There are two viable tools for making box joints in the home workshop, the router and the table saw. Both require the use of a special jig to form and space the fingers of the joint.
While many woodworkers like to cut their finger joints with a router, my prefered method is using a shop made jig mounted in the miter slot of my table saw. It's quick, accurate and once you gain a bit of experience, very easy.
This follow up article to building the easy-set box joint jig will show you how to fine tune the jig and help guide you through the process of making great looking box joints for your own workshop projects.
Although used frequently for creating decorative boxes, finger joints are easily adapted to the construction of furniture, cabinetry, and other forms of woodwork. You'll find many ways to put this table saw jig for box joints to work once you become skilled in using it.
But since there are many variables that can effect the quality of your box joints, I recommend you make a few simple boxes out of cheaper material like pine before moving on to more expensive hardwoods.
Once you have a few notches on your belt, the process of making box joints becomes second nature and any problems that arise are easily understood and corrected.
The box shown on the left is made from yellow cedar and it's construction is fully detailed below, from cutting the fingers to gluing up the joints. But first we'll make a test cut to determine how close our set-up is and whether or not we have to adjust the box joint jig.
In the article on building the box joint jig, we set the initial spacing of the jig by moving the main fence exactly twice the thickness of the indexing pin. This should be pretty close (within a few thousandths) to where we want to be, but the only way to tell is to make a test cut and join the two pieces together to check the fit.
Ideally, your test cuts should be made with the same wood you'll be using to build your box. The test pieces should also be the same width and thickness as those for the box, with the length being long enough to safely clamp to the jig's fence. You'll need at least two pieces to make a full test cut.
To make a test cut, read and follow the directions below (under "Cutting the fingers") to cut a full set of fingers to form one corner of a box. The two pieces should fit together with a slight friction fit, with no gaps between the fingers. It shouldn't take more than a few light taps with a mallet to close the joint completely.
It's important to understand what's going on when the fingers of the joint don't fit together like they should. Most problems that occur when making box joints are related to finger width.
Loose fingers with gaps. Move the main fence toward the right of the table saw, widening the fingers.
Tight fingers. Move the main fence toward the left of the table saw to make the fingers narrower.
When the fingers of the joint are loose, this simply means that the spacing between the index pin and the outside teeth of the dado blade is too small. This space determines the width of the fingers, so to correct the problem, move the main fence to the right to widen this spacing. Of course, the opposite is true if the fingers are too tight and the fence must be moved to the left to narrow the spacing.
The real beauty of the easy set box joint jig lies in being able to confidently adjust the spacing in very small increments to get a perfect fit. By confidently, I mean there's no fear in trying to improve on the fit because you can easily return to the previous setting.
Micrometers are excellent for making tiny adjustments of .002" or less.
When making box joints in expensive hardwoods, I'll use a micrometer to adjust the jig by as little as .001" to find the fit I want. The micrometer is more accurate and easier to use than digital calipers.
To adjust the jig, I loosen the two screws that secure the main fence to the back fence a quarter turn with one hand while holding the two fences together between the thumb and fingers of the other.
A light tap with the screwdriver on the edge of the main fence (in the direction needed) is usually sufficient to move the fence a small amount. Retighten the screws and take a new measurement with your digital calipers or micrometer.
Only move the fence .002" at a time, making a test cut after each adjustment to check the finger fit. This may seem like a tiny amount, but it really doesn't take much to make a big difference in the fit of your box joints.
With the jig properly adjusted, we can now move on to making box joints for our project box.
There are a few things that should be considered when cutting the stock for your boxes...
Once you've dialed in the box joint jig by making test cuts and have adjusted the jig for a perfect fit, we can start making box joints with the wood we set aside for our project.
I don't know about you, but I'm making a storage box that will hold the change gears for my metal lathe. The lid will hold the few tools I need to install them on the lathe.
Both boards in each corner are marked with the same number. Write them on the top edge, up far enough to evade the blade.
With the four sides cut to size, set them up the way you want them to look as a finished box. For what I'm making here it's not real important, but if it's going to be a fancy box made out of some exotic wood, the colors and grain orientation will probably matter.
The tops of both sides in each corner are marked with a number or letter to make sure the fingers of the joints are cut in the proper sequence and that everything lines up after the fingers are cut.
Although it's hard to see in the above photo, the corners are marked 1 to 4, starting from the right upper corner and going counter-clockwise. When making the box joints, these numbers will always be facing the right side of the box joint jig. Do the same for the pieces that will form your box lid.
Before you start forming the fingers, keep in mind that consistency plays a key role in making box joints that fit together well. I start and stop the jig in the same place, push the jig though the blade at the same speed and clamp the board to the jig for each finger I cut. If there's any skill involved in making box joints, this is it.
To set up the first cut, butt the board up against the pin and clamp it to the fence.
Each box you make will have a front, back and two sides. Although it doesn't really matter, I like the front and back boards to start with a full finger at the top and have the sides with a matching notch at the top.
Starting with one of the front/back boards, place it up against the indexing pin with the marked side facing the pin and clamp it to the fence. Make sure that the end of your board is flat against the table and the pin, then run it through the dado blade.
Set the notch you just cut over the index pin and cut the second notch. Keep moving the board along until you reach the end, then flip the board over end for end ( marked end still facing pin) and repeat the process. Do the same to the other front/back board.
Setting up for the first cut in one of the side boards using the top finger (first one cut) from a front/back board as a spacer.
To set up the first cut for the side boards, take one of the front/back boards you already cut and place the first notch you cut over the index pin. The numbered marks will be facing the left.
Butt one of the side boards up against the front/back board with it's marks facing right and clamp it down. Now remove the front/back board and run the jig through the blade. This will cut a notch on the top end of the box side which will match the finger on the top of the front and back boards.
Now slide the cut notch up against the pin to make the second cut. After the second cut, simply index the notch for the rest of the cuts like you did with the first board. Repeat this process for the other end, as well as the remaining side board and your box is finished.
Slide the cut notch against the pin to make the second cut in the box side.
Making box joints in the lid
The lid is cut opposite from the box. On the box, the numbered marks represent the top edge. On the lid, these numbers will represent the bottom edge.
In order to keep the flow of fingers consistant where the top edge of the box meets the bottom of the lid, the sides of the lid will start with a full finger, instead of the front and back as in the box.
Simply put, start the cutting process detailed above with a side board against the pin, instead of a front or back board. When the side boards are cut, use the first finger of one of the side boards as a spacer for the front/back boards of the lid. All you're really doing is reversing the order in which the boards are cut.
Trimming off the excess material.
The extra material can now be trimmed from the box and lid pieces, bringing them to their final dimensions. Make sure that the edge with the number you marked earlier is facing the fence.
I simply lined up the edge of a finger with the outside edge of the blade and locked down the fence. This will leave me with a full finger on the top and bottom when the box is assembled.
Dry fitting assemblies before gluing them up is a common woodworking procedure and should be performed regardless of the simplicity of the project. A dry fit will reveal little problems before they become bigger problems.
A dry fit will reveal any problems before glue-up. Note that the lid is on top of the box in this image.
Before attempting a dry fit though, the box pieces should be sanded to remove any irregularities from the finger cutting operation. Do not sand between the fingers, only on the sides of the pieces.
I'll usually sand them down to 150 or 300 grit, depending on the type of wood, the finish that will be applied and what the boxes will be used for.
After sanding, the corners should fit together easily with a few light taps of a rubber mallet. The fingers must seat fully into their notches and everything should be square. If it takes too much effort to seat the fingers, you'll likely encounter problems once the glue is applied. The reason is that the glue itself takes up some room, but it will also sometimes swell the wood, making it difficult to close the joints.
A useful work-around for this is to glue up your test cut before you even cut the fingers for the box. If everything works out in the test cut glue-up, it should be fine for your project. If it's hard to close with the glue applied, you might want to re-adjust the jig slightly for narrower fingers, make a new test cut and re-glue. I'll sometimes do this when working with a wood species I've never used before.
Apply glue to the tops of each finger.
You have to work somewhat fast when applying the glue, especially if there's a lot of fingers on a wide board. Spreading glue on just the tops of the fingers will save some valuable time and allow you to finish quicker.
I use regular yellow wood glue for most of my glue-ups, except for outdoor projects like window boxes which need a waterproof glue like Tightbond III.
Applying painter's tape to the inside of the box corners can save a bit of clean up and help prevent the glue from soaking into the wood if you're going to apply stain later on. I haven't bothered with it here, but it works pretty good.
The first two clamps are in place. The wood blocks spread the pressure evenly.
When you have all the corners glued and tapped into place, start clamping right away before the glue gets a chance to set.
Use wood blocks and clamps at both corners of each joint, tightening them evenly a little at a time to allow both sides of the corner to close fully and to keep the box from racking out of square.
If the box is fairly long on one side, use a spacer block on the inside of the box between the long ends to keep the boards from caving inwards under the pressure of the clamps. Do not overtighten the clamps, moderate pressure is adequate.
Make one last check and make sure your box is square and that all the joints are fully closed before leaving it to set up. Glue up the lid next and you're done.
Although I've no doubt made it sound more complicated than it really is, making box joints is not difficult. It takes a bit of practice to get proficient at it, but what doesn't.