Servo - September 2009

then I drilled and tapped additional holes to connect the blade to the

gear. Volià! A sturdy hub to mate the two objects together.

After some decorations, the robot named “Pacman” was ready just in time for the competition. While it was not the best, it was a

favorite to the audience, as they chanted “Wakka wakka wakka!”

Pacman charges up his blade and tests his metal against the titanium tank Fir Darrig. Photo by Kevin Berry.

when Pacman was called to compete. The serpentine belt system worked like a charm, the robot drove with great control, and I was relieved to know that I had survived the two-day gauntlet with a record of five wins and two losses.

Robot building is similar to school work. There are deadlines, there are excuses, and there are lessons to be learned in both. Just remember your robot building basics and there will be no pressure when your robot is due! SV

MAN UFACTUR IN G:

RioB tz Combat Tutorial

Summarized — Materials

● Original Text by Professor Marco Antonio Meggiolaro; Summarized by Kevin M. Berry

Professor Marco Antonio Meggiolaro of the Pontifical Catholic University of Rio de Janeiro, Brazil, recently translated his popular book, the RioBotz Combot Tutorial into English. In June and July, SERVO summarized Chapter 2, “Design Fundamentals.” This month, we continue the series with a summary of the first part of Chapter 3, “Materials,” focusing on commonly used metals in combat bot building. Marco’s book is available free for download at: www.riobotz.com.br/en/tutorial .html, and for hard copy purchase — at no profit to Marco — at www.lulu.com/content/7150541. All information here is provided courtesy of Professor Meggiolaro and RioBotz.

Mechanical Properties

There are several terms needed to understand the properties of materials. Most materials start off deforming in an elastic manner, meaning they spring back after

being bent. This is dependent on the stiffness, or “Young Modulus.” After some point, the elasticity is lost. The material begins to yield, meaning it no longer springs back to its original shape. This is called “plastic deformation.” It continues yielding (bending) until it reaches a value called the ultimate strength, where it breaks. Ductility is another term related to how well it deforms without breaking, related to the above terms in a way requiring a lot more explanation that we can do here.

The above values are usually tested in a slow manner. Dynamic, or “fast” loads use the terms resilience and impact toughness. Resilience is how much impact energy it can take before it starts to yield, or bend. Toughness tells how much impact energy it absorbs before breaking.

How do we use this information? A tough material is not necessarily resilient, and vice versa. For instance, the stainless steel (SS) type 304 — the most used SS —

tolerates large deformations but it is easily yielded. It is very tough, being good for armor plates that can be deformed. However, it has low resilience, and thus it should not be used in shafts (which should not get bent or distort) or in wedges (because if their edges are bent or nicked they lose functionality).

On the other hand, the steel from a drill bit, for instance, is very hard, with a very high yield strength, and thus it has a high resilience. However, its impact toughness is low. This is why drill bits do not make good weapons for combat robots, because they easily break due to impacts. Titanium is an excellent choice for use in combat robots because it is very tough (good for armor) and resilient (good for wedges) at the same time.

Fracture toughness is another useful term which means how well it resists propagation of cracks. This is related not only to the material itself, but the geometry of the actual piece being used. Thinner plates, for example, can deform