Box joints: finger width, fit and layout

A box joint, or finger joint, interlocks square fingers cut in the ends of two boards. It is one of the strongest corner joints for boxes and drawers because every finger adds two long-grain glue faces, and it is easy to cut accurately with a simple jig on a table saw or router table.

Choosing a finger width

The finger width is usually set by the cutter you have: a dado stack, a straight router bit or a single saw blade. Common widths are 1/4, 3/8 and 1/2 inch.

• Proportion. Fingers roughly equal to the stock thickness look balanced: 1/4- or 3/8-inch fingers on 1/2-inch drawer sides, 3/8- to 1/2-inch fingers on 3/4-inch stock.
• Strength. Narrower fingers mean more glue faces and a stronger joint, but more cuts and more places for error.
• Appearance. Wide fingers read as bold and rustic; narrow fingers read as fine.

Laying out whole fingers

A box joint looks best when both edges of the corner end with a full finger, and when the top and bottom match. That happens when the width of the boards is an odd multiple of the finger width.

board width = finger width × an odd number

A 4-1/2-inch drawer side with 1/2-inch fingers has nine fingers: five on one board and four on the other, with full fingers at the top and bottom of the first board.

If the width is not a multiple, you can:

• Adjust the part width to the nearest odd multiple, which is easiest when designing from scratch.
• Change the finger width so it divides the part evenly.
• Accept a partial finger at one edge, ideally the bottom, where a drawer bottom groove or a base molding hides it.

Finger length

Fingers are as long as the mating board is thick. Many makers cut them 1/64 to 1/32 inch long and sand or plane them flush after glue-up; this is easier than trying to hit the thickness exactly. Set the blade or bit height from an offcut of the actual stock, not the nominal thickness.

Tuning an indexing jig

The usual jig is a sled or mitre-gauge fence with a key, a small strip exactly as wide as the cutter, fixed at a set distance from the cutter. You cut a slot, move the board so the slot drops over the key, and cut the next slot.

The width of the fingers left behind is the gap between the key and the cutter. That gap sets the fit:

• Joint too tight: the fingers are wider than the slots. Move the key slightly toward the cutter.
• Joint too loose: the fingers are narrower than the slots. Move the key slightly away from the cutter.

Any error repeats once per finger. A key 0.005 inch too far from the cutter adds up to 0.045 inch across nine fingers, which is enough to stop the joint going together. Make the adjustment tiny, cut two test pieces from scrap of the same thickness, and repeat until they slide together with hand pressure.

The first board starts with its edge against the key, so it begins with a finger. The mating board has to begin with a slot instead; jigs offset that first cut in different ways, so follow your jig’s method and check the pair on scrap before cutting project parts.

Drawer bottoms and grooves

A groove for a drawer bottom that runs the full length of a side will show at the end of one finger. To hide it:

• Plan the groove so it runs inside a finger on the side and exits through a slot on the front and back, where it is covered by the mating finger.
• Or stop the groove short of the ends.
• Or run it through and glue a small filler into the exposed end after assembly.

Gluing up

Box joints have a lot of glue surface, which is good for strength and hard on working time. Use a glue with longer open time for big boxes, spread it on the finger cheeks with a small brush, and have clamps ready before you start. Clamp gently across both directions and check the box for square by measuring the diagonals.

In GrainDraft

Box joint is one of GrainDraft’s eight joint families. You set a finger width and the band of the joint; complementary fingers connect the two boards, and the shop packet prints which pieces to cut away and which to keep. Supported box joints can also be exported as Shaper Origin cut files. Because the finger layout follows the modelled part width, it is worth choosing part widths that are odd multiples of the finger width while you design.

Related: sizing a mortise and tenon, dadoes and rabbets in plywood.