Woodworking joints

A worker uses a mortising machine to shape timber framing joints
Joinery is a part of woodworking that involves joining together pieces of wood , to produce more complex items. Some wood joints employ
fasteners, bindings, or adhesives, while others use only wood elements.

The characteristics of wooden joints – strength, flexibility, toughness,appearance,etc,derive from the properties of the joining
materials and from how they are used in the joints.

Therefore, different joinery techniques are used to meet differing requirements. For example, the joinery used to build a house is different from that used to make puzzle toys, although some
concepts overlap.

List of wood joints

1. Butt joint
The end of a piece of wood is butted against another piece of wood. This is the simplest and weakest joint.

2. Bridle joint
Also known as open tenon, open mortise and tenon, or tongue and fork joints, this joint is where the through mortise is open on one side and forms a fork shape. The mate has a through tenon or necked joint.

Bridle joints are commonly used to join rafter tops, also used in scarf joints
and sometimes sill corner joints in timber framing.

3. Dowel joint
The end of a piece of wood is butted against another piece of wood. This is reinforced with dowel pins. This joint is quick to make with production line machinery and so is a very common joint in factory-made furniture

4. Miter joint:

Similar to a butt joint, but both pieces have been bevelled (usually at a 45 degree angle). 

5. Finger joint

A corner joint with interlocking fingers. Receives pressure from two directions.

6. Dovetail joint
A form of box joint where the fingers are locked together by diagonal cuts. More secure than a finger joint.

7. Dado joint
Also called a housing joint or trench joint, a slot is cut across the grain in one piece for another piece to set into; shelves on a bookshelf having
slots cut into the sides of the shelf, for example.

8. Groove joint
Like the dado joint, except that the slot is cut with the grain.
9. Tongue and groove
Each piece has a groove cut all along one edge, and a thin, deep ridge (the tongue) on the opposite edge. If the tongue is unattached, it is considered a spline joint .

10. Mortise and tenon
A stub (the tenon) will fit tightly into a hole cut for it (the mortise). This is a hallmark of Mission Style furniture, and also the traditional method of jointing frame and panel members in doors,windows, and cabinets. This joint is a good strong joint to use.

11. Birdsmouth joint
Also called a bird’s beak cut, this joint used in roof construction . A V-shaped cut in the rafter connects the rafter to the wall-plate.

12. Halved joint
A joint in which the two members are joined by removing material from each at the point of intersection so that they overlap.

13. Splice joint
A joint used to attach two members end to end.

14. Pocket hole joinery
A hidden screw is driven into the joint at an angle.
Traditional ways of improving joints

A doweled joint
Dowel : A small rod is used internal to a joint both to help align and to strengthen the joint.

Traditional joints are used with natural timbers as they do not need any other materials other than the timber itself. for example: Butt joints.
Dowel joints are also useful for pegging together weaker, cheaper composite materials such as laminate-faced chipboard, and where limited woodworking tools are available (since only simple drilled holes are needed to take the dowels).
Nontraditional ways of improving joints

Biscuit joints: A small ‘biscuit’ is used to help align an edge or butt joint when gluing.

Domino joiner : A trademarked tool similar to a biscuit joiner, where a piece larger than a biscuit has some of the advantages of dowels,
and some of the advantages of biscuits.
Many wood joinery techniques either depend upon or compensate for the fact that wood is anisotropic : its material properties are different
along different dimensions.

This must be taken into account when joining wood parts together, otherwise the joint is destined to fail. Gluing boards with the grain running perpendicular to each other is often the reason for
split boards, or broken joints. Furniture from the 18th century, while made by master craftsmen, did not take this into account.

The result is this masterful work suffers from broken bracket feet,
which was often attached with a glue block which ran perpendicular to the base pieces. The glue blocks were fastened with both glue and nails,
resulting in unequal expansion and contraction between the pieces. This was also the cause of splitting of wide boards, which were commonly
used during that period.
In modern woodworking it is even more critical, as heating and air conditioning cause major changes in the moisture content of the wood. All woodworking joints must take these changes into account, and allow for the resulting movement.

Strength of Wood
Wood is stronger when stressed along the grain (longitudinally) than it is when stressed across the grain (radially and tangentially). Wood is a natural composite material; parallel strands of cellulose
fibers are held together by a lignin binder. These long chains of fibers make the wood exceptionally strong by resisting stress and spreading the load over the length of the board. Furthermore, cellulose is tougher than lignin, a fact demonstrated by the relative ease with which wood can be split along the grain compared to across it.
Different species of wood have different strength levels, and the exact strength may vary from sample to sample.

Dimensional stability

Timber expands and contracts in response to humidity , usually much less so longitudinally than in the radial and tangential directions. As
tracheophytes, trees have lignified tissues which transport resources such as water, minerals and photosynthetic products up and down the plant.

While lumber from a harvested tree is no longer alive, these tissues still absorb and expel water causing swelling and shrinkage of the wood in kind with change in humidity.

When the dimensional stability of the wood is paramount, quarter-sawn
or rift-sawn lumber is preferred because its grain pattern is consistent and thus reacts less to humidity.

Materials used for joining

Metal plates are often incorporated into the design where the timber alone would not be strong enough for a given load. Pin-connected post and beam house framing

Joints can be designed to hold without the use of glue or fasteners; a pinned mortise and tenon is an example of this.
Glue is highly effective for joining timber when both surfaces of the joint are edge grain. A properly glued joint may be as strong or
stronger than a single piece of wood. However, glue is notably less effective on end-grain surfaces. Animal glue is soluble in water,
producing joints that can be disassembled using steam to soften the glue.

Various mechanical fasteners may be used, the simplest being nails and screws . Glue and fasteners can be used together.

 

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