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Slip Frame Joints

Slip joints are essentially open-ended mortise-and-tenon joints, and resemble simple lap joints. Essentially, they are formed by creating a slot along the end of one board, the same width as the tenon of the other board which is formed normally.

The mortise slot is relatively easy to make on a tablesaw using a mortising jig, or even an appropriately-configured router. Traditional mortises require the use of a drill-press or hand-chisel. One disadvantage of this joint is that the cheeks must be clamped together during glueup, in addition to the regular clamping required to keep the mortise against the shoulder of the tenon.

Tapered Slip Joint

When applying a veneer to the face of a frame, the joint line where the grain changes direction may move at different rates, and may eventually cause damage that will be visible in the veneer. To prevent this, the tenon is cut so it is almost full-thickness at the shoulder, and slightly narrower than normal at its end. The slot is tapered to match the tenon. The purpose of this construction is to reduce the amount of wood movement at the transition point where the two grain-directions meet, and distribute the stress over a wider area. This is usually sufficient to prevent any visible damage to the veneer surface.

Mitered Slip Joint

A slip joint with the appearance of a mitered joint can be made by first mitering the slot of the slip joint, then by cutting the tenon on an angle to match the miter so the shoulders have a mitered appearance, but the tenon itself ends square. A blind mitered slip joint can be constructed by stopping the mortise before it becomes a complete slot, and trimming the tenon to match.

Bridle Joint

A bridle joint is formed by creating a groove on both sides of a board and inserting it into the slotted end of an open-ended slip joint, giving the appearance of a T-joint. This joint offers great load-carrying capacity, as the vertical member can support the weight of the crossing board. To hold maximum weight, the slot should be two-thirds of the board’s thickness. For maximum twisting strength, the slot should be one-half of the board’s thickness to make the (total of the) slip’s walls and the middle tenon the same thickness.

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Mortise And Tenon Frame Joints

Mortise-And-Tenon joints are an extremely old construction technique that has stood the test of time and is still being used today. Examples of this ancient joint is found in Egyptian furniture thousands of years old. It can produce joints that are extremely strong, and the technique can be scaled up or down in size with great success.
The joint has two basic components: a mortise which is essentially a hole cut into one board, and a tenon which is a tongue that is shaped onto the end of another board which will fit into the mortise. The cheeks are the sides of the tongue or mortise, and the shoulder is the portion of the tongue-board that rests against the mouth of the mortise board. The length of the tenon is the distance from the end grain to the shoulder. The width of the tenon is the distance from tenon edge to tenon edge. The thickness of the tenon is the distance from tenon face to tenon face. The mortise depth is the distance from the mouth of the mortise to the bottom of the mortise. The width of the mortise is sized to receive the tenon’s thickness. The mortise length is sized to accomodate the tenon’s width.
A very important consideration for the design of mortise and tenon joints is wood movement. By its very nature, mortise-and-tenon joints involve cross-grain joinery, which introduces the risk of joint failure due to seasonal wood movement. To accomodate for this, various factors should be considered when designing the joint to optimize the joint strength.

Tenon Length

If the tenon is to be full-length, it should be wedged from the exposed side. If it is not to be visible, it should extend approximately half-way into the stile, or three-quarters if the stile is narrow.

Tenon Width

The optimal tenon width is the full width of the rail, however this leads to problems with wood movement for wide rails. Multiple tenons, equally spaced, produce a design that retains the tenon’s strength yet resists against wood movement by distributing the stress.

Tenon Thickness

If cutting the mortise and tenon by hand, the mortise walls and the tenon should all be of equal size (one-third the stock thickness). This is to prevent accidental splitting of the boards during the construction of the mortise or tenon. If cutting the mortise and tenon by machine, very little stress is applied and therefore a tenon thickness of one-half the stock’s thickness can be used. which gives equal strength to the tenon and the mortise walls once the joint is glued.

Through Mortise-And-Tenon Joints

This is an extremely common joint that is formed by cutting the mortise completely through the stile and sizing the tenon to match, flush with the far side of the stile.
The end of the exposed tenon can be further secured by adding a very narrow slot, either diagonal or straight across the width of the tenon, then inserting a solid wedge into the slot once the joint has been assembled. This can greatly strengthen the joint’s holding power, but care should be taken to not split the joint apart by using an overly thick wedge. See wedged mortise-and-tenon joints below

Blind Mortise-And-Tenon Joints

The blind mortise-and-tenon joint gives the outward appearance of a butt joint, however has all the strength and advantages of a mortise-and-tenon joint. The mortise does not extend completely through the stile, and therefore the tenon is not visible once the joint has been assembled. This is the most commonly-seen version of the mortise-and-tenon joint in use today.

Haunched Tenon Joints

A haunch is a short tongue that protrudes from the rail’s shoulder, between the rail’s edge and the tongue’s edge. When a mortise and tenon joint is constructed with a stile that has a groove through which a tenon is cut, such as in frame-and-panel construction, the normal technique for forming a mortise and tenon would leave a void at the end of the slot, and this void would be visible on such assemblies as panel doors. In order to compensate for the slot, a haunched tenon is constructed so the haunch fills the groove at the tenon’s edge. Depending on the final use, the rail can have a haunch on only one side, or both sides, as required.

Wedged Mortise-And-Tenon Joints

Mortise-and-Tenon joints can be further strengthened by the addition of a wedge. A thin kerf slot is cut into the end of the tenon, then after the tenon is inserted into the mortise, a wedge is inserted into the slot to secure the joint. Wedged joints such as these may not even require any glue, especially if the mortise is tapered to be wider at the wedge end, so that the joint can not be pulled apart by brute force.
The slot can be cut across the tenon’s width, along the tenon’s width, or even diagonally, to provide a variety of detail, however to produce the strongest joint, the finished joint should force the tenon apart in the direction towards where there is the most wood so that the wood around the mortise won’t split under the tension. The slot should be terminated by a hole, slightly wider than the slot’s kerf, to help prevent the wood from splitting further into the rail. A good technique to use is to drill the oversized hole first, then cutting the slot just as far as the hole.
A fox-wedged tenon is a variation where a wedge is used on a blind tenon. This variation is deceptively simple, yet frustratingly difficult to execute perfectly because by its very nature, once you have begun assembly you can never disassemble it, even if the assembly goes wrong! The idea is to create a stopped tapered mortise, wider as it gets deeper yet which is only one-half to two-thirds the stile’s width. The tenon is cut square, slightly shorter than the mortise is deep. The trick is to determine how wide the bottom of the mortise actually is, and to calculate the appropriate wedge thickness so that the tenon ends press against the sides, yet allow the wedge to be inserted completely. The assembly process involves placing the wedges loosely in the slots, carefully inserting the tenon into the mortise, then forcing the tenon into the joint which will force the wedges into the slots. Once this process is begun, the tenon starts to widen, and the joint is impossible to separate successfully, but if done successfully, you are left with a very strong mechanically bound joint that will last you for many years without any visible fastener, and optionally without any glue either.

Pegged Mortise-And-Tenon Joints

As an alternative to wedges, a pinned or pegged mortise-and-tenon joint is extremely strong. After glue-up, drill one or more evenly spaced holes from face-to-face through the stile, close enough to the rail to pass through the tenon, about half-way down its length. Then glue and insert dowels or pegs. These pegs can be made of the same wood to hide the reinforcement, or they can be of a decorative contrasting wood. After the glue has set, the pegs can be trimmed flush with the face.

Loose Tenon Joints

Loose tenon joints are constructed by mortising both the side of the stile and the end of the rail, and then inserting an appropriately sized tenon during glue-up. Even though the tenon is not integral to either piece, it still creates plenty of long grain to long grain glue surface to create a very strong joint.
Mortise-And-Tenon With Stuck Molding Joints
When the stiles and rails have a decorative profile, resembling a molding, cut directly onto the wood’s edge, an option to produce a flush joint edge is to carefully chisel away the profile from the tenon area but leave a mitered section at the ends. Then form a similar miter on the tenon’s shoulder near the ends. When the two pieces fit together, the gluing surfaces will be flush and a neatly mitered joint will be remain.

Mortise-And-Tenon With Mitered Shoulders

Producing a curve on the inside corners of a frame joint involves more than simply using wider stock for the rail, and hollowing out the edges to produce the required curve. Doing so would leave tiny slivers of wood that would be prone to breakage, and would also waste an enormous amount of wood to construct.
A far better option is to produce two matching pieces that each make up half of the curve. The grain on each piece would be in the direction of the rail or stile to which it will be attached, and the joint between the two will be a simple miter. A simple way to accomplish this is to take a square piece of wood, the size of the curved section, cut it diagonally and then flipping one half over and then re-gluing them together. Once the glue has set, cut the board diagonally from corner-to-corner again, but between the other corners this time. This will give you two blanks that can be placed into the inside corners of the mortise-and-tenon joint and shaped at will. An even better suggestion is to shape the two (or more) corners simultaneously before attaching them so that you can ensure the curves are identical.

Double Mortise-And-Tenon Joints

For rails that are more than ten times their thickness, multiple tenons should be used. This means that a standard 3/4-inch-thick board move than 7 1/2-inches thick should be treated this way. The proportion between the tenon and the spaces between is the key, and not the actual number of tenons. It is recommended that the space be divided into thirds, two-thirds being tenon and one-third being space, equally distrubuted along the end of the rail. If the rail is of a material that is prone to cupping or warping, you can leave a short haunch between the tenons and notch the mortise to match.

Groove-And-Stub Tenon

For light-duty frames which are anchored and do not need to support any weight, the tenon can be extremely short, such as just short enough to fit into a panel groove. This joint provides long grain to long grain, but it is very minimal.

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Lap Frame Joints

Extremely easy to form, lap joints also have the benefit of being extremely strong. By notching one board and placing it another board within the space, the walls of the notch prevents the other board from twisting free. Also, the gluing surface is entirely long grain to long grain, producing a very strong bonding surface to complemetn the mechanical interlocking of the joint.

By its nature, lap joints result in joints that form an L, X, or T, however there are many variations on how to accomplish this.

The depth of the joint is the depth of the notch measured from the face of the board to the bottom of the notch. When a lap involves the end of a board, the width is the width of the board. The length is the distance measured from the end of the board (the end grain), along the cheek, to the shoulder. For the portion of a lap that occurs in the middle of a board, the width is measured along the direction of the grain, the cheek is the bottom portion of the notch, and the shoulders are the two sides of the notch.

Half-lap joints are formed by both mating pieces receiving identical-depth notches, usually half the depth of the boards being joined.

Full-lap joints are formed by having only one thicker board receiving a notch the entire depth of the second board. The second board sits completely within the notch.

End Lap Joints

These joints are formed by notching the end of two boards, then overlapping the ends so the cheeks meet and the shoulders are square to the cheeks. It is essential that the cheeks be flat or the joint will not mate properly, which will weaken the joint.

T-Lap Joints

These joints involve one board receiving a rabbet at one end, and another board receiving a notch somewhere mid-section.

Cross-Lap Joints

These joints involve both boards receiving a notch somewhere along its length into which one board is placed into the other, notch-to-notch. This is an extremely strong joint due to its mechanical interlocking.

Pocket Lap Joints

A pocket lap is a joint in which one board receives a rabbet at its end, but its length does not reach entirely across its mate. Its primary use is when you do not want to expose the end grain of the joint.

Mitered Half-Lap Joints

A mitered half-lap joint is one in which one board receives a normal rabbet at its end, but is then mitered to remove half the cheek. The second board would need to be measured from the first, then notched appropriately. This joint will give the appearance of a mitered joint, without any of the failings of a normal flat miter joint.

Dovetailed Half-Lap Joints

Though the standard half-lap joint provides a very strong mechanical joint, the dovetailed half-lap improves upon the design by preventing the lap from being pulled out due to the dovetail-shaped lap. To create the joint, first create the end lap one one board and trim the cheeks to a dovetail shape. Once done, simply transfer the shape to its mate and notch it accordingly.

In addition to its added strength, this joint benefits from its dovetail-like appearance.

Dovetail-Keyed Half-Lap Joints

This variation turns the end lap into a dovetail pin instead of the tail. The joint not only resists racking, but it also prevents one board from being twisted free from the other. The only direction the board can possibly be forced is out the direction it was inserted.

To produce this joint, a normal lap is created at the end of a board, then the sides of the lap are slanted to be wider at the cheek, and narrower at the face of the board. Again, keeping the standard dovetail angle of 8° to 12° is optimal for strength. Once the end lap has been created, the notch can be formed with matching angles. The least error-prone method to create this joint is to use a router to create both the end lap sides and the notch, then trimming the shoulder with a chisel.

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Flat Miter Frame Joints

Miter joints are beautiful to behold when properly constructed. There is no visible end grain, and the seam where the two boards are joined produces a gentle change in grain direction that can be very pleasing.

Miter joints have several disadvantages to consider, some of which can be countered by using an appropriate option. The joint is essentially end grain to end grain which produces a relatively weak joint to begin with. Fasteners are an option however they must be driven into end grain, and the wood ends get progressively thinner which weakens the wood being held. And finally the assembly process can be irritating since clamping miter joints only seems to cause the boards to slide against one another instead of holding in place for the joint to set.

All of these disadvantages, however, can be mitigated by the use of an appropriate fastener to provide the structural strength.

Nailed Flat Miter Joint

Nails can be driven from both ends through the side of one board and into the end grain of the other board. Nailing into both directions prevents the boards from simply being pulled apart, and using two nails from each direction, each at skewed angles, further reinforces the joint. This option should only be used when joint strength is not critical.

Doweled Flat Miter Joint

Dowels can be used to align the two boards being joined. The holes can easily be drilled and aligned by the use of an appropriate jig, however the difficulty is in producing an accurately angled hole that aligns with its mate. Any mismatch in the alignment or angle of the holes will result in a joint that will not close.

Biscuited Flat Miter Joint

Biscuits can be used as a floating tenon to join a flat mitered frame joint. These biscuits can help in the alignment of the joint during assembly, and produce a relatively strong joint since the long grain of the boards is glued to the neutral grain of the biscuit.

Splined Flat Miter Joint

An alternative to a biscuit is to use a spline. It is a relatively simple matter to cut the slots into the ends of the two boards, then inserting the spline during the glueup process. If the spline is full-length, the spline can be cut to length after the glue has dried.

If the spline slot is stopped before it reaches the board’s edge, you can create a full-blind or half-blind spline to hide the spline inside the joint. This gives you the strength of the spline without the visual interuption that the spline end would show. If you do decide to use a blind spline, cut the spline slightly shorter than the slot length to allow for some adjustement and fine-tuning during glue-up.

For maximum strength, the grain of the spline (if it has any) must run perpendicular to the miter. If the spline grain is oriented in the same direction as the joint cut, the spline will be prone to split along its full length. If hardboard or plywood is used, this is not a concern since they are considered grain-neutral.

Splined Four-Way Flat Miter Joint

In order to produce a joint where four different boards meet at a point, each board would first receive two 45° miter cuts to form a point on one end, then the tip would receive a slot that would be as deep as the point, but would not reach the board’s edges. During glueup, a single square spline (slightly undersized!) would be inserted into the slots and the four points joined together. Then it is simply a matter of gluing the joint and clamping the boards with minimal force to prevent shifting.

Flat Miter With Spline Key

In this joint a spline is inserted diagonally from edge to edge across the joint. Though the spline slot can be cut and the spline inserted before glue-up, it is far simpler to glue a simple flat miter joint with glue only, then once the glue has set you can cut the slot for the spline and insert the oversized spline key for a second glue-up. Once the glue has set, you can trim the spline to the appropriate size. This joint produces an abundance of long grain to long grain gluing surface, which produces a very strong joint.

If the spline material has a grain, then the grain should be aligned perpendicular to the miter, running from outer edge to outer edge. This will minimize any effect produced by wood movement.

As with most splines, the spline material can be of a matching or contrasting colour, depending on the effect you desire.

Flat Miter With Feather Keys

An alternative to using a single spline key is to use multiple feather keys. These keys are essentially made of thin veneer and are inserted into the very narrow kerf of specialty handsaw blades.

Flat Miter With Dovetail Keys

A dovetail key adds mechanical interlocking to the concept of a simple spline key or feather key. The wedge shape of the dovetail key prevents the joint from being separated, and the gluing surfaces presented are all long grain to long grain.

This joint is easiest to make by first producing a simple glued miter joint, then cutting the slot for the key with a router and an appropriate dovetail bit, or cutting the slot by hand. Once the slot is cut, you can then proceed to cut an appropriately sized angled wedge. You should make the wedge longer than you need so that you can trim the ends after assembly. Once you have made the dovetail key, simply insert the wedge with plenty of glue and clamp the joint until dry.

The limiting factor for the size of the key is the width and depth of the miter corner. The dovetail key must leave enough continuous wood to support the two slot walls, and therefore the width of the slot should not be more than two thirds of the frame’s thickness, or one half of the frame’s thickness if the frame is made from a soft wood. The angle of the key’s sides should conform to the dovetail standards of 8° to 12°, since this provides the most strength without removing excessive material which would weaken the joint.

As with the spline key, the grain of the dovetail should run perpendicular to the miter cut.

The beauty of this joint is it leaves a dovetail detail visible from either end of the joint, and this can be highlighted by the use of contrasting coloured material for the key.

Flat Miter With Butterfly Keys

The butterfly key is an hourglass-shaped key that is inserted into the face material of the joint, spanning the two boards. The shape of the key prevents the two boards from being separated, and adds a decorative visual detail to the joint, especially if the key is made of a contrasting material.

The slot for the key is cut after glueup, using a router and/or a chisel. The narrow portion of the key should be perfectly aligned along the joint line. After the slot has been cut, a key can be cut to the exact shape then glued into the slot before being trimmed flush with the surface.

As with the spline key, the grain of the butterfly should run perpendicular to the miter cut, from wide portion to wide portion.

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Cope-And-Stick Frame Joints

Contemporary frame-and-panel construction involves detailing the inner edges of the stiles and rails with decorative profiles (shapings), along with a slot cut beneath the profile to accomodate the floating panel. The traditional method of joining the rails to the stiles was to use a stuck mortise-and-tenon joint, however modern production requires a less time-consuming, more economical joint. The advent of shaping machines such as the router gives us the ability to quickly, easily, and most importantly accurately produce a joint suitable for profiled framing joints.

The cope-and-stick joint (or cope-and-pattern joint) is formed by with two different profiles. The first pass through the shaper adds the interior profile to the stiles and rails. Once all the pieces have been shaped, the ends of the rails receive a second pass that gives them an inverse of the first profile. This allows the ends of the rails to be neatly inserted into the profile of the stiles, and essentially forms a groove-and-stub-tenon joint. This produces only slightly more glue surface than a groove-and-stub-tenon joint, however it adds a great deal to the visual effect of the panel.

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Butt Frame Joints

Butt frame joints are formed by simply butting the end of one board against the edge of another board. The contact is entirely end grain to long grain with no mechanical interlocking. This situation forms a relatively weak joint

In order to overcome some of its weaknesses, fasteners are frequently used to secure the joint.

Nails and Screws

Nails and screws are intended to have approximately two-thirds of their length embedded in the “second” material, with one-third of the material being supported by the nail’s head. Because of this, nails and screws only become an option if the frame members are relatively narrow, otherwise an alternate joint would be more suitable.

The end piece can be toenailed to the edge piece. This involves driving nails diagonally from edge-to-end through the end-board, and into the edge of the second board. The nail should start far enough from the end to prevent splitting the wood and provide enough material to hold the butt joint together, yet still embeds two-thirds of its length into the second material. It is recommended that a pilot hole be drilled through delicate woods to prevent splitting.

A nail or screw can also be driven through the opposite edge of the edge board, entirely through the edge board, and into the end grain of the end board. For nails, a sufficiently long nail can be used so that the nail can reamain flush.

For screws, the screw hole should first be drilled and counterbored to a sufficient depth to allow the screw’s shoulder to reach the far edge of the material, or the diameter of the screw hole should be large enough to prevent the screw from gaining purchase. The end piece should also receive a pilot hole, but its diameter must be appripriately sized for the screw shaft, in order to maximize grip. This technique allows the screw to apply maximum pulling force to keep the joint tight.

Doweled Butt Joint

A dowel can be used as a fastener in butt frame joints. The dowel forms a loose tenon that is glued into matching holes drilled into the end of one board and the edge of the other board. Doweled joints should have a minimum of two dowels to prevent the joint from twisting itself free. If accurately drilled, the dowel provides positive registration during assembly, however the strength of the joint is somewhat suspect: the surfaces are long grain to long grain in the side piece, but long grain (dowel) to end grain in the end piece. Therefore it is recommended that the dowel in the end piece be fixed in place by short brads driven into the dowel ends through the face or edge of the end piece.

The most difficult aspect of this joint is the accurate alignment of the holes, and this can be mitigated by the use of a special-purpose drilling machine or a suitably configured drilling jig.

Biscuited Butt Joint

Biscuits can be used as a loose tenon in butt frame joints. They have the distinct advantage of being easy to produce, and since the biscuit slot is slightly longer than the biscuit, the joint can be finely tuned during assembly to overcome any alignment errors. This joint is commonly seen in cabinetry to join the rails and stiles in place of traditional mortise and tenon joinery.