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A scarf joint, or scarph joint, is a method of joining two members end to end in woodworking or metalworking. [1] The scarf joint is used when the material being joined is not available in the length required. It is an alternative to other joints such as the butt joint and the splice joint and is often favored over these in joinery because it yields a barely visible glue line.
The joint is formed by cutting opposing tapered ends on each member which are then fitted together. When working with wood, this gives better long grain to long grain gluing surface, which yields a stronger joint than would be achieved with a simple butt joint. The tapers are generally cut at an angle between 1:8 to 1:10. The ends of a plain scarf are feathered to a fine point which aids in the obscuring of the joint in the finished work, while in other forms of scarf the ends are frequently cut to a blunt "nib" which engages a matching shoulder in the mating piece.
Where scarfed joints are used in the restoration of vintage aircraft most developed countries will only issue an airworthiness certificate if all such joints have used an angle no less than 1:8.
Determination of the maximum axial force for two pieces joined by adhesive can easily be determined using two equations that can be derived from the geometry of the problem by breaking the axial force component into a tensile force and shear force normal and parallel to the scarf joint. Shear strength is assumed to be equal to σ/2. The following equations need to be adjusted if the shear strength is greater than σ/2. The two equations that give a maximum axial force are F=σ/sin(α)^2 and F=σ/sin(2α), where α is the angle from the horizontal to the joint. Both should be evaluated for a given problem, and the smaller F of the two is the magnitude of the maximum allowable axial force. The first equation accounts for failure in tension. The second equation accounts for failure in shear. Some special angles should be noted or the graphs of two equations should be compared on the same plot. The joint is weakest at α=90° due to tension limits and 45° due to shear limits. However, α=45° will be stronger than α=90° if shear strength is greater than σ/2. The joint is strongest between these two angles at 63.4°. The joint becomes stronger than 63.4° at 25.4°. At a shallow enough angle, strength of the joint continues to increase and failure will occur anywhere in the two pieces, most likely outside the joint.
In woodworking, there are two distinctly different categories of scarf, based on whether the joint has interlocking faces or not. A plain scarf is simply two flat planes meeting on an angle relative to the axis of the stock being joined, and depends entirely on adhesive and/or mechanical fasteners (such as screws, nails, or bolts) for all strength. Interlocking scarf joints, such as a hooked, keyed, and nibbed scarves, offer varying degrees of tensile and compressive strength, though most still depend on mechanical fastening to keep the joint closed.
The plain scarf is not preferred when strength is required, so it is often used in decorative situations, such as the application of trim or moulding. The use of modern high-strength adhesives can greatly increase the structural performance of a plain scarf.
The keyed-hook scarf is common in ship and boat-building, [2] as well as timber framing and wooden bridge construction. In large timbers such as these the scarf is virtually always secured with through bolts, and is frequently reinforced externally with iron or steel fishplates, and/or strapping.
A scarf joint may also be used to fix problems caused when a board is cut too short for the application. The board can be cut in half with a tapered cut yielding a scarf joint. When the joint is glued together, the tapers are slid against each other so that the two sections are no longer in line with each other. This has the effect of making the board longer. Once the glue has set, the board can be planed down to an even thickness, resulting in a longer but thinner board.
In traditional timber framing there are many types of scarf joints used to join timbers.
Joinery is a part of woodworking that involves joining pieces of wood, engineered lumber, or synthetic substitutes, to produce more complex items. Some woodworking joints employ mechanical fasteners, bindings, or adhesives, while others use only wood elements.
A composite material is a material which is produced from two or more constituent materials. These constituent materials have notably dissimilar chemical or physical properties and are merged to create a material with properties unlike the individual elements. Within the finished structure, the individual elements remain separate and distinct, distinguishing composites from mixtures and solid solutions.
In continuum mechanics, stress is a physical quantity that describes forces present during deformation. For example, an object being pulled apart, such as a stretched elastic band, is subject to tensile stress and may undergo elongation. An object being pushed together, such as a crumpled sponge, is subject to compressive stress and may undergo shortening. The greater the force and the smaller the cross-sectional area of the body on which it acts, the greater the stress. Stress has dimension of force per area, with SI units of newtons per square meter (N/m2) or pascal (Pa).
In engineering and materials science, a stress–strain curve for a material gives the relationship between stress and strain. It is obtained by gradually applying load to a test coupon and measuring the deformation, from which the stress and strain can be determined. These curves reveal many of the properties of a material, such as the Young's modulus, the yield strength and the ultimate tensile strength.
In mechanics, compressive strength is the capacity of a material or structure to withstand loads tending to reduce size. In other words, compressive strength resists compression, whereas tensile strength resists tension. In the study of strength of materials, tensile strength, compressive strength, and shear strength can be analyzed independently.
A lap joint or overlap joint is a joint in which the members overlap. Lap joints can be used to join wood, plastic, or metal. A lap joint can be used in woodworking for joining wood together.
A dovetail joint or simply dovetail is a joinery technique most commonly used in woodworking joinery (carpentry), including furniture, cabinets, log buildings, and traditional timber framing. Noted for its resistance to being pulled apart, also known as tensile strength, the dovetail joint is commonly used to join the sides of a drawer to the front. A series of pins cut to extend from the end of one board interlock with a series of 'tails' cut into the end of another board. The pins and tails have a trapezoidal shape. Once glued, a wooden dovetail joint requires no mechanical fasteners.
Stress–strain analysis is an engineering discipline that uses many methods to determine the stresses and strains in materials and structures subjected to forces. In continuum mechanics, stress is a physical quantity that expresses the internal forces that neighboring particles of a continuous material exert on each other, while strain is the measure of the deformation of the material.
A bolted joint is one of the most common elements in construction and machine design. It consists of a male threaded fastener that captures and joins other parts, secured with a matching female screw thread. There are two main types of bolted joint designs: tension joints and shear joints.
Glued laminated timber, commonly referred to as glulam, is a type of structural engineered wood product constituted by layers of dimensional lumber bonded together with durable, moisture-resistant structural adhesives so that all of the grain runs parallel to the longitudinal axis. In North America, the material providing the laminations is termed laminating stock or lamstock.
A finger joint, also known as a comb joint, is a woodworking joint made by cutting a set of complementary, interlocking profiles in two pieces of wood, which are then glued. The cross-section of the joint resembles the interlocking of fingers between two hands, hence the name "finger joint". The sides of each profile increases the surface area for gluing, resulting in a strong bond, stronger than a butt joint but not very visually appealing. Finger joints are regularly confused with box joints, which are used for corners of boxes or box-like constructions.
In engineering, shear strength is the strength of a material or component against the type of yield or structural failure when the material or component fails in shear. A shear load is a force that tends to produce a sliding failure on a material along a plane that is parallel to the direction of the force. When a paper is cut with scissors, the paper fails in shear.
A box joint is a woodworking joint made by cutting a set of complementary, interlocking profiles in two pieces of wood, which are then joined (usually) at right angles, usually glued. The glued box joint has a high glued surface area resulting in a strong bond, on a similar principle to a finger joint. Box joints are used for corners of boxes or box-like constructions, hence the name. The joint does not have the same interlocking properties as a dovetail joint, but is much simpler to make, and can be mass-produced fairly easily.
A butt joint is a wood joint in which the end of a piece of material is simply placed against another piece. The butt joint is the simplest joint. An unreinforced butt joint is also the weakest joint, as it provides a limited surface area for gluing and lacks any mechanical interlocking to resist external forces.
A splice joint is a method of joining two members end to end in woodworking. The splice joint is used when the material being joined is not available in the length required. It is an alternative to other joints such as the butt joint and the scarf joint. Splice joints are stronger than unreinenforced butt joints and have the potential to be stronger than a scarf joint.
Wood glue is an adhesive used to tightly bond pieces of wood together. Many substances have been used as glues. Traditionally animal proteins like casein from milk or collagen from animal hides and bones were boiled down to make early glues. They worked by solidifying as they dried. Later, glues were made from plant starches like flour or potato starch. When combined with water and heated, the starch gelatinizes and forms a sticky paste as it dries. Plant-based glues were common for books and paper products, though they can break down more easily over time compared to animal-based glues. Examples of modern wood glues include polyvinyl acetate (PVA) and epoxy resins. Some resins used in producing composite wood products may contain formaldehyde. As of 2021, “the wood panel industry uses almost 95% of synthetic petroleum-derived thermosetting adhesives, mainly based on urea, phenol, and melamine, among others”.
A fracture is any separation in a geologic formation, such as a joint or a fault that divides the rock into two or more pieces. A fracture will sometimes form a deep fissure or crevice in the rock. Fractures are commonly caused by stress exceeding the rock strength, causing the rock to lose cohesion along its weakest plane. Fractures can provide permeability for fluid movement, such as water or hydrocarbons. Highly fractured rocks can make good aquifers or hydrocarbon reservoirs, since they may possess both significant permeability and fracture porosity.
Hankinson's equation is a mathematical relationship for predicting the off-axis uniaxial compressive strength of wood. The formula can also be used to compute the fiber stress or the stress wave velocity at the elastic limit as a function of grain angle in wood. For a wood that has uniaxial compressive strengths of parallel to the grain and perpendicular to the grain, Hankinson's equation predicts that the uniaxial compressive strength of the wood in a direction at an angle to the grain is given by
Anchor bolts are used to connect structural and non-structural elements to concrete. The connection can be made by a variety of different components: anchor bolts, steel plates, or stiffeners. Anchor bolts transfer different types of load: tension forces and shear forces.
This glossary of woodworking lists a number of specialized terms and concepts used in woodworking, carpentry, and related disciplines.
