Laser Cut Finger Jointed Boxes

One of the first real production projects we did was laser cut lamps for Christmas gifts. The ones we modeled ours after used a miter joint but we didn’t have a good way to cut a consistent angle across the length of the wood pieces. We opted instead for a finger or box joint.

One of the things that made the decision easier was this great piece of software called BoxMaker web based tool BoxMaker by Rahul Bhargava. You simply enter the overall dimensions you want your box to be, as well as the material width (thickness), the length of the notches/fingers, and cut width (kerf).

From there it outputs a PDF file that looks like the image on the right. The PDF format is based on Postscript and is therefore vector based. That means you can open the PDF in a vector program like Adobe Illustrator or Corel Draw and get the shapes as vectors, ready for cutting on the laser.

You will notice that there are only really two different pieces. The top and bottom are one design, and the four sides are another design. The position of the notches and shape of the top vary depending on the box size and notch length.

For the lamps, I made a box that was the same size as the overall lamp, and then took parts of the output and merged them into my lamp drawing.

A few tips:

• The cut width or kerf, is the width of the “blade” of whatever tool you’re using to cut. The laser kerf is approximately 0.005″. The material width refers to the material thickness. Materials vary, so I recommend measuring the thickness with a caliper, otherwise your notches could be too deep and the fingers will stick out slightly. When using acrylic, be sure to measure the material without the masking paper. Two layers of masking can add significantly to the material thickness.
• The application warns you that narrow notches can be delicate, and simply refuses to allow you to use notch widths equal or less to the material thickness. I find this annoying personally, warn me, sure, but let me do what I want within the limits of the software.

At some point, you will want to be able to draw notches yourself, whether to create thinner ones than the program allows, or to support odd shapes, etc. The trick is:

• Notches will be (1 x kerf) larger than what you designed them to be
• Fingers will be (1 x kerf) smaller than you designed them to be

You would expect it to be (2 x kerf) but the kerf is split evenly on either side of the line being cut. So for the laser, with a kerf of 0.005″, if I want the fingers and notches 0.25″ wide, I need to draw the fingers 0.255″ and the notches 0.245″ so in the end, they will both be 0.25″ and will fit together.

In the diagram to the right, you can see why this is (you may need to click the image to view it full size. The black line is what we told the laser to cut. The red line is the kerf, so everything red will be cut away and we will be left with the yellow areas only. As you can see, by making the notch smaller than it needs to be, and the finger larger than it needs to be, we end up with two yellow parts that fit together perfectly.