Scarph joint

A scarf joint (also known as a scarph joint) is a method of joining two members end to end in woodworking or metalworking. 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.

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 fastening (screws, bolts, etc.) for all strength. Hooked, keyed, and nibbed scarfs are some of the many example of interlocking scarfs, offering 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, 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.

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.

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Wikipedia