Making Things Move: DIY Mechanisms for Inventors, Hobbyists, and Artists - Dustyn Roberts (2010)
Chapter 2. Materials: How to Choose and Where to Find Them
Once you’ve decided to make something that moves, you’ll need to find parts and materials to build it. Aside from the usual constraints of availability and budget, how do you choose materials for your project? What’s the difference between types of wood or kinds of aluminum, and are there any other options?
In this chapter, you’ll learn about the various types of materials, how to use them, and where to find them. But first, we need to talk about how materials are described.
In order to choose materials for projects, you need to learn how to describe materials and how strong they are. Each type of material is characterized by material properties.
A material property is just something about the material that’s the same regardless of its size or shape. For example, density is a material property, but weight is not. Density is equal to mass divided by volume, so no matter how much stuff you have, that ratio stays the same. However, the more stuff you have, the more it will weigh, so weight is not a distinguishing material property.
Another useful material property is yield strength. A material that yields, or stretches, before it breaks is called ductile. One that breaks right away is called brittle.
As an example, take a paperclip and try to bend one of the legs just a little, so it returns to the original shape. This leg will have deflected, or deformed, but because it returned to the original shape, it hasn’t actually yielded yet. Now take that same leg and bend it a lot—way out to the side. It stays in its new position, and returning it to the original shape would be hard (if not impossible). At this point, the paperclip has yielded and deformed in a way that is not temporary.
If you can imagine paperclips made of different materials yielding at different angles, you can see how yield strength is a good material property to use when comparing the strength of materials relative to each other. Look on the MatWeb site (www.matweb.com) to see the yield strength (among many other material properties) of just about any material you might want to use.
Material Failure: Stress, Buckling, and Fatigue
The yield strength is the name given to the specific stress a material can experience before it gets, um, stressed out. Stress is just a force applied over a certain area, commonly expressed in pounds per square inch (psi). The stress at which a material actually breaks is called the ultimate strength.
There are actually four different kinds of stresses, and therefore four different ways a material can fail (see Figure 2-1):
FIGURE 2-1 Tension, compression, shear, and torsion
1. Tension Tension is a fancy word for stretch. Think of the chains that hold up a child’s swing on a playground. These chains are in tension when someone sits on the swing because they are being stretched. If someone very heavy sits on a swing designed for a two-year-old, the chain will break or fail in tension. The force or weight of the person divided by the cross-sectional area of the chain is the stress the chain feels in tension.
2. Compression Compression is a fancy word for squish. When you look at material properties on a website like matweb.com, you will sometimes see two different numbers for tensile strength and compressive strength. Consider this when you are designing support structures for your mechanisms— especially ones that could hurt someone if they break. There’s a reason why buildings don’t have foundations made of cheese: The compressive strength just isn’t high enough. The force of the building divided by the area of the foundation is the total stress the cheese would feel.
3. Shear Shear stress is what’s happening to the box in Figure 2-1, where the force is coming from the side of the box instead of in line with it. Try to avoid this situation in your designs, because the shear strength is only half the tensile yield strength in things like metal bolts.
4. Torsion Torsion is a fancy word for twist. The hex keys in Figure 2-2 failed in torsion when I tried to use them to unscrew a bolt that was glued too tight. The hex key twisted out of shape before I could get the screw unstuck.
FIGURE 2-2 Allen keys that failed in torsion
Special cases of failure also include buckling and fatigue. Buckling happens when something is too long and skinny, like a column, and doesn’t even get a chance to squish before it gives out. For example, you can probably balance your coffee mug on an empty toilet paper roll, but if you tried to balance it on one drinking straw, that wouldn’t work out so well.
Fatigue failure is what happens when you bend a paperclip back and forth a bunch of times until it snaps. A single back and forth bend isn’t enough, but after 20 or so, the paperclip gets stressed out and breaks from fatigue.
How to Tolerate Tolerances
The description of most raw materials will tell you what the tolerance is on the length, width, diameter, or some dimension of the part. So what is a tolerance?
The tolerance of a part dimension is the range of values a thing could actually have when you get it. For example, you may think you need a half-inch-diameter aluminum rod, but you don’t really mean 0.5 in. That implies that you want a rod that is 0.500000 in, or perfectly 1/2 in. There are two problems with this:
1. You probably don’t want a 1/2 in rod. You want one a little smaller or a little bigger.
2. No manufacturing technique is perfect, so there is no machine that exists that can make you a perfect 0.500000 in rod.
You need to figure out the range that’s acceptable to you. Let’s say you need this 1/2 in rod to fit in a hole in a wood block you measured with your calipers that has a diameter of 0.517 in. You need the rod to slide in and out and spin freely, so you want to leave some clearance, or space, between the rod diameter and the hole diameter (see Figure 2-3).
You look on the McMaster-Carr website (www.mcmaster.com) and find a 1/2 in rod that says the diameter tolerance is ±. 005 in and the length tolerance is ± 1/32 in. This means that your rod diameter could be as big as 0.505 in or as small as 0.495 in, and McMaster-Carr is not guaranteeing anything more specific than that. Luckily, either one of those will work for you in this case, because they are both smaller than your 0.517 in hole. The length tolerance is a wider range, but length matters less to you for your specific application than diameter, so you decide that anywhere in that ± 1/32 in length range is just fine.
TIP When looking for just about anything—from raw materials to project supplies and components—McMaster-Carr (www.mcmaster.com) is a good place to start. It’s like a giant online hardware store with an enormous selection of parts and materials paired with the best user interface for a website of its kind. It also has helpful descriptions of materials and uses, with detailed pictures of the majority of the products. I will mention it frequently in this book, and call it McMaster for short.
FIGURE 2-3 Tolerances of a shaft and hole combination
You will almost always find that more precise materials with tighter tolerances are more expensive. It takes more work to make a metal rod that is 0.5 ± 0.001 in than it does to make one that’s 0.5 ± 0.010 in. If you don’t need perfection, don’t ask for it! You will pay the price if you do. Words like precision ground and tight will get you tighter tolerances and very straight, flat material, but only spring for them when you really need them. On the other hand, oversize parts guarantee they will always be bigger than the standard dimension you choose.
There are three main types of solid materials: metals, ceramics, and polymers (plastics). Three other categories are useful to define: composites (including wood), semiconductors, and biomaterials.
Metals can be pure, as in the case of pure aluminum or iron, or alloys, like steel. An alloy is just a mix of two or more metals. Metals are generally good conductors of heat and electricity. They tend to be strong yet ductile, so you’ll find them in a wide variety of shapes and sizes.
Your local home-improvement store should have a good selection, and you can look to more specialized metal distributors, depending on the sizes, shapes, and materials you need. In addition to our staple supplier McMaster, here are some other places to look:
• Metalliferous (www.metalliferous.com) sells a good assortment of metals in sizes and shapes common for hobbyists.
• At Metal Supermarkets (www.metalsupermarkets.com), you can get small pieces and quantities of just about any metal cut to the size you need, while you learn from their helpful material descriptions.
• OnlineMetals.com is another great resource, and you can get a 5% discount if you access the store through the Maker Shed site (www.makershed.com/v/metals/).
• Speedy Metals (www.speedymetals.com) has a good selection and good prices, but searching the site is less beginner-friendly than on the preceding sites.
Although we’ll cover different types of metal here and talk about finding the raw materials, don’t discount sets like Erector Set and VEX Robotics Design System for structural metal parts that are easy to configure. 80/20 Inc ) and Item () sell grown-up versions of these configurable structural systems, and MakerBeam (www.makerbeam.com/) is an emerging open hardware version of the same.
Steel has excellent strength, but this comes at the expense of high density. Out of the materials we’ll cover, steel is one of the more difficult to work with in its raw form because drilling holes in it or shaping it takes patience. However, it’s great for screws, shafts, and bearings, and as sheet metal.
Steel is a mix of mainly iron and carbon, with other elements like chromium, sulfur, and manganese thrown in to create different combinations of strength and other material properties. It conducts electricity and heat well.
Stainless steel resists rusting and is generally not magnetic, but it is difficult to sharpen. Plain or carbon steel will rust, but it is easier to sharpen, so it is often used to make cutting tools.
If you go to mcmaster.com and type “steel” in the Find box, you’ll get a screen that shows a bunch of different shapes, dozens of alloys, over a dozen finishes, and something about specifications. So how are you supposed to make sense of all this?
First, realize that you don’t need to find the perfect material—just something that satisfies your criteria. As a rule of thumb on McMaster, start with the characteristics that are most important to you, and then keep narrowing down the options until you reach a manageable number of options.
Let’s assume you’re looking for some thin, steel sheet metal you can bend and drill holes in for an automatic electro-mechanical toilet paper dispenser you have in mind. In this case, you start by choosing the Sheets, Bars, and Strips icon. You don’t want anything fancy, so choose Plain on the next screen under Type. You also think you want something about 12 in long, so choose 12 in the Length section, but you’re not picky about the other dimensions.
Now scroll down and read through some of the material descriptions. It looks like the first one, General-Purpose Low-Carbon Steel, is easy to bend, so you choose that one. You want your toilet paper dispenser to be about 2 in wide, so you choose that option. Uh-oh—unfortunately, the thinnest piece at that width is 1/8 in thick, which you won’t be able to cut or bend by hand. So you back up and try choosing a thickness you know you can cut and bend first, and decide on 0.020 in. Phew! Now your job is easy, because there is only one option, a 12 × 8 in piece that comes in packs of five.
TIP Sometimes sheet metal is sold in a gage thickness, not a decimal thickness like 0.020 in. The higher the gage number, the thinner the material. For example, a 0.030 in thick steel is also called 22 gage, and a 0.060 in thick sheet is about 16 gage. To convert from one to the other, check eFunda’s handy online calculator at www.efunda.com/designstandards/gages/sheet_forward.cfm.
Local metal shops are another great resource, but they may not have convenient online stores. Don’t be afraid to pick up the phone and ask questions about what size/thickness/gage/alloy of steel would be best for your project. Most suppliers will be happy to help you—after all, it is in their best interest to have an informed customer. Also check out plumbing pipes, fixtures, and electrical conduit as sources of steel tubing that are much less expensive than more precision extruded material.
Aluminum is less dense than steel and is known for having a great strength-to-weight ratio. It’s much easier to bend and drill than steel, and won’t rust like plain steel.
A good multipurpose aluminum alloy is 6061. If you want something strong but still lightweight, alloy 7075 is about twice as strong as 6061 and just as light. It’s actually stronger than a few steel alloys, and is used in aircraft and aerospace mechanisms. This means, of course, that it’s generally more expensive, so choose the alloy that is most appropriate for your application.
NOTE The naming schemes for alloys of most metals (aluminum 6061 and 7075, 303 stainless steel, and so on) are confusing and not necessary to understand or memorize. I’ll mention the most common alloy names, but other than those, just read the descriptions of the material options you have once you have narrowed down your options using other variables that are important to you. McMaster often includes suggested uses in the descriptions, like “excellent for sheet metal work,” “use for gears,” or “light structural applications.” Follow that advice, or give the store you are ordering from a call for help with selection.
Aluminum extrusions are made by pushing hot metal through a shaped orifice, just as you did with the Play-Doh Fun Factory when you were little, only the metal is heated to a much higher temperature to be formable. Extrusions can be flat bars, L-shaped angle stock, or C-shaped channels.
Shelving standards (the rails you can nail into a wall, and then attach brackets and shelves to) are readily available at home-improvement stores and can be used as structural components. Aluminum angle and shelf brackets are good for mounting motors and joining pieces together.
Aluminum is a good conductor of heat, but it’s a poor conductor of electricity relative to copper.
Copper is a good conductor of electricity and very cheap, so it’s used a lot in wires, printed circuit boards, and other electronic equipment. It is also used extensively in the plumbing industry. Copper tubes can be assembled by brazing (see Chapter 3) to create small art pieces, sculptures, or stands. Copper is relatively soft, so you will rarely see it used for structural or high-strength parts, but you will see it a lot in sculpture and decorative work. When exposed to air for long periods of time, it will get a light-green coating, or patina, as on the Statue of Liberty.
These are two common copper alloys:
1. Brass Mix copper and zinc together, and you get brass. It’s stronger and more durable than copper, and good at resisting corrosion from the atmosphere and water (including saltwater). It’s not generally used for structural parts, but is common in furniture and architectural design.
2. Bronze Mix copper and tin together, and you get bronze. The alloy is harder and stronger than copper, and more corrosion-resistant but softer than brass. Bronze is also used in bushings because of its relatively low friction. (If you don’t know what a bushing is, no worries—you’ll learn in Chapter 7.)
Silver is actually a better conductor than copper, but it is much more expensive, so it is used only in very high-end electronics. It is also soft and easy to form, making it a favorite of jewelry makers.
Mixing silver with a bit of copper yields sterling silver, an alloy that is even easier to work with. Silver is not used structurally because it is too soft and too expensive.
Ceramic compounds fall between metallic and nonmetallic elements (oxides, carbides, and nitrides). Examples include clay, glass, diamonds, precious stones like amethyst, and your favorite coffee mug.
Ceramics are hard, but if you’ve ever dropped a coffee mug and seen it shatter, you know they are also brittle. Unlike metals that stretch and yield before they break, ceramics just break. They are good insulators of electricity and heat, and resist high temperatures and corrosion well. For our purposes, you might see them used only as insulating spacers (standoffs) that keep electrical components safe.
Plastics and rubbers are types of polymers. Foams, like neoprene, also fall into this category. They have low densities, so they are relatively light.
Thermoplastic materials can be molded and remolded when heated, and will return to original form. This quality makes thermoplastics, like soda bottles, recyclable.
Thermoset plastics cannot be remolded and thus can’t be recycled. Examples are bowling balls, football helmets, and epoxies.
As with all materials, plastics come in a variety of shapes and sizes. The following are some common types and uses (thanks to Peter Menderson and his materials class notes and resources at http://itp.nyu.edu/materials/):
• ABS is tough and impact-resistant. A lot of small appliances are made from this. It’s softer than acrylic and easy to machine. It’s what LEGOs and the original printing material used for Makerbot’s CupCake CNC are made of.
• Acrylic (trade name Plexiglas) is a clear, hard plastic commonly used in laser cutters and model making. You can cut thin sheets just by scoring it with a hobby knife and then snapping it apart.
• Delrin is tough, easy to machine, and low friction (although not as low as Teflon). It is used commonly in gears and bearings.
• Nylon is similar to Delrin and good for general-purpose wear applications.
• PETG bends easily and is a cheaper alternative to polycarbonate.
• Polycarbonate (trade name Lexan) is shatter-resistant, has excellent clarity, and has high-impact strength. It is porous, so it will absorb moisture from the air.
• Polyethylene comes in a wide range of grades and properties. It also vacuforms well (see Chapter 9 for a description of vacuum forming).
• PVC plastic is the white material usually used for plumbing pipes and fittings. Although easy to saw, cut, and drill, it is particularly environmentally unfriendly because of toxins released in its manufacture and disposal. You cannot laser cut it, and you should use safety gear (mask, goggles) even when cutting or drilling it.
• Styrene is easy to machine, low cost, and flexible. It comes in thin sheets that can be cut with a sharp knife.
• Teflon is slippery, so sheets and tubes are used for bearings and sliding surfaces.
• Rubber parts are used as shock absorbers and stoppers, as well as in O-rings and gaskets to seal mechanisms from the elements.
The type of plastic you’re looking for, and how much you need, will help determine where you can find it. As always, starting at the McMaster site is convenient. However, plastics are so popular that you can likely find what you need at a local arts-and-crafts store like Pearl (www.pearlpaint.com) if you’re looking for common shapes and sizes like sheets or rods.
Foams, rubbers, and plastics for molds and casted parts fall in this section as well. Insulation foam (the harder, pink and blue kind) and RenShape foam are popular for prototyping. Smooth-On () makes dozens of materials for prototyping, sculpting, and model making. The Compleat Sculptor (www.sculpt.com), based in New York City, is one of Smooth-On’s distributors, and a one-stop shop for everything you could ever need for molding and casting. We’ll talk more about this process in Chapter 9.
For more do-it-yourself (DIY) versions of shaping your own plastic parts, check out ShapeLock Design Plastic and Sugru. Both of these products are plastics that you can mold and shape by hand, and then let harden at room temperature. Depending on your application, don’t overlook Tupperware or toy sets like LEGO and K’NEX for structural parts and housings.
Composites are similar to alloys in that two or more materials are mixed together to combine the favorable characteristics of each. However, metal alloys mix only types of metal, while composites can mix materials across different material groups, such as fiberglass and carbon fiber. For example, composites are being used increasingly in the newest airplanes, including the Airbus A380 and Boeing 787. Composites are lighter weight but still strong, so the plane uses less fuel to cover the same distance. Rapid prototyping companies like Solid Concepts (www.solidconcepts.com), which we’ll talk about more in Chapter 9, can use composites of aluminum, nylon, and glass to create small functional parts for prototype designs.
Wood is actually a natural composite, made of strong cellulose fibers (think of them as straws) held together by a stiff material called lignin (think of this as the glue around the straws).1 It’s relatively easy to work with and generally low cost.
In general, harder woods are stronger, but more difficult to work with. Soft woods like balsa are very light and easy to work with, but also very weak and split easily. Wood tends to split along the grain (that’s why those disposable wooden chopsticks pull apart so easily).
Composite woods like plywood, medium-density fiberboard (MDF), chipboard, oriented strand board (OSB), particle board, and Masonite are popular due to their low cost and availability. All are combinations of wood chips or particles and binding agents.
• Plywood is made of alternating layers of large wood chips at right angles to each other that are held together with glue, so it is stronger than the sum of its parts and tends not to split like pure woods. Aircraft-quality plywood, available from hobby stores, is higher quality than home-construction plywood.
• MDF is made with fine sawdust combined with a wax and resin binder, and formed under pressure, which makes it stronger and denser than plywood, but also heavy.
• Thin Masonite is very popular to use with laser cutters. It is more environmentally friendly than some of the other composites because it is made from natural materials and doesn’t used formaldehyde-based resins to bond the wood particles.
• Bamboo is even more environmentally friendly. Because it grows so fast, it’s easily renewable. A few companies make bamboo plywood (try www.plyboo.com for a sample), but it may be a few years before prices come down and bamboo becomes more popular with hobbyists.
Natural woods have a wide range of properties as well. Maple, cherry, and oak are very hard. Spruce and balsa are very soft. They also come in a variety of shapes and sizes, from sheets to more common structural forms like 2 × 4 boards.
The lumber section of your local home-improvement store, arts-and-crafts stores, and hobby stores are good places to find wood materials. Look for wood with few or small knots, dings, or cracks. Look down the board length to check for warp, and avoid wet boards. You may also find scrap wood outside these stores, and some of that might be suitable for your projects.
To construct structures from wood, make through-holes (holes that go all the way through the piece) whenever possible, and use nuts and bolts instead of screwing or nailing directly into the wood and risking a split edge. The farther from an edge the hole is, the less likely the wood is to split.
Wood glue works very well when two pieces of wood are glued along the grain (not end to end!). When glued along the grain, the glue has a chance to seep in and grab onto the straw-like fibers, and will actually reinforce the wood. When glued end to end, the glue does not have a chance to seep in and grab anything; it just makes a very weak joint. Keep in mind that wood glues do not accept stains, so be careful not to leave any glue smudges on your work if you plan to stain the final piece. Natural woods accept stain much better than composite woods, so keep this in mind if aesthetics are important to you. See Chapter 3 for more on wood screws and glues.
Paper and Cardboard
Paper, cardboard, and foam-backed boards are also made of wood. Although most of you might not consider paper an engineering material, you can actually do a lot with some thick card stock. Just check out the paper animation kits at Flying Pig (www.flying-pig.co.uk).
Paper or cardboard is also great for quick prototyping to get your head around your ideas—kind of like LEGOs, but easier to cut. You can even cut out pieces of paper or cardboard first, and then glue them to your final work material to use as a cutting guide. Get a good X-Acto (utility) knife, a cutting mat (from an artists’ supply store, not a kitchen store), and a metal ruler to use as a straight edge. You can substitute old magazines for a cutting mat, but the mat makes a great work surface in general, so it’s a good investment.
To find paper and cardboard, your local arts-and-crafts store is a good place to start, followed by office-supply stores. You can find posterboard and index cards at most drugstores.
Foam core is made of stiff foam sandwiched between two stiff paper boards. It’s lightweight, easy to cut, easy to glue, and easy to find. Your local arts-and-crafts store, as well as just about anywhere with a stationery or school supplies section, will carry it.
Semiconductors have electrical properties halfway between electrical conductors (metals) and insulators (ceramics). These materials have made integrated circuit chips possible, and enabled the electronics and computers we take for granted every day.
You won’t work directly with semiconductors in this book, but you will use components made from them (like transistors), so it helps to know what they are.
A biomaterial is a nonliving material used in a component that interacts with the human body. Biocompatibility is the ability of a material to avoid rejection by the body.2
Some common biomaterials are titanium, stainless steel, PMMA (the pinkish translucent plastic used in dental devices like retainers), Teflon, and silicone. For our purposes, these will come into play only if you are designing wearable technology or something that people will interact with for long periods. In those cases, you need to pay attention to biocompatibility to avoid problems with skin allergies and to enhance comfort. For example, if you’re designing a metal frame knee brace that generates energy when you walk, realize that up to one in four women have metal contact allergies (less than half that many men), and you should put a biocompatible pad of neoprene (or something similar) on any part of the frame that rests on skin.
Project 2-1: Different Diving Boards
In this project, we’ll put a fixed weight on the end of each of four different materials and see how they behave based on their material makeup and geometry.
• Four ~12 in long strips of different materials, such as the ones used for Figure 2-4:
• Wooden paint stirring stick
• Brass 1/16 × 1/4 in cross section
• Brass 1/32 × 1/2 in cross section
• Steel 0.020 × 1/2 in cross section
• Three C-clamps
• Scrap 1 × 3 wood board or hardcover book
• 4 oz clay blocks (or other small, fixed weights)
• Duct tape
FIGURE 2-4 Material testing
1. Allow about 10 in of each material to hang off the edge of a table. Clamp the wooden paint stick to the edge.
2. Clamp the remaining three materials underneath the book or scrap wood piece.
3. Duct tape the clay blocks to the ends of each material.
4. Notice how small changes in geometry as well as material affect how far the clay droops at the ends of the diving boards. Always keep material and orientation in mind when designing structures for your projects!
1. Michael R. Lindeburg, Mechanical Engineering Reference Manual for the PE Exam, 12th Edition (Belmont, CA: Professional Publications, 2006).
2. Buddy Ratner, Allan Hoffman, Frederick Schoen, and Jack Lemons, eds., Biomaterials Science: An Introduction to Materials in Medicine (San Diego, CA: Academic Press, 1996).