Can I Rely on Glue Alone for a Strong Wood-to-Wood Joint? By Louis Scrivani Information used from myself as well as the web.
Modern adhesives are very strong, but smart joinery — a DOVETAIL, mortise-and-tenant joint or the equivalent — still makes all the difference.
Question With the new glues I don't need to make a mortise and tenon - I could use glue, right? Wrong, that is just one part of the picture.
Question: I thought the glue was stronger than the wood isn’t that right?
In most instances even the weakest cheapest glue can be used to glue wood together and the wood will break first. But if that was true 100% of the time then I could just glue every newel post I install right to the floor without any other means of joinery. Try this - make a raised panel door and do not cope and stick it - just glue the butt ends of the stiles to the rails. It will stay together, but for how long? Drop it on the floor. What happens? Glue does not provide a good enough bond over time. Dovetails, dados, rabbets, lock miters, box joints, mortise and tenon, dowels - they all have a place. Glue can and should be used with them all, but glue should never be used alone except to glue up panels, butcher blocks, tabletops etc.
Question: OK, so that’s your opinion. The fact is if glue is stronger than wood then the joint should hold if it is just glued and the wood around it would give and break.
Answer: The end grain of wood bonds very poorly. The purpose of joinery is partly to provide a mechanical fastening, but even more so to provide sufficient long grain-to-long grain gluing surface. This is why you can edge glue wood all day, but a butt joint of end grain to long grain, where you would typically use a mortise and tenon, will fail rather easily.
Before I get into any further detail let’s talk about a bit of the history of glue. The History of Glue
Glues are part of a larger family called adhesives. Most adhesives are chemical based, while glue is generally made from organic compounds. These terms, however, are used loosely today and most adhesives are still referred to as “glue.” The earliest glues were made from various plant-based materials. The history of glue goes much farther back than most people think. The earliest evidence of the use of glue can still be observed in cave paintings made by our Neanderthal ancestors in Lascaux, France.
These early artists wanted their work to last and mixed glue with the paint they used to help the colors resist the moisture of the cave walls. But this information is not important for this article.
We must take into consideration what we are gluing together as well as the durability that we wish of it and last but not least expansion of the wood. I think most crafts people, would all say what they build they wish to last for ever. Ok then if we agree with that statement then we must take into consideration grain type, since for a glue, to work properly grain direction is a very important factor as well as how much of it.
A long rain to long grain glue joint is ideal, where a long to short grain is ok and a short grain to short grain is only used depending on the glue to hold it together. In that case we must help that type of joint, with such things as dowels, biscuits, splines and so on. Since those additions help with area of wood to be glue that can be used as well as grain, glue area.
For example: think of the grain as straws the area that you use to draw up the drink, would be like the short grain of wood. The side of the straw would be like the long grain of wood. Now that you have the image in your mind, take a hand full of straws in your hand grasping the bunch in the center portion of the straws, now where you are holding them that would be like the long grain of wood. So if you were to add glue to it would seep into and around the straws and bond very well as well as strong. Now take those same straws and apply glue on the end , the location of where you would draw up the liquid when drinking The glue would travel down the straw holes and have very little grasping strength to the wood, if we were talking of the grain in this example. So in this case the strength is purely dependent on the glue area and not the wood. Now we also have to take into consideration expansion of the wood. Wood expands outward,, not the length of the grain. So if your making a box and even though the joints are glued the grain, is traveling in the same direction all around the box. As the wood expands the entire box is going to expand at the same rate. Saying that is why if you just pick two different types of wood, without taking into consideration expansion rates of the types of wood into consideration it could cause problems. The location of where the project will be is important as well. If the project is going to be in a house where the temperature is constant the expansion and contraction of the wood will be consistent, outside it will be very erratic and become a victim to consistent movement. So building a piece of furniture takes a lot of thought, before you start picking wood, choosing joints, glue choices and even type of finish. I will try to touch on all of these points in the information to follow.
**Wood Movement and How It Affects Your Woodworking Projects Dealing with Moisture Content, Wood Expansion and Contraction
This end table by Richard Slosky demonstrates one way to attach a table top while allowing it to be able to expand and contract with changes in moisture content. Small L-shaped "buttons" are screwed to the bottom of the table top while an expanded slot on the inside of the table skirt accepts the leg of the button. As the top expands and contracts over time, it's able to move without putting stress on the table joinery or the table top - thereby letting this table last for many, many years. First a word of caution. Here and in other publications precise numbers are calculated or given for different wood properties. These numbers are always based on some averages that have been developed in laboratory testing, sometimes over many years. Wood is a very complex material with many variables that will affect moisture content and wood movement. It is not possible to predict accurately the movement of a single piece of wood. The average of a quantity is much more predictable. Still the averages can be applied to most practical situations.
For most of us it is adequate to know that a table top will move about .5 inches. It is inconsequential to calculate that it may move .457” or .514”. It is far more important to understand that it WILL move.
Wood expands and contracts with changes in the surrounding humidity and to a lesser degree the temperature. More humid air will cause wood to expand; drier air will cause wood to contract. This movement cannot be stopped. You can learn what to expect and techniques to cope with the movement.
Moisture content is expressed as a percentage and calculated as follows:
Moisture content = Weight of the water in the wood Weight of the wood oven dry
In trees moisture content will range from 30% to more than 200%.
Water is present in wood as liquid or vapor inside the cells (free water) and water bound chemically to the cell walls (bound water). Visualize a hand full of straws or tubes. Water in the tubes is free but there is also water bound inside the walls of each tube. The point at which the cell cavities are empty but the walls are still saturated with water is called the Fiber Saturation Point. In wood this averages about 28% but will vary by a few points depending on wood species, specific tree and even individual boards. As wood dries below this moisture content it shrinks until the moisture content reaches equilibrium with its surrounding environment.
Drying lumber, either naturally (air drying) or in a kiln is the process to lower the moisture content to the surrounding environment. For much of the United States, the minimum moisture content of thoroughly air dried lumber is 12% to 15%. Kiln dried hardwood will usually be less than 10%.
Dried lumber has many advantages over green lumber for Woodworkers. Removing excess water reduces weight, thus shipping and handling costs. Proper drying will help confine shrinking and swelling of wood in use to manageable levels. As wood dries, most of its strength properties increase. Properly dried lumber can be cut to precise dimensions and machined more easily and efficiently; wood parts can be joined more securely or fastened together with nails, screws, bolts, and adhesives. Warping, splitting, checking, and other harmful effects of uncontrolled drying are largely eliminated; and paint, varnish, and other finishes are more effectively applied and maintained.
The objective is to bring the moisture content of wood, as close as possible, to the level the finished product will experience in service. Acquire your lumber in advance and give it time to acclimate to the environment in which it will be used or worked. Sometimes it may be necessary to “stack and sticker” lumber to allow it to properly reach equilibrium with the environment.
Finished projects will continue to absorb or give off moisture with changes in humidity and temperature of the surrounding air. The wood will always undergo changes in moisture content and dimension. Consequently doors and drawers may stick during periods of high humidity. Molding joints may open. Solid wood panels in raised panel doors may shrink exposing unfinished wood along the edges of the frame. Furniture moved from one part of the world to another may change enough in dimension that it must be reworked to function correctly. And, during a dry night a poorly designed furniture panel may explode with a bang!
Direction of Movement
Wood does not move in all directions equally. The greatest movement is across the grain. There is very little movement along the length.
Again a word of caution. We can predict, with a high degree of accuracy, the movement in a large quantity of wood. It is not possible to accurately predict the movement of a single piece of wood. Some of the variables to consider: •Whether the piece is flat sawn or quarter sawn. More on this later. •Sapwood content. Sapwood will usually change moisture content more rapidly than heartwood. •Grain structure-open or closed pores. •Defects in the wood. •Changes in grain direction. •Consistency of moisture content throughout the board. It is likely to have a higher moisture content in the center than on the surface. •Tension in the wood. •Location of the wood in the tree. •Age of the Wood. •Size of the piece
Wood technologists refer to flat sawn lumber as “tangential”. The saw blade cuts a kerf on a line tangent to the annual growth rings. Quarter sawn lumber is “radial”. The saw blade cuts a kerf from the pith across the annual growth rings. Lumber is seldom completely tangential or radial but ever changing throughput the cutting process. Look at the end of a board; if the angle of the rings to the face is 0 to 30 degrees the board is flat sawn; if the angle is 60 to 90 degrees the board is quarter sawn. In between, 30 to 60 degrees, is bastard or rift sawn. Flat sawn lumber has tapered cathedral grain, while quarter sawn lumber has long straight grain lines.
Some species-White Oak, Lacewood, Sycamore etc.- have large rays that are exposed when quarter sawn producing a dramatic pattern. Similarly, woods with interlocked grain-Sapele, African Mahogany etc. - produce a striped ribbon effect when quarter sawn.
Movement across the grain is greatly influenced by how a board was sawn. On average flat sawn hardwood will shrink 8% from the Fiber Saturation Point to oven dry while quarter sawn hardwood will shrink only 4%. There are tables for tangential and radial shrinkage by species but for most applications knowing that quarter sawn lumber moves only half as much as flat sawn is sufficient.
On some projects the orientation of the cut will make a difference but for most applications either flat sawn or quarter sawn is satisfactory. Here are some characteristics of each:
Flat sawn , Shrinks and swells less in thickness
Quarter sawn, Shrinks and swells less in width
Grain presents a cathedral pattern. Figure patterns resulting from annual rings and are displayed more conspicuously
Grain presents a pattern of long straight lines. Raised grain caused by separation in annual rings is not as pronounced. Figure patterns resulting from pronounced rays, interlocked grain, and wavy grain are brought out more conspicuously
Knots are round or oval; boards with round or oval knots are stronger than boards with spike knots
Knots are spike shaped; boards with spiked knots are weaker than boards with round or oval knots
Is less susceptible to collapse in drying
Cups, surface-checks, and splits less in seasoning and in use
Shakes and pitch pockets, when present, extend through fewer boards
Does not allow liquids to pass through readily in some species
Holds paint better in some species
Standard sawing pattern for most species. Generally available
Commercially sawn in only a few species. May be difficult to obtain
Coping with Wood Movement
wood movement in frame and panel doors cabinet doors an example of frame and panel construction, one method of dealing with wood movement in solid-wood furniture. The doors consists of a frame made of rails and stiles, and then there's a solid wood panel that's attached to the frame in a hidden groove. The panel is not glued in place, it simply floats within the frame. This method allows the overall door to stay the same size over the years while allowing the panel to expand and contract as needed.
Take wood movement into account in the design of your project. Experience taught woodworkers long ago how to deal with dimensional change due to change in moisture content. The answer was joinery that allowed for wood movement. Despite today’s super-strong adhesives and moisture fighting finishes that is still the solution. Here are some techniques.
Acquire your lumber in advance and give it time to acclimate to the environment in which it will be used or worked. It may be necessary to “stack and sticker” lumber to allow it to properly reach equilibrium with the environment before it is used.
Consider plywood. Plywood is stable; it does not expand and contract like solid wood. Many hardwood species are available or sheets can be made using veneer. Flexible paper back veneer is easy to work with and readily available. Cabinets and furniture cases are good candidates for plywood construction.
Plan the joinery to avoid cross-grain assemblies. Cross-grain joints constantly pull in different directions weakening the joint over time. Design joints so the grain runs in the same direction on both pieces.
Allow tops to move freely. Attach tops with Figure 8 Connectors, Z clips, shop made blocks or elongated screw holes. All of these methods will securely attach the top but all it to move across its width. A 36” wide top, made with flat sawn lumber, can move more than an inch with a 10% change in moisture content.
Use the nickel-dime approach. When fitting cabinet doors or drawers build to the normal humidity in your area. During periods of low humidity allow a reveal the width of a nickel. Conversely, during periods of high humidity allow for a dime reveal.
Use elongated holes for screws. When attaching A cross-grain board glue and screw the front few inches but secure the remaining length with screw in slots. This will allow the back to move but keep the front firmly aligned. the slot should be across the grain to allow the wood to move.
Use “frame and panel” construction. Panels may be plywood or solid wood with a raised center field. Most folks like the look of raised panels better. If using solid wood panels, DO NOT glue it to the frame. The panel must be free to float and change dimension. A small spot of glue in the center of the width will keep the panel centered.
When making a drawer for a project make the back of the drawer short enough so the bottom of the drawer can slide into the dados that you made around the bottom or near the bottom to allow the bottom to slide in. Also the bottom board should not be glued for the same reason. Also the grain should be situated so the expansion of the wood would allow movement under the front board of the drawer.
**Apply a finish. Apply an equal number of coats to ALL surfaces to equalize the loss or gain of moisture. No finish will block moisture transfer; they just slow it down. Penetrating oils provide the least protection. Epoxy offers the greatest.
The tendency of wood to contract and expand, shrink and swell cannot be stopped. You must plan for it. Design and build with dimensional change in mind. This could have been a article by itself, but that is all part of building.
Now that we have an idea of the role the glue, wood movement and wood choices has in bonding two pieces of wood we can talk of different types of joints, in a combination with the glue would be a stronger or not so strong joint. I will not talk of every joint, if you think there is a joint missing that should be present, please feel free to add it to the list. First the mortis and tenon, I think it is agreeable that it is one of the strongest joints. This Joint has been used for thousands of years by woodworkers around the world to join pieces of wood, mainly when the adjoining pieces connect at an angle of 90°. In its basic form it is both simple and strong. Although there are many joint variations, the basic mortise and tenon comprises two components: the mortise hole and the tenon tongue. The tenon, formed on the end of a member generally referred to as a rail, is inserted into a square or rectangular hole cut into the corresponding member. The tenon is cut to fit the mortise hole exactly and usually has shoulders that seat when the joint fully enters the mortise hole. The joint may be glued, pinned, or wedged to lock it in place.
The dovetail joint, is another one of those top strongest joints, in combination with glue it is perfect.
A dovetail joint or simply dovetail is a joint technique most commonly used in woodworking (carpentry) including furniture, cabinets, log buildings and traditional timber framing. Noted for its resistance to being pulled apart, called tensile strength. The dovetail joint is commonly used to join the sides of a drawer to the front. A series of pins cut to extend from the end of one board interlock with a series of tails cut into the end of another board. The pins and tails have a trapezoidal shape. Once glued, a wooden dovetail joint requires no mechanical fasteners. The dovetail joint probably pre-dates written history. Some of the earliest known examples of the dovetail joint are in furniture entombed with mummies dating from First Dynasty of ancient Egypt, as well the tombs of Chinese emperors. The dovetail design is an important method of distinguishing various periods of furniture. In Europe the dovetail joint is also called a swallow-tail joint or a fantail joint.
The butterfly is another strong join, it can take a rather weak joint and turn it into a extremely strong joint.
A Butterfly joint is type of joint used either to hold two or more boards together or to keep two halves of a board that have already started to split from splitting further.
A butterfly joint resembles two dovetails connected at the narrow part. A negative of the hole is cut out of the board the butterfly will be placed in and the butterfly is then fitted, keeping the joint together. The wood used for the butterfly is usually a contrasting wood, often walnut.
Butterfly joints are common in the work of George Nakashima.
A finger joint or box combing or box joint is a woodworking technique used to join two pieces of wood at right angles to each other. It is much like a dovetail joint except that the pins are square and not angled and usually equally spaced. The joint relies on glue for holding together as it does not have the mechanical strength of a dovetail.
The joint is relatively easy to make using a table saw or a wood router with a simple jig.
The full lap is a very basic method of joining two members and requires little or no joinery skills to make. It is made by just over lapping or sandwiching, two pieces of wood together .It requires some form of mechanical fastener to be effective. It offers no resistance to racking but some resistance to twisting and shearing depending upon the fastener used.
Large framing members in light frame construction are often joined by lapping – A variation of the lap joint there is a half lap joint. 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.
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.
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.
Mitered half lap, This is a variation of the end lap which shows a miter on the face of the finished work.
The mitered half lap is the weakest version of the joint because of the reduced gluing surface.
A miter 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 degree angle. It is often used in making picture frames. It is a nice looking joint, but unless reinforced with a dowel or biscuit or a spline or a fastener of some sort, it is very weak since it only depends on the glue to adhere it.
A rabbet (also known as rebate) is a recess or groove cut into the edge of a piece of wood. When viewed in cross-section, a rabbet is two-sided and open to the edge or end of the surface into which it is cut.
An example of the use of a rabbet is in a glazing bar where it makes provision for the insertion of the pane of glass and putty. It may also accommodate the edge of the back panel of a cabinet. In woodwork, it is one way of joining two pieces of molding that meet at an angle. (Compare the cope cut.)
It is also done in pipe work, to take ducts around a corner, or in pipe organs, to fit a large bass pipe into a small space. (The same cut is used to shape of the resonators of certain reed ranks-e.g. see Great ranks and Pedal reeds of Ashton Hall organ-but only the other kind are called "mitered pipes". And, "mitered pipes" are sometimes constructed without miter cuts.
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 favored over these in joinery because it yields a barely visible glue line.
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.
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 molding. 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 ship and 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 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.
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.
Bevel lap splice joint
The bevel lap splice joint 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.
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.
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 fascia.
The joint is usually made by machine.
Tongue and groove, Each piece has a slot (the groove) cut all along one edge, and a thin, deep ridge (the tongue) on the opposite edge. The tongue projects a little less than
the groove is deep. Two or more pieces thus fit together closely. Such a joint should not be glued as shrinkage would pull the tongue off. The tongue and groove could be cut in a number of ways.
The dado joint is much stronger than the
butt joint and creates a more professional appearance. into it. Used when building shelf's in a cabinet for example. A groove is made in the stile of the cabinet and the rail is fitted into that groove, very strong downward pressure since the shoulder of the groove is holding it.
The stopped dado joint has a neater appearance than the regular dado joint because the front edge is uncut. Thus, the grooved area is not visible from the front side. Use ordinary wood adhesives, wood screws, nails or dowels to hold the two pieces of wood together in a stopped dado joint.