Selecting a construction material for your next 'bot

Choose From Wood, Plastic, Metal, or Composites

Once you know the type of robot you wish to build (see Exploring Robot Locomotion Systems) it’s time to turn to both the method of construction, and the materials used, for building robots. It may be a surprise to some, but building a robot is a tad more complicated than going out to the garage and cutting up a hunk of pine.

To begin, you have a choice of building the robot from scratch, using raw materials like plywood or sheet metal. Or if you prefer, you can adapt some ready-made product to serve as the base of your robot. Inexpensive house wares, hardware items, and toys can be used in various creative ways to make robot building faster and more economical.


“Scratch-Build” Using Metal, Plastic, Wood, and Composites

When someone “cooks from scratch” it means to prepare food using basic ingredients like flour, milk, butter, baking powder, salt, and garlic — in this case, garlic-flavored pancakes. Similarly, in robot construction, building from scratch means using raw materials such as plastic, wood, and metal to concoct a robot body. More often than not, the body of the robot is cut from a larger piece of material, and shaped into the desired form. Round, oval, and square robot bodies are common.

Do note that I make a distinction between scratch building and “adapt building,” the latter (my own made-up phrase) taking commercially-available products and adapting them to construct a robot. This technique is covered separately, below. Because toys can be a goldmine in adapt building.

Note:
Building from scratch assumes the availability of at least a minimum set of tools, including a saw, drill, and screwdriver, and of course, a place to use them. As such, scratch building is best-suited for those robot builders who have these tools available, and a workshop to ply their craft. Those with limited tools and workshop area should consider building their robots from adapted parts or toys. This may seem obvious, but keeping this recommendation in mind helps match the robot-construction method with the available resources.


Choosing the Material

Raw construction materials should be chosen for their suitability for the job, as well as their machining and shaping requirements, not by their price or availability. Bear in mind the tools and skills you have, and match the materials to them. A cheap piece of old wood is hardly a good choice for a robot body, though it may be free and no further away than the shed. The wood may be warped and weak or full of termites; if your time and temperance matter to you, avoid materials that cause extra work and frustration. 

Wood

is reasonably inexpensive and can be worked using ordinary tools. Avoid soft “plank” woods, like pine and fir, because they are bulky for their weight. Instead, go with a hardwood such as ash or birch, but stay away from the very dense hardwoods. Oak is too heavy, and its density makes it a chore to cut and drill.

I prefer hardwood plywoods designed for model building. Though more expensive — about $5-8 for a 12″ x 12″ square — the plys are sturdy and won’t delaminate. This makes the wood resistant to cracking and flaking. Thickness ranges from 1/8″ to over 1/2″, with 1/4″ being a good compromise for typical applications. Hardwood plywood is available at better-stocked hobby stores, but most folks I know purchase it mail order from specialty retailers. These include woodturning supply, model shipbuilding, and taxidermy catalogs. Examples include Constantine’s Wood Center (www.constantines.com) and Penn State Industries (www.pennstateind.com).

For a unique look, consider using a so-called exotic wood meant for millwork, and which is available from woodturning supply catalogs (see above). These include bubinga, teak, walnut, and rosewood. These are hardwoods designed for turning on a lathe, or for carving or milling. They tend to be quite expensive, $8 for a 1″ x 1″ x 36″ length. Still, they are well suited for such roles as sturdy risers that separate the decks of a multiple tier robot. And, when properly sanded and finished, they have the look of fine furniture, adding a certain uniqueness to your robot.

Not all hardwoods are “hard” — the term actually applies to the wood from a deciduous (loses its leaves every season) tree. Softwoods come from non-deciduous, or evergreen, trees. With this in mind, cottonwood (Populus fremontii) and balsa (Ochroma pyramidale) are both “hardwoods” because they are from deciduous trees. Yet they have low densities, so they are somewhat soft and easy to work with. Balsa is commonly used to make model airplanes because it’s strong and lightweight. It can be useful in robotics as a way to create quick engineering samples, as well as struts for structural strengthening. 

Plastic

is the material of choice by manufacturers because it can be readily molded to shape. The typical plastic molding process involves molten plastic injected under high pressure into specially-made metal forms. Injection molding is a manufacturing technique that is not readily adaptable by the amateur robot builder. Instead, our plastic material of choice is the raw shape: sheet, bar, rod, and so forth, which are then cut into the desired form. These sheets, bars, rods and more can be purchased at home improvement stores, specialty plastics retailers, and sign makers.

There are literally thousands of plastics, and each is designed to be best suited for a particular application. For robotics, there are just a small handful of plastic materials that are both affordable and readily available. These same plastics also tend to be the ones most easily worked using standard shop tools.

  • Acrylic is used primarily for decorative or functional applications, such as picture frames or salad bowls. It can also be used in low-stress structural applications, such as robot bodies, as long as limitations for weight and impact shock are observed.
  • Polycarbonate is similar in looks to acrylic, but is considerably stronger. This plastic is a common substitute for window glass; because of its increased density, it’s much harder to work with.
  • PVC is familiar to anyone who has installed a water irrigation system. Though white pipe is a common form for PVC, it’s just one of many. PVC plastic is routinely available in all shapes, including colored sheets of various thicknesses. PVC sheets are often made using a gas expansion process that “bulks up” the plastic, making it lighter. These “expanded” yet rigid sheets are used to cut out shapes for making signs. Expanded rigid PVC can be cut, drilled, and even sanded like wood — in fact, this material is often used as a substitute for wood trim.
  • Urethane resin is a common component in “casting” plastics, such as that used with fiberglass. While you can buy already-cured bars, rods, and other shapes of urethane, you will more likely use the liquid resin to mold your own shapes.
  • Acetal resin is referred to as an “engineering” plastic. A typical application is turning or milling parts that do not require the strength of steel or aluminum. Acetal resin (sometimes referred to as Delrin, a popular trade name for an acetal resin product) is softer than metal so the work goes faster. Yet the plastic is surprisingly dense and strong. If you have a small lathe you can use acetal resin to make components for your robot. Even if you don’t have a lathe, you can cut the raw acetal resin — it comes in rods, bars, and sheets — into useful shapes.


Rigid expanded PVC is an ideal material for constructing robot bases.

Acrylic, polycarbonate, PVC pipe, and liquid urethane resin are commonly available at home improvement stores. Expanded rigid PVC sheets and acetal resin can be purchased from a plastics specialty retailer. Check the Yellow Pages under the various Plastics headings. 

Metal

is the archetypal material of a robot, though it’s not always the ideal choice. Metal is among the most expensive materials for robots — in terms of both cost and weight — and is harder to work unless you have the proper tools and skills. That said, metal is a must if your robot will be bashing other robots in a death-match contest, or is made for rugged outdoor use, or any of a number of other valid reasons that call for a strong body in a small package.

For robots, aluminum and steel are the most common metals. Aluminum is a softer metal and is therefore easier to work with, but steel is several times stronger. In any case, because of the inherent strength of metal, robot bodies can be made using sheet, bar, rod, channel, and other shapes. There are two general approaches to metal construction in robots:

  • A frame provides the base of the robot, and lends its support. The frame can be flat or box-shaped. A flat frame has four corners and four sides, and provides a convenient platform for motors and other components. A box-shaped frame is just what it’s name implies: a 3D box with six faces. The box frame is particularly well suited for larger robots or those that require extra support for heavy components.
  • A shaped base is a piece of metal cut out in the shape of the robot. The metal must be rigid enough to support the weight of the motors, batteries, and other parts without undue bending or flexing. For small tabletop robots, a thickness of 22 or 24 gauge steel, or 1/32″ aluminum, is usually sufficient. The weight of the robot increases dramatically with thicker sheets. Some very capable robots are basically a piece of sheet metal on wheels, with a laptop PC resting on top.

Composites

as used here, takes three primary forms:

  • Any laminated material, typically in sheet form, that combines wood, paper, plastic, or metal, relying on the intrinsic properties of each in order to increase rigidity and/or strength. A common laminate composite is foam board (such as Foamcore brand), which is made by sandwiching a core of springy foam between two pieces of stiff paper. Other laminates might combine wood and metal, plastic and paper, or most any other combination.
  • Any material using fiberglass and a resin. Sometimes a filler of metal, fabric, or carbon is added to the resin to give it extra strength.
  • Any material using carbon or graphite for strength (these materials may or may not contain other components). A good example are carbon composite tent poles that are both lightweight, flexible, and incredibly strong.

Why use a composite? Because of its weight-to-strength ratio. The strength of composites is relative — some composites, like foam board, are lightweight but not very strong, and are best used for creating mockups, or for reinforcing wood or plastic parts. You can work it with just a knife and straight-edge. Other composite, are both lightweight and very strong (i.e. carbon composites), but require specialty tools for cutting and drilling.

Two disadvantages, at least of the stronger composites, are cost and availability. Most composite materials are only available from specialty retailers and industrial suppliers.