Wood Joints

     Trees seldom grow into the shape of furniture! It's up to the designer and woodworker to assemble wood pieces into a functional piece of furniture. Assembling these pieces of wood into furniture is called joinery. A majority of the tools in a workshop are devoted to joining wood pieces.

     I use most of the following wood joints in furniture construction projects. Some of my tools are solely made to create one specific joint.

Wood Joints

Description

Biscuit Joint

picture of a biscuit joint

     The biscuit joint is made by
  1. Matching two pieces of wood
  2. Cutting slots into each piece
  3. Selecting a biscuit (football-shaped piece of hardwood)
  4. Applying glue to the joint surfaces and biscuit
  5. Press-fitting the pieces of wood together
  6. And clamping the joint until the glue sets
     This makes a really strong joint. You'll need a portable biscuit cutter or a router table with a bit which will cut the thickness of the biscuit. You can buy different sized biscuits to best fit the dimensions of the wood joint.

Box or Finger Joint

picture of a box joint

     A box joint is made by cutting slots into the ends of adjointing wood pieces. This job can be performed using a
  • Special router bit equipped with several evenly spaced cutters.
  • Straight router bit and a special jig set for cutting individual slots into each piece of wood.
  • Table saw with a special jig set for cutting individual slots into each piece of wood.
     The special router bit will cut multiple slots with each pass. However, multiple slot cutters have a tendancy to pull the wood piece further into the cutting assembly. You should clamp the wood to the miter guage as you feed the wooden pieces. Also, use a wooden backer to ensure clean cuts. The special router bit has height limitations; meaning you can't use it to cut slots along the edge of a 6" piece of wood. It works well for cutting slots for a 2 1/2" high jewelry box.

     After you're happy with the fit of each joint, apply glue to the top and bottom of each finger. The box joint's purpose is to provide more wood surface for glue applications - resulting in stronger joints. I use a small spatula to smear glue on each wooden surface. Use a clamp to hold the joints until the glue sets. If you're building a small box, ensure the box is square before the glue sets.

     I bought a special jig for my router table but I haven't used it. I intend to use it on my next box-building project. I have never used the table saw to cut finger joints.


Butt Joint

picture of a butt joint

     The butt joint is the absolute weakest of all joints. I don't use it in designing or building furniture. Maybe it could be used in building a dollhouse. It's being shown here just for illustration.

Dado Joint

picture of a dado joint

     Dado joints have multiple applications in building furniture. For example, dadoes are frequently used when installing shelving. Cutting a dado into the carcass provides a long ledge to support a shelf. Combine that with glue and you have a strong joint. It does have limitations. By cutting a dado into the carcass, you weaken the carcass. Also, the depth of the dado may not be sufficient to fully support the shelf; especially if it's carrying a heavy load. Finally, a dado used to support shelving means that the shelving heights are fixed.

     I typically use a portable router to cut dadoes for shelving support. Layout is critical. If possible, I will align both sides and cut the opposing dadoes at one time. If the sides of the shelving are narrow, cut shelving dadoes in a wide piece and then rip it!

     I incorporated dado joints when framing an outdoor play kitchen. I cut a dado into a 4 x 4" support post, applied glue to the dado cut, and press fit a 1 x 4" cross piece into the dado. This was followed by adding two 1 5/8" Torx screws.


Dowel Joint

picture of a dowel joint

     Dowel joints can be used in building furniture. I've seen dowels as small as 1/8" in diameter. I suppose you could use dowels over an inch in diameter; especially if you were framing a barn! Here are two methods of installing dowels.
  1. Put the two pieces of wood together and mark on both pieces where you want to place the dowel. Then carefully drill holes into both pieces and hope the holes match perfectly. There are dowel jigs you can buy to help align the holes with one another. This method is perferred when you don't want any part of the dowel to show.
  2. A second method is to put the pieces of wood together and drill one hole through the first piece and into or through the other piece of wood. This guarantees a perfect fit but one or both ends of the dowel will be exposed using this method.
     I use the second method of dowel installation.
  1. Clamp the wood pieces together
  2. Drill a hole through one piece and into the other
  3. Cut a piece of dowel rod longer than the total hole depth
  4. Apply glue to the dowel
  5. Using a hammer, tap the dowel into the hole; stopping when it has reached the bottom of the hole
  6. Using a fine finish pull saw, cut the excess dowel even with the wood piece
  7. After the glue has set, sand the dowel end so that it is perfectly even with the wood piece
     I think it's important that you use a dowel made from the same wood as the wood pieces you're joining. E.g., If you're joining two oak pieces, then you should use an oak dowel. I typically use a dowel to reinforce mortise and tenon joints where additional stress is expected. I have also used dowels to reinforce butt joints and molding corners.

Full Dovetail Joint

picture of a full dovetail joint

     A full dovetail joint is one where the dovetails (pins) are exposed on both pieces of adjoined wood. I think they're called dovetails because the pins are flared on the ends. (Maybe resembling a dove's tail feathers?) When cut accurately, this is an attractive joint and it is very strong! I've never used a full dovetail joint but it does have application in building furniture. Making drawers is probably its primary use. There are two methods of creating a dovetail joint.
  1. By hand using a small dovetail saw and wood chisels
  2. Using special tools such as a dovetail machine or router table jig

Half Dovetail Joint

picture of a half dovetail joint

     A half dovetail joint is one where the dovetails (pins) are exposed on only one piece of adjoined wood. For instance, maybe you prefer not seeing the dovetails on a drawer front. When cut accurately, the half dovetail joint is attractive and it is very strong! I frequently use half dovetail joints on drawers and on small pieces such as tabernacle and jewelry cabinet carcasses.

     There are two methods of creating a dovetail joint.

  1. By hand using a small dovetail saw and wood chisels
  2. Using special tools such as a dovetail machine or router table jig
     Knowing my penchant for using tools, you should have already concluded that I rely on a dovetail machine. However, cutting accurate dovetail joints using a machine is dependent upon
  • Accurate thickness of wood being used
  • Setting up the wood pieces correctly in the dovetail machine (wood pieces aligned properly on the machine before clamping)
  • Having the correct depth set on the router bit (using a portable router and dovetail bit)
Unless you do these things correctly, you will not get a close fitting joint!

Half Lap Joint

picture of a half lap joint

     A half lap joint is sometimes used in building furniture and furnishings. It is useful when two pieces of wood cross one another and you need to maintain the same overall thickness or maintain points of origin on the same plane. The half lap joint can occur on the ends of two pieces of wood (as shown in the picture) or they can occur anywhere along the wood pieces. I use half lap joints when building wooden crosses.
  1. Select two wooden pieces which are of the same thickness
  2. Cut dadoes on both pieces of wood. The width of each dado cut should match the width of the adjoining wooden piece. (It helps to have both pieces the same width, too.)
  3. Apply glue to one dado cut
  4. Press fit the two pieces together and clamp the joint
     After the glue sets, You'll have a very tight and strong joint.

     You can use half lap joints when building a frame. Frame examples are: a pet gate, picture frame, or framing for a cabinet. For these applications, I recommend using glue and another fastener such as screws, bolts, or dowels.


Miter Joint

picture of a miter joint

     A miter joint is easy to make and assemble. Examples of its use include picture frames, wood trim on a cabinet top, cabinet doors, and drawer fronts. Both pieces of wood should be the same width.
  1. Set your miter saw on a 45° angle and cut both pieces. (Make sure that the length of each piece is correct. Otherwise, your frame will not be square!)
  2. Test the result to ensure the fit is perfect. (If not, your miter saw sucks and you may have to sand one or both ends to get the desired fit. Again, be careful not to change the overall length of each piece.)
  3. Apply glue to one piece and press the adjoining pieces together.
  4. Clamp the joint until the glue sets.
     This joint is not much stronger than a butt joint. However, most molding joints do not have external stresses being applied and a mitered joint is perfectly acceptable in those applications. I have used miter joints when applying molding to drawer facings on a dresser and night stand. I also use mitered joints when attaching a molding to the border of cork trivets.

Mortise and Tenon Joint

picture of a mortise and tenon joint

     This is my favorite wood joint. Mortise and tenon has the distinction of being named the strongest woodworking joint. I use this joint when assembling cabinet doors, cabinet facings, sofas, chairs, kitchen mallets, table legs, pet gates, etc. Here are two methods of creating mortise and tenon joints.
  • The purist method
      Using a hand drill, make a series of holes along the edge of a piece of wood. The depth of the holes should be slightly deeper than the proposed tenon. These holes will remove a majority of the wood which must be taken from the slot (mortise). Ensure the length of the mortise is less than the width of the adjoining piece of wood.
    1. Clean up the slot with a chisel; making it rectangular in shape. This mortise will accept the tenon.
    2. Using a hand miter saw, cut the ends of the adjoining piece of wood. The remaining wood (tenon) should be almost the size of the hole (mortise) made on the adjoining piece of wood. Most tenons will extend from the exact center of the end of the piece of wood.
  • Joe's method
    1. Set the mortiser machine to the desired depth and distance from the edge of the piece of wood.
    2. Cut a series of square holes to form the mortise. Ensure the length of the mortise is less than the width of the adjoining piece of wood.
    3. Set the router table bit to the desired height and width and cut dadoes into the end of a scrap piece of wood. (The wood scrap must be of the same thickness as the piece planned for use.)
    4. Turn the scrap wood over and dado the other side. This will create the tenon.
    5. Test fit the tenon to ensure the router bit is set to the correct height. Make further adjustments and cuts as necessary.
    6. Once satisfied, cut the tenon in the adjoining piece of wood.
    7. Trim the sides of the tenon so that it covers the ends of the mortise. Notice in the picture that when the tenon is inserted into the mortise, the mortise will become invisible because the tenon's shoulders will overlap all sides of the mortise.
  • Apply glue to all sides of the tenon and insert it into the mortise.
  • Clamp the joint until the glue sets.
     I have also used a table saw with a special tenon jig to create tenons. Unless the tenon will be longer than 1 inch, I prefer using the router table method of creating tenons.

     I've also pinned a mortise and tenon joint with dowels to decrease the possibility of the tenon breaking free of the mortise.


Sliding Dovetail Joint

picture of a sliding dovetail joint

     A sliding dovetail joint is one where a dovetail (pin) is cut along the end of a board and a corresponding slot is cut in an adjoining board. This joint is very strong and it is designed to prevent the boards from separating.

     I have used this joint to attach fronts to the sides of drawers and attaching legs to a stool's central support piece. Here is how I make this joint:

  1. Using the router table with a straight router bit, cut a dado along the line where you will make the dovetail cut. This is to remove some of the wood so the dovetail bit will have less work to do. Make all your line cuts at this time.
  2. Switch to the dovetail bit and cut dovetail slot; following the dado cut.
  3. Maintaining the same height of the dovetail bit, adjust the router fence and cut a dovetail into the end of a scrap piece of wood. Make sure the thickness of the wood scrap is the same as the wood piece you're planning to use!
  4. Using a series of progressive cuts, continue cutting the scrap piece until its dovetail exactly in the dovetail slot.
  5. Cut a dovetail slot into the end of each board.
  6. Apply glue to the dovetail end (pin) and slide the board into the slot.
  7. Clamp the joint to ensure a 90° angle with the adjoining pieces as the glue sets.

Spline Joint

picture of a spline joint

     The spline joint is strong and has several applications in building furniture. Let's assume you're building a box that is 18" long, 16" wide, and 3" high.
  1. The spline joint begins with a miter joint. Cut a 45° miter on both ends of the four wood pieces.
  2. After cutting the pieces, dry fit the pieces into a box shape to ensure proper fit.
  3. Glue the joints and clamp. I use a ratcheting band clamp for this job. The band clamp applies even pressure to all four corners of the box.
  4. After the glue sets, remove the clamp.
  5. The next step is to cut slots for the splines. For this task, I use a table saw with a spline jig.
  6. Set the saw blade to the correct height and cut a slot in each corner of the box. Depending on the width of the box, you may need to cut multiple slots in each corner of the box. E.g., for a box 3" high, I would use two splines. (A picture frame or door would only require one spline.)
  7. Hardwoods such as oak, walnut, poplar, and maple are good choices for splines. The splines should be the thickness of the slots cut into the box corners.
  8. Layout triangular shapes on the spline wood and cut the triangles on a band saw.
  9. Apply glue to both sides of the triangle and insert it into each slot. The base of the triangle goes in first.
  10. After the glue sets, cut the excess spline material on the bandsaw.
  11. Sand the remainder so that the splines are even with the sides of the box.

Tongue and Groove Joint

picture of a tongue and groove joint

     Tongue and groove joints have been around for many years. I have used tongue and groove joints when building a table, entertainment center, and end table. Anything which involves aligning boards side-by-side is a good candidate for tongue and groove joinery. For example, when building a kitchen table, I decided to build the table top out of 6 and 7 inch wide oak boards. Here are the steps:
  1. Plane the boards to a uniform thickness. This is done on the planer.
  2. Cut the boards to a specified length. I normally add 1/2 inch to the desired length. This is done on the 12" miter saw.
  3. Rip a small amount off the edges of each board. This was done on the table saw. (I don't have a jointer/planer tool.)
  4. Place the boards side by side and move them about until you get a desirable grain and color pattern.
  5. Mark the ends of the boards; a "T" for tongue near one side and a "G" for groove on the other side. This is to avoid a mistake during the next step.
  6. Insert a groove making router bit on the router table and cut the edge of the boards marked with a "G".
  7. Insert a tongue making router bit on the router table and cut a tongue on a piece of scrap wood. (Scrap piece must be the same thickness as the table top boards.)
  8. Check the scrap wood piece with the grooves made earlier to ensure the assembled boards will be at the same height.
  9. After you're satisfied with the setting of the tongue making router bit, cut tongues into the board edges marked with a "T".
  10. Apply glue to both sides of the tongue and along its edge. Then fit the board's tongue into the adjacent groove. You may need to use a rubber mallet to persuade the edges to fully seat. Continue this step until all boards are assembled.
  11. Using pipe clamps, apply lots of pressure to the assembly. I typically use three or four pipe clamps on a four foot long table top.
  12. After the glue sets, remove the clamps and begin sanding. This is where I wish I had a drum sander!
     The tongue and groove making router bits I use are a set which makes a 1/2" long tongue and corresponding groove. This creates a very strong joint along the entire edge of each pair of boards.

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Updated 03/28/2018
Joe R. East, Jr.