Woodworking Joints

Joinery often refers to the part of woodworking that involves the joining together of parts of wood.

Traditional wood joinery techniques address the distinctive material properties of wood, often without resorting to mechanical fasteners. There are different types of joinery for different structures. For example the joinery used to build a house is different from that used to make puzzle toys, although some concepts overlap.

Properties of wood

Wood is anisotropic: its material properties are different along different dimensions. It is strong when stressed along the grain (longitudinally), but weak across it (radially and tangentially). It expands and contracts in response to humidity. This change is very small longitudinally. It is considerable, but unequal, in the radial and tangential directions. The frame and panel constructions of doors and cabinets is not purely decorative. The panel would be fragile without the support of the rails, whose grain runs perpendicular to that of the panel. But, if the rails were directly fastened to the panel, the difference in the rate of expansion across and along the grain would rip the two apart. When properly constructed, the panel is free to expand, while still supported by the frame.

Materials used for joining

• Joints can be designed to hold without the use of glue or fasteners.
• Glue is highly effective for joining wood when both surfaces of the joint are edge grain. A properly glued joint may be as strong as a single piece of wood. However, glue is ineffective on end-grain surfaces. Compared to a mortise and tenon, a dowel joint is a poor joint because it does not address these properties. Much of the surface of the hole of a dowel joint is end-grain, to which glue adheres poorly. In a mortise and tenon, most of the surface of the joint is longitudinal-grain. Animal glue is soluble in water, producing joints that can be disassembled using steam to soften the glue.
• Various mechanical fasteners are used, the simplest being nails and screws. Glue and fasteners can be added together.

Types of joints

Some types of joints used include:

• Biscuit join
• Bridle
• Butt
• Butterfly
• Dowel
• Coping
• Cope and stick
• Dado (US) or Housing Joint (UK)
• Dougong
• Dovetail
• Finger (US) or box combing (UK)
• Lap (halving joint)
  • Cross-lap
  • Halved joint
  • Dovetail-lap
  • End-lap (corner halving joint)
  • Middle-lap (Tee halving joint)
• Miter (mitre)
• Mortise and tenon
• Pocket-Hole Joinery
• Rabbet (rebate)
• Scarf (scarph)
• Splice joint
• Tongue and groove
• Frame and Panel (rail and style)

different types of joints

Bridle joint	Dowel joint	Coped joint	Dado
Dovetail joint	Finger joint or box combing	Lap joints	mitre (miter) joint
Mortise and tenon joint	Rabbet or rebate	Scarf joint	Splice joints

Styles of Joinery

Two of the most common traditions of joinery are Chinese and European. The Japanese and the Chinese in particular developed hundreds of types of joints and their furniture was often held together without glue or nails. The Chinese have been using this method for the last seven thousand years.

Bevel

A beveled edge refers to an edge of a structure that is not perpendicular (but instead often at 45 degrees) to the faces of the piece. The words bevel and chamfer overlap in usage; in general usage they are often interchanged, while in technical usage they may sometimes be differentiated as shown in the image at right.

Bridle Joint

A bridle joint is a woodworking joint, similar to a mortise and tenon, in that a tenon is cut on the end of one member and a mortise is cut into the other to accept it. The distinguishing feature is that the tenon and the mortise are cut to the full width of the tenon member.

 

Corner bridle joint

 

The corner bridle joint (also known as a slot mortise and tenon) joins two members at their respective ends, forming a corner. This form of the joint is commonly used to house a rail in uprights, such as legs. It provides good strength in compression and is fairly resistant to racking, although a mechanical fastener or pin is often required. The bridle joint is very popular in workbench construction.

Corner bridles are often used to join frame components when the frame is to be shaped. Material can be removed from the joined members after assembly without sacrificing joint integrity.

 

T-bridle joint

 

A variation of the bridle joint is the T-bridle, which joins the end of one member to the middle of another.

Applications

• Legs or stiles to rails
• Frames

Methods

Corner bridles can be cut by the following methods:

• Hand saw and chisel
• circular saw with multiple passes (depending on width and depth)
• dado set in a single pass
• electric router using a straight or rebate bit
• spindle moulder

Butt joint

A butt joint is a joinery technique in which two members are joined by simply butting them together. The butt joint is the simplest joint to make since it merely involves cutting the members to the appropriate length and butting them together. It is also the weakest because unless some form of reinforcement is used (see below) it relies upon glue alone to hold it together. Because the orientation of the members usually present only end grain to long grain gluing surface, the resulting joint is inherently weak.

Applications

• Carcase and frame assembly in cabinet making
• Face frame construction
• light frame building construction (including platform framing and balloon framing) where all joints are nailed butt joints.
• Curtain wall systems and other glazing applications where minimization of mullions is desired.

Methods

The butt joint is a very simple joint to construct. Members are simply docked at the required angle (usually 90°) and required length. One member will be shorter than the finished size by the thickness of the adjacent member. For enclosed constructions, such as four-sided frames or boxes, the thickness of the two adjacent members must be taken into consideration.

For example: when constructing a four sided box made from 19mm thick material that is to be of finished size 600mm x 600mm, two of the members will be docked at 600mm and two will be docked at 600mm - 19mm - 19mm = 562mm.

Reinforced butt joints may require additional steps which are detailed below.

In solid timber, docking of members for a butt joint is usually a crosscut, so a crosscut saw is best for this job. When working with sheetgoods, there is no distinction between crosscut and rip cut.

The joint members can be docked by any of the following methods:

Carcase Butt Joints

• Crosscut saw
• Circular saw and straightedge
• Table saw

Frame Butt Joints

• backsaw
• Miter saw
• Table saw using a mitre gauge, crosscut sled, or sliding table attachment.

Reinforced Butt Joints

To overcome their inherent weakness, butt joints are usually reinforced by one of the following methods:

Nailed Butt Joint

This is the most common form of the butt joint in building construction. Members are brought together and a number of nails are driven in to hold them in place. The technique of skew-nailing is applied so that nails are not parallel to each other and so resist the pulling apart of the joint. This form of butt joint is rarely used in furniture making.

Applications

• framing in building construction, such as platform framing
• Basic or temporary box/cabinet/frame making

Dowel Reinforced Butt Joint

A dowel reinforced butt joint

The dowel reinforced butt joint or simply dowel joint has been a very common method of reinforcing butt joints in furniture for centuries. They are common in both frame and carcase construction. Dowel joints are popular in chairs, cabinets, panels and tabletops. They are also used to assist with alignment during glue up.

The technique consists of cutting the members to size and then drilling a series of holes in the joint surface of each member. Holes are often drilled with the assistance of a dowelling jig which aids in accurate hole placement - accuracy is paramount in this technique to ensure members line up perfectly in the completed joint. The holes are drilled such that there are corresponding holes in each member into which short "Dowel" title="Dowel">dowels are inserted with some glue. The joint is brought together and clamped until the glue has dried.

This produces a joint which is much stronger than a butt joint without reinforcement. The dowels offer some holding strength even after the glue has deteriorated. Over time, dowels may shrink and become loose. They take on an oval shape in section owing to the different rate at which wood moves with different orientations of the grain. Loose dowels allow the joint to flex, although it may not fall apart. This phenomenon is evident in creaking chairs and wobbling book cases. For this reason, dowel joints are not preferred for high-end furniture.

Applications

• Frame joinery (eg. face frames, web frames, door frames, table legs to aprons, chair legs)
• Cabinet carcase construction (eg. carcase sides to top and bottom, fixed shelving/partitions)
• Panel assembly (for alignment)

Biscuit Reinforced Butt Joint

A biscuit reinforced butt joint

The biscuit reinforced butt joint is a fairly recent innovation in butt joint construction. It is used primarily in carcase and frame construction. The biscuit is an "Oval" title="Oval">oval shaped piece of specially dried and compressed wood, usually beech, which is installed in matching mortices in both members of the joint in a similar fashion to a loose or floating tenon. Biscuit joints are common in both frame and carcase construction. They are particularly convenient for panel glue ups as they facilitate alignment of panel members.

To create the mortice for the biscuit, a biscuit joiner is usually required. There are other methods of cutting the slot, such as a slot cutter bit in a router, but the biscuit joiner is the most common. Accuracy is not as important in the creation of these mortices as the biscuit joint is designed to allow a bit of flexibility during glue up. The mortice must be located the correct distance from the face of the joint in both members but the width of the mortice is not as critical.

When the mortices have been cut, the biscuit is inserted with some glue and the joint is brought together, aligned and clamped. The biscuit absorbs some moisture from the glue and swells up in the mortice, creating a tightly fitting joint.

Biscuits are available in a range of sizes for different purposes. It is also common to use more than one biscuit side by side in a joint when members are thick.

Applications

• Frame joinery (eg. face frames, web frames, table legs to aprons, chair legs)
• Cabinet carcase construction (eg. carcase sides to top and bottom, fixed shelving/partitions)
• Panel assembly (for alignment)
• Attaching face frames to cabinets

Screwed Butt Joint

A screwed butt joint

The screwed butt joint uses one or more screws inserted after the joint has been brought together. The screws are usually inserted into an edge on the long grain side of one member and extend through the joint into the end grain of the adjacent member. For this reason, long screws are required (usually 3 times the thickness of the member) to ensure good traction. These joints may also be glued although it is not necessary.

In solid timber work it is common to counterbore a hole in the frame or carcase member to conceal the head of the screw. This also allows more of the body of the screw to penetrate the adjacent member for greater traction. After the screw has been driven into the joint, the counterbore can be filled with an appropriately sized piece of dowel or a wooden plug cut from an offcut of the same timber using a plug cutter.

There are also commercial systems available for screwed butt joints in which a plastic cap is provided with the screw to be fixed to the head of the screw after it has been driven home. Counterbores are not required for these fasteners. This system is more common with manufactured board products.

The screwed butt joint is common in both frame and carcase joinery. Modular kitchens make regular use of this fixing method.

Applications

• Frame joinery (eg. face frames, web frames, door frames)
• Cabinet carcase construction (carcase sides to top and bottom, fixed shelving/partitions)

Butt joint with Pocket Hole Screws

A Butt joint with Pocket Hole Screws

This is a variation of the screwed butt joint in which the screws are inserted into pocket holes drilled in the rear face of one of the joint members. The screws extend into cross grain in the adjacent member, so much shorter screws can be used. This method is preferred when the edges of the frame will be visible.

The pocket holes require two drilling operations to be completed. The first is to counterbore the pocket hole itself, which houses the screw head within the member. This hole is stopped 1/4" (5mm) or so from the edge of the frame member. The second step is to drill a pilot hole concentric with the pocket hole which extends through the edge of the member. The pilot hole allows the screw to pass through the member and into the adjoining member. This two stage drilling operation may be performed with two different sized drill bits, however there are special stepped bits available to perform the operation in a single pass. The drilling operation is often facilitated by a pocket hole jig which allows the user to drill the pocket hole at the correct angle and to the correct depth.

Applications

• Frame joinery (eg. face frames, web frames, door frames)
• Attaching face frames to cabinets

Pocket-Hole Joinery being used to assemble a simple 'T-Joint'.

Knock-down Fasteners

Knock-down fasteners are a hardware device made for the purpose of constructing butt joints that can be assembled and re-assembled repeatedly (hence knock-down). This type of fastener is very popular in flat-pack furniture, which is typified by items such as book cases and wall units that come in a package of pre-cut and pre-drilled components ready to assemble by a novice. They are also very common in modern modular kitchens.

Knock-down fasteners usually consist of cam dowels locked by cam locks, also known as conformat fasteners, installed in adjacent members. The members are brought together and the joint is secured by turning the cam-lock.

Specialist tools and jigs are often required for the repeatable installation of knock-down fasteners (but not for assembly of prefabricated flat-pack furniture), so they tend to be limited to those who are making mass-produced items. However, there are applications in which the hobbyist can benefit from the range of fasteners that are available. They are easier and require less skill to install than some of the other more traditional techniques.

Knock-down fasteners are typically used for carcase joinery; furniture designs using them are usually of frameless construction.

Applications

• Wide application in cabinet making depending on type of fastener: particularly in carcase construction (eg. Carcase sides to top and bottom, fixed shelving/partitions, drawer boxes, counter tops to carcase)

Lap joint

In woodworking or metal fitting, a lap joint describes a technique for joining two pieces of material by overlapping them. A lap may be a full lap or half lap.

In a full lap, no material is removed from either of the members to be joined, resulting in a joint which is the combined thickness of the two members. In a half lap joint, material is removed from each of the members so that the resulting joint is the thickness of the thickest member. Most commonly in half lap joints, the members are of the same thickness and half the thickness of each is removed.

Full lap

The full lap is a very basic method of joining two members and requires little or no joinery skills to make. It requires some form of mechanical "Fastener">fastener to be effective. It offers no resistance to racking but some resistance to twisting and "Shear_stress" title="Shear stress">shearing depending upon the fastener used.

Large framing members in light frame construction are often joined by lapping - for example bearers supporting joists.

Applications

• Bracing where bracing members overlap
• Temporary framing
• Some applications in timber frame construction

Half lap

Clockwise from top left: Half lap, mitred half lap, dovetail lap, and cross lap

Half lap joints are used extensively in construction and cabinetry for framing. They are quick and easy to make and provide reasonable strength through good long grain to long grain gluing surface. The shoulders provide some resistance to racking. They may be reinforced with dowels or mechanical fasteners to resist twisting.

Applications

• Frame assembly in cabinet making - particularly when frame members are to be shaped after joining.

Methods

Half lap joints can be cut by the following methods:

• electric router using a straight or rebate bit
• circular saw with multiple passes (depending on width and depth)
• dado set in a single pass
• spindle moulder
• hand saw and shoulder plane or chisel

There are four basic types of half lap joint:

End lap

Also known simply as a Half lap, it is the basic form of the lap joint and is used when joining members end to end either parallel or at right angles. When the joint forms a corner, as in a rectangular frame, the joint is often called a corner lap. This is the most common form of end lap and is used most in framing.

For a half lap in which the members are parallel, the joint may be known as a half lap splice. This is a splice joint and is an alternative to scarfing when joining shorter members end to end.

Both members in an end lap have one shoulder and one cheek each.

Applications

• Internal cabinet frames
• Visible frames when the frame members are to be shaped

Cross lap

This form of the half lap is used when one member crosses the other. The main difference between this and the basic half lap is that the joint occurs in the middle of one or both members, rather than at the end. The two members are at right angles to each other and one member may terminate at the joint, or it may carry on beyond it. When one of the members terminates at the joint, it is often referred to as a Tee lap or middle lap. In a cross lap where both members continue beyond the joint, each member has two shoulders and one cheek. For a Tee lap, one of the members has only one shoulder.

Applications

• Internal cabinet frames
• Simple framing and bracing

Dovetail lap

This is a Tee lap in which the housing has been cut at an angle which resists withdrawal of the stem from the cross-piece.

Applications

• Framing applications where tension forces could pull the joint apart

Mitred half lap

This is a variation of the end lap which shows a mitre on the face of the finished work.

The mitred half lap is the weakest version of the joint because of the reduced gluing surface.

Applications

• Visible framing applications where a mitred corner is desired

Mitre Joint

Mitre joint (pieces ready to be joined).

A mitre or miter joint is a joint made by beveling each of two parts to be joined, usually at a 45° angle, to form a corner, usually a 90° angle. It is often used in making picture frames.

For miter joints occurring at angles other than 90°, for materials of the same width and height (i.e. pieces of a picture frame border) the proper cut angle must be determined so that the two pieces to be joined meet flush (i.e. one piece's mitered end is not longer than the adjoining piece). To find the cut angle divide the angle at which the two pieces meet by two.

Technically two different cut angles are required, one for each piece, where the second angle is 90° plus the aforementioned cut angle, but due to angular limitations in common cutting implements (hand circular saws, table saws) a single angle is required and is used to cut the first piece in one direction and the second piece in the opposite direction.

In woodwork, it is one way of joining two pieces of moulding that meet at an angle. (Compare the cope cut.)

It is a popular method of joining as it is easy and attractive. By itself, it is one of the weakest joints to choose, but it can be strengthened with a spline.

Mortise and tenon

Diagram of a Mortise and Tenon Joint

Simple and strong, the mortise and tenon joint (also called the mortice and tenon) has been used for millennia by woodworkers around the world to join two pieces of wood, most often at an angle close to 90°. Although there are many variations on the theme, the basic idea is that the end of one of the members is inserted into a hole cut in the other member. The end of the first member is called the tenon, and it is usually narrowed with respect to the rest of the piece. The hole in the second member is called the mortise. The joint may be glued, pinned, or wedged to lock it in place.

A mortise is a cavity cut into a timber to receive a tenon. There are several kinds of mortises:

• Open mortise - a mortise which has only three sides. (See Bridle joint).
• Stub mortise - a shallow mortise, depth depends on the size of the timber; also a mortise that does not go through the workpiece (as opposed to a "through mortise").
• Through mortise - a mortise which passes entirely through a piece.
• Wedged half-dovetail - a mortise where the back is wider, or taller, than the front, or opening. The space for the wedge initially allows room for the tenon to be inserted, the presence of the wedge, after the tenon has been engaged, prevents its withdrawal. Sometimes called a "suicide" joint - since it is strictly a "one way trip".
• Through wedged half-dovetail - a wedged half-dovetail mortise which passes entirely through the piece.

A tenon is a projection on the end of a timber for insertion into a mortise. Usually the tenon is taller than it is wide.

There are several kinds of tenons:

• Stub tenon - a short tenon; depth depends on the size of the timber; also a tenon that is shorter than the width of the mortised piece so the tenon does not show (as opposed to a "through tenon").
• Tusk tenon - a kind of mortise and tenon joint that uses a wedge-shaped key to hold the joint together
• Through tenon - a tenon which passes entirely through the piece of wood it is inserted into, being clearly visible on the back side
• Teasel tenon - a term used for the tenon on top of a jowled or gunstock post, which is typically received by the mortise in the underside of a tie beam. A common element of the English tying joint
• Top tenon - the tenon which occurs on top of a post.
• Feather Tenon - a round shouldered machined fillet or feather which is glued into a machine (router) made slot or mortise on each side of the joint.

Generally the size of the mortise and tenon is related to the thickness of the timbers. It is considered good practice to proportion the tenon as 1/3rd the thickness of the rail, or as close to this as is practical. The haunch, the cut away part of a sash corner joint that prevents the tenon coming loose, is one third the length of the tenon and one sixth of the width of the tenon in its depth.

Various mortises

A traditional through, wedged, mortise and tenon joint

A stub tenon corner joint

A haunched stub tenon corner joint

A foxtail wedged tenon joint

A pinned corner tenon joint

A modern feather tenon joint

History

This is an ancient joint, and has been found in archeological sites in the Middle East, Europe and Asia.

Mortise and tenon joints of tie beams and cross beams from the Yingzao Fashi, published in 1103 Song Dynasty Chinese scholar-b ureaucrat Li Jie (1065-1110).

In traditional Chinese architecture, wood components such as beams, brackets, roof frames and struts were made to interlock with perfect fit, without using fasteners or glues, enabling the wood to expand and contract according to humidity. Archaeological evidence from Chinese sites show that by the end of the Neolithic, mortise and tenon joinery was employed in Chinese construction.

Scarf Joint

A scarf 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 favoured over these in joinery because it yields a barely visible glue line.

The scarf joint is not preferred when strength is required, so it is most often used in decorative situations, such as the application of trim or moulding. The scarf joint is commonly used in construction fit out tasks, including fitting of skirting, picture rails, dado rails or chair rails, handrails etc..

Traditionally, the scarf joint was common in boat-building and forge welding. Often, especially in shipwrights' work, the basic wedge was modified to give some degree of tensile strength to the joint. The joint was finished by bolting it or perhaps strapping it together. Iron bolts and straps were normal in carpentry while copper clench bolts were used in wooden shipbuilding. A competently made forge weld held itself together without reinforcement.

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.

Applications

• Moulding, trim other decorative applications where the material is not available in the lengths required
• Lengthening a board that has been cut too short
• Guitar headstocks, to reduce the wood required to have a tilt back headstock on a straight wooden neck.

Methods

The joint is formed by cutting opposing tapered ends on each member which are then fitted together. When working with "Wood" title="Wood">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 the tapers are feathered to a fine point which aids in the obscuring of the join in the finished work. A related joint is the hooked splice joint which resembles the scarf joint except that the tips of the tapers are not feathered in this way but are blunt.

• Hand saw
• Mitre saw
• Electric router and scarfing jig

Splice joint

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 unreinforced butt joints and have the potential to be stronger than a scarf joint. They are more visible than a scarf joint but may be preferred when more strength is required.

Splices are therefore most often used when structural elements are required in longer lengths than the available material. The most common form of the splice joint is the half lap splice, which is common in building construction, where it is used to join shorter lengths of timber into longer beams.

Applications

• Joining structural members end to end

Types of splice joints

Half Lap Splice, Bevel Lap Splice and Tabled Splice Joint

There are four main types of splice joints: Half lap, Bevel lap, Tabled, and Tapered finger.

Half lap splice joint

The half lap splice joint is the simplest form of the splice joint and is commonly used to join structural members where either great strength is not required or reinforcement, such as mechanical fasteners are to be used.

The joint is cut as for a half lap.

Bevel lap splice joint

The bevel lap is a variation of the half-lap in which the cheeks of the opposing members are cut at an angle of 5 to 10 degrees, sloping back away from the end of the member, so that some resistance to tension is introduced. This helps to prevent the members from being pulled apart.

Tabled splice joint

A wedged tabled splice joint

The tabled splice joint is another variation of the half lap. The cheeks are cut with interlocking surfaces so that when brought together the joint resists being pulled apart.

Tapered finger splice joint

The tapered finger splice joint requires a series of matching 'fingers' or interlocking prominences to be cut on the ends of opposing members. The joint is brought together and glued, with the fingers providing substantial glue surface.

This joint is commonly used in the production of building materials from smaller offcuts of timber. It is commonly found in skirting, architrave, and fasia.

The joint is usually made by machine.