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Dovetail Case Joints

Dovetail joints are among the most aesthetically pleasing joints, and if correctly constructed they can also be the strongest. The locking of the pins into the tails provides a very strong mechanical joint that has many advantages. Dovetail joints also allow for the expansion and contraction of the wood, without compromizing its structural integrity1. When joining large expanses of wood, such as for case sides, this is extremely desirable. The dovetail joint also allows woodworkers to create projects that are made entirely of wood, with no hardware visible.

The strength of the dovetail joint comes from two things: the mechanical interlocking of the pins and grooves, and the incredibly large gluing surface that the crenelated ends provide. The joint has incredible resistance to racking (bending of the joint), and even without glue the two boards can not be separated, except in the direction the pins were first inserted. This makes the joint ideal for high-stress uses such as attaching drawer fronts to the drawer cases: Every time the drawer is pulled open or slammed shut, the drawer front is undergoing extreme stresses. The dovetail joint ensures the drawer front can never separate from the case under normal usage.

The terminology of dovetails can be little confusing, and there are some variations. I will try to walk you through the basics before detailing some of the more innovative variations that are available.

Terminology

Pins and Tails

The easiest way to remember which is which is to remember that it is the pins that do the sliding. The pin slides into the triangular space between the tails on the other board. A board will usually have multiple pins across its length that fit into matching spaces between tails on the other board. The pin or tail at the very outside edge of the board is either a “half-pin” or “half-tail” since it will only have a slope on one side.

Spacing

The distance between the pins does not need to be uniform, however for visual appeal some form of pattern is highly desirable.

Angle

The angle of the pin and tail sidess must match if the pieces are to fit together, however the angle used can vary greatly. For most standard applications, an angle from 8º to 10º is ideal. If the angle is too steep, the ends of the tails are liable to break off during assembly or during the rigors of use. If the angle is too shallow, the ends of the tails do not exert enough pressure to hold in the pins, and the joint can be considered a box joint, which does not have the same mechanical benefits of dovetails.

Through Dovetails

This is the most basic form of dovetail. The end-grain of both pins and tails are visible from the outside surfaces of the joint since the pins go completely through the tails spaces, and the tails reach through the pins.

Decorative Dovetails

Decorative dovetails add even more visual appeal to the beautiful dovetail by varying the spacing, length, or shape of the pins and tails. These can be hand-cut, or they can be made using one of many router jigs. Some of the jigs available can even make exotic shaped pins.

Mitered Dovetail Shoulders

Instead of using a standard half-pin and half-tail at the edge of the board, the half-pin and half-tail can be mitered to give the finished joint a clean mitered look when viewed from the edge, instead of the butt joint appearance it normally has.

Half-Blind Dovetails

Half-blind dovetails can only be seen from the side. This is because the pins are not cut completely through.

Traditionally, half-blind dovetails were cut by hand, though most are now cut using a router. Because of this, half-blind dovetails bear the stigma of being a machined joint, and even if the craftsman put in hours of work to hand-cut the joint, viewers would be hard-pressed to tell the difference. The evidence is undeniable prior to assembly though: routed half-blind dovetails have rounded slots between the pins and one side of the tails are rounded. Hand-cut half-blind dovetails have squared angles everywhere.

Blind Dovetails

Full blind dovetails give all the mechanical strength of dovetails, without their outward appearance. The joint effectively appears as a mitered corner. This is achieved by not cutting the full depth on the pins or tails.

Blind Dovetails With Lap

A slight variation on the blind dovetail is to create a lap on the tail board. The pin board is made as a half-blind pin board would normally be made, but the tail board is created with a lap that extends to cover the full end of the pin board, instead of being mitered as a full-blind dovetail would.

Sliding Dovetail

This joint is used to attach the end of one board to the middle of another board. Its advantages are that it allows for the removal of the sliding board, mechanically locks in the sliding board, and allows for natural wood movement.

The end of the sliding board receives a tongue with angled sides, and a matching slot is cut into face of the other board.

Tapered Sliding Dovetail

A variation of the standard sliding dovetail is the tapered sliding dovetail. The groove is slightly tapered, being wider where the tongue is inserted, and narrower at the far side. The dovetail tongue also receives a matching taper, leaving the tongue slightly thinner on the side that is inserted first into the slot. If this taper is accurately cut and matched on both pieces, the sliding dovetail produces a joint that has mechanical locking, and is held together tightly by the compression of the taper.

This joint is notoriously difficult to calculate how far the sliding board will travel before being stopped by the joint’s taper. It is therefore recommended that you cut the sliding board slightly wider than required, then cut the dovetail tongue and the groove, test fit the pieces, then cut the sliding board to the size measured on the assembled project.

Sliding Half-Dovetail

This is identical to a sliding dovetail joint however the slope is only on one side of the tongue and slot. In order to provide mechanical strength, the side of the board that receives the slope must be the same side of the board that will receive the force. For example, if a sliding half-dovetail is used on a horizontal shelf, books and other items are placed on the top of the shelf, and therefore the top of the tongue and top of the slot should receive the slope. Which side of vertical boards should receive the slope is slightly more difficult to determine, however a basic rule of thumb is that the slope should be on the side that is towards the center of the project if it is to provide maximum support.


1 Only if properly constructed. If both pieces have no grain like MDF or plywood, this is not an issue. For wood with grain, the matching dovetails must be made on matching sides with the same direction of grain. For example, end grain tongues matching to end grain grooves, or long grain tongues matching to long grain grooves. If you mix the grain directions (end grain tongues to long grain grooves), the expansion of one piece will be perpendicular to the expansion of the other piece, and the joint is liable to fail (separate or crack). Keeping the grain directions matched will prevent this.

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Dado Case Joints

A dado is simply a rectangular groove that is cut across the grain of a board. The joint formed by placing an intersecting board into that groove.

The dado joint allows the load on the board to rest along the full length of the dado, thereby giving it considerable load-bearing capacity. Because the end of the board is entirely encased by the dado’s sides, the board can not cup or tilt. The dado joint does not offer any protection against the shelf pulling out of the side unless glued or fastened in some manner, and because the joint involves end grain, the gluing strength is limited.

The depth of the dado has only minimal impact on the strength of the joint. The joint is designed to support a load or to control the movement of one board relative to another. If the joint must prevent the shelf from separating, then a sliding dovetail would better suit your purpose. With that in mind, as little as 1/8″ is adequate for hardwood shelves, 1/4″ for manufactured woods (plywood, mdf, particleboard, etc.)

The depth of the dado is how deep the groove is, or the distance from the face of the board to the bottom of the dado. The width of the dado is the distance between the two sides of the dado.

Through Dado

Dados that extend from edge to edge are referred to as through dados. These are the easiest form of dado to construct, however the dado is visible when viewed from the edge. This can be concealed by applying a face frame or other trim after the joint has been constructed.

Stopped Dado and Blind Dado

Stopped dados start at one edge but stop before they reach the other edge. Blind dados both begin and end shy of the edge. The benefit of these options is that the dado itself is not visible from one (stopped) or both (blind) sides. The board that fits into the dado must have one or both ends notched, and the notch should be oversized so the board has a bit of play when it is fitted in the dado to allow you to adjust the board flush with the case edge. Wood movement is not an issue since the grain direction is identical.

Dado-And-Rabbet

This joint has is formed by one board having a narrow dado, and the mating board having a rabbet cut to form a tongue which is sometimes called a barefaced tenon because the tenon is flush with one of the board’s faces.

For shelves it is recommended that the rabbet be cut so the remaining tongue is at the bottom of the board. This will increase the load the shelf can support. If the tongue was at the top of the board, the board would have a tendancy to split if overloaded.

If used to join case sides to case tops or bottoms, you have a choice for orientation. If you have the dado on the top/bottom board and the rabbet on the vertical (tongue flush with inner face for strength), then the joint will resist the vertical board from separating, but the joint will be visible from the side. If you construct the joint with the dado on the vertical board and the rabbet on the horizontal board (tongue flush with the bottom face for strength), the top of the case can support more load, however it does nothing to hold the side, but the joint is only visible from the top.

If you stop the dado before it reaches one edge and trimming the tongue to match, you prevent the joint from being visible from one edge. If you do the same for both edges, it becomes a blind dado-and-rabbet. These variations offer the benefit of hiding shrinkage and gaps.

Dado-And-Tongue or Tongue-And-Dado

Essentially a dado-and-rabbet with the tongue no longer flush with one face, the tongue-and-dado can better resist racking because it has two shoulders.

If you stop the dado before it reaches one edge and trimming the tongue to match, you prevent the joint from being visible from one edge. If you do the same for both edges, it becomes a blind dado-and-rabbet. These variations offer the benefit of hiding shrinkage and gaps.

Dado-And-Spline

This joint is ideal for MDF and particleboard since they do not have any grain, and therefore do not lose anything by having the tongue formed by a spline instead of being an integral part of the board.

This joint is essentially a dado-and-tongue, however the tongue is formed by a spline that is inserted at glue-up time. The spline used should be at least as hard as the wood it is supporting, otherwise the spline will become the weakest part of the joint. The spline should extend into approximately 1/3 of the side’s thickness, and it should extend about twice that distance into the horizontal board. The spline should not extend much further than this into either piece because it risks weakening the boards . If the spline does not extend far enough into the boards, the spline will not provide enough holding strength to support the shelf load. Also, the spline should be located below the center of the shelf board since it can then support a heavier load before the board splits.

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Corner Block Case Joints

Corner block joints are intermediaries. Instead of connecting two boards directly together, each board is connected to the corner block instead.

The corner block does not offer any structural improvements over other joints, however it does offer some design possibilities that might otherwise be unavailable, especially if the boards being joined together are manufactured woods such as plywood or veneered, where you might wish to hide the edges of the boards. The corner block can be molded or shaped, can be made of a different material or color than the main boards, or can even be a different thickness than the boards which are connected to it, possibly producing a proud corner.

The most common material for the corner block is solid wood. Though manufactured wood can be used, the shaping and stylistic benefits of corner blocks are lost since the surface of the block would still need to be covered in some way.

If a solid wood corner block is used, the proper orientation of the grain, relative to the case sides, is crucial. Misaligning the grain directions can cause joint separation and failure, and may even damage the wood of the case sides.

If the case side panels are made from manufactured material (such as plywood, MDF or particleboard), the block’s grain should be aligned parallel to the direction of the board’s edge (regardless of the direction of the grain on the surface veneer on plywood). This is because manufactured boards will not experience any significant movement due to seasonal moisture content fluctuations, but the hardwood corner block will. By aligning the corner block’s grain to run along the edge of the case panel, the expansion will not run along the length of the panel, and so it will have no effect. If the grain were to run in any other direction (out from the edge of the panel instead of along the length of the edge), the seasonal movement of the corner block would make it wider or narrower than the panel it is joined to, and the joint would fail.

If the case side panels are made from solid wood, the grain of the corner block must run in the same direction as the grain in the panels. Aligning the corner block in any other direction will cause the joint to fail due to seasonal wood movement. Note that this also means that the grain on the two panels being joined (through the corner block) must also have their grain running in the same direction.

Corner Block With Tongue-And-Groove

The tongue can be formed on either the side panel or the corner block, but it is often easiest to place the tongue on the corner block since forming details on the ends of long side panels can be difficult. If the corner block is used to join solid wood panels, it is recommended that the grain on the corner block run diagonally, from one tongue to the other. This is to provide maximal strength to each tongue so it can better resist the shearing force once it is mated with the panels. If the corner block is being used with manufactured boards, the block’s grain is running parallel to the edge of the panel and this is not a concern.

Corner Block With Splines

A full-length, stopped, or blind spline can be used to attach the corner block to the side panel. This eliminates the complexity of forming tongues on the narrow corner blocks, and it can also be accomplished with just one setup of the tablesaw or router table.

Corner Block With Biscuits

Instead of using long splines, biscuits can be easily substituted.

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Butt Case Joints

Butt joints are one of the weakest forms of case joints, but they are the easiest and quickest to make. A butt joint is achieved whenever you join the straight and square end grain of one board to face grain of another board. End grain is notoriously difficult to glue or fasten, and butt-joints present the smallest amount of surface area to be used for gluing. In order to achieve any significant strength and sturdiness, you must reinforce the joint.

Glued Butt Joint

As its name implies, the glued butt joint is held together by the force of the glue adhering to the end grain of one board and the face grain of another. The face grain of most wood species providing a very good glue bonding surface. The end grain, however, is notoriously weak and is prone to failure.

Butt Joint with Glue Block

Glue blocks are square or triangular wedges of wood that are glued to the hidden side of the joint. These blocks can run the full length of the joint, or only be used in strategic locations. One problem with case joint glue blocks, though, is that their constructions usually has the glue block’s grain oriented perpendicular to the case boards’ grain, resulting in a cross-grain construction. This can cause problems when wood movement is involved.

Butt Joint with Fasteners

Nailed Butt Joint and Screwed Butt Joint

Nails or screws can be used to fasten a butt joint. Inserting the nails or screws at alternating angles strengthens the joint and prevents the boards from separating. In softer woods that have trouble holding screws or nails in the end grain, you can reinforce the end grain portion with a strategically placed dowel which provides enough cross-grain to increase the holding power of the screws.

Dowelled Butt Joint

Dowels are a decorative replacement for nails or screws. Again, they can be inserted in alternating angles, increasing their holding power. Blind dowels can be created by using stopped holes on the face grain board, however these holes are difficult to produce without the aid of specialized tools, since doing them by hand or even a drill-press is prone to uneven hole depths, or mis-aligned holes.

Using a contrasting wood type for the dowel can produce an attractive decorative element to the workpiece.

Biscuited Butt Joint

Biscuits serve two purposes in case joints: They provide positive registration between boards during assembly, and they provide a mechanical brace against shifting in the finished product. Biscuits are essentially thin oval wafers that are inserted and glued into a matching pair of slots that are cut into the two pieces to be joined.

When the biscuits are used at the top of an upright board, offset the location of the slots slightly by 1/4 to 1/8 of the upright’s thickness towards the hidden side of the vertical. This is because if the joint should fail, the damage done to the wood will be in a hidden location, and more easily repaired invisibly. If the biscuit were placed in the center, there is an equal risk that the damage would be exposed.

When the biscuits are used to secure a horizontal shelf to an upright member, the biscuits should be offset slightly by 1/4 to 1/8th of the shelf’s thickness towards the bottom of the shelf. When the shelf is loaded with weight, the biscuit will be applying force towards the top of the shelf. By shifting the biscuit’s location downwards, you thicken the amount of wood that is resisting that force.

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Box Case Joints

Box joints are the “poor man’s dovetail joint”. It has relatively good mechanical strength, but does not have the interlocking capabilities given by the sloping of the dovetail pins and tails. The box joint does still provide an incredible amount of gluing surface, making it a strong joint.

The basic box joint is formed by creating a regular series of slots and fingers, the length of which is the same as the stock’s thickness. It is recommended that the width of the fingers also be the same as the stock’s thickness since this gives the optimum strength for the least amount of cutting. Having thicker fingers actually reduces the gluing surface, weakening the joint. Having narrower fingers, however, can dramatically increase the gluing surface, at the cost of more labour to construct the joint.

In terms of aesthetic appeal, the box joint does suffer somewhat by its patterened appearance, but this can be overcome by some creative alternatives.

Decorative Box Joint

The standard box joint has all its fingers and slots the same width. The joint can be given some visual excitement by varying the width and spacing of the fingers that make up the joint. This does increase the difficulty in cutting the joint, and measurements must be carefuly made because the joining boards will no longer have identical setups.

Angled Box Joint

If the joint being formed does not conform to a 90º angle, an angled box joint can be created. This is accomplished by placing the board on the required angle while passing it over the tablesaw/router while creating the slots. Just remember to keep the same side “forward” as you pass it over the cutter in order to create a matching angle.

Half-Blind Box Joint

When you want the box joint to be visible from only one side, you can take advantage of the half-blind box joint. This is formed the same way as the half-blind dovetail, but is considerably easier to cut, and it can even be done on a tablesaw.