Some assembly required
CSC 297 Robot Construction: Assembly
Some assembly required
Robots are made of many parts, generally connected to each other. Connections may be moveable or rigid. Moveable connections, which allow signficant motion between components in one or more degrees of freedom are handled in the section on joints. This section handles the case of separate components that need to be rigidly attached to each other so as to form the equivalent of a single element.
There are two basic methods of connecting parts to each other. The first is to use discrete mechanical components (such as nuts and bolts) to fasten the main components together. These fasteners may themselves bear significant loads. The second is to bond the component materials directly together at a microscopic level, via either material fusion (welding) or micro-mechanical/molecular forces using glues or adhesives.
A distinction also needs to be made between load-bearing and
mounting connections.
Some connections need to transfer significant loads between structural
components.
In such cases, the connection should be as strong as the structural
elements, at least in the directions of applied stress.
Other connections simply serve to hold non-load-bearing components
(e.g. circuit boards or sensors) in place, and do not need to match
the component strength, but only resist forces that might displace the
component.
Joinery is the art of structuring the parts of an assembly so that loads are effectively transferred between components. The term is most often associated with woodworking, but the basic principles are important for assembled artifacts made of any materials. Ideally, the major components of a structure could be placed in position, and the whole system loaded to spec without the need for any glue or fasteners. Usually, this is not quite possible. If nothing else, vibration and incidental forces tend eventually to move components out of alignment. However, it IS often possible to arrange matters so that the primary loads are transmitted directly through component-to-component contact without large stresses on any connecting mechanism. For example, if a thrust needs to be transmitted from one beam to another, the contact between the beams should be designed to be statically stable in the direction of that thrust, with any connecting mechanism serving only to stabilize the contact.
Classic furniture makers and timber framers were masters of the joiner's art. Prior to the industrial era, threaded fasteners were unavailable, and glues were relatively weak. Wood joints such as dovetails and mortise-and-tenon were designed so that the major loads were supported by the wood itself. Glue and nails typically only held things in place. These joints also had to deal with the fact that wood is strong only in one dimension (along the grain). Many woodworking joints have analogs used in metal, plastic, and composite fabrication.
When forces DO need to be transmitted through a connection, The attachment needs to designed to take the load. When the elements themselves are of appropriate size, this means that the joint has to be as strong as the components. Such joints often entail more or larger connectors than the beginner might expect (take a look at the number of bolts or rivets connecting overpass beams next time you are on the highway).
Glues and adhesives in particular are limited in their ability to carry tensile and sheer loads. As a rule of thumb, glues or adhesives should never be relied upon to carry a structural tensile load, and flexible adhesives should never carry a static sheer load. There are exceptions, but they require careful analysis.
The following describes a few of the most basic joints. A detailed discussion of joinery, which is usually considered part of structural design is beyond the scope of this section. Whole books have been written on the subject of wooden joints alone. The same is true of bolted, riveted, and welded joints for metal. The budding roboticist would do well to study references on engineering design. (add pointers)
These are connections where the pieces push or "butt" directly against each
other, as in a stack of blocks.
Forces at the joint are strictly in compression.
Connectors usually do not bear structural loads, but serve only to
stabilize the joint.
Common angle brackets, for example, are intended to be used
primarily as stabilizers, not as load-bearing elements.
They are often misused in this respect by amateur builders.
Butt joints can be reinforced using pins, dowels, screws, or nails, which
can allow them to bear minor loads in directions other
than compression.
For major non-conpressive loads, it is better to use another joint.
Wikipedia on butt joints
These are connections made by overlapping structural elements
and fastening them together with pins, bolts, glue, or other methods.
Separation forces are generally sheering and taken by the
connectors, which must be adequately strong.
In bolted lap connections, the forces on the bolts may be primarily
tension as they provide compression that allows the sheer forces
to be transferred by friction between the primary elements.
Properly designed lap joints can serve as general connections,
supporting any forces that could be born by the components if
they were formed as a monolithic unit.
In general, the length of lap joint should be at least 3-4 times the thickness
of the (thinner of the) lapped elements for maximum strength.
Wikipedia on lap joints
The socket or peg-in-hole principle is one of the oldest known methods of
joining structural elements.
When the hole and peg are square, they are often referred to as mortise
and tenon repectively.
A simple, snug-fit peg-in-hole connection will strongly support any load that
does not tend to extract the peg.
If the hole is relatively deep (at least 2 times the diameter of the
peg) then there is a lot of surface area that can be exploited
to resist withdrawal by gluing or pinning the peg in place, or subjecting
it to compressional friction.
Such a joint can be as strong as the peg itself.
Since the peg can be as large as the connected component,
properly designed socket joints can support components up to their
load limits.
Wikipedia on mortise and tenon joints
Flanges are peripheral rim or ridge elements attached to or incorporated into
the body of structural components, particularly pipes, to allow them to be
fastened, often via bolts, to other components.
Flanges can be load-bearing or simply stabilizing.
When load-bearing, they must be carefully designed and connected.
Wikipedia on flanges
Gussets, or more properly gusset plates, are shaped plates placed
on one or both sides of a joint to fasten and strengthen it.
They may be pinned, riveted, bolted, screwed, glued or welded to or through
the underlying components.
Gusset plates can be viewed as externally implemented lap joints.
Properly designed and implemented, they can transfer general loads
up to the strength of the components.
Wikipedia on gusset plates
There exist a great variety of general- and special-purpose
auxiliary connectors
designed for holding primary components together in different ways.
Such components may be called brackets, ties, hangers, fittings, couplings,
and other names.
Some are intended for stabilizing applications, e.g., generic
angle brackets.
Others are designed to carry structural tensile or sheer loads.
Joist hangers for example, are designed to transfer
joist loads to beams, but only in one specific direction
(downward against gravity).
Structural auxiliary connectors must be employed with care.
Not only must they be designed for the particular load to be carried,
but they must be placed correctly, and attached with sufficient
fasteners of the correct specification.
Using the wrong nails with joist hangers, or subjecting them to
a tensile load can result in the unexpected collapse of a floor or deck
(and has).
Many methods for mechanically fastening components
to each other have been developed over the ages, from wooden pegs and ropes
to nails, rivets, bolts, screws and velcro.
Unlike fusion and bonding techniques, mechanical fasteners sometimes
permit easy disassembly and reassembly.
Many can transfer structural loads.
On the down side, use of mechanical fasteners is often more complex
than simple bonding methods.
They often work in conjunction with brackets and other auxiliary components,
and require additional design and fabrication steps.
Wikipedia on fasteners
Threaded fasteners essentially work by twisting a helical "male" element (e.g. a bolt or screw ) into a corresponding "female" receiver (e.g. a nut). This results in an intricately interlocked connection that resists movement in any direction other than the exact rotation that assembled it, and which can be fully as strong as the elements composing it.
The helical form and various uses of it (e.g. for wine presses) were known in ancient times, certainly by the first century AD. However, prior to the late 1700s, screws were rarely used as fasteners due to the conlexity of manufacture. It was only the development of efficient mass-production that allowed screws to be produced cheaply enough for their intrinsic advantages as fasteners to be exploited. Standard shapes, pitches, and sizes were developed that allowed screws to be used as interchangable elements. By the beginning of the 20th century, threaded fasteners were universally employed in almost every fabrication process involving assembly of separate components.
Screws can be loosely divided into two classes: "self-tapping" units such as wood screws and sheet-metal screws that effectively cut their own receiver, and "machine screws", that generally require a pre-tapped hole or nut. In high-load applications, machine screws are generally used since cutting receiving threads in hard and strong materials requires specialized tooling and processes.
Screws can transfer mechanical loads in three ways. First, since they resist being pulled out, they can transfer high tensile and compressive loads along the screw axis. Second, they can transfer sheer loads by acting as pins or dowels in their holes. If high loads are to be transmitted, this requires snug, precision fitting into the holes. Third, when appropriately tightened or "torqued" they can produce high clamping forces between fastened elements which allow sheer loads to be transmitted by friction between the clamped components. This mechanism is employed in many load-bearing applications to prevent high local forces from developing where a bolt contacts the edge of a hole, possibly initiating failure. Clamping friction load transfer permits non-precision holes to be employed, which also allow small adjustments to be made before tightening. Its is the reason threaded fasteners often need to be torqued to specification.
Some common threaded fasteners useful in robot construction are
described below.
More extensive chart from Bolt Depot
Various machine screws
Machine screws (or bolts) are the fastener most commonly encountered in robot construction. They have a uniform (male) thread cut along a straight shaft. These threads engage with a corresponding (female) thread cut in a nut or pre-tapped in a hole. The difference between "screws" and "bolts" is not clearly defined. Generally speaking, fasteners labeled bolts tend to be larger, have heads that permit tightening with a wrench, and often thread into a nut rather than a tapped hole.
The shapes and sizes of the threads are highly standardized, and components meeting the same specification are interchangable. The thread on a machine screw is described by its outer diameter, which is the diameter of the shaft it is cut on, and by its pitch, which specifies the distance between threads. The latter may be specified directly, or in threads per unit distance. There are both English inch (SAE) and metric (ISO) standards for specifying machine screw threads.
The most common SAE (English) standard is UTS (for Unified Thread Standard).
It specifies diameter as a number #000 - #16 for
smaller sizes, and in (fractional) inches 1/4 - 1+ for larger sizes.
Thread pitch (spacing) is given in threads per inch (tpi), and there are
course and fine (and sometimes extra fine) standards for each diameter.
Thus, for example, a screw might be specified as #8-32,
meaning number 8 diameter with 32 threads per inch,
which is the coarse standard.
Odd number diameters (e.g. #5), and numbers above 12 are unusual.
Wikipedia on UTS
For each screw size there are standard hole sizes for tapping and clearance.
The following table provides a summary.
UTS Tap and clearance drills from LittleMachineShop.com
The ISO metric standard specifies both the outer diameter and the pitch
in millimeters. As with the English system, there is a set of preferred
diameters, with coarse and fine thread pitches for each diameter.
Thus the designation M5x0.8 refers to a metric screw with an outer diameter
of 5mm and a pitch of 0.8mm (coarse thread standard).
Wikipedia on ISO screw threads
ISO Tap drills from LittleMachineShop (pdf)
Metric and ISO drill charts from Kasthuri
Screws and bolts come with a wide variety of head shapes, designed
for a variety of drives.
Common head shapes include flat, round, oval, and pan.
Common drives are flat slot, Phillips, hex socket, and Torx.
Large bolts commonly have an external hex head to allow tightening
with a wrench.
Chart of screw heads from Eagle Fastener
Wikipedia list of screw drives
Screws are also rated by strength. Common English (SAE) ratings are Grade 2 (ordinary steel), Grade 5 (high strength) and Grade 8 (highest strength). Grade 5 and 8 have distinctive markings on the head. These marks are sometimes counterfeited, creating serious risks to life and property.
An analogous and more directly quantitative strength rating system is defined by ISO for metric fasteners. The rating consists of two numbers separated by a decimal point, e.g. 5.8. The first number is the ultimate tensile strength in units of 100 megapascals. The second is the ratio of yield strength to tensile strength. Thus ISO 5.8 means the fastener has a tensile strength of 500 MPa and a yield point of .8 times that amount (i.e. 400 MPa).
Some sheet-metal screws
A variety of wood screws
Various washers
Sheet-metal screws are pointed, self-tapping screws with a full-length thread intended for holding thin sheets of metal together, or attaching other objects to sheet metal. They are also useful for attaching objects to soft materials such as wood or flexible plastic. They most frequently have a pan head, and a slot or phillips drive. In the SAE (English) system they are described by a diameter number (#0 - #14) and a length.
Wood IS useful for robot construction, particularly for amateurs. Wood screws are the preferred hardware for connecting pieces of robot wood to each other (as opposed to nails) since they allow for accurate, controlled alignment and disassembly. Wood screws are pointed, self-tapping screws, usually with a non-threaded shank, which is intended to pass through the attached piece and allow the threads to pull it tightly against the substrate.
Wood screws are available in a wide variety of sizes, head styles, and drives. In the SAE system they are given a diameter number (#0-#14) and a length. Long wood screws called deck screws are made of hardened steel and often protected against corrosion. They make decent "universal" fasteners. Large wood screws called lag screws or lag bolts have diameters sized in fractional inches (1/4, 3/8, etc.)
Wood screws should always be inserted into a pilot hole to ensure
good alignment, and to prevent
splitting of the wood and excessive drive forces.
A shallower, larger hole is sometimes co-bored for shank clearance.
Even with piloting, drive forces are often large.
If more than a few screws are used, or if they are large or long,
use of a head that allows a power drill drive is a very good idea.
Torx drive heads, available on deck screws, are far superior to
Phillips (or flat) drive heads in this respect as they require less downward
force when being inserted, and are far less likely to strip out.
Inserting a long deck screw with a phillips head can require
body-weight-level downward force to prevent stripping.
Lag screws generally have a hex head, which can be driven with a wrench
or a power driver with the correct adaptor.
Wood screw pilot and shank clearance holes from Sizes Inc.
Washers are flat rings, usually of metal but sometimes other materials, that are used in conjunction with threaded fasteners. They are placed between a screw head or a nut and the bearing material to distribute loads and/or prevent damage to the underlying surface. They can be particularly important when the underlying material is softer than the fastener, and when the fastener is highly torqued. Lock washers are specially shaped to prevent vibration from loosening and backing out threaded fasteners. They are usually not needed in highly-torqued metal-to-metal applications, but can be crucial in snug-tightened situations, or where the underlying material can deform and reduce fastener tightness (e.g. with wood).
Rivets are used to permanently attach two (occasionally more) pieces of
sheet material together.
They consist of a headed pin that is placed through aligned holes
in the materials to be joined, and whose far end is then mushroomed over
or "headed" to press the materials together and prevent removal of
the fastener.
Rivets were once widely used in heavy metal construction, and the noise
produced by hammering during the heading process was in famously deafening.
Thanks to reduced costs, rivets have been replaced with threaded fasteners
in many modern applications, but they are still occasionally used where
a permanent attachment with a clean surface appearance is needed.
Low-strength hollow fasteners commonly known as "pop rivets" that can
be installed with a simple hand tool are widely available, and are
sometimes useful for quick assembly or repairs.
Wikipedia on rivets
Two pieces of material can be joined by placing cylindrical pins or dowels of a strong material through aligned holes in the parts. They are commonly used to align components and to prevent them from moving with repect to each other. Pins can bear large shear loads, and can be assembled and dis-assembled more quickly and easily than threaded fasteners, often without special tools. They are often used in applications that require this.
Assorted nails.
Top: spiral and ring-shank;
center-left: common; center: roofing; center-right: finish;
bottom: box.
A variety of secondary fasteners are used to prevent pins from falling or backing out of their holes. These include cotter pins, roll pins, R-clips, and split rings placed through a hole in the end of the main load-bearing pin. Some pins contain spring-loaded catches or latches of various sorts.
Pins can also be held in place by glue or friction. Dowels used in woodworking are an example of this. With metals, pins can be precisely machined to be very slightly larger (e.g. a thousanth of an inch or less) than the holes they fit in. The resulting compression fit can be extremely secure, even exceeding the tensile strength of the fastener. Assembling such extreme friction fits sometimes involves differential heating of the components (e.g. cooling the pin in liquid nitrogen). They are effectively permanent.
Nails are essentially a form of friction pin used mostly for wood,
designed to be inserted quickly by hammering.
A nail in wood holds by elastically displacing the wood fibers it passes,
which then press stongly on the sides of the nail producing high frictional
forces that resist withdrawal.
Nails are not recommended for robot assembly: they are imprecise,
non-removeable, involve violent installation forces, and can distort or
damage components they pass through.
They also have a tendency to work loose in moving systems.
NO NAILS IN ROBOTS. Use glue or threaded fasteners
Wikipedia on nails
Rope and string, together referred to as "line", are occasionally useful to the robot builder, especially for bundling and securing tubes and cables to keep them organized and prevent flopping. Line made from a number of natural and synthetic fibers is available. Natural fibers (cotton, sisal, linen, hemp) are durable, flexible, and hold knots well. They are subject to decay in damp environments. Synthetic fibers (nylon, polyester, polypropylene, and blends) are stronger than most natural ones and resistant to decay, but also more slippery, springy, and difficult to tie. Some of them lose strength (melt) at relatively low temperatures, and some degrade in the presence of sunlight over the course of a year or so.
Using line requires some elementary knowledge of knots. The robot builder should be familiar with at least the square knot (for tying bundles, NEVER for joining lines), the sheet bend (for joining lines), the bowline (for making a loop), and the clove hitch (for attaching a line to a round post or bar).
Zip ties are a modern convenience that make tying bundles and some other things quick, easy, and permanent. Rubber bands and wire twist ties can also be useful for quickly restraining loose conponents. Ordinary rubber bands should not be employed in any role where they will be subject to tension for more than a week. Streching the rubber even moderately makes it susceptible to oxidative degradation, which causes cracking and disintegration. Special rubbers are available for situations where continuous tension is encountered.
Miscellaneous other fasteners, including ones used
primarily in clothing such as zippers, snaps, buttons,
hooks-and-eyes, etc. have potential uses in robot construction.
"Velcro" in particular has a unique combination of useful characteristics,
being rapidly removeable and replaceable,
secure against creep, and arbitrarily adjustable.
And don't forget sewing - an ancient and time-honored method for securely
fastening flexible materials together.
A variety of adhesives
Glues and adhesives are materials that are applied in a layer between two surfaces to create a bond between them. When used correctly, the strngth of the bond is proportional to the surface areas involved. Usually there is a small gap between the joined substrates that is filled with the adhesive material. The strength of the bond thus cannot exceed the ultimate strength of the adhesive material.
Adhesives attach to the substrate either through interlocking micro-mechanical structures produced when the adhesive penetrates surface roughness, or through attractive forces between adhesive molecules and the substrate surface. Often both effects are present. Mechanical attachment will clearly be weaker than the bulk adhesive. Molecular forces can exceed bulk strength, but usually do not.
Compared to most engineering materials, bulk adhesives are relatively weak, with tensile strengths limited to a few thousand psi. They are thus not a good choice for transferring structural loads unless the joint is carefully designed to distribute the transfer load across an area much greater than the structural cross-section. Such joints exist, (e.g. a post glued into a deep hole) but must be fully understood if used. Another strength issue is that the discontinuity in material properties between adhesive and substrate makes rigid glues susceptible to brittle fracture, especially along the bond plane. In general, glues and adhesives should be used for stabilizing purposes rather than for transmission of loads.
The distinction between glues and adhesives is a bit fuzzy. The term "adhesive" is often used generically for any applied, layered substance that functions to bind two surfaces together. In more specific usage, adhesive tends to be applied to substances that remain more or less flexible in the finished bond, while glue is applied to substances that set up hard and rigid. Some flexible adhesive bonds are subject to creep - that is they will slowly deform when subjected to continuous stress, sometimes at very low levels. Thus materials specifically labeled as adhesive should be used with caution in applications that involve static forces. Finally, the term adhesive is sometimes used technically for substances where the primary bonding forces are intermolecular in nature.
Because adhesives work via intimate interaction with surfaces, substrates must be carefully prepared before bonding. Dust, dirt, grease and oil, and other contaminants, even in very small amounts can cause total failure of a bond. Appropriately cleaning surfaces is thus important. Many adhesives do not fill gaps well, so there must generally be a good fit between mating surfaces. Adhesives that bond primarily via micro-mechanical mechanisms may require that surfaces be roughened. Adhesives that bond via intermolecular forces sometime require chemical pre-treatment of substrate surfaces.
PVA glues
These are your "ordinary" white and yellow glues, known by brands such as Elmer's, Titebond etc. They are typically composed of an emulsion of polyvinyl acetate (PVA) and other ingredients in water. Elmers white glue is mostly PVA. "Yellow" glues such as Titebond contain other aliphatic resins to increase water resistance and modify other properties such as tack, and are thus sometimes refered to as "aliphatic resin" glues. The yellow color is dye added for identification/marketing purposes, and is not due to functional ingredients.
PVA glues bond through mechanical adhesion, and are designed for use on rough, porous materials such as wood, paper and cardboard. They set rigidly through absorbtion and evaporation of water, though the adhesive material often remains somewhat flexible, and can creep under heavy, sustained loads. The glues vary in degree of water resistance once set, from easily softened (e.g. Elmers school glue) to nearly waterproof (e.g. Titebond III wood glue). However, none should be used for permanently moist or wet joints.
Most hard-setting glues, including PVA, are stronger than wood in its weak direction (across the grain), and when side-grain is glued to side grain, the wood will generally fail before the glue. Along the grain, glue is not nearly as strong as wood, and hence it should not be used to attach end grain to end grain in a load-bearing application. Certain joints (e.g. lap or finger joints) can be used to glue wooden load-bearing elements end to end, but this requires careful design, and failure can still occur due to a glue-starved joint or brittle fracture.
PVA glues have little gap-filling ability, and as a general rule, gaps should be kept under 1/100 inch. When gluing wood, it is often necessary to apply clamping pressure in order to maintain close contact while the glue sets. Otherwise, the glue may not fill, or may run out of small gaps, resulting in a weak joint.
Use of PVA glues on materials other than paper and wood products can be problematic. Cotton or wool fabrics and leather usually glue well; synthetics are iffy. A useful rule of thumb is that porous materials that absorb water can be glued successfully. Fully waterproof materials, rough or not (e.g. metals, plastic, granite, porcelain, rubber) usually do not glue well.
Cyanoacrylate glues
Superglues are amazing. They bond "instantly" (within a few seconds), are strong, water-resistant, and adhere well to most materials with the exception of slippery plastics. Adhesion is primarily through strong molecular attraction between the triple-bonded nitrogen atom in the CA molecule and surfaces. The glue itself consists mostly of cyanoacrylate monomer that polymerizes in place when it encounters traces of moisture or other contaminants on the surfaces to be bonded. Very clean, smooth surfaces (e.g. glass, some metals, and plastics) may be slow to bond. So-called "accelerators" are available to increase bonding speed in such situations, but they can also weaken the bond.
The original formulations of superglue were very fluid with little or no gap-filling ability, and worked best on non-porous materials in extremely close contact. On wood and other porous materials, the glue tended to wick into to substrate, leaving insufficient adhesive to create a good bond. Modern formulations come in a variety of viscosities from thin to gel; the thicker ones can be used on porus materials like wood, and have some gap-filling properties. However, close-contact bonds are still stronger. The most liquid formulations have the ability to wick into hairline fractures, making them useful for repairs where an object has cracked, but not separated into pieces. CA can be used to glue rubber with a bond as strong as the rubber itself.
Polymerized CA is hard and rigid, making it prone to brittle fracture when used to connect rigid materials with a higher elastic modulus than the glue itself (e.g. metals and ceramics). It is particularly prone to sheer failure, especially when impact forces are present. It does not bond well to slick polymers (polyethylene, polypropylene, nylon, delrin, teflon, etc.) or to hard, ultra-smooth surfaces (e.g. glass and polished metals).
Cyanoacrylate bond skins instantly and strongly, so care must be taken not to touch surfaces with uncured CA adhesive. This property has made it useful in medical applications, e.g., for wound closure in place of sutures. The fumes are somewhat irritating to the eyes and respiratory system, and repeated contact with sensitive skin can cause chemical burns. The polymerization of CA is an exothermic process, and a large amount in contact with porous organic material such as paper or cotton (e.g. spilled material soaked up with a rag) can get very hot as it sets, in some cases enough to produce ignition and fire.
CA has a relatively short shelf life. Opened containers may degrade in as little as a month, and even unopened containers start to degrade after a year or so. Old CA glues should be discarded so no-one uses them by accident and gets an unpleasant surprise when their "super" bond suddenly fails.
Epoxy glues
Epoxies are a large group of resins that set vias a polymerization reaction involving an epoxide group (a three-membered ring of and one oxygen and two carbon atoms). They are generally formulated as two components that must be thoroughly blended together before use. The resulting mixture has a specified "working time" ranging from a few minutes to a few hours before it sets up. Cured epoxy is a very durable material: strong, rigid, waterproof, heat resistant, and unaffected by most chemicals.
Epoxy adhesives can be used to join a very wide range of porous and non-porous materials, including wood, composites, many plastics, brick, stone, ceramics, and sometimes metal and glass. Slick plastics (polyethylene etc.) cannot be glued, and flexible materials such as rubber can be problematic. Formulations exist designed specifically for metal and glass. In general, gluing these materials is successful only in cases where large surface areas are involved because epoxy, though strong for a homogeneous plastic, is much weaker and less rigid than metal or glass. Epoxy is not a substitute for welding or threaded fasteners in load-bearing metal.
Epoxies are nearly unique among high-performance adhesives in having good gap-filling abilities. They can make sloppy joints sound. This should not be taken as an excuse to make sloppy joints, but it is worth remembering
Partly because of their good fill characteristics, epoxies are widely used in the production of composites, especially fiberglass and carbon fiber. Since fibers are very long compared to their cross-section, a fiber surrounded by epoxy can resist being withdrawn strongly enough that it will break before coming loose. Composite materials can thus be made that approach the one-dimensional strength of the fibers, which can be many times higher than that of the homogeneous material. [As an example, homogeneous UHMWPE (ultra high molecular weight polyethylene) has a yield point of perhaps 10,000 psi. Fiber made by aligning the molecules can have a tensile strength above 300,000 psi, as high as the strongest steels and about 1/10 the weight.]
Because they typically require intimate mixing of two chemical components, epoxies are more difficult to use than other adhesives. Failure of epoxy adhesives is often due to inadequate mixing, which must be down to the molecular level for good performance. Since the resin molecules are too large to diffuse quickly, this must be done mechanically. Material mixed in a container should be stirred at least 50-100 times. Theoretically, a uniform folding factor of 25 is sufficient to provide molecular-scale contact in a 1 cm scale volume, but hand-mixing is rarely uniform, so we go for the overkill. Even then, material near the surface of the mixing vessel is sometimes inadequately blended, so scraping out the last bits is inadvisable.
Uncured epoxy resins, though not particularly toxic, can irritate the skin (and of course the eyes) so, as with almost any reactive chemical, reasonable handling precautions are in order.
These are much-hyped new adhesives based on polyurethane resins that polymerize when exposed to traces of moisture in the substrates. In theory, they are strong, waterproof, and chemically resistant. However, in current formulation, there is a foaming action as the material sets. This tends to push the resin out of the bonding space, and, because of embedded bubbles, reduce the amount of resin participating in the bond.
I have personally found these glues difficult to use. In tests I have made with wood, the strength of the bonds has been far below those made with yellow wood glue and epoxy. Similar difficulties are widely reported in online forums. I suspect there are methods by which these adhesives can be employed successfully, but since other glues can do everything advertised for polyurethanes and more reliably, I find it difficult to recommend them for general use at this point.
The only exception I can think of offhand would be applications where a sizable gap needs to be filled and only a weak bond is needed, as in stabilizing masonry structures. In this case the foaming fill action could be an advantage, especially in large constructions where enough epoxy to fill the gaps would be prohibitively expensive.
Adhesives in general should not, of course, be consumed. They are generally indigestable (with the exception of animal glues and vegetable pastes), and may be toxic. Polyurethane glues however, present a unique hazard in this respect: because of their foaming action and subsequent adhesion and polymerization, they can produce severe intestinal blockage that requires emergency surgery to correct. Not fun. Don't eat.
Sovent-based glues
There is a class of cements used with certain plastics that work differently than most glues. Rather than attaching to a surface and forming a bonding layer between substrates, they partially dissolve the surfaces to which they are applied. The dissolved material on surfaces unites, forming, sometimes together with cement material, a continuous structure. When the solvent disperses, generally by evaporation and diffusion into the substrate, the formally separate pieces are a contiguous unit of plastic material. The result is analogous to welding where components are joined by melting together the material from which they are made, hence the process is referred to as "solvent welding".
The most familiar glue of this type is the plastic model cement used by hobbyists to assemble injection-molded polystyrene model kits. The primer and cement used to join PVC plumbing compenents also works in this manner. These cements have some plastic material pre-dissolved in the solvents to provide some gap-filling ability.
If the components fit very closely, a bond can be created using solvent alone. One of the most effective ways of joining polycarbonate is to shape the pieces so they are separated by no more than one or two thousandths of an inch, and then apply a methylene-chloride containing solvent to the edges of the dry-assembled joint. The solvent wicks into the hairline gap by capillary action dissolving the polymer on either side, which then merges together. The resulting joint is almost invisible and can be nearly as strong as the underlying plastic, (which for polycarbonate, is quite remarkable.)
Some care is needed in use of solvent-welding compounds. The solvents are often toxic if inhaled, so they should be used with good ventilation. They are also often flammable. Because they dissolve the substrate, misapplied cement will scar and can damage components. It cannot be gooped on and cleaned up. Too much cement can also deeply soften the surfaces of components resulting in deformation when they are pressed together. Because of the mode of action, parts to be joined must fit well. Too wide a gap, and dissolved material cannot bridge it.
Despite the difficulties, solvent welding, when applicable, is the preferred method of gluing plastic components together. Plastics to which the technique can be applied include polystyrere, acrylic (Plexiglass), PVC, and polycarbonate (Lexan). Slick plastics (polyethylene etc.) generally cannot be bonded by this method.
There is a large class of adhesives that consist of sticky polymeric materials dissolved in a volatile organic solvents. This formulation resembles some solvent welding adhesives, but the primary mode of action is different. Applied between substrates, the solvent vehicle evaporates leaving behind the adhesive material in drasically thickened form. This bonds the components together. These adhesives are often called "universal" because they can be used on almost any sound substrate. Some will even stick to slick plastics, at least to some extent. Some are also reasonably good at filling modest gaps.
Modern "universal" adhesives include brand names such "Liquid Nails" and "Goop". Older examples include substances such as pitch and tar and various natural gums. The class can also include contact cements, which are materials that are spread on one or both surfaces and allowed to dry for some specified amount of time. When the prepared surfaces are touched together, a strong bond forms instantly.
Sounds like the perfect glue-all. Unfortunately universal adhesives have two significant limitations. The first is that the bond formed tends to be significantly weaker than that formed by an adhesive specially suited for the application. The second is that many materials that have universal adhesive characteristics remain slightly plastic even when the solvent has evaporated. This can be an advantage if flexible materials need to be bonded. However, under continuous stress, such bonds may "creep", resulting in eventual distortion or failure.
As with solvent-welding compounds, the solvents in universal adhesives tend to be toxic and flammable, so suitable precautions should be observed.
Silicone compounds
Finally, there is a class of adhesives that set to form elastomers, that is, rubbery substances. Various silicones (e.g. "Silicone Seal"), commonly used to form a waterproof seal around bathtubs and other plumbing fixtures are most familiar. Other elastomeric adhesives are based on latex or polyurethane. The bond formed by elastomers tends to be comparatively weak, but they have two remarkable advantages. The first is that they can adhere tightly to smooth, waterproof materials such as metal, glass, and porcelain. The second is that they form a bond that is very flexible, but not subject to creep. The bonds can also be completely waterproof, gas-tight, and resistant to organic solvents and most other chemicals. Some silicones can resist temperatures of over 300 degrees C, and remain flexible below -40 C.
In addition to sealing gaps, elastomeric adhesives are commonly used for assembling aquaria, forming gaskets in-situ, and embedding electronics and other solid-state equipment to protect against water, weather, and other environmental hazards. Like other adhesives, elastomers can be irritating to the skin in uncured form. Some silicones emit irritating, and potentially corrosive vapors (e.g. acetic acid) during the curing process. This could be an issue if they are applied against corrodable metals such as bare aluminum.
Hot glue is produced by melting a stick of gummy plastic material in the nozzle of a "glue gun", from which it can be gooped onto and between various surfaces to hold them together when it rehardens. It is widely used by "craft" hobbyists and by schoolchildren to quickly assemble "art" projects. It is also useful for quickly assembling shipping boxes and gluing them shut. It bonds with low to modest strength to many materials, but not stongly to anything. It fills gaps, but with little integrity, and not neatly as it is nearly impossible to smooth.
Hot glue forms a relatively low-quality bond, and encourages sloppy design and assembly. Its general use in robot building is strongly discouraged. If glue is an appropriate method of joining components, then there is a suitable high-quality adhesive. Use it.
Duct tape (and adhesive tapes in general) make use of pressure-sensitive adhesives. These are substances that when placed between two surfaces, adhere strongly under light pressure with no drying, setting, or polymerization time required. Many can be modeled as extremely viscous liquids that immediately wet the substrate surfaces.
Tape has a reputation somewhat like hot glue in that it can be used for sloppy assembly of carelessly made components. However, unlike hot glue, adhesive tapes have some unique, useful properties. In applications where tape can be wrapped around a circumference, the connection can be relatively strong. Tape is useful for bundling wires, cables, and tubes, and attaching them underlying structures so they do not flop about. It can be used as a temporary connection while more permanent ones are installed. It can be used to cover or seal holes (e.g. to prevent escape of liquid material while it sets) Tape is commonly used to mask surface areas to prevent contamination by paint, adhesives, or other substances. Double-sided tapes are useful for temporarily attaching parts to each other if they need to be worked on together (e.g. matching holes need to be drilled).
Hot glue is the road to perdition (in your author's opinion), but every robot constructor should have a few rolls of electrical tape, duct tape, and masking tape on hand.
Wikipedia article on adhesives (same link as at top of section)
Soldering, brazing, and welding are techniques for joining metals with a direct metal-to-metal bond produced through a melting process. Soldering and brazing employ a filler metal with melting point lower than the substrate to create the bond. Welding creates a bond through melting and fusion of the substrate itself.
Of these processes, soldering is the only one likely to be much employed by the amateur robot constructor, and only for constructing circuitry. That however, is crucial.
There are many methods of connecting electrical components to each other: twisting wires, wire nuts, plugs, sockets, and electrical connectors of all kinds. None of these, however can approach soldering in terms of good electrical connectivity, strength, and permanence. Soldering is the only technique robust enough to allow a complex, low-voltage circuit with hundreds or thousands of connections to operate reliably. For this reason, non-soldered connections are limited to situations where they are absolutely needed for modularity or component replacability.
Non-soldered connectors used in electronic systems are special-purpose, precision-engineered components, generally plated with gold and/or other noble metals. There are special sockets designed to make integrated circuits replaceable, and special plug/socket connectors for various communication cables. However, any circuit made from discrete components should be soldered together wherever possible. A robot constructor needs to learn to solder.
Like any other task, doing a good job soldering requires decent tools. For electronic work, this means a good, fine-tipped soldering iron, preferably termperature-controlled, and a well-lit work-station equipped with a variety of adjustable clamps and clips for holding components precisely in position while they are soldered. It also requires the correct solder.
Solder comes in a variety of types and sizes. There is solid core, rosin-flux core, and acid-flux core. For electronic work, solid or rosin core is used. Rosin-core is somewhat easier to use since the rosin acts as a flux, cleaning the surfaces of oxides and improving their "wettability" by the solder. Available sizes range from .015 inches (.38mm) to .093 inches (2.3mm). A good rule of thumb is to use a solder with a diameter close to or slightly smaller than the wires being soldered.
Solder is available in a number of different formulations. The largest component is generally tin, a relatively expensive metal, which accounts for the high cost of a pound of solder. The biggest distinction is between leaded and lead-free varieties. Environmental and consumer-safety regulations require the use of lead-free solder in commercial products. However, leaded solders remain easier to use than the best non-leaded alternatives, and are thus still available for hobby and prototyping use.
Common leaded formulas are 60-40 and 63-37, referring to the percentage of tin and lead respectively. 63-37 is a "eutectic", meaning it has the lowest melting temperature of any tin-lead combination (183 C = 361 F). The most common lead-free solders contain around 95% tin, 4% silver, with the remainder copper and sometimes other elements.
Soldering circuit components is a skill that develops with practice. The following can serve as a starting point.
Every now and then you will discover you have soldered a connection incorrectly. This connection now needs to be "desoldered". Desoldering is harder than soldering because all the solder has to be removed before the connection will come loose. There are two basic tools that are useful. The first is a desoldering pump, basically a spring-loaded reverse syringe that can be cocked and triggered. You apply the iron to the joint, and when the solder melts, put the pump tip as close as possible to the joint and trigger the suction. If you are lucky, the molten solder is all sucked away and the joint can be disassembled.
The other tool is desoldering braid or ribbon, basically very fine copper wire braided into a flat cord. Applied to molten solder, with the iron beside or behind it, it wicks the solder away by capillary action. After use, the solder-saturated section of braid is cut off and discarded.
Even when you have removed as much solder as possible, residual bits will sometimes hold the joint together. In this case, you need to reheat the joint while applying disassembly forces to the component. Often the component is hot at this point, so you need to use some tool other than your fingers. Very small needle-nose pliers can be helpful.
It is possible to solder sheet and other thin metal components to each other. The most common example is connecting copper plumbing components. Commonly soldered materials include copper, brass, and tinned steel sheet. Aluminum cannot be soldered, and untinned steel or iron requires special procedures. Soldering sheet-metal requires careful preparation and pre-fluxing of the surfaces, and a high-power heat source - generally a torch or very large iron. Few, if any, robots will require soldering of non-electrical components.
Brazing is essentially soldering bulk-metal components to each other using a brass or other copper-based solder. It usually used on iron and steel. It requires components to be brought to orange heat at the joint which needs a torch, usually oxy-acetylene, though rumors persist that it can be done with a propane torch. Surface preparation, fluxing, and adequately fast heating are required, otherwise oxides form and prevent bonding. Again, not a technique likely to be needed by the budding robot constructor.
Welding is joining bulk metal components to each other by melting and fusing the metal in the vicinity of the joint. It is an important industrial fabrication process, as it can produce a permanent connection that is of the same strength as the underlying material. Most metals can be welded, including steel, stainless, and aluminum, but many require special procedures - e.g. use of inert gases to exclude atmospheric oxygen and nitrogen. Copper alloys are sometimes problematic simply because copper conducts heat way from the weld zone so efficiently.
Welding requires considerable knowledge, skill, and specialized,
high-power equipment to carry out.
Not a job for a beginner.
If something absolutely has to be welded, there are shops that
will do it for a price.