Tongue & Groove Joints
When joining two matched boards into one, a tongue and groove joint is much stronger than a simple butt joint. The tongue and groove joint can be easily created using a router with a matched set of router bits, or on a table saw using a stacked dado blade set.
To begin, cut all of the boards you want to use for your tongue and groove assembly to a length of at least one inch longer than the desired finished length using a circular saw, or compound miter saw.
Prepare One Edge of Each Board
With all boards cut to the proper length, the next step is to joint one edge of each of the boards. This can be easily done with a jointer.
If you don't have a jointer, there is another option. Try using a table saw jointer jig. This simple jig allows you to create a smooth, flat edge that is perpendicular to the flat sides of the board using only your table saw.
Jointing one edge of the boards is vital to having a clean, seamless tongue and groove joint.
Rip the Opposite Edges
With one edge of each board jointed (either with a jointer or using a table saw jointer jig), the next step is to rip the opposite edges so that they're perfectly parallel to the jointed edge.
Set up your table saw fence so that the blade will take off a minimal amount off of the width of the boards. Then, align the jointed edge against the fence and rip each board accordingly.
Both edges of the boards should now be flat, parallel and square to the faces (assuming, of course, that the faces of the boards are parallel and flat) of the boards.
Before beginning to make your tongue and groove joints, it is advisable to lay out the boards to try and find the combination that will look the best. Place the boards in an order that will align similar colors, grain patterns and knot locations that will look the best. Also, to avoid issues with cupping or warping down the line, alternate the end grain patterns (as shown in the image above) so that one board's grain faces up, the next down and so on.
You can also number the boards left to right at this point, so you know which way is up on each board and the order that you've decided upon. Simply write the numbers 1, 2, 3, etc. on the end grain of each board. You can also label them A, B, C and so on.
Cut the Grooves
With the boards matched and aligned, it's time to begin creating the grooves. Always create the grooves first, as it's much easier to take a little extra off of the tongues to ensure a good dry-fit than it is to take more out of the grooves.
When creating the grooves, you'll want to remove the center one-third of material from the edge of the board. For the remainder of this article, we'll assume you're working with one-by material (1x6 or 1x4 stock, for instance). This means that you'll cut 1/4" out of the middle of each board for the grooves.
Set up your table saw with a stacked dado blade set for a 1/4" width. On most stacked dado blade sets, this will likely be only the two outer blades (with no chippers or spacers). Adjust the depth of cut to 1/2" and set your fence so that it is 1/4" away from the right outside edge of the blade.
You will want to cut one edge of each inside joint of your layout with a groove. It doesn't matter whether you cut the left or right side with the groove - just be consistent.
Place the board with the face flat against the fence and the edge to receive the groove flat on the table. Holding the board securely against the fence and the table, run the edge of the board all the way through the blade. This will give a 1/2" deep, 1/4" wide groove across the edge. However, to ensure that the groove is perfectly centered, turn the board around and run the same edge through the blade (leading with the opposite edge of the board this second time). This will ensure that your groove is perfectly centered on the edge of the board.
Cut the Tongues
Next, change your stacked dado blade set on your table saw to a 1/2" width, and adjust the depth of cut to about 3/16". Clamp a clean, flat sacrificial board onto the fence (making sure that your clamps are at least an inch off of the table surface), and then slide your fence over so that the sacrificial board is just barely touching (or just a hair away from) the dado blade.
Locate the tongue side of each board from your layout and lay one side of the board flat on the table and against the sacrificial fence. Turn on your saw and push the board completely through the blade. This completes one-half of the tongue. Then, flip the board over (so that the opposite face is against the table), hold it against the sacrificial fence and cut the opposite side of the tongue.
If all went according to plans, you should have a tongue that is a little too fat to fit into the corresponding groove. This is by design. Determine how much more you need to take off (conservatively), and raise your blade a little higher before running both sides of the tongue through the blade and testing the fit again.
Continue this procedure until your tongues fit snugly (but not too tightly) into the grooves. There should be no play, but the fit should not be so tight that you have to force the boards together after you apply glue (in an upcoming step).
To ensure that your board stays flat on the table and securely against your fence, you might consider clamping a featherboard against both your fence and your table to hold the board in place across the blade.
Dry Fit the Joints
After all the tongues have been cut to match the grooves, dry fit your entire layout of all of your boards with the completed tongue and groove joints just to check and see how all of the joints will look when the boards are clamped together. This is the final visual check before assembly, so this is the time to correct any errors or address joints that are too tight.
NOTE: You may now remove your stacked dado blade set from your table saw, and install a clean, sharp all-purpose saw blade for use in an upcoming step.
Clamp the Assembly
After verifying that all of the tongue and groove joints fit to your satisfaction, it's time for some assembly.
Apply a thin layer of glue to all edges of the first groove before sliding the tongue into place. Immediately clean off any glue that squeezes out of the joint, to prevent any finishing issues later on. Repeat the procedure with the second groove, and so on, until all tongues are fitting within their appropriate grooves.
Next, place the entire assembly into a few clamps to hold the joints closed tightly. Be sure not to tighten the clamps so tightly that you squeeze all of the glue out of the joints or cause the joints to misalign. You want to use only enough pressure to hold the joints closed completely, but no more.
Allow the assembly to dry completely, typically at least 24 hours.
Trim Off One End
After allowing the tongue and groove assembly to dry in the clamps for 24 hours, it's time to trim the ends.
Remove the assembly from the clamps. With a square, align a straight-edge square to the sides of the assembly (perpendicular to the tongue and groove joints). Clamp the straight-edge in place so you can trim off just enough of the bottom edge of the assembly with a circular saw to ensure that the bottom will be square to the joints.
Trim the Remaining Joint Edges
With the bottom edge of the tongue and groove joint assembly cut square to the sides, you can now trim the remaining three sides with your table saw to the final size your woodworking plans require.
Begin by adjusting your table saw fence to the height your assembly requires. Place the assembly flat on the table and the square end (that you cut in the previous step) against the fence, and cut off the opposite end of the assembly.
Next, if you need to make any cuts off of the sides to get your assembly to the proper width, adjust your fence accordingly and make your cuts parallel to the joints. It is a good idea to make a cut on each side (taking an equal amount of stock off of each side) rather than removing the entire amount off of one side. This is in the event that, if any of your joints show after applying your finish, the assembly will look balanced.
With the assembly cut to size, move on to sanding. Start with a relatively coarse grit in your random orbital sander and work up to finer grits until the entire assembly is flat, smooth and looks like one solid board before applying the finish of your choice.