Designing Box Joints

copyright 2016

Rick Forney

Introduction

Box joints (also known as finger joints) consists of rectangular fingers and notches on the edges of panels. When two panels are joined together, interlocking the fingers, it gives a single-row checker-board look, which can make a very strong joint.

Box joints have been used for over a century, mainly used on shipping containers and storage boxes.  Because of this, they were not thought of as being a nice and elegant joint, just very functional.  Then along came cardboard, a cheep and sturdy product.  Wooden boxes with box joints lost their popularity.  Now people think of them as visually appealing, interesting, and artistically useful, along with being very functional and strong.

Designing Box Joints

A good box joint design will be strong, and aesthetically pleasing.  More elaborate box joints will have angled sides, and/or varying widths of the fingers.  More artistically pleasing boxes have one side being of a light color wood, and an adjacent panel being of a dark color wood. Most of the time, the fingers of the box joint will all have the same width.

Always make sure the dimensions of the project material (length, width, material thickness), and the kerf width (1/8”, 3/32”, 1/4”, 0.275”, etc.) of all tools are known.  Assumptions will lead to headaches, scrapping parts, and having to start over in the design and manufacturing of the project.  So measure before designing and using the material and cutting tools.

Know the dimensions (height, material thickness) of the sides of the box.  The height of the box will determine the width of the fingers of the box joint, or vise-verse.  Normally, all of the fingers have the same width, and the number of fingers will be a multiple of the height of the box.  For the project to look symmetrical, the width of the fingers should be the same as the thickness of the wood.  Therefore, the height of the box is then governed by the width of the fingers.  And the width of the fingers are the same as the thickness of the material.  

Know the width of the saw blade kerf.  Do not assume knowing the width of the blade kerf. This will lead to either loose joints, or tight joints that are very difficult to assemble.  If unknown, then make a test slot in a scrap of wood using the blade, and measure the width of the slot.  

As always, rules are made to be broken.  The artist may want to vary the width of the fingers to have a special appeal.  Elaborate box joints can vary the widths of the fingers.  It is best to have the fingers that are at the ends of the joint edge be the same width as the material thickness.  These special cases will be discussed later.

Box Joint Ratio

The “box joint ratio” is the ratio between the material thickness and the width of the fingers. This is critical for functionality and being symmetrically appealing. During the manufacturing of the adjoining sides, the fingers and corresponding notches must be exactly the same width. The examples given here have all the fingers for their respective box the same width.  

As an illustration, the material thickness for each of these examples is 1/4”, but the idea of the proper box joint ratio should be the same.  

Figure A

Also, the boxes were designed to have a sliding lid, see figure A.  A sliding lid with the lid removed prominently shows problems that can arise.

In the manufacturing of a sliding lid box,  a 1/4” would be trimmed from the top of the front panel, allowing the lid to slide free, and be removed.

Figure B

In figure B, the fingers are 1/4” wide.  The finger width are then the same as the material thickness.  It also gives an appealing look of symmetry, with all fingers and notches the same width. 

After trimming 1/4” off the top of the front panel, a full finger on the sides is left, giving a since of symmetry.

When the width of the fingers are the same as the material thickness, then the box joint ratio is 1:1.

Figure C

Figure C shows a box joint ratio of 1:2.  The width of the fingers are twice that of the material thickness.  With the material being 1/4” thick, the fingers are 1/2” wide.

Ripping off 1/4” off the top of the front panel exposes only half of the top notch.  This isn't very symmetrical, but it is still functionally acceptable.

If this box would not have a sliding lid, and the front panel was not trimmed, then the symmetry would be complete and it would be aesthetically pleasing.

The end fingers of this 1:2 box joint ratio makes this box joint aesthetically lazy as compared to the 1:1 ratio.

Figure D

Figure D shows the material thickness being twice that of the finger width, making a box joint ratio of 2:1.  The fingers are 1/8” wide.

When the top of the front panel is trimmed 1/4”, the top fingers of the sides are not attached to anything.  This makes them structurally week, and can easily break off. The top fingers should be trimmed off, but it would give the box an aesthetically incomplete feeling.  

If this box would not have a sliding lid, and the front panel was not trimmed, then the symmetry would be complete and it would be aesthetically pleasing.

This ratio is functionally acceptable, but it makes the box joint aesthetically busy, which may be the desired effect.

Figure E

The previous three examples shows good matching box joint ratios between the material thickness and the finger width.  The ratios match well with no ill effects.  The ratio between the material thickness and the saw blade kerf should be a whole multiple: i.e. 1:1, 1:2, 2:1, etc.

Box joint ratios with fractions leaves very thin elements that can give problems.  Figure E shows a ratio of 1:1.1, and what can happen when the ratio is not a well rounded multiple.  There is a sliver of material left on the top finger after the slot for the top lid has been cut.  The saw blade kerf was 0.275”, thus defining the finger width, and the material thickness was 1/4” (0.25”). This sliver would give problems and may break off.  It would give cause for concern and make a bad quality product.

Dimensioning the Box

The dimension of the wooden box is left to the maker.  The box could be a square cube (where all of the sides have the same dimensions), or the front being square, and the sides longer (like a sliding pencil box, see figure A).  The maker can also make a series of nesting boxes (like the Matryoshka nesting dolls).  The maker may want to follow the golden dimensions rule, where one edge of a rectangle is 1.618 times longer than the other (i.e. one edge is 5” and the other is 8-3/32”).

The dimensions of the box should follow the box joint ratio to make a well manufactured box and an aesthetically pleasing box, with good symmetry.

Box Joint Jigs

There are many box joint jig designs on the internet.  Some are simple, and can only be used for a fixed finger dimension (width and height).  These simple jigs are great for getting familiar with box joints.  However, I recommend manufacturing a box joint jig that utilizes a lead screw, such as the “Ed Stiles” or the “Lynn Sabin” box joint jig.  They are easy to make, give good results, and allow the maker to be versatile with the design of the box joint without making another box joint jig.

The lead screw is used for moving the part, allowing the width of the fingers to be varied over the width of the part.  As long as the first and last fingers follow a good box joint ratio, there should not be any problems.  Varying the width of the fingers should produce some visually interesting products.