How to cut box joints by hand

How to make a box joint using hand tools, part 1

Sometimes we call them finger joints, but I guess those are other type of wood joints and that the actual name of what I will make in this post is box joints. And here in Spain we sometimes also call them finger joints o uniones tipo dedos o uniones de dedos. Maybe the right name in Spanish should be “uniones de lazos rectos”. Well, I have to say that recently I was given a (well deserved) little bit of a telling-off, explaining to me that box joints in Spanish are called uniones de lazos rectos. The truth is that I have seen these woodworking joints with different names, but definitely the names that I like the most and that make it clear what kind of wooden joints we are talking about are “box joints” in English and “uniones de lazos rectos” in Spanish. And that’s what we’re going to see in this post, how to cut a box joint. Or at least how I cut them. And I admit I am not very skilled making these kind of woodworking joints.

To cut a box joint in two boards with the same width and the same thickness I start by marking the thickness of one of the boards on the end of the face of the other board, and on both sides (1). Now I have to mark the dimensions of the pins I will cut on the line (2), distributing the pins in such a way that they are symmetrical with respect to the center of the line (the center of the board). With the help of the square I draw the perpendicular in the end of the board (3).

Mark the size of the pins of the box joint

And I continue the line on the end perpendicularly to the corner (4), and trace the perpendiculars to the end on the other face of the board (5). Just to avoid mistakes when cutting this box joint, I have to mark which are the pins I want to keep and which are the sockets I want to cut (6). And all this with using a pencil as sharp as possible.

How to cut the pins of the box joint

Now, with the fingernail on the line, I place the blade of the tenon saw close to the fingernail, so that the saw blade is just to the right of the line and on the socket I want to cut (7). And I am going to cut with the saw tilted back almost until I reach the horizontal line that marks the height of the pins (8). I then cut the other end of the socket with the saw to the left of the other line, making both cuts from this face of the board for all the sockets. And I rotate the board in the homemade bench vice (9). With the board turned, I cut again on the same cuts I already made, but this time with the tenon saw horizontally. I must make sure that I do not cut the horizontal line that marks the height of the pins (10). Now with a fret saw I cut a few millimeters above the horizontal line, thus removing most of the wood from the sockets I want to make (11 and 12).

With the chisel, mark the base of the sockets.

To finish working on this board I just need to clean the sockets between the pins. I lay the board flat on my workbench, and with a chisel I start marking on one face of the board right on the line that tells where the base of the sockets is.

Cut three-fourths of the board thickness.

And now with the flat side of the chisel facing the board, and with it perfectly vertical, I’m going to drive the chisel halfway through or a little more than half the thickness of the board. I start at one end and work my way along the line, overlapping one cut with the previous one. With a sharp chisel and the help of the wooden mallet, the chisel enters the wood without any problem.

Mark and finish cutting the sockets with the chisel.

I rotate the board and mark the line between the pins on the other side. This time, when I drive the chisel into the wood, I reach the cut I had made from the other side of the board. This way I can remove the wood I had left at the base of the sockets when using the fret saw. Due to lack of expertise and perhaps because the wood is very dry, or maybe the wood is not the most suitable for cutting these box joints, sometimes there are some “holes” in the center of the base of the sockets caused when the wood breaks. It is not something that worries me too much, since those “holes” will be hidden when making the box joint with the other board.

Use the chisel to clean the base of the sockets.

To finish cleaning the holes, with the flat side of the chisel always resting on the base of the sockets, I carefully pivot and move the cutting edge sideways and move it forward, cutting the protruding wood little by little.

Use the chisel to adjust the sides

And on the sides of the sockets, with the flat part of the chisel against the corresponding pin, and the chisel perfectly vertical, I will retouch the cut to leave it clean and as vertical as possible.

Draw pins on the other board to make box joint.

Now I can use the pins and sockets I cut on this board to draw the sockets and pins on the other board with which I want to make the box joint. But that will be in the next woodworking post in this blog. Or else, in the following video I show the complete process with all the steps I follow to make this wood joint.

-> Go to the second part.

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