Have you ever wondered what materials are used to make a robot? The world of robotics is fascinating and diverse, with various materials coming together to create these innovative machines. In this article, we will explore the different materials used in constructing robots, highlighting their unique properties and contributions to the overall design. From metals and plastics to electronics and sensors, delve into the intriguing world of robot construction and gain a deeper understanding of the science behind these impressive creations.
If you are serious and want to learn all there is to learn about Robotics, please check out our complete guide on Where to Begin With Robotics.
Want to learn more why the materials used to make robots are diverse and range from metals and plastics to electronic components and sensors? Continue reading.
Metals are robust, rigid, durable, resistant to heat, and isotropic (their properties have no directional dependence). No other common class of material can compare to this set of qualities. They are frequently heavy and moderately to extremely challenging to construct. Forging, bending, machining (sawing, drilling, milling, turning), and grinding are the usual methods used to shape metals. Some can also be cast, but this usually necessitates some additional machining. Engineering materials are often alloys containing additional elements because metals are typically surprisingly soft in their pure state. Less than 1% carbon in steel, a few percent copper in copper, and a few percent magnesium in aluminum can produce an order of magnitude increase in yield strength over the pure material.
Metals’ unique ability to combine strength (hardness) and toughness is essentially unmatched. Other extremely strong materials can be difficult to work with because they have a tendency to be fragile. Metals have this characteristic because they bend before they break. The yield point, or practical upper limit of the forces that can be applied to a metal structural member, is the stress at which a metal deforms. However, this deformation significantly reduces the stress concentration at the leading edge of an impending fracture and stops the fault from spreading brittlely. Some plastics also possess this quality, but their yield points are substantially lower.
Some of the metals that are most frequently used as structural materials are those listed below.
Robotics With Steel
The most accessible and affordable metal, steel, is also one of the strongest. At 30,000 to 50,000 psi, unhardened mild steel gives. It can be easily strengthened to over 300,000 psi for tooling and 100,000 psi for structural reasons. It has a density that is around eight times that of water (7.9 gm/cc) and a melting point that is white hot, at 1400 degrees C (pure iron melts at 1530 C).
Ordinary or low-alloy steel is mostly made of iron, with a tiny amount of carbon (less than 1%) added to make it harder. Additionally, it frequently contains 1% or less manganese as well as trace levels of silicon and other metals. Steel alloys are frequently identified by a 4-digit AISI/SAE code, such as 1015 or 1040, or an analogous 5-digit UNS number (10150, 10400). The first two AISI numbers designate a broad type; for example, the number 10 in the number 1015 denotes common low-allow carbon steel. Carbon is represented by the final two figures in hundredths of a percent.
Generally speaking, the more carbon a steel contains, the more it can be hardened through heat treatment, which is the process of heating it to high heat, rapidly cooling it (typically in water or oil), and occasionally followed by a milder re-heating or “tempering” process, which makes the hardened material less brittle. When steel is “annealed,” it is heated to a red or orange temperature and then slowly cooled (over the course of hours or even days), returning it to a (relatively) soft state. Steel grades into the substance known as cast iron when the amount of carbon in the steel rises to one percent.
There are two types of steel stock: hot-rolled and cold-rolled. The least expensive type of roll is usually hot roll, which has a thick, black “mill scale” oxidation on the surface. It might also have uneven surface hardness as a result of the roll’s quick cooling after contact. These characteristics can be hard on tooling and lead to faster wear or breakage than anticipated. Cold rolling offers a more polished surface and a more consistent consistency because it is worked in a cooled state. Due to work-hardening effects, it tends to be slightly stronger and harder than hot roll but can still be machined.
For our purposes, the only kind of steel that can be worked is mild or annealed steel. With a little work, it can be machined. High-speed steel (HSS) tools can travel across the metal surface at a rate of up to 100 square feet per minute, according to machinists’ tables, but this is a bit of a maximum when proper cooling, lubrication, and feed rates are used. Beginners should aim for 25 SFM if they want to keep their tools in good condition. Although it seems slow, it will work.
If you have a welder, joining steel is simple, and the welded connections that result can be just as strong as the steel itself. It can also be linked using the lower-temperature brazing technique, but you usually still require a welding torch for that. Low-temperature soldering can be used to join tinned sheet steel (the material used by the old tinsmiths), but it requires a lot more power and practice to become proficient.
Steel will be useful for fasteners, bearings, springs, and other small parts of our robots, but it is probably too strong for most of the structural parts.
Aluminum has a density of 2.7 gm/cc and a yield point of 10,000–40,000 psi, making it about one third as heavy and one third as strong as steel. It typically melts at temperatures just above red heat, or 600 C (pure aluminum melts at 660 C). It typically takes a high polish and is malleable and ductile in nature. Engineering aluminum is typically an alloy with small amounts of other metals that add hardness and strength. Copper, magnesium, silicon, and manganese are typical additions.
Although aluminum does not rust, it can occasionally corrode in moist settings, particularly if the water contains acidic or basic contaminants. Anodizing is a general term for surface treatments that can offer significant protection. Aluminum is chemically unstable when it comes to air and water, yet it self-protects by covering itself with a very hard, tiny layer of aluminum oxide. Fortunately, it is quite difficult to ignite in air, even in fine shavings, despite the fact that it will burn intensely in pure oxygen.
Aluminum is the preferred structural metal for our robots since it is MUCH SIMPLER to produce than steel. With HSS tools at 100 SFM, it can be sliced with ease. If the tooling is not crisp, aluminum has a propensity to “gall,” producing a rough surface. Additionally, it occasionally “melts” or adheres to the tool’s cutting edge. A piece of hard plastic, soft metal, or even a fingernail may be used to snap off these accumulations from cooled tools, but you must be careful not to chip the cutting edge or injure yourself in the process. Aluminum cannot be soldered together and can only be welded using specific methods. Thus, threaded fasteners are typically used in the assembly of components.
The price of aluminum is almost twice that of steel for minor components. Large components have a bigger difference. Even though bulk materials are often cheaper, they are often more than made up for by how easy they are to work with.
Aluminum is non-magnetic and a superior electrical and thermal conductor. Due to the difficulties in creating reliable connections, its usage as an electrical conductor is often restricted to applications where very large conductors are required (it cannot be soldered, and contact surfaces sometimes become resistive and heat up). With the exception of applications requiring corrosion resistance or involving temperatures exceeding 300 °C, it is the material of choice for heat sinks and heat exchangers.
Bronze, Brass, and Copper in Robotics Materials
Copper alloys such as brass and bronze are occasionally used in construction. Bronze is an alloy of copper and other elements, most frequently tin, but occasionally silicon, aluminum, or other metals. Brass is an alloy of copper and zinc. They have a density of 8.4 gm/cc, which is comparable to steel, and a yield point of between 25,000 and 60,000 psi, which is typically half to two thirds of steel’s strength. Both melt at an excellent red-orange temperature of 900 C. At 1080 C, pure copper melts and turns bright orange.
Non-magnetic, non-sparking, and some of them particularly corrosion resistant, are all characteristics of copper alloys. When these qualities are necessary enough to justify the price, which is 4-5 times that of standard carbon steel in small amounts and more in large pieces, they are employed in place of steel. Due to its self-lubricating qualities both against itself and steel, brass is occasionally used in bearings. Both aluminum and a greased steel-on-steel bearing will quickly self-destruct (which is generally not hard enough for bearing purposes in any case).
The majority of popular brasses are easy to machine.Bronzes can be worked in a variety of ways, from easily to difficultly. Soldering allows the joining of several copper alloys.
Due to its extreme softness, pure copper is rarely employed in structural applications, but it is almost always used as an electrical conductor and occasionally in heat exchangers in places where aluminum would corrode. It is used in plumbing because it is resistant to corrosion, strong, and heat (compared to plastics), and because it is easy to solder.
Robotics & Stainless Steel
Since iron makes up the majority of the material, stainless is technically a type of steel. But nickel and chromium are present in quite high concentrations (10%–20%), along with additional elements like molybdenum or vanadium occasionally. These offer very strong corrosion resistance. Its density, melting point, and strength qualities are comparable to those of carbon steel. It is possible for stainless to be magnetic or not. It’s important to keep in mind that non-magnetic materials can become magnetic when bent or distorted if you’re counting on their non-magnetic qualities. Compared to copper, aluminum, and even normal carbon steel, it is a rather poor conductor of heat and electricity. However, compared to the human body or insulating materials, it is still a really good conductor, so use the same caution with electricity as you would with any other metal.)
Stainless steel is primarily employed as a structural material when corrosion resistance, high strength, and temperature resistance are required. The surface of machined stainless steel can be polished to a mirror finish. It is easy to clean and resistant to chemicals and scratches. It is occasionally employed in situations where looks are crucial due to these characteristics. For minor components, stainless typically costs two to three times as much as carbon steel, and even more for large ones.
Machine work with stainless steel is notoriously challenging. When cut, it quickly work-hardens even after being annealed. It frequently chips or breaks tooling. If you must machine it, use carbide tools and cut it at a speed of 1/4 to 1/3 that of cutting carbon steel with a similar hardness. Stainless steel cannot be brazed or soldered and requires specialized welding methods. Welded joints that are properly created are of a high caliber.
Titanium in Robotics
Titanium is a rare metal that is occasionally used in high-tech products. It has a density of 4.3 g/cc, which is roughly half that of steel, yet some alloys may be manufactured to have yields of 150,00 psi. It has a high melting point (1670 C), is highly corrosion resistant, and is inert to living things. Stainless steel is more expensive, harder to machine, and only possible to cast, forge, or weld using very specialized machinery. Turnings are a fire hazard because thin shavings can ignite and burn in the air. It will also burn in carbon dioxide and even pure nitrogen, making it difficult to put out a fire of this nature (dry sand will work).
When biological inertness is crucial or a very high level of strength and light weight are required for an application, titanium is typically the only material chosen (e.g., some aerospace applications). Most likely, we won’t need to use it in this course.
Other Types of Metals Used in Robotics
Magnesium is identical to aluminum in quality, but it is lighter (density 1.7 gm/cc), more expensive, more prone to corrosion, and its shavings burn hotter in the air (machining it is a fire hazard). Only when the reduced weight surpasses all the drawbacks is it employed. Most likely, we won’t apply it.
References to magnesium
Sometimes Zinc is used to make inexpensive metal parts for things like toys, locks, automobiles, and poor-quality machinery. Components may be swiftly die-cast at low temperatures, which is an advantage. The material is nearly as heavy as steel (density 7.1 gm/cc), but it is also brittle and not very heat resistant for a metal.
At low temperatures, Lead is likewise simple to cast, and some of its alloys have stronger mechanical qualities than zinc. Due to its high density (11.3 gm/cc), it is also frequently used to create weights. Lead is present in some of the best low-temperature solders. Unfortunately, some of its constituents are poisonous and can harm children’s developing brains when consumed or inhaled in small amounts. As deteriorating paint crumbled and peeled into living spaces, the widespread use of lead carbonate as a white pigment in house paint during the 19th and early 20th centuries led to widespread exposure. Due to the widespread knowledge that lead is exceedingly harmful, the majority of its uses have been abandoned. Due to its ease of use compared to all lead-free alternatives so far discovered, some electrical solder still includes lead. Batteries for automobiles continue to consume a lot of this material. Only use lead solder in areas with adequate ventilation. Avoid eating solder or batteries, and wash your hands immediately after using them.
The main reason Mercury (not a structural metal!) is fascinating is that it is a liquid at room temperature. In addition to mercury switches, which offer a very nice spark-free, position-sensitive electrical contact, it was commonly utilized in domestic thermometers. Lead is substantially less poisonous than mercury, yet mercury itself is dangerous because it can be ingested and it slowly evaporates into the atmosphere. A mercury switch can break when it is dropped, sending tiny mercury droplets racing over the floor and into every crevice and nook. We would probably be shut down permanently, and UR would have to send a decontamination crew. Therefore, I won’t be using the outdated mercury switches that we originally received as robot orientation sensors. Mercury is denser than lead (13.5 gm/cc), which is an interesting statistic. Google “mercury”
With a density of 19.1 gm/cc, Depleted Uranium performs even better for weight than lead or mercury. It is utilized in military armour-piercing bullets as well as to ballast the keels of some high-performance sailboats. It is regrettably slightly radioactive and can result in additional cancers if consumed in a bio-available form, as well as lung cancer if inhaled as dust. If fine shavings are burned in the air, uranium oxide will burn and release a fine, inhalable smoke. To use it in a robot, we would need to obtain specific clearance.
Plastics in Robotics
Materials with an astounding range of qualities are made possible by plastics, which are mostly products of applied chemistry from the 20th century. But they can be made to have almost any other material’s mechanical properties, except for extreme strength and heat resistance, where metal is still better than any plastic.
Plastics are polymers, which are composed of lengthy chains of repeating subunits. The majority of the time, these building blocks are organic chemicals, although certain plastics also use silicon and other uncommon chemical compounds. Monomers are the substances that represent a component subunit. But a polymer’s properties are most closely linked to the properties of its chains, such as their length, local flexibility, and whether or not they are crosslinked.
In general, plastics are impervious to water, corrosion, and chemical attack, especially from acids and bases. Organic solvents can be used to soften or dissolve some. Numerous plastics are capable of being rendered transparent, and these materials make up an important class of optical materials. They are often very poor heat conductors and excellent electrical insulators.
UV radiation, which has enough energy to break the polymeric chains and disrupt carbon-carbon bonds, is capable of photodegrading the majority of plastics. When exposed to external light, they deteriorate from weeks to years, becoming cloudy, yellow, feeble, and eventually disintegrating. UV inhibitors are applied to plastics made for outdoor use to halt the process. Some of these can withstand ten years or longer in the sun. In the long run, though, the only way to stop damage is to keep UV radiation from getting in.
Many polymers lose all or most of their structural strength at temperatures over 100 °C, and the majority soften at relatively low temperatures. Above 300 °C, a plastic’s effective mechanical properties are rarely retained.
In construction, plastics are frequently employed as hard bulk materials. The best engineering plastics are an order of magnitude weaker and lighter than mild steel, respectively (yield strength 3,000-12,000 psi). This makes them an appealing substitute in weight-limited applications. They are used as stretchable films and fibers as well. They can have a strength to weight ratio that is far higher than steel in this form. Polymers are also used to make most elastomers and a large group of semi-rigid materials, which we will talk about separately.
The plastics that are most frequently used as rigid structural materials are listed below.
Mostly, polystyrene is listed here as a plastic that should not be used in robotics. It is inexpensive, rigid, and simple to inject mold. It is frequently used as enlarged “styrofoam” and in low-cost toys and plastic model kits. Although it can dissolve in acetone and several other organic solvents, it is chemically fairly inert. With plastic model cement, this makes bonding quick and reliable. Styrene is a hard plastic, but it has a relatively low yield point (about 3,500 psi), making it susceptible to brittle fracture. With some caution, it can be machined. Slow drilling is necessary to avoid melting or breakage. When tapped or if screws are overtightened, it is prone to breaking. Polystyrene is mostly used in robotics as a surprisingly affordable source of intricate pre-made pieces salvaged from toys or model kits. Parts from many mass-produced hobby kits were used to embellish the starship models in the original “Star Wars” film. Styrofoam materials can occasionally be beneficial for structural purposes when very light components are required. Such pieces can be surprisingly strong for their weight if made with a tensile skin.
Sometimes referred to as acrylic or polymethylmethacrylate, is frequently sold in sheets and is used as a break-resistant alternative to glass windows. Although plexiglass is less likely to fracture under force than sheet glass, it is not a very durable industrial material. Although it has an extremely high yield point (about 10,000 psi) and a high absolute tensile strength, it is exceedingly brittle, similar to styrene. Strong localized pressure, such as that from a threaded fastener, or a violent hit, will both produce fracture. It has a propensity to fracture or melt if drilled too quickly, just like styrene. It’s a suitable material for tiny to medium-sized transparent panels that aren’t meant to withstand a lot of impact. When it comes to bars, the price of Plexiglass is about the same as that of aluminum, but it is much cheaper when it comes to large sheets used for windows.
Polyvinylchloride or PVC
This is a chemical. It is most often recognized as the soft, flexible plastic used to cover wires and as the material used to create inexpensive raincoats, inflatable toys, and imitation leather. However, without plasticizers, it is a solid, stiff substance. Many plastic plumbing parts are constructed of PVC. It is far more impact resistant than plexiglass and has a yield strength of about 7,000 psi. Before breaking, it also deforms by 10% to 20%. It is a reasonable structural material because of these qualities. It may be permanently cemented together using solvent welding cements and machines. PVC is the most affordable, structurally functional plastic, costing less than half as much as aluminum stock in tiny pieces and significantly less when purchased in bulk. It is available in big sizes as rods or sheets. PVC pipes are often used as building materials and are easy to find in the plumbing section of hardware stores.
This stands for Acrylonitrile-Butadiene-Styrene, is a copolymer made of three distinct monomers. It is an illustration of an engineered plastic in which monomers are mixed to create a substance with novel, desirable qualities. In this instance, a stiff styrene/acrylonitrile copolymer is interlaced with polybutadiene to create a synthetic rubber. Although not brittle, the result has the stiffness of polystyrene, and polar attraction from triple-bonded nitrogens in acrylonitrile provides additional inter-(and intra-) molecular bonding.The resulting plastic has a yield of around 5,000 pounds per square inch and is very impact resistant. ABS demonstrates significant ductile deformation, stretching by at least 20% before breaking. It is the material used to make LEGO bricks.
ABS operates effortlessly and smoothly. It holds threaded fasteners effectively and can be tapped for them. Effective glue or solvent welding are both options. At -40 C temperatures, it keeps its strength and impact resistance. The drawback is that it loses strength over 80 °C, making it unsuitable for hot environments. Additionally, it is not easily accessible in transparent form. ABS is roughly as expensive in small quantities as metal. It is much less expensive in huge commodity forms (like certain plumbing tubing).
Also referred to by the brand name “Lexan,” is the traditional miracle “indestructible” material. It is exceptionally resilient to impacts, with a yield point of roughly 10,000 psi, and can deform up to 100% before breaking. Similar to how metal is created, sheet material can occasionally be cold bent. It is frequently used for “bulletproof” windows and is available in a crystal-clear form. It can be softened with acetone and a few other solvents, but acids, bases, alcohols, and most oils are not able to break it down.
Generally speaking, polycarbonate machines work well. Since it has a tendency to melt when drilled, drilling should be done slowly and regularly with the tool removed to prevent chips from being stuck between the drill and the hole wall, where they would quickly transform into a gooey mess. While it can be glued, it won’t adhere as firmly as PVC. One of the more expensive plastics is polycarbonate, which costs 1.5–2 times as much as a modest amount of aluminum. It is not frequently utilized in residential plumbing or other high-volume consumer applications, unlike PVC, plexiglass, and ABS, so cheaper commodity forms are not typically available.
Acetal, also referred to by the brand name Delrin, is essentially a polyformaldehyde with a straightforward backbone made of alternating carbon and oxygen atoms. It is extremely chemically inert, somewhat ductile (up to 60% deformation), and strong (yielding 10,000 psi). Acids, bases, and all conventional solvents have no effect on it. It is suitable for low-pressure bearings and sliding parts due to its low coefficient of friction and resistance to abrasion. Even though it breaks easily when hit, it doesn’t break easily when stresses are applied gradually.
Acetal is a material that can be machined easily, is tapped, and holds screws well. The low coefficient of friction may be problematic in applications where screws have a propensity to get loose. Although it is easier to drill through because of its slickness than stickier plastics, it is still possible to melt or burn a hole if the drill speed is too high. Acetal cannot be properly attached due to its inertness, while some flexible adhesives might be able to create a weak bond. Acetal is reasonably priced; in tiny quantities, it costs a little less than aluminum. It is not a typical household item.
Polyethylene (PE) and Polypropylene (PP)
These materials are widely available, reasonably priced polymers with comparable quality. Both materials have a high degree of ductility and a final bulk strength of about 5000 psi, making them rather soft. They are slick, incapable of adhering, chemically inert, and resistant to all conventional solvents. They are frequently employed as containers for various kinds of chemical reactants, from powerful acids to paint thinner, because of their qualities. Both plastics will slowly diffuse through small, non-polar organic molecules, making them unsuitable for highly hazardous liquids. Both plastics are extensively utilized as single-use food and beverage containers. Cutting boards made of thick slabs of both PP and PE can occasionally be purchased as consumer goods.
Although they are far softer and more flexible than the other engineering polymers we have explored, both plastics have sufficient mechanical strength to be helpful in structural applications. With reasonable care, they can be machined. Due to the material’s elasticity and softness, some safety measures are necessary. Materials must be firmly anchored and tools must be extremely sharp to prevent material from bending away from the tool. Both materials cannot be bonded, and flexible adhesives only hold sporadically. Only marginally more expensive than PVC in price are PE and PP.
Actually, there are three types of polyethylene. When used in large quantities, low density polyethylene (LDPE) is rather brittle and prone to fatigue and impact fracture. Its suitability as a structural material is thus constrained. High density polyethylene (HDPE) is significantly stronger and more impact resistant. It is highly sturdy and is frequently used for big plastic containers like trash cans and chemical barrels. The creation of Ultra High Molecular Weight Polyethylene (UHMWPE) is very new. One of the best known polymers for impact and abrasion resistance, It has been applied to joint replacement implants and ice-free skating rinks.
A fiber with the trade names Spectra and Dyneema can be made from UHMWPE. When the extremely long molecules are aligned, this fiber has absolutely unbelievable qualities. Only in laboratory environments have metals ever surpassed their absolute tensile strength, which can be greater than 350,000 psi and is stronger than very high-strength steel cable. It is ten times stronger than steel cable in terms of strength-to-weight ratio. The fiber loses strength at 100 C, which is the only drawback. It is utilized in additional military applications as well as bulletproof garments. Amazingly, you can get this material as fishing line at a reasonable price at your neighborhood sporting goods store.
The innovative fiber’s initial trade name, nylon, was created by DuPont. Currently, the phrase is used to refer generally to synthetic polyamide polymers. Although the word is typically linked with fibers and cloth, structural components can also be made from bulk solid nylon. The tensile strength of this material can reach 12,000 psi, and it can withstand significant plastic deformation before breaking. It lacks ABS’s or polycarbonate’s impact resilience, and after being repeatedly stretched to its elastic limit, it gradually ages and fractures. It can be continually stressed practically indefinitely as long as it is kept well below the elastic limit. It is slick, resists abrasion, and can be easily injected molded into extremely accurate forms. It also has a low coefficient of friction. It is the perfect material for plastic gears and other moving parts because of this combination of qualities. Nylon gears are much less expensive to build than metal ones. Most of the gear trains and sliding parts in consumer electronics today are made of molded nylon.
As with plastics, nylon can be machined, drilled, and tapped with the usual safety measures against melting. It is unglueable. It costs slightly less than the price of aluminum in tiny quantities, making it reasonably priced.
PolyEthyleneTErephthalate is a polyester plastic, commonly referred to as PETE. It is best known as the incredibly durable material used to make Coke bottles, but it also serves as the foundation for Mylar film and Dacron polyester fiber. Although it is occasionally used in bulk, robotics is particularly interested in using it as an extremely robust sheet and film material. Stretching the amorphous plastic in both the “x” and “y” directions results in strength. The polymer molecules are stretched out as a result, aligning them with the sheet surface. Long molecules’ attraction to one another must be broken by further deformation along their entire length, not simply locally, as it would be the case if they were coiled up like spaghetti at random. The two extended directions of the film have a lot of strength and stiffness as a result. It’s possible to create PETE in a form that can tolerate temperatures higher than 200 °C. Plastic food containers that can be heated directly in an oven have been made using this material.
PETE film has been utilized in countless applications, ranging from packaging to magnetic tape substrates to balloons, due to its strength, durability, and chemical inertness. Mylar’s well-known glossy sheen is created through the vacuum deposition of aluminum vapor. The film can be securely attached for assembly using the right adhesives.
According to technical definitions, a composite is a structural material made of two or more simpler substances, frequently with highly different qualities, combined (in a macroscopic fashion) to produce a material with combinations of attributes not found in any of the components alone. For example, fiberglass, which is very strong and hard to break, is made by mixing plastic resins that are not very strong but are flexible with very strong but fragile glass.
The majority of engineering metals are technically composites under the description given above. Alloys can be observed to be made up of various crystal phases that are interlocked and cemented together when they are carefully polished, etched, and magnified somewhat. The desired qualities of alloys are mostly the result of the interaction of these components’ various mechanical properties. We will use the term in its more general sense for things made by putting together parts that are more obviously different from each other.
Non-isotropy in mechanical qualities, such as strength, is a feature of many composites. Fishing rods made of carbon composite can withstand lunges from hooked marlins that would bend steel in the same way, but they will fracture if they are stepped on. This is caused by a composition technique that is frequently used, in which a sturdy but brittle material is produced into thin threads and joined along their length by a weaker, more flexible “glue.” The mechanical concentration of stress at a fracture tip supported by a strong bulk of material behind it causes brittle fracture to spread. A stressed bulk element’s (small) distortion can be easily tolerated by individual fibers. Any concentrated stresses cause the softer material to give locally, dispersing the stress along the fiber’s length and preventing the bulk mechanical leverage that leads to brittle fracture. Since the weak glue keeps the fibers together edge-to-edge for a considerable distance, the composite retains the majority of the strength of the fiber material along the fibers. However, the composite can only be pulled apart across the fibers with the strength of the glue.
Modern composite engineering is a distinct field unto itself. Only a select few materials, such as wood (yep, wood), fiberglass, potentially pre-made carbon fiber components, and glass-filled polymers, are likely to be used in the construction of our robots.
Wood in Robotics
Although wood may not seem like the right material for sleek, contemporary robots, it offers some benefits. The most notable of these are the low cost, vast availability, and ease of shaping with readily available tools. However, wood is a great structural material on its own. It is a natural composite made of cellulose fibers and lignin that has a stiffness to weight ratio comparable to that of steel or aluminum. This is crucial for constructions that have enough extension to flex under their own weight. Wood is more durable than most plastics and stronger than many in terms of absolute strength (usually several thousand psi along the grain). Its low propensity to “creep,” a creeping deformation under load that troubles many plastics, is more significant once it has dried. More securely than almost any other material, wood may be bonded together in a variety of ways. It can be fastened with nails, screws, glue, bolts, dowels, wedges, and dovetails. There are ways to assemble things without utilizing external fasteners that are just as safe. Additionally, it can be polished to have almost any desired appearance.
Yes, there are issues with wood. Only in the direction of the grain does it have strength. It is often much weaker in compression and barely 1/100 as strong in tension perpendicular to the grain. For structural purposes, wood should be regarded as having practically little tensile strength across the grain. The two-dimensional strength of plywood, which is created by adhering thin sheets with the grain angled at various angles, is good. There is currently no robust isotropic wood-based material. Although slightly isotropic, particle board is weak in all directions. Although fibers might theoretically interlace in all three directions, trees have not successfully done so on a large scale. Although some “unsplittable” woods, like elm, are well known, they are actually not even close to being isotropic.
Even within the same board, wood might have knots, splits, flaws, uneven grain, and varying strengths and densities. And as the humidity changes, it moves. On a humid day, a snug fit on a dry day can give way. Usually, gaps develop in initially well-fitting assemblies. Flat boards can now bend or twist. Uneven motion is present. Wood doesn’t really alter in size along the grain. It’s possible for the grain to shrink by up to 5% across, especially if it was slightly green to begin with. A big part of woodworking is making sure that things still work even though wood naturally moves.
For our robots, the most useful kinds of wood are probably small bars and dowels, which can be used to make decently light beams, and some of the thinner plywoods, which can be used as mounting surfaces and simple boxes.
Glass Fiber with Carbon Fiber
A matrix of oriented fiberglass filaments is impregnated with a liquid plastic resin, typically epoxy, and then cured to form a strong, hard material. Glass filaments can be fashioned into cloth or a felted mat for two-dimensional strength or longitudinally for a material with primarily one-dimensional strength like wood. Although they are possible, three-dimensional weaves are rarely used and typically only in pricy, exotic applications. In its strong directions, cured fiberglass composite can be as strong as medium steel but is much lighter (density of about 2 gm/cc).
An acceptable DIY material is fiberglass. Automobile parts stores, craft and hobby stores, and even some hardware stores carry glass cloth and epoxy resin. It is among the simplest methods for producing curved custom parts. The materials’ risks are minimal: the resins may irritate skin, and glass fibers may occasionally pierce the skin and cause a rash and irritation. Both must be kept completely away from the eyes, and dust from sawing or sanding should never be inhaled or allowed to collect in an area where it might be disturbed and spread into the air. Wet-sanding techniques should be utilized if curing fiberglass needs to be abraded.
With the exception of using ultra-strong carbon fibers created by pyrolizing specific orientated polymers in a furnace in place of glass, carbon composites, also known as “graphite,” are comparable to fiberglass. It is more expensive and stronger. Glass is safer to work with than carbon fiber. Unlike glass, which can (very slowly) dissolve in the body, the rigid fibers easily penetrate the skin and never disappear. Although the majority of industrial fibers are much larger than the.5 micron diameter that is most harmful to the lungs, inhaled carbon fiber is still a potential carcinogen. In any situation, safety measures need to be implemented. Never saw or sand pre-fabricated carbon composite parts unless you have respiratory protection and containment mechanisms in place to prevent residues from leaking into the environment.
To boost their yield strength, plastic resins are occasionally combined with (relatively short) fibers made of harder or stronger materials. Although chopped glass fiber is the filler that is most usually employed, other materials like ceramics, metals, and even other plastic fibers have also been used. Such materials are not as strong as manufactured composites utilizing the same components since the fibers are short and typically do not have coordinated orientation (e.g., fiberglass). Glass-filled plastics, however, are typically twice as strong as unmodified resin. This usually means giving up ductility and sometimes impact resistance. Glass fill, on the other hand, can make brittle resins more impact resistant by “bridging the gap” and stopping cracks from spreading.
Ceramics, or heat-fused refractory, totally oxidized materials, can be compared to artificial igneous rocks. They are usually created by heating specifically formulated mineral mixes to high temperatures in a furnace, where they adhere to one another and do not dissociate even after the substance has cooled. High pressures or particular atmospheres may also be used throughout the process, and the mixes may or may not totally melt. Pottery is a classic example that has been around for a very long time.
Ceramics have not historically been considered as materials for structural engineering. with the exception of bricks, which were not typically considered to be pottery. This began to change midway through the 20th century, when builders started employing glass as a load-bearing material. High-tech ceramics are now crucial structural components in a variety of products, including gas turbine blades, electrical insulators, and bearings. In several of these materials, the brittleness related to glass and porcelain has been considerably decreased. Ceramics can do much better than metals in terms of strength, hardness, and resistance to wear, even though they aren’t as strong as good metals yet, especially at high temperatures.
Ceramics are challenging to work with due to their abrasiveness. They are frequently produced straight in their finished state or simply require a final polishing procedure. With the exception of the occasional glass window, we won’t have many reasons to use ceramics as structural components in our robots.
The most accessible variety of glass is common “window glass,” which has the famous fragile characteristics of tending to break into very large, very sharp fragments. Glass is an extremely durable substance by nature. Its brittle fracture, when a crack begins and mechanically concentrates tension at its tip, is the cause of its fragility. Due to the fact that glass is essentially non-ductile, the tension cannot be reduced by plastic deformation, and the fracture spreads quickly. Microscopic scratches are a common feature of glass surfaces, and when that surface is put under tension, these scratches act as fracture starting sites.
Glass can be made less brittle using a variety of “tempering” techniques. In essence, tempering creates tension inside by compressing the external layer. Because the surface is not under tension when the glass is stressed, minute scratches are less likely to develop into fractures. On the other hand, as is seen in car side windows, if a chip or scratch penetrates the compressed surface layer, the pre-existing stress in the interior causes the entire volume to quickly fracture into little fragments. In general, these tiny fragments pose less of a risk to individuals than big, pointed shards.
Initially, glass was tempered by rapidly cooling the surface while it was still hot and plastic. The inside, which is still plastic, accommodates local contraction as the surface layer freezes. The surface layer is then compressed as the inner shrinks as it cools. The only shapes that this method actually works well with are simple ones like sheets and curved surfaces.
More recently, “chemical” tempering techniques that can be applied to any shape have been introduced. Here, glass in its solid state is put in a chemical bath that switches out some mobile elements in the glass matrix with bigger equivalent atoms, such as sodium in soda-lime glass (e.g., potassium or cesium). The surface layer is compressed because these require a little more room. Usually, this is carried out at temperatures that are high enough to make the atoms more mobile but not so high as to weaken the glass. To temper soda-lime glass, one of the simplest procedures is to submerge the pieces for 8 to 24 hours in a bath of molten potassium nitrate (approximately 350 C = 660 F). Try this nearly, but not quite, in your home oven. Molten KNO3 dissolves with the evolution of oxygen and nitrogen only slightly above its melting point, causing strong reactions with organics and metals (at 400 °C).Hot spots in your home oven will be disastrous.
Recently, items like “gorilla glass” (used in smartphone screens and other uses) have been created using extreme tempering. These thin sheets can be twisted twice without breaking and, essentially, don’t seem like glasses at all.
It is fairly simple to “cut” untempered glass sheet (such as window glass) by scoring and shattering it with a low-cost glass cutter. By using abrasive grinding and polishing, larger materials can be formed into more intricate shapes, such as extremely precise optical components, although this requires specialized wet-process equipment. If the abrasive can be kept moist without posing an electrical threat or endangering the machinery, minor grinding can be done on a regular abrasive wheel or a stationary belt sander. Thermal stress is usually the cause of glass breakage during dry machine grinding. Using a hand sanding block and fine (>= 220 grit) sandpaper, you can smooth out sharp edges. There are specialized diamond and carbide drill bits available that, with a little caution, can be used to create circular holes in glass sheet. A usable disk may also be produced by the diamond hole saw. They are always used with liquid coolants, such as water. Tempered glass is typically impossible to cut and can only be lightly polished and ground. Prior to being tempered, it must be formed.
Home centers and hardware stores sell window glass at reasonable prices. Plate glass comes in a variety of thicknesses and colors and can be purchased from specialty commercial and auto glass shops. For an additional fee, they can also buy tempered glass sheets that are cut to size, have holes, and have polished edges. High-quality glass lenses, ports, and filters can be purchased from optical shops like Edmund Optics for a very high price (and at an even higher price, some components made from non-glass materials such as sapphire and silicon).
Rubbery materials are called elastomers. Their ability to experience significant deformation under low stress and then quickly return to their former shape after the load is removed is one of their special mechanical properties, and it is also one of the most valuable. Elastomers have a relatively high compressive modulus in the bulk, similar to liquids. Elastomers experience little volume change under uniform (isostatic) pressure. Due to their very low shear moduli, which effectively enable them to flow under stress, they exhibit an array of intriguing features. Sometimes elastomers are modeled as very surface-tensioned liquids. This is a passably accurate representation of a spherical rubber ball. To accommodate alternative shapes, some adjustments must be made.
Most elastomers are polymers, which are frequently cross-linked collections of loosely coiled chains. They can be manufactured to be extremely resistant to chemical attack and are often watertight. They produce excellent seal and gasket materials due to their elastic and conforming characteristics. A rotating shaft can be safely sealed against pressures of several thousand psi using rubber O-rings. They enable hydraulic pistons. Elastomers are also utilized in flexible connectors, mounts that reduce vibration and shock, and surfaces that need to be soft to the touch, highly frictional, and have good grip. Many elastomeric parts can be bent and twisted several times without losing their resilience or original shape (think tires).
It is incredibly difficult to form cured elastomers. They cannot be cut precisely because they distort when under mild force. They are often abrasion resistant, and if possible, abrasive machining frequently results in an excessively harsh surface. As a result, the majority of elastomeric parts are produced in their final state. Unvulcanized rubber is an example of an elastomer that can be melted and molded, while other elastomers begin as liquid resins that are polymerized right in the mold.
Elastomers are employed in robots to create friction on grippers or traction on the ground. To reduce impact pressures and prevent damage to and by parts that contact the environment, they are employed as soft contacts. For strong (and damage-free) interaction with objects, they provide passive compliance in joints and manipulators. And when it’s necessary to keep liquids or dust inside or outside, elastomers are used as seals.
Any elastomeric material is frequently referred to by the general term “rubber.” The phrase more particularly refers to polymeric elastomers with carbon-based backbones, and even more precisely, to those made from the original substance. A variety of plants produce latex, a milky sap that is used to make natural rubber. The tropical rubber tree (Hevea brasiliensis), a type of euphorbia, is the source of commercial rubber, although many other plants, including milkweed and dandelions, generate rubbery latex. Its primary function in nature seems to be to permanently gum up an insect’s mouthparts so that it cannot consume the plant. When latex is exposed to air, it changes into a soft, flexible material that you can heat and reshape.
When latex is heated with a small amount of sulfur, it becomes significantly stiffer, tougher, and more abrasion resistant, according to Charles Goodyear.The tire industry was created when he gave the procedure the name “vulcanization.” Chemical crosslinking of the polymer chains by sulfur makes the substance more robust and rigid.
Premium tires, particularly those for trucks, are still produced using natural rubber. But the demand for rubber increased during and after World War II, and this led to a lot of research into synthetic substitutes. Today, there are hundreds, each with a unique set of special qualities. Based on polybutadiene, the first commercial synthetic success was created (butane with two double bonds). Cross-linking is caused by the second double bond, which makes the chains coil into springy coils.
Numerous shapes of pre-formed rubber are available. Rubber bands, sheet material, corks, and O-rings are examples of frequently used forms. In a variety of machines, large, particularly formed portions, appear as isolation mounts. Curved rubber is challenging to shape, although sheet material can be cut with scissors, and there are specific instruments for cutting round holes in corks or other solid shapes. It adheres effectively to rigid substrates with special cement and can be secured with superglue. Superglue does not work well for rubber-to-rubber contacts that will be stretched since they are hard.
Elastic polymers with a silicon-containing backbone are known as silicone rubbers (instead of or in addition to carbon). Keep in mind that these polymers are different from elemental silicon, a fragile, silvery substance used to make computer chips.
Some silicones are offered in an uncured form that becomes a rubbery solid when exposed to air (e.g. “silicon-seal”). These components can be used to build robots in a variety of ways. They are perfect for securing gaskets and creating waterproof or even airtight junctions between constructed components. Additionally, they can be used to “glue” materials together that ordinarily cannot be joined, particularly glass and metal. Although the connections are not particularly strong in an absolute sense, their flexibility precludes fracture failure, making them quite secure for holding components in place against light stresses.
Tensile Elements That Can Bend
In the same sense as rigid solids and elastomers, tensile components can be thought of as a separate material class. They are flexible otherwise but only have strength under stress in one dimension. Although solid wires are rarely used, multi-strand twisted or braided cable is the most common type. The more sophisticated examples include chains and flexible belts.
The majority of the materials used to make structural cables are stranded steel wire, often known as wire rope, and a small number of polymer fibers, such as polyester (Dacron), aramid (Kevlar), and UHMWPE (Spectra, Dyneema). Although nylon and polypropylene are often used for rope, most structural applications find them to be too elastic. Chains are usually made of steel links, while belts are usually made of a mix of tensile fibers and elastomers.
In robotics, tension factors are mostly used in two ways. In stiff structures, the first is used merely as a tensile reinforcement. A rectangular frame is fragile and prone to diagonal skewing failure. The structure is sturdy in two dimensions, very light, and reinforced by two diagonal cables. Although the racking (twisting) failure mode still exists in three dimensions, robust 3-D structures can be created by joining such 2-D panels. The other usage is for tendons, belts, or chains in transmissions. Transmissions are a different topic, but some of the easiest and most effective transmissions use tension parts that can bend.
We at Awe Robotics want everyone to be able to create the robots of their dreams, so I’m writing this tutorial in the hopes that it will serve as a good place to start. The traditional method of purchasing a microcontroller board and all the components separately can be a worthwhile learning experience, but it may also not be feasible for many people, be more expensive, and prevent you from creating a truly stunning robot.
Regardless of your skill level, a 3DoT board will have you covered. Controlling the robot with the supplied iPhone or Android app can also be a terrific option. However, if you really know what you’re doing, a bespoke PCB will be the best option if you’re searching for the maximum performance.
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