Joinery The process of connecting or joining two pieces of wood together through the use of various forms of wood joints. In basic materials processing, common forms of joinery include dovetail joints, mortise-and- tenon joints, biscuit joints, lap joints, and spline joints.
Butt Joint An easy but often weak technique for joining two boards together simply by gluing and pressing two flat surfaces together. Typically made by gluing an end to an adjoining flat surface.
Biscuit Joint A butt joint that is reinforced with football- or lozenge-shaped wooden "biscuits." Biscuits are usually made from compressed wood, frequently birch wood. When the biscuit comes into contact with glue in the biscuit slot, it swells thus creating a tighter joint. Sometimes called a plate joint.
Dado Joint A joint where one piece is grooved to receive the piece which forms the other part of the joint. Dado (definition) A groove which is cut across the grain to receive the butt end or edge of a second piece.
Dovetail Joint Joining two boards in which alternating slots (or tails) and protrusions (or pins), each resembling in shape the v-shaped outline of a bird's tail, are snugly fitted together, thus increasing the gluing area. Produces a joint that, even without glue, can be difficult to pull apart. Regarded as one of the strongest and most reliable forms of wood joinery.
Lap Joint A joint where one piece of wood is crossed over another.
Miter Joint The woodworking joint created when two boards are cut at an angle to one another. The most common miter joint is the 45-degree miter such as the cuts used to build square or rectangular picture frames.
Mortise-and-Tenon Joint A joinery technique where the cut end (tenon) from one board fits into the matching opening (mortise) of another. Mortise (definition) An opening chiseled, drilled or routed into a board to receive the end of an intersecting board. The opening or socket that receives the tenon in the classic woodworker's mortise-and-tenon joint. The female part of a mortise-and- tenon joint. Tenon (definition) The end of a board, cut to a specific size and shape, that is inserted into the mortise, or opening, in a second board. The male part of a mortise-and- tenon joint.
Rabbet Joint A joinery technique where an “L” groove across the end of the edge of one piece of wood fits into a edge or end of another board with an “L” groove. Rabbet (definition) A rectangular, stepped recess cut along the edge of a section of wood. (May be used as a verb or noun.)
Scarf Joint A joinery technique where two wedge- shaped pieces have been cut to correspond to one another.
Finger Joint A joinery technique used mostly in industry where small “fingers” are cut into corresponding pieces that will be joined together. Finger joints are used to making wide boards, in extending the length of dimensional lumber, and in laminated construction.
Key (or Biscuit) A small, flat lozenge-shaped dowel for edge or corner- jointing. Wood biscuits are fitted into slots that are created with a biscuit jointer. Dowel pin Pegs of wood that fit into two matching holes to strengthen a joint. Spline A thin piece of wood that fits in the mating grooves cut into two pieces of wood. Joinery Reinforcements
Wood Fasteners What fastener is best to use? There are a few questions you need ask to determine which type of fastener you need to use. What materials need to be joined? Wood, metal and masonry all require different fasteners. What is the thickness of the material to be joined? For a secure connection, the fastener must be the correct length. What weight or strength requirements are there? From framing a house to hanging a picture, there's a fastener for the job. How permanent will the connection be? If the work will be disassembled at some point, use a screw. Will the work be indoors or outdoors? There are specific fasteners for both types of applications.
Nails The nail has been around for thousands of years. The first versions were heat- forged. When machinery entered the picture, they were cut from sheets of metal. Today, most nails are drawn and cut from rolls of wire.
Nail Size Nails are sold by weight or length. Wood nails are measured, or sized, according to length. This is expressed by the letter "d" (called penny). The symbol is English, signifying a pound in weight, related to the weight in pounds of 1000 nails. Sizes run from 2d (2-penny/1" long) to 60d. Nails generally are 1" to 6" in length, usually getting thicker as they get longer. Nails larger than 6" are sometimes called spikes.
Nail Styles Nail points vary, but the four- sided diamond point is the standard point found on most nails. Nail heads also vary. Smaller heads can be driven in and painted over. Large framing nails have corrugated heads to reduce the danger of a hammer slipping and causing injury or damage. Regular wood nails are often referred to as wire nails.
Common Nail Thick, heavy-duty, general-purpose nail. Large, flat head for performing rough work such as construction framing.
Box Nail Short, thin shaped nail with a blunt tip. Used to fasten smaller stock when common nails are too large.
Finishing Nail Small nail with cupped head for fastening trim when nail heads should not show. Can be countersunk with a nailset, then filled over.
Brad Smaller version of the finishing nail up to about 1" long. Used for detail work such as attaching molding or trim.
Casing Nail Similar to finishing nail, but thicker and heavier. Used to attach case molding or rough trim where strength and concealment are required.
Tack Very short nail with relatively large head and very sharp point. Used to fasten upholstery or carpet or to perform other light fastening jobs.
Upholstery Tacks Short nails with ornamental or colored heads. Used for attaching upholstery where fasteners will show.
Screws For fastening, screws are stronger than nails. They can be removed with less damage to the material (especially wood) than nails. When using with wood, best practice is to pre-drill a “pilot” hole to guide the screw into position.
Screw Slot Types Slotted Conventional single-groove screwhead. Applied with a flathead screwdriver. Phillips Cross-slotted screwheads with U or V-Shaped slots of uniform width. Driven with a Phillips screwdriver. Torx™ Require special drivers with six point heads. Commonly used in electronics, metal or automotive applications.
Screw Head & Thread Types Head Types Oval Lower portion is countersunk and top is rounded. Easier to remove & better looking than flathead screws. Round Used where the fastened piece is too thin to permit countersinking. Also used on parts that may require a washer. Flat Used in applications where the head needs to be flush with the surface. Slotted and Phillips type are available. Thread Styles Fine-thread Work best for hardwoods. Coarse-thread Intended for soft woods.
Wood Glues & Adhesives By definition, glue is made from animal byproducts such as skins and bones. An adhesive is any product that joins materials together. Today, the words "glue" and "adhesive" have become interchangeable. Natural Glues are made from animal byproducts (hide glue and casein) or plant sources (paste, cellulose and rubber). Synthetic Adhesives include polyvinyl acetate (PVA), aliphatic resin, contact cement, hot melt, and polyurethane. Whether natural or synthetic, the bond is formed as the liquid dries and hardens. Adhesives are often used in conjunction with fasteners to strengthen wood joints.
Animal Glues Used mostly for furniture making and woodworking. They are easy to work with, but less resistant to heat or cold. Hide glue Made from bones, hoofs and skin from animals and fish. Dry form is mixed and heated (in a glue pot) into a gelatinous mixture that is applied with brush. Casein Made from milk, powdered and reformulated with water. Creates a waterproof bond.
White Glue (polyvinyl acetate) For wood, engineered wood, paper, crafts and simple projects. Most commonly available in plastic squeeze bottles. Begins to set within one hour, dries clear. Has easy clean up, doesn't stain, nontoxic, nonflammable. Is not waterproof. Requires work to be clamped for best results.
Yellow or Carpenter’s Glue (aliphatic resin) For woodworking. "Refined" version of PVA. Packaged in squeeze bottles. Varieties typically come in yellow or brown. Begins to set within 15 minutes. Water-resistant nontoxic, nonflammable. More resistant to temperature and water than white glue. Work needs to be clamped for best results. Used for indoor and outdoor applications.
Contact Cement For bonding wood veneer and plastic laminates to countertops. Water-resistant. Applied to both surfaces needing to be bonded. After a designated curing time, the two work pieces are put together. Forms an instant bond, leaving no margin for error. Water-based versions are nonflammable.
Hot Melt For crafts, general projects and repair. Requires an electric glue gun. Adhesive is a 2-4" glue stick that is inserted into the gun. Heat converts solid to liquid for application. Begins to set in about a minute as it cools and solidifies. Fills gaps for good adhesion.
Polyurethane For woodworking. Synthetic plastic-based material. Requires moistening of one or both sides of the materials to be joined. Creates a strong bond. Can be difficult to clean up.
Metal Shop: Fasteners, Joinery, & Adhesives Manufacturing Technology
Fasteners Metal assemblies are often held together with fasteners, hardware devices that mechanically join or affix two or more objects together. Assembling with most types of fasteners allows components to be repeatedly assembled and disassembled. This is important where a product is expected to undergo modifications, repairs, or where it may provide access into an assembly.
Bolts Threaded shafts that use a threaded nut to fasten metal together. Bolts are sized by length and thread. Bolts are stronger than screws. Bolts are classified by the type of head. Stove bolts and machine screws (actually bolts) are turned with a screwdriver. Hexagonal- and square-head bolts are held in place with a wrench while the nut is turned to tighten.
Types of Bolts Carriage bolt Smooth round head & course thread that starts part way down the shaft. Usually used to attach a wooden part to metal. Machine bolt Hexagonal head & only partially threaded. Used for precision attachment using threads to secure materials together. Tap bolt Similar to a machine bolt but the whole body is threaded. Stove bolt Round or flat head with course thread along the whole body. General purpose fastener used when precision fit is not necessary. Stud bolt No head and threaded on both ends. One end is driven into material & the other end is left exposed so that other parts can be fastened to it.
Types of Machine Screws Machine & cap screws Come with a variety of head & thread types. Used for precision fit into thread holes in metal. Setscrew Made with square heads or no heads. Typically used for safety reasons to hold a sleeve, collar or gear on a shaft to prevent relative motion. Thumbscrews Has one or two wings or a knurled head. Used where a screw must be turned by hand using the thumb and a finger.
Sheet Metal Screws Short thick screws that are self-threading (cut or form their own threads as driven into soft metals). Used in the economical assembly of sheet metal. Threaded all the way down the shank. Come in a variety of head types depending on application.
Lag Screw Bolt is a bolt head with a screw body. Has either a square or hexagonal head. Used in fastening where maximum holding power is needed (i.e.- holding a vice to to a work bench).
Nuts Type of hardware fastener with a threaded hole. Usually hexagonal to permit tightening with a wrench but may also be square, knurled, winged or otherwise shaped. Along with a bolt, nuts are designed to capture and fasten objects together.
Type of Nuts Machine screw nut (Hex nut) Square or hexagonal shaped with fine or course thread. Jam nut (Lock nut) Thinner than an ordinary nut. Used as a lock to keep another nut from loosening. Castle nut Has slots cut into the top of the nut that extend upward making it look like a castle. A hex nut with a slightly reduced slotted cylindrical section on one end. Used with a cotter pin to prevent loosening. Wing nut A nut with two thin flat wings. Used in place of a regular nut and can be turned with the thumb and forefinger.
Washers Placed under the bolt head or the nut for a firmer fasten. Designed to protect the surface under a bolt or nut. Used to spread load of a mechanical connection out over a greater area.
Type of Washers Plain washers Circular, small flat piece to widen the bearing surface of a bolt head or nut. Measured by the diameter of the bolt that fits into it. Lock washer Used to lock a nut or screw in place, prevent it from moving from vibrations. Helical spring - looks like a coil from a spring that tightens when applied to prevent movement. Toothed – has teeth that wedge into bearing surface when applied to prevent movement.
Pins Used to hold mechanical parts together or limit travel of moving parts. Cotter pin Made of soft wire. Placed through a hole in a bolt behind a castle nut to prevent the nut from turning. Tapered pin Used to hold a collar or pulley against a shaft. Roll pins Made from sheet steel that is rolled into a tube. Driven into holes slightly larger than a standard hole size so they grip tightly when pounded in.
Keys Used to keep pulleys and gears from moving on shafts. Half the key fits into the keyway (a slot on the shaft), the other half fits into a slot that is on the pulley or gear. Square key Most commonly used. Gib-head key Toothed key that is useful when you need to remove the key from one side of the pulley or gear. Can be removed with a wedge. Woodruff key Semicircular in shape and fits a matching semicircular pocket in the shaft. Key becomes locked in position and cannot be knocked loose due to vibration.
Joining Metal As with wood, there are many ways of joining metal permanently. The method used will depend on the function of the product, the strength needed and the quality of the product. There are several ways of joining metal permanently. riveting soldering and brazing welding The later two of these techniques rely upon heat. With soldering and brazing, the two metals are joined by melting a second metal between them. With welding, the two metals are melted and fused together.
Rivets Metal pins that look like bolts with no threads. Used to hold pieces together permanently. Used when fastening metals together that are not easily welded, or where welding is not practical.
Rivet Characteristics May be either solid or tubular. Made from different materials such as soft steel, aluminum, copper, and brass. Come with a variety of different shapes and heads. For application, rivets are placed in holes, pre-drilled in materials and fastened together. With solid rivets, the headless head is pounded to form a head. Hollow rivets are clinched at the headless end with a special riveting tool.
Spot Welding Form of resistance welding done with a spot welder. High current at a low voltage passes through a spot on two pieces of metal (usually sheet metal) for a short period of time. Resistance to the flow of current through the metal at the spot causes heat, which melts the metal and makes a spot weld. Most frequently used to weld metal joints but sometimes used to weld sheet metal to small diameter rods or small flat bars.
Soldering Process of fastening two metals together with solder, a nonferrous metal that has a lower melting point than the parts being joined. Parts being joined are heated until the solder, when brought into contact with them, melts and flows between the surfaces. When the solder solidifies, it adheres (sticks) tightly and forms a strong bond between the two surfaces.
Soft Soldering Occurs at temperatures below 800 degrees Fahrenheit. For general work, a solder called rosin core (60% tin, 40% lead) is often used. Solder often comes in a coil of wire 1/16” in diameter but can come in other pre-cut shapes, sizes, and forms. Heat for soft soldering is applied using soldering gun or a soldering copper.
Hard Soldering If solder melts above 800 degrees Fahrenheit, it is called hard soldering. Used where a strong joint is needed or where the parts will be used in greater heat than the melting point of soft solder. The most widely used hard solders are silver alloy solders that come in ribbons, sheets, wire, or pre-cut pieces of various shapes and sizes. Often used in jewelry and art metalwork for joining copper, silver, and gold. Heat for hard soldering is applied directly with the flame of a torch.
Brazing Hard soldering processing where the filler material flows into the joints using capillary action (the natural tendency of a liquid to be drawn in between two close fitting surfaces). Filler material used is brazing rods (60% copper, 40% zinc).
Adhesive Bonding of Metals Process of fastening parts of metal products together permanently with non- metallic materials. Often used as an alternative to mechanical fasteners. When using adhesives, the entire joint must be given even more consideration than when using mechanical fasteners. Unlike a bolt or rivet, an adhesive's properties may change depending on where it is used. Light-gauge materials are often good candidates for adhesive bonding.
Advantages of Using Adhesives to Join Metals Many adhesives easily join dissimilar materials if proper surface treatments are used. Adhesively joined structures and products are inherently smooth. Exposed surfaces are not defaced, and contours are not disturbed as with other types of fastening systems. This is important both to function and appearance. Adhesives are sometimes used with mechanical fasteners for sealing flange joints or holding the parts together while the bond forms. Thin or fragile metal parts can be bonded. Adhesives do not usually impose heavy loads on materials, such as in riveting, or localized heating, such as in soldering or welding.
Cyanoacrylates (superglue) Adhesives developed for production situations requiring instant bonds and immediate handling strength, particularly when bonding rubber, metals and plastics. These adhesives are especially well-suited for trim attachment and light assembly. Cure through reaction with moisture held on the surface to be bonded. Good environmental resistance; therefore, they offer excellent resistance to weathering and aging.
Epoxies Synthetic adhesive made of two components, a liquid resin and the hardener to convert the liquid resin into a solid. Create superior bonds for a wide variety of materials including metals, rubber, and plastics. Widely used in the automotive industry. Many cars and light- and heavy-duty trucks feature body panels bonded with epoxy adhesives. Excellent environmental and chemical resistance. They resist the effects of dilute acids, alkalis, solvents, greases, oils, moisture, sunlight and weathering. Flexible cure rates. Allow great versatility in formulation since amount of hardener can be adjusted to increase work time or decrease set time. Low shrinkage and good creep properties.
Urethanes Primerless adhesive that is ideal for prepared metals, rubber and fiber reinforced plastics. Create strong flexible bonds that bring excellent structural integrity to assemblies made of plastic, metal, foams and elastomers.
Anaerobics Specialty adhesives designed to cure on metals in the absence of oxygen. Primarily used to anchor threaded joints and shafts against breaking free due to vibration. Can eliminate the need for lock washers and press fitted metal joints. Often known as "locking compounds.” Based on synthetic acrylic resins.
Snap-Fits Molded-in pieces in a plastic assembly that are designed to form a mechanical joint system where part-to-part attachment is accomplished with locating and locking features to connect components together. Provide for the economical and rapid assembly of plastic parts.
Types of Snap Fits Snap-fits are used to assemble parts of all shapes and sizes. A snap fit with a tapered finger provides more uniform stress distribution and can be used over and over again. There are three types of snap-fits: cantilever, torsion, and annular.
Use of Snap-Fits Snap-fits are often used for high-volume production. In many products, snap-fits are designed for one- time assembly without any provision for nondestructive disassembly. For example, snap-fit designs, such as those used in the plastic shell of small kitchen appliance or a child’s toy, are not designed to be taken apart without destroying the product. Where servicing is anticipated, snap-fits can be designed that allow for release of the assembly with an appropriate tool. For example, snap-fit designs, such as those used in battery compartment covers for calculators and radios, are designed for easy release and re-assembly over hundreds or even thousands of cycles.
Cantilever Snap-Fit Consists of a protrusion (some type of bead or hook) at one end of the beam and a structural support at the other end. Most common type of snap-fit used for plastic assembly. Commonly used in applications such as toys or battery compartment doors. Most cantilever snap-fits are usually a one-time use but some designs can be used more than once.
Cantilever Snap-Fit (continued) Snap fits that are intended for one time use have a sharp edge, or tang, that holds the part in place. Snaps intended for limited use will have a rounded tang to allow the snap feature to be pulled off yet still have holding ability. With this type of snap fit, the plastic does not experience a lot strain, so multiple flexes are possible without damaging the plastic beam. It also has a built-in stop, so the beam cannot be flexed too much and damaged.
Torsion Snap-Fit Spring-loaded lever that snaps into place when the mating part is pressed into place. The torsion snap can be released by pressing down on the lever. This design can be used for frequently assembled and disassembled parts, or to provide constant pressure to the assembly. The complexity of this type of snap-fit, prevents it from being widely used in industry. As its name implies, the torsion snap-fit relies for its spring effect on twisting rather than flexing like the other types. It is less common than cantilever or annular snap-fits but it is particularly useful when you want to be able to release the catch easily and often. For example, a torsion snap fit can be a good way of fastening a hinged lid on a box or container.
Annular Snap-Fit This type of snap fit is best for assembling cylindrical or ring-shaped joints. Classic examples include ballpoint pens with snap-on caps, the child- resistant caps on medicine bottles, and cottage cheese container lids. Generally stronger, but needs greater assembly force than their cantilevered counterparts. Annular snap-fits are basically interference rings. There is a smaller-diameter male component (plug) which has a bump or ridge feature around its circumference. The ridge diameter of the plug is slightly larger than the inside diameter of its mating tube-shaped female hub.
Benefits of Snap-Fits An integral element of the plastic part – no other components are needed. Can replace screws, nuts, and washers. Easy automation can reduce assembly costs. No other fastener, adhesive, solvent, welding, or special equipment is needed. Design can minimize the risk of improper assembly. Can be designed to engage and disengage.
Factors Regarding Snap-Fits Snap-fits that are assembled under stress will allow creep, a plastic's deformation under load (tension, compression or flexure) over time. It is difficult to design snap-fits with hermetic seals. If the beam or ledge of the snap-fit relaxes, it could decrease the effectiveness of the seal. Snap-fits can be damaged by mishandling and abuse prior to assembly. The key to successful snap-fit design is to have adequate holding power without exceeding the elastic or fatigue limits of the material.
Press Fits Press fitting refers to two parts being pressed together, making an interference fit. An interference fit occurs when the inner diameter of the hole is slightly less than the outer diameter of the part being inserted. When the two parts are pushed together, they stick. A common example is the forced insertion of a metal pin or shaft that is slightly larger than a plastic hub or boss it is inserted into. Press fitting is a simple, low- cost method for assembling parts or components.
Use of Press Fits Used in the telecommunications and computer industries, as well as in automobiles, airplanes, office equipment, and consumer goods. Press fitting is a lower quality fitting process. However, once a press fit is in place, it will not come loose. The particular application dictates whether a press fit or other fastening method is used. Press fits are sometimes used to get a complete alignment between two pieces. Press fits are also used to prevent bearings from spinning. It is a good fastening method for components that undergo temperature fluctuations, such as automotive assemblies. Regardless of the temperature, the interference fit or force between the two parts is always there.
Living Hinges Thin sections of plastic that connect two segments of a part to keep them together and allow the part to be opened and closed. Typically these are used in containers that are used in high volume flexing applications such as toolboxes, fish tackle boxes, file card boxes, etc. The materials used to make a living hinge are usually a very flexible plastic such as polypropylene and polyethylene. These can flex more than a million cycles without failure.
Bosses Projection designed into a plastic part to add strength, facilitate alignment during assembly, or to provide for fastening. Bosses are used for the purpose of registration of mating parts or for attaching fasteners such as screws or accepting threaded inserts (molded-in, press-fitted, ultrasonically or thermally inserted).
Hot Gas Welding Most widely applied in the fabrication of plastic assemblies. Involves the use of various butt joints & hot gas from a welding torch to melt filler material between pieces of an assembly to create a weld.
Ultrasonic Welding The most common thermal method for joining small and medium-sized parts of amorphous and crystalline plastics. The process normally lasts less than 2 seconds and forms a continuous, leak-proof joint that often is as strong as the base material. Done using equipment that applies high-frequency energy (20 to 40 KHz) directly to the interface between parts.
Laser Welding Involves use of a laser to melt the bond line between two parts to form a weld. This method is a fast, economical, and safe way to weld compatible plastics having similar melt temperatures.
Vibration Welding Done by parts being rubbed together to create frictional heat. Rubbing usually involves amplitudes of 0.1- to 0.2-in. and frequencies of 120 and 240 Hz. It creates strong joints and works best with large parts that have irregular joint interfaces
Spin Welding Joins parts with circular joint surfaces using relatively simple equipment, sometimes just a drill press. Involves holding one part firmly and pressing a rotating part against it at a steady pressure. The weld usually forms in less than 3 seconds.
Mechanical Fasteners Readily available, easy to install, and usually allow for nondestructive disassembly. Often have high assembly costs and require that extra parts be stocked. Most fasteners used with metals also work with plastics (e.g., screws, bolts, nuts and lock washers). When selecting metal fasteners, be aware that these components can overstress plastic parts. This can be prevented through proper design, using the appropriate fastener and torque-limiting drivers.
Threaded Metal Inserts Permanently installed in molded bosses, eliminate the need for a nut, simplifying the assembly. Can include female threads, threaded male studs, locating pins, and bushings. Ultrasonically-installed inserts are especially popular, because the surrounding plastic melts around the insert, make it strong and relatively free of stress.
Self-Tapping Screws Cutting-style screws are best because they act like thread-cutting taps and remove material, without generating high stresses on plastic materials as do forming-style screws. Screws with multiple lobes and those with alternating thread heights offer excellent holding power and reduced stress levels.
Metal Rivets Allow for fast, permanent assemblies. Should have large heads to spread the load. Rivet should be formed against the metal part of an assembly or against a metal washer if both parts are plastic.
Stamped Sheet Metal Fasteners Provide light-duty threads or push-on assemblies. Push-nuts, for example, are simply pressed onto plain, molded plastic studs or bosses in permanent assemblies. Easy to install, inexpensive, and vibration- proof. Another such fastener, boss caps (cup-shaped parts pushed onto a plastic boss), add partial metal threads for self-tapping or sheet metal screws, and reinforce the boss against the expansion forces of the screw.
Molded Plastic Screws, Rivets and Other Similar Fasteners Used for light-duty plastic assemblies, especially where appearance is important, such as to attach trim and faceplates.
Solvent Bonding A liquid applied just before assembly dissolves the joint surfaces. This is enough for a weld to remain after the solvent evaporates. This method is limited to compatible materials that dissolve in the same solvent or solvents. The chemical resistance of many plastics limit this method from being used.
Adhesive Bonding Occurs when a third substance bonds a plastic to another plastic or to metal, rubber, ceramic, glass, or wood. Adhesives frequently used with thermoplastics include epoxy, acrylic, polyurethane, phenolic, rubber, polyester and vinyl. Cyanoacrylate (superglue) adhesives are popular because they work rapidly. Many adhesives contain solvents that partially dissolve the plastic surface, which improves adhesion. Surface preparation is also critical for successful adhesion. Many materials must be roughened or etched to eliminate overly smooth surfaces. They also may need thorough cleaning because grease, mold release compound, and other contaminants can spoil a bond.