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)

Thrust and butt connections

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

Lapped connections

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

Other Joinery Components

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).

Mechanical Fasteners

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 (Screws)

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.

Wikipedia on screws

Types of threaded fasteners

Some common threaded fasteners useful in robot construction are described below.
More extensive chart from Bolt Depot


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

Ties etc.

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.

Glues and Adhesives

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.

Wikipedia on adhesives

List of Common Adhesives

Additional Reference Material on Adhesives

Wikipedia article on adhesives (same link as at top of section)

Wikipedia list of glues

Soldering, Brazing and Welding

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.

Electrical Soldering

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.

Wikipedia on solder

How to Solder Circuits

Soldering circuit components is a skill that develops with practice. The following can serve as a starting point.

  1. If you are using leaded solder, make sure you are working in a well ventilated area. Don't inhale smoke or fumes. This is good advice for any soldering job.
  2. Allow soldering iron to come to full temperature. This can take a couple of minutes, depending on the iron. Generally, the lowest temperature that works well is preferable to reduce the danger of thermal damage to components. If you have a thermally controlled iron, 600 F is a reasonable starting point for normal-sized through-hole components. Adjust up or down as appropriate.
  3. Clip, clamp, or otherwise secure the components to be soldered in position, making sure the surfaces to be soldered are in contact with each other and will stay so when contacted by the iron. Soldering requires two hands, so you can't use one to hold things together.
  4. "Tin" the tip of the soldering iron by melting a bit of solder directly on to it, or by dipping it in tinning paste. If the tip will not tin, it should be cleaned using a damp sponge or abrasive pad. Periodic cleaning on a damp sponge during the soldering operation will help keep everything going smoothly. The thin layer of molten solder on the tip of the iron improves thermal contact when the iron is placed in contact with the surfaces to be soldered.
  5. Holding the soldering iron in one hand, and the solder in the other, bring the iron into contact with the metal to be soldered. Hold it there for a second or two, to bring the substrate above the melting point of the solder. With your other hand, touch the solder to the joint close to the iron, but not directly to the iron itself, since you want to make sure the metal is hot enough to melt the solder (otherwise it won't stick). Aiming at the angle between the iron and the substrate can be a good strategy. If all goes well, the solder will flow beautifully into and around the joint, showing concave surfaces everywhere. Remove the solder and the iron.
  6. If the solder "balls up" into a bead, or displays convex surfaces around the joint, all is not going well. The most frequent cause is that the joint is not hot enough. Reposition the iron for better contact, re-tin the tip, or increase the soldering temperature. If none of this works and the joint involves large wires, it is possible the soldering iron cannot generate sufficient heat. In this case it may be necessary to resort to a higher-wattage iron or a high-power "soldering gun"
  7. When soldering components to printed circuit board, the components are inserted from the non-plated side and soldered from the metal-plated side. The solder should fill the hole and wick up around the lead. After the joint has cooled, the leads should be trimmed down to the solder with special flush-cutting nippers.
  8. Power everything done when you are done. Fires have resulted from soldering irons left on.


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.

Soldering Bulk Metal

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.

Wikipedia on Soldering


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.

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