Building the Frame (Armor Style)
-Jeffrey Scholz
Last updated - January 2007

DISCLAIMER: Combat robotics is a dangerous sport by its nature. Extreme caution should be taken when performing any operation explained on this site. Any injury or death resulting from the use of these pages is the sole responsibility of the user and not totalinsanity.net. By undertaking the construction of a combat robot you assume all responsibility for your actions. When building a combat robot, always make sure you are, or are with, a responsible adult.

Build Responsibly.

Prerequisite: Armor/Materials Tutorial

After you have decided what "type" of robot you will build, (i.e., lifter, wedge, spinner, etc) you next must decide what kind of frame it will have and how to design it.

Types of Frames

1. Tubular frame. This is what you see on race cars. The frame is made up of steel (or titanium if you can afford it) welded together to make an outline of the robot's shape. Sheets of metal are fastened to the tubes as armor.

Example:

2. Armor/Baseplate Frames. This frame consists of a whole bunch of armor plates screwed together.

Example:

3. One pieces chassis. Used only in the insect classes, this construction method involves milling the frame out of one block of aluminum or plastic so that the motors and electronics have slots for them to fit in.

Example:

4. Composite frames. Composite frames have foam bodies to create the shape and have carbon fiber and/or Kevlar epoxied to the foam.

Example:

To make tubular frames, one piece chassis, or composite frames requires learning welding, machining, or composites respectively. However, someone with a jigsaw and a drill can design and assemble an armor frame easily, so we'll focus on that.

Armor frames consist of three major components: the base plate, the sidewalls, and the lid.

The Base Plate

The base plate holds everything, the motors, the batteries, the electronics, and probably the weapon. Thus, an effective base plate requires rigidity, but not cutting resistance since weapons cannot really reach it. This makes carbon fiber the ideal material. Of course, carbon fiber gets amazingly expensive in larger weightclasses, so you can substitute aluminum, titanium or magnesium in bigger bots.

Because of carbon fiber's high price tag, you might consider using garolite, available on Mcmaster Carr. Garolite is not as light and stiff as carbon fiber, but costs 90% less. Builders have used garolite base plates up to the 12 lb class with success.

After selecting the material, you'll want to select the correct dimensions. The base plate needs to be large enough to accommodate all the components you place on it, but not larger than necessary because the extra size adds weight. First, lay out all your components on paper or on a computer program to decide the correct length and width. Let's say that you decided the base plate will be one foot by one foot. (This will be an imaginary 12lb robot)

1 foot = 12 inches
1 foot * 1 foot = 144 inches2

Correctly choosing the thickness requires you to balance between required strength and weight allowance. Increasing the thickness makes the base stronger, but too much thickness saps up your weight budget. You can judge the best thickness by checking out other people's robot's in the same weightclass. Or, if you have already decided how the weight will be distributed among the components of the robot, you can work backwards from there. Let's say you limited the base plate to 12oz and decided to use 2024 aluminum.

2024 Aluminum has a density of 1.616oz/in3

Volume * density = weight.

Volume = weight/ density

The weight is 12oz and the density is 1.616oz/in3, so the volume will be 7.43 in3

Volume = length * width * height (for a rectangular solid). The "height" is the "thickness"

We already know the length and width…

7.43 = 12 * 12 * thickness

divide both sides by 12*12…

thickness = .052 inches.

.052 inches of aluminum will not be sufficient. We have two options: make the base plate smaller or use a different material.
Let's say we managed to rearrange the components to cut the size down to 8in by 8in.

7.43in3 = 8 * 8 * thickness

thickness = .116"

That's almost 1/8th of and inch, which is acceptable. .Since suppliers do not sell .116" aluminum, let's choose .125", which is the same as 1/8th inch and see what happens.

Volume = 8 * 8 * .125

Volume = 8 in3

Weight = volume * density

12.982 = 8 * 1.616

That's more that what we wanted, but only by .982 oz.

Let's say we substituted carbon fiber of an equivalent thickness.

A piece of .125" thick carbon fiber (available on the Robot Marketplace) has a weight of 0.11oz/in2. Note that carbon fiber is weighed by weight per square inch, not cubic inch.

Weight = surface area * oz/in2

7.04 = 64 * 0.11

Wow, that saves 5.942 oz! Also, 1/8th carbon fiber resists bending better than 1/8th inch aluminum. Of course, carbon fiber costs more.

If you don't want to calculate the density yourself, you can enter the dimensions and material here on this online calculator.
http://www.indmetals.com/tools_mtl_wt_calc.asp
Use these same calculations for calculating the weight the side walls and the lid.

Side Walls

Because they will take most of the hits from spinner bots, these parts need to be as thick as possible. Additionally, tapped (i.e. has screw threads) holes will need to go through the middle of the side walls so that the top plate and base plate can be screwed on. Unless you badly need extra weight, avoid using lexan because it can crack around the mandatory screw holes and does not resist sharp spinning blades well. If you must use lexan, try putting steel or titanium over it. This increases the armor's resistance to blades digging in while taking advantage of polycarbonate's impact resistance. Plus, the outer metal distributes the impact over a wider area. Aluminum is a good material for the sidewalls since it is lightweight and accepts tapping well. Unfortunately, titanium is extremely difficult and expensive to tap.

This picture illustrates how side walls work. The two pieces of carbon fiber in the picture are the baseplate and lid. The white translucent "thing" is the sidewall. You can see the screws inside this material.

Lid

Unless your robot is invertible, has a very low profile (which allows opponents to drive on top of you), supports the weapon, or encounters a lot of hammer bots, your lid does not need strength; it only needs to keep the wires and electronics inside the frame and not hanging out. Lexan is a good, lightweight material to use because it allows you to see into your robot and does not interfere with radio signals.

Conclusion

Armor frames are the cheapest and easiest type of frame to build. Construction only requires a saw to cut the pieces out, a drill, and a tap(to make the screw threads in the side wall). The base plate needs to be a firm, stiff foundation for the robot. The side walls need to be thick enough to take a beating and hold the screws. These will be the heaviest part of the robot's frame. Generally, the lid only needs to keep stray wires inside the robot. Considering the simplicity and strength, it is no surprise armor frames are the most popular construction method for combat robots today.

There is so much more to designing the frame than just predicting how much it will weigh. Other factors, like ground clearance, wheel protection, frame stiffness, additional support, serviceability, gyroscopic precession(what makes vertical spinning robots tip, see our veteran section for more), and the number of screws need to be factored in. However, that discussion is for another article.

Material Densities:

Aluminum 6061
1.568 oz/in3
Aluminum 2024
1.616 oz/in3
Aluminum 7075
1.616 oz/in3
Garolite
1.161 oz/in3
Lexan
0.6944 oz/in3
Magnesium
1.024 oz/in3
Steel
4.528 oz/in3
Titanium
2.608 oz/in3
UHMW
.5504 oz/in3
Carbon Fiber
Check with supplier