Yet Another Box Joint Jig - Free Plan

Like most everyone in the world, I've posted a few of my projects on LumberJocks.com. So far, the most popular one has been this box joint jig. The basic idea came from Matthias Wandel's original box joint jig, but instead of a crank like Matthias used, I put a knob with a dial on mine to move the workpiece back and forth. I thought this was a little bit simpler and a little more versatile than Matthias's version with the crank.

The jig is built by adding a sliding carriage to the rear fence of a standard crosscut sled. To use it, you clamp the your workpiece to the sliding carriage, and then turn the knob to precisely position the carriage for each successive cut. Each mark on the dial represents 0.002" of movement, so with a little planning, you can make any sort of box joint you want. More on that later.

The Details

If you want to build one of these yourself, the following pictures link to drawings that give the dimensions of the jig as I built it.  The table on my saw is about 27" wide, and the blade is about 16" from the left side of the table.  If your saw is substantially different, you may need to adjust the dimensions of the jig to suit your saw.  In particular, you must make absolutely sure that your saw cannot cut into the jig's metal lead screw.

Rear Fence

Bearing Block

Carriage

Lead Screw

Dial

Building the Jig

To build the jig, start by making a standard crosscut sled for your table saw.  Make the rear fence 2-1/4" tall and 1-1/2" thick, as shown in the first drawing above.  Make sure the rear fence is square to the saw blade.  This video shows a quick and straightforward way to square the fence on a crosscut sled.

Next, find a bearing to support the end of the lead screw near the dial.  Ideally, the bearing should have an inner diameter of 1/4" to match the 1/4-20 threaded rod used for the lead screw.  If you happen to have a bearing with a larger inner diameter, you can make it work with a bushing as shown in the photo nearby.  My bearing happened to have an outer diameter of 1-3/8".  Anything smaller than 1-1/2" or so would work.

When you have found a suitable bearing, make the bearing block shown in the second drawing above, except that the hole should fit your bearing.  Center the hole in the bearing block, and size it so that your bearing fits tightly into the hole.  Attach the completed bearing block to the left end of the crosscut sled's rear fence, as shown in the first two photos above.

Now make the carriage as shown in the third drawing above.  Use extra care when making the little hooks that ride over the fence.  You want the carriage to slide freely on the fence, but with as little play as possible.  There's nothing special about the big mortise that houses the lead screw;  it was just easier for me to make it that way than to try to drill a long hole lengthwise through the carriage.  (I guess there's also some comfort in being able to see the lead screw, in order to make double dang sure that you're not going to cut into it with the saw blade.)  Embed a 1/4-20 nut in the end of the carriage as shown in the drawing and in the first photo above.

Important:  When you have the carriage complete, add a block of wood to the backside of the rear fence as shown in the second photo above.  The purpose of this block is to protect your fingers from the saw blade at the end of each cut.  The block must fully enclose the blade as it comes through the back of the fence.  You will need to notch the top corner of the block as shown to allow the carriage to slide freely back and forth.  Do not use the jig without this block in place.

To make the dial, click on the image above to open a .PDF file that contains a full sized image of the dial.  When you print the image, the diameter of the dial should be 3-1/2".  Paste the printed image onto a piece of 1/8" plywood (or something similar), cut out the circle, and drill a 1/4" hole in the center.  Next make a knob of some sort, and cut the lead screw to length from 1/4-20 threaded rod stock.  Attach the knob and the dial securely to one end of the lead screw.  The knob and the dial must not be allowed to rotate on the lead screw.

Next, thread the free end of the lead screw through the bearing and install a washer and a pair of jam nuts as shown in the photo nearby.  When you're using the jig, you'll need to apply slight pressure to the carriage to ensure that the jam nuts ride tight against the bearing.  If you want, you could add some sort of spring arrangement to take care of this automatically.

Finally, position the carriage over the rear fence and thread the lead screw into its embedded nut by turning the dial.  Check one last time that your saw can't cut into the lead screw, and you're ready to go.

Measuring Your Kerf Width

With the jig complete, you can now make precisely spaced crosscuts by clamping your workpiece to the carriage and then turning the dial to move the workpiece after each cut. Before you can do much of anything useful, though, you need to know the width of the kerf that's taken by your particular saw blade. If you have a dial caliper or one of those fancy electronic ones, this is fairly easy to measure, as follows:

1. Use your saw to rip a piece of scrap two or three inches wide. Make sure the edges are both straight and parallel to one another, then use the caliper to take a precise measurement of the width of the scrap.
2. Rip the scrap down the middle into two pieces.
3. Reassemble the two pieces next to each other, and measure their combined width.
4. Subtract the combined width of the two pieces from the width of the original piece. This difference is the width of the kerf that was taken by your saw blade.

If you don't have a caliper, you will have to take as good a guess as you can, then make some trial and error adjustments later when you actually go to use the jig.

Cutting a Simple Box Joint

Probably the simplest box joint to cut is one where the fingers are the same width as the saw kerf.  That would be roughly 1/8" for a normal blade, or whatever you want if you are using a dado stack.  In any case, the joint involves a series of cuts that are evenly spaced by twice the width of the saw kerf.  So let's suppose that your kerf is 0.132" wide.  That means you want to move the carriage by twice that amount, or 0.264", after each cut.  So how do you do that?

If you're a math whiz (or maybe a machinist), you might realize that each dot on the dial represents 0.002" of carriage movement, and that the numbers on the dial represent thousanths of an inch.  From that, you could figure out how far to turn the dial based on the numbers, but doing so would involve some error-prone arithmetic for every single cut.  Fortunately, there's an easier way that's based on the pattern of colored arcs and dots on the dial.

Here's all you have to remember:

• Rotating the dial by one dot's worth moves the carriage 0.002".
• Rotating the dial by one arc's worth moves the carriage 0.010"
• Rotating the dial one full turn moves the carriage 0.050"

So, in our example, to move the carriage by 0.264", you would turn the dial five turns to move it by 0.250", then one arc to bring it to 0.260", then two dots to reach 0.264".  So "five turns, one arc, and two dots" is all you have to remember (or write down), and it's the same for every cut.  This is actually a lot easier to do than it is to explain.  Once you've played with it a while, you'll do it without thinking.

Cutting More Complicated Joints

With the jig, you're not limited to any particular finger width or spacing, although more complicated joints do involve a little bit of planning.  But the fundamental process is straightforward:  Figure out how you need to space your cuts, turn the dial to put the workpiece exactly (!) where you want it, and have at it.