieved by a crowned pulley, or by selecting an alternate belt type, such as a V-Belt