One of the things I really like about box joints is how great they look on a small project. With their rows of evenly spaced pins, box joints have a traditional, almost old-fashioned appeal.
Box joints aren’t just good looking. They’re also very strong. And box joints are simple to cut — all you need is a table saw.
With all that box joints have going for them, they sound like the perfect joint. But there is one thing I don’t like about them — and that’s the time it takes to cut them. Even on a small project, it can be a tedious process.
Stacked Cutting – To save some time, I tried something a little different when I cut the box joints for a recent project. I stacked the mating corners together as I cut the joint. This process worked great. I was able to get a good, tight fit in about half the time.
The reason this stacking process works is that unlike most other woodworking joints, both halves of a box joint are identical. As you can see in the drawing below, the only difference is that the pins on one piece are offset from the pins on the other so they mate up with the slots.
Even though both parts of the joint are cut at the same time, the trick to getting a good fit with box joints is precision and consistency.
The best way to achieve this is to use a jig. Once the jig is adjusted properly, cutting the joints is almost automatic. (If you don’t already have a box joint jig, check out the easy-to-make jig shown below.)
Of course, adjusting the jig is where the challenge lies. Most box joint jigs use an index key to determine the size and spacing of the slots and pins. But in order to get a perfect fit, you have to spend a little bit of time adjusting the position of the index key in relationship to the blade of your saw. And the best way to do this is to cut some box joints on a couple of test pieces before moving on to your actual workpieces.
Setting Up the Jig – Before you can use the jig, you’ll need to cut a slot for the key that matches the width of the pin, as in Figure 1. Then before you use the jig for the first time, you need to make an initial cut through the front face of the jig with the key spaced approximately the width of the key away from the dado blade as in Figure 1a. To set the height of the blade, I use a piece of the same stock that I cut my workpieces from and set the blade slightly higher (about 1⁄32") than the thickness of my workpieces. This way, the pins will stand a little proud, and you can sand them down flush later.
Test Pieces – Rather than diving right in and cutting the box joints on my workpieces, I like to start with a couple of test pieces. But it’s important that these test pieces be the same thickness and width as your actual workpieces.
Why? Even though stacking the workpieces helps with getting a good, consistent fit, I find it best to check the entire setup by cutting a full-width test joint.
Step 1. After adjusting the spacing to match the approximate width of the key, set the first test piece against the index key and cut a slot.
Step 2. Next, flip the first test piece around and set it over the key. Butt the mating test piece against the first and cut another slot.
Step 3. Now you can cut the remaining slots by moving both pieces together as you cut all the remaining slots across both pieces.
Test the Cuts
The first step is to cut the slot at the bottom of the first workpiece, like you see in Step 1. Then before cutting any more, flip the test piece around and fit it over the key. Next, butt the mating test piece against it, like you see in Step 2, and cut the open slot at the end. Now you’re ready to cut all the box joints. In Step 3, you can see how I stacked both test pieces together before cutting all the remaining box joints. This way, any slight variation in pressure or position is accounted for on both pieces at the same time, resulting in a more accurate fit on wide workpieces. Once you’re done, try fitting the pieces together and compare the results with the drawings below. If the pieces fit together perfectly on the first attempt, great. You’re ready to move on to your actual workpieces. But chances are that you will have to make an adjustment to your jig by moving the fence (or the index key). On our jig, this is just a matter of loosening the screws at the back of the fence, nudging the fence over a bit (Figure 1b) and then tightening the screws back down. The difference between a joint that fits perfectly and one that is too tight or too loose is only a few thousandths of an inch, so move the fence in very small increments. After each adjustment, make another series of test cuts. You may need to repeat this process a few times to “zero in” on the perfect fit.
Checking the Fit
Loose. A gap between the pins indicates the index key needs to move away from the saw blade.
Tight. If you can’t fit the pins together at all, you’ll need to slide the index key toward the saw blade.
Short Pins. Raise the dado blade if the pins don’t extend slightly past each other when mated together.
Perfect and Proud. With a perfect-fitting box joint, the pins will fit smoothly together. Then just sand them perfectly flush with the surface.
Cutting the Box Joints
Now that you have the jig adjusted for a perfect fit, you’ve completed the most time-consuming and challenging part of the box joint process. But there are a couple things to keep in mind when it comes to cutting the box joints on your actual workpieces.
Visual Appeal – When building with box joints, it’s more important to me for the project to be visually accurate.
The important thing is that there’s a full pin (or slot) at the top and bottom of each part.
Extra-Wide Pieces – To do this, I typically start with workpieces that are extra wide and then trim them down after the box joints are cut, see Step 4 below. This way, no matter what the actual size of the pin or slot ends up being, I always end up with a full pin (or slot) at the top.
Step 1. After labeling all the parts of the box, begin by setting the bottom of one of the side pieces against the key. Hold it tightly in place against the front face of the jig and then cut the first slot.
Step 2. Now, flip the side piece around and fit the slot you just cut over the index key. Next, butt the bottom of the mating piece (the front or back) against the side piece and cut the first slot.
Step 3. At this point, stack the two pieces together so the slots fit over the index key, then clamp them together. Now cut the next slot. Repeat this process until all the slots on this end are cut.
Step 4. After repeating the process for all the corners, you can trim the waste at the top of each piece to leave a full pin and slot. Then lay out and cut the grooves for the top and bottoms of the compartments.
Sure, there are times when a workpiece has to be a specific size, like when building a drawer that has to fit an opening. But even in these cases it’s still best to start with an oversize piece. Once you’re done cutting the box joints, simply trim the pieces so the pin (or slot) is the same size at the top and bottom.
Label Pieces – To keep myself organized, another thing I like to do is label all the parts and number the joints at the bottom of each piece (Figure 2) so I can be sure to stack the proper parts together with the correct edge against the index key on the jig, as in Steps 2 and 3.
Cutting the Joints. – When cutting the box joints, the workpieces can “ride up” the dado blade slightly, resulting in slots of uneven length. So I make sure to keep downward pressure on the workpieces. Unfortunately, even if your box joints fit perfectly, you may run into another problem — chipout.
Chipout. – The jig helps take care of this problem, but there are a few other things you can do to minimize it. First, make sure your blade is sharp. And don’t push the pieces through the blade too quickly. If you’re getting a lot of chipout, try slipping a scrap piece of hardboard behind the workpiece so that each cut is backed up completely.
Smooth Assembly. – After cutting the slots on all the parts, you’re ready to assemble the box. This can get a little frantic, so I take a few steps to minimize any problems.
First, to make cleaning up any glue squeeze-out easier, I tape the inside edges of the pieces (Figure 3 below). When the glue is dry, you can carefully “peel away” any excess.
Because you have to spread a lot of glue, I like to buy myself time by using white glue instead of yellow glue. White glue sets up slower, which helps when there are a lot of box joints, like on the sharpening box.
An “acid” brush (available at most hardware stores) makes it a little easier to spread the glue (Figure 3a). And go light on the glue. Even a little bit will create a strong hold. And when you’re ready to clamp up the box, check the information below for a few tips.
Clamping Tip
Clamping up a project with box joints can be a little tricky. Because the ends of the box joint fingers stand proud of the sides, you can’t really apply clamps directly on the corners. The answer is to use clamping blocks that will allow you to position the clamps close to the joints (photo above).
Keep in mind that each joint needs to be clamped in two directions in order to pull the pieces together. So you’ll need a minimum of four clamps for even the smallest project. Bar clamps work well because they allow you to concentrate the clamping pressure in a straight line. For small projects like the sharpening box, I use band clamps.
On the sharpening box, the top and bottoms help keep everything square. If your project doesn’t have a top or bottom, it’s a good idea to use a squaring form made from a piece of foam or particle board to keep things aligned.
Finally, to hide the holes where the grooves for the drawer bottoms come through, cut some plywood plugs (they’ll match the plys) and glue them in place (Figure 4).
