Introduction to making box joints
In the history of woodworking joinery, box joints are a fairly recent arrival. That’s because before the circular saw blade, cabinetmakers didn’t think that the square-shouldered joint was worth the effort. They viewed box joints as harder to cut than dovetails. Plus, the joints required glue. True to its name, the box joint found its first practical use later in mass-produced boxes and crates for storage and shipping purposes.
Because the fingers multiply the gluing surface area, box joints are super strong, making them the perfect choice for utility boxes and tool chests. Their decorative appearance is a bonus, lending a unique design element to jewelry boxes. The best thing is that with the right jig, you can fashion box joints quickly and easily with either the table saw or router. Since the table saw offers more flexibility and variety, we’ll explore this option first.
Box Joints with a Table Saw
To make box joints at the table saw, you’ll need three items. First is a dado set which will enable you to make almost any size joint. Second, you’ll need a zero-clearance insert to prevent tear-out. Last but not least, you’ll need a box-joint jig. You can buy a ready-made jig, but you may want to start off with a shop-made version that can be fastened to your miter gauge. Once you have these basic items, you’ll be ready to get started. In the shop we will be using an Incra brand jig.
Dado set specifically made for cutting square box joints. This set will make either 1/4" or 3/8" width joints. Because they have a flat kerf the joint can be cut in a single pass. More information can be found on the manufacturer's website: http://www.freudtools.com/products/product/SBOX8
Shopmade zero clearance table saw inserts greatly improves the safety and efficiency of the table saw. Table saws come with an insert. However, they are generic and often have a gap alongside of the saw blade which can lead to tear-out and also cause more sawdust to fly. With a Dado blade you will also need an insert with a wider than normal opening, to match the cut width of the blade. You can make your own zero clearance table saw inserts from material readily available in the woodworking shop. Baltic birch plywood, solid wood, or MDF can be used. The choice is yours. When you have zero clearance table saw inserts, you will be working more confidently as well as more safely.
INCRA I-BOX JIG can be used to make joints on the table saw & router table. It can create a variety of box joints & finger joints. Its revolutionary design allows faster setup, greater versatility, and improved user safety compared to traditional shop made jigs. Additionally it can be used to cut square moldings and even wooden hinges. More information can be found on the manufacturer's website: https://www.incra.com/precision_fences-ibox.html
Gluing Box Joints
Gluing box joints can be time consuming due to needing to apply glue to each finger. Small boxes can generally be clamped in two directions but large boxes can quickly prove challenging to keep square while applying good pressure. For this reason we use plastic corner cauls and ratchet straps. Uniformly spaced pressure points clamp individual box joint fingers at the corners to give you the tightest joints possible. Simply align the pressure points with box joint fingers and tighten in place with one or more ratchet straps. Slots on the outside of the cauls are made to hold pennies firmly in place to support your straps at varying heights. Plastic cauls stand 6-1/4" (1/4" Cauls), 6-3/8"(3/8" Cauls) and 6-1/2" (1/2" Cauls) tall.
Gluing up box joints can be a tedious job, but there are a few things you can do to make the task easier. The first is making sure the joint fits together well. Joints that are cut properly should come together smoothly. The big advantage of box joints is the large amount of gluing surface the interlocking fingers create. So it only takes a small amount of glue to create a rock-solid joint. In fact you don’t need to put glue on every surface. I like to use an artist’s brush to apply a small amount of glue to the top of each finger. As you assemble the joint, glue spreads to other surfaces. But there are a couple of ways to control it. You can place masking tape next to the pins before you glue. This will trap any squeezeout on the inside corners where it’s tough to get at.