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Tongue-And-Groove Edge Joints

The tongue and groove joints offer a means of registering the joint edges during assembly. They are often used without any glue, allowing the boards to expand and contract without any negative effects. As long as the contraction of the board is less than the length of the tongue, the joint will not be exposed, and the panel will retain its intended appearance.

The joint is formed by having one piece having a groove, or slot, cut the length of the edge. This groove is most often one third of the wood’s thickness and is placed in the center of the edge, producing two walls of wood that are the same thickness. The other piece has the sides of the stock removed, leaving a tongue that is precicely the width of the groove formed on the first piece.

The recommended length of the tongue depends on the width of the stock used to form the panel. For panels formed with stock less than 3 inches wide, the tongue length is not that much of a factor. For these panels, the tongue only needs to be as long as they are thick. This will produce a tongue that appears square when viewed from the end. For panels that are formed with wider stock, it is recommended that you make the tongue’s length at least half the stock’s thickness.

The groove should ALWAYS be slightly deeper than the tongue is long, by as much as 1/16″ for 3-inch wide boards. The reason for this is two-fold. First is to prevent problems during assembly. If the tongue length is cut exactly to the groove depth, then the slightest piece of sawdust or imperfection in the wood will keep the two pieces from mating properly. The second is because of the effects of seasonal expansion and contraction. If one panel expands at a slightly different rate than its neighbour, the tongue from one piece can actually push its neighbour away, and break the joint.

When all the tongue and groove boards in a panel are assembled, there is often a slight difference in height between the panels, or the panels may separate slightly due to seasonal changes, and that can produce an effect that is undesireable for some people. In those cases, you can add a tiny bevel on the edge of every board. This will produce a v-groove effect between each board, and it will camouflage the uneven height, at the expense of having a visible groove.

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Shiplap Edge Joints

Shiplap joints are formed by cutting identical rabbets into opposite faces of adjoining boards. This produces a joint where the rabbets overlap, preventing gaps between the boards from being visible. Shiplap joints are often referred to as a poor-man’s tongue-and-groove since the visual effect is very similar to tongue-and-groove, however less work is involved.

The area where shiplap joints are inferior to tongue-and-groove, though, is that shiplap joints do not keep the boards flush with one another. However if the boards are to be fastened at regular intervals along their length and the wood is relatively stable, shiplap joints can save you much time.

To hide any uneven boards, you can add an edge detail like a chamfer of bead to the end of each board, or you can cover the exposed edges with a decorative molding.

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Routed Edge Joints

A router can greatly speed up the process of preparing edges for joining, however much effort can also be wasted unnecessarily. As has been stated, a simple glued joint is very strong, and is sufficient in most situations.

Where a router excels, though, is for creating interlocking joints which also increase the wood surface to be glued.

Glue Joints

These router bits give a simple tongue-and-groove style interlocking edge that increases the gluing surface and when the boards are mated together, positively align the boards to be glued. These bits are only applicable when the edges to be joined are exactly the same thickness. With the router bit adjusted to the correct height, first rout one of the edges face up, then rout the second panel with the face down. The pieces can now be joined edge-to-edge with both faces in the same direction.

Finger Joints

These router bits produce a joint that is similar to the glue joint, however it has many more projections. It is produced in the same way as glue joints are. The advantages of finger joints is that the surface area in the joint is increased, increasing the gluing surface.

Lock Miter Joints

A variety of router bits are available that produce with one (sometimes two) passes through the router, a joint that locks together with another edge, giving a more mechanical connection that a simple glued joint could provide. These joints are often used for edge-to-face joints or edge-to-edge joints. The edge-to-edge joints are produced in the same manner as glue joints above, however the edge-to-face joint has the second piece routed in a vertical position, with the profile being produced on the board’s face instead of the edge. Care should be taken to correctly orient the boards before routing.

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Rabbet Edge Joints

A rabbet is formed by removing a section of wood from the edge of a board. When you place another board into this recessed section, a rabbet joint is formed.

Single-Rabbet Joint

This joint is produced when only one of the pieces has a rabbet. Traditionally, rabbet is placed on the back side of the face that is the most visible. This produces a joint that has a clean and undisturbed face, and the small amount of edge grain is only visible from the lesser-viewed side.

One option to camouflage this end grain is to add a chamfer to the edge. By making this chamfer the exact depth of the exposed end grain, the end grain is beautifully camouflaged. A stopped chamfer, as shown in the photo to the right, adds further aesthetics to the joint.

Another option to camouflage the end grain is to actually reveal more end grain! By cutting the length of the rabbet slightly shorter (by the width of the exposed end grain), the corner produced is attractive and can be emphasized with a contrasting stain or paint.

Double-Rabbet Joint

A double-rabbet joint is one in which both boards to be joined receives a rabbet. If both boards to be joined are exactly the same thickness, this type of joint can be faster and easier to produce since the rabbet is a consistant depth, and every board receives an identical rabbet.

This type of joint resembles a shiplap joint, but with the length of the rabbet being exactly 1/2 the stock’s thickness.

Rabbet-And-Groove Joint

A double-rabbet joint is one in which both boards to be joined receives a rabbet. When the two pieces are joined together, the two pieces are effectively locked together and this provides some additional strength to the joint against racking. The second rabbet does not need to be large, and even a single saw-kerf is sufficient to produce the desired strength.

This joint can also be accentuated by various end grain details to hide the separating line between end grain and face grain.

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Mitered Edge Joints

Edge miter joints offer the ability to completely hide the end grain of the joined pieces, providing a smooth, clean surface to view. The largest problem with miter joints is that they are very difficult to assemble. There are, however, some variations that offer some assistance in assembly.

Glued Edge Miter Joints

Edge miter joints actually expose more wood surface, and therefore the glued joint produced can actually be stronger than a glued butt joint. If more strength is required, you can use small glue blocks, or a continuous glue block. Cut these blocks so the grain runs in the same direction as the pieces they are going to join, so that the two boards and the block expand and contract identically.

Fastened Edge Miter Joints

Using some sort of mechanical fastener such as screws or nails can hold edge mitered joints together, however this can ruin the esthetics of a piece, unless the fastener can be painted over or camouflaged.

Biscuited Edge Miter Joints

Using biscuits in combination with an edge miter joint is an ideal way to have an easy assembly process while having the beauty of nearly-invisible miter joints. The biscuits don’t make the joint stronger (in fact, they can weaken it), however this is a small price to pay for side-stepping the frustration of assembling edge miter joints.

When the joint is assembled, the fit of the biscuits will determine how well the joint keeps its shape during glue-up. By choosing a biscuit that has a snug fit into the biscuit slot, you reduce the sideways movement that can lead to misalignment under clamping pressure. You can improve this by placing the biscuits even closer, as close as 6 inches if required.

Splined Edge Miter Joints

A full-length spline is a convenient substitute for a biscuit jointer, and can even be more accurate. A hardwood spline is ideal, but if you intend to use a hardboard spline, measure its thickness before making any cuts: hardboard is often narrower than its stated dimension. Cut the slot into both pieces of wood no more than 1/3 in from the inside corner. Cutting it any closer to the point will leave very little wood intact to give mechanical strength to the joint.

Remember, a spline will not increase the strength of the joint, however it can make the joint weaker. Its primary purpose is to provide positive registration for the assembly and clamping process

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Face-To-Face Butt Edge Joints

These joints are the simplest of all edge joints. Their primary purpose is to produce wood stock that is thicker than can be economically purchased by gluing together two or more flat boards.

The key to successful face-to-face joints is to have the surfaces flat and clean. When the two pieces are glued together, any voids between the pieces may be visible at the edge, and will reduce the effectiveness of the glue holding them together.

An important precaution to note is that attempting face-to-face joints with different wood species is a recipe for disaster, and should only be attempted when attaching a thin layer to a heavy/stable layer. An example would be to apply a veneer to a plywood base. This is because the different species of wood will expand and contract at different rates, and will cause the wood to warp or ripple.

No mechanical fasteners or fancy joints are required for a successful face-to-face joint.

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Edge-To-Face Butt Edge Joints

The joints used to connect the vertical components of cases, cabinets and furniture require strength and often a measure of ease of assembly. The mating edges are long grain to long grain and are therefore present the strongest joints. And since the grain of the mated surfaces run parallel to one another, there is no worry regarding cross grain joint instability.

Glued Edge-to-Face Joint

This joint is the simplest to form, since all you require are two pieces that are butted together. If the exposed end grain contrasts significantly with the exposed face of the glued-on piece, the appearance may suffer, but for most woods this will not be much of an issue.

Care must be taken to carefully align the two pieces, since any misalignment or warping in the woods can reveal a slight lip along the edge. This can either be planed or sanded off, or it can be hidden by adding a v-groove created by adding a bevel to the edge of the two pieces.

Fastened Edge-to-Face Joint

Fasteners can be used instead of, or in addition to, glue. Clamping may only be required until the fasteners are attached, after which the clamps can be removed. The fasteners will provide enough clamping force to hold the joint until the glue, if applied, can cure.

Note that for most solid wood joints, mechanical fasteners are not required.

Biscuited Edge-to-Face Joint

Biscuits offer the benefit of aligning pieces during the assembly process. Well aligned biscuit slots can produce a project that is quick and easy to assemble. Mis-aligned slots, however, can lead to raised edges that may be troublesome to remove or sand smooth.

Once again, please note that biscuits do not offer any mechanical strength. Their only purpose is to aid in the assembly of the project. In fact, biscuits are weaker than most woods they are used in joining.

Splined Edge-to-Face Joint

A splined joint is essentially a full-length biscuit. Both parts have a groove that accepts a spline. Variations are a through spline, where the spline is visible when viewed from the end of the wood, or a blind spline, where the spline slot has been cut short, and no spline is visible from the end of the joint.

Splines do not offer much strength, however they can be a great aid in speeding the assembly of a project.

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Edge-To-Edge Butt Edge Joints

Many woodworkers find it difficult to believe, however the simple glued edge joint is remarkably strong. Having a good clean and flat edge mating with another similar flat edge produces a joint that is just as strong as the surrounding wood.

The use of joining devices such as splines, biscuits, dowels or keys can actually weaken some joints especially when the material of the device is weaker than the wood to which it is attached. Though they do not impart any strength to the joint, these devices do offer one important advantage: They give the woodworker a handy alignment device that can speed assembly, or maintain alignment during the gluing and clamping process.

Biscuited Edge-to-Edge Joint

Biscuited joints use a series of small slots cut into the edges of both boards to be joined, into which small oval or disk shaped piece of wood or other fibre which are placed. The biscuits maintain an almost perfect surface alignment between the two pieces of wood, yet allow for end-to-end movement between the two pieces. This end-to-end movement helps to make up for slightly mis-aligned slots in adjoining pieces.

Use the largest biscuit that will fit the slot, since a loose biscuit will allow the two wood pieces to drift. It is the stiffness of the biscuit in the slot that gives the alignment assistance that you are looking for.

Splined Edge-to-Edge Joint

This is a variation of a biscuit joint. Instead of using numerous smaller biscuits, splines are essentially a single long biscuit used to align the two pieces. The spline is a piece of plywood or hardboard that is placed in a slots that are cut in the adjoining edges. These slots can be stopped so they do not show if the ends are to be exposed. One thing to keep in mind is to ensure the spline is slightly narrower than the depth of the slot. Making the spline exactly the depth of the slot can lead to splitting of the wood as the surrounding wood shrinks, but the hardboard spline does not (hardboard does not shrink in the same manner as hardwood). A gap of 1/32 (1/64th on either slot) is sufficient to prevent this problem.

Dowelled Edge-to-Edge Joint

Dowelled edge-to-edge joints are another variation on the biscuit joint, but use dowels instead of biscuits. The dowelled joint is prone to errors associated with the difficulty in drilling perfectly perpendicular holes in the narrow edge of panels. This problem can be managed through the use of specialty drilling rigs that drill the alignment dowlel holes in a regular pattern, but these machines are beyond the reach of most hobyists, and other simpler techniques produce similar or better results. As with splines, ensure that the holes drilled for dowels are slightly deeper than the dowel is long.

Butterfly Key Edge-to-Edge Joint

Butterfly keys are a traditional japanese jointing technique, and it provides a very decorative way to edge-join boards. It adds visual appeal to the joint, and depending on the size of the key, the key can be used without adding glue to the joint which offers the possibility of disassembling the boards.

The keys themselves can be made of wood, or even from certain metals such as brass. Note that certain metals are not suitable for use because they will rust in the presence of the natural moisture in the wood.

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Butt Edge Joints

The three cornerstones of edge joints, commonly referred to as “butt joints” are the Edge-To-Edge, Edge-To-Face, and Face-To-Face.

Each of these joints are formed by when each piece of wood is machined straight and square before being fixed to its mate.

  • Edge-to-edge – Where the narrow surface of one piece is joined to the narrow surface of another piece. These joints are commonly used to create wide boards out of smaller pieces.
  • Edge-to-face – Where the narrow surface of one piece is joined to the wide surface, or face, of another piece. These joints are often called corner edge joints since they are used in connecting the columns or vertical components of cabinets to each other.
  • Face-to-face – Where the wide surface, or face, of one piece is joined to the face of another piece. These joints are primarily used to create thicker pieces of wood from thinner stock. You would use these joints to form bedposts or thick table legs.