Improved Box Joint Jig

My last jig worked so well I just had to build another, bigger and better 🙂  The silly thing is I have barely built enough boxes to justify the first one, but I  just couldn’t help myself. I’m an engineer at heart, and tinkering with designs is my idea of fun.

So here the design is in Sketchup in all its glory. I still haven’t found anyone else who’s used a similar design; not sure if that puts me in the inventive or naive category. Like my last attempt, this design works with just a single blade rather than needing a dado stack. This post will try to explain it’s operation a bit more than my last one.

(Left) Front view (Right) Rear view

I won’t bother writing too much about the build, as really it’s simple enough and not so different in principle to the last one. Plywood cut into an array of rectangular shapes, glued and screwed were appropriate to assemble. Plastic runners again, and some hand made adjustment knobs for the two stops. A few photos says enough for a build report. I did switch to a hardwood ply this time, as I felt it would be more stable which I wanted for this design. The whole thing is a little bigger, heavier, and so more robust and accurate.

All the various pieces cut to size, coming out of a single sheet of marine ply.
Most pieces glued and screwed together, taking advantage of square and accurate cuts to keep the overall jig square
Screws and no glue to attach these stops after it’s all assembled, otherwise you can’t get it together or apart again.

The build took longer than I  expected, as there was a lot of sanding and scraping involved to get the sliding parts to move smoothly enough. I resorted to just sliding parts back and forth dozens of times to wear them in. But in the end this thing actually looks  pretty impressive, and it works better than my first design.

The jig from the back, looking pretty awesome. 

So anyway, here’s how this sucker works. First understand the three main parts. There’s  base sled, and inner sled, and a cradle, or at least that’s what I’m calling them.


Base Sled: This is basically an cross cut sled with delusions of grandeur. It has a couple of adjustable stops built into the back, which restrict the lateral movement of the inner sled. Unlike my previous design, the stops are at the back so are protected from sawdust build up which could effect the accuracy of the cuts.

Inner Sled: The inner sled nests into the outer sled, and moves sideways between cuts to create the width of cut you need for the box join. A tiny wooden key is glued in place, sized to about the saw kerf (about 3mm square) and placed just off to the side of the centre. The strips at the back hook over the outer sled keeping it square and allowing it to slide left and right.

Cradle: The cradle is really an optional part that slips over the inner sled to help line up the parts between cuts. You can use it to clamp the boards, or just to help keep a the boards lined up nicely against the key on the inner sled. 

    Now onto the actual  operation. So after a few test cuts to get the stops adjusted, it’s just a matter of repeated cuts. For each slot you use one side of the prior slot to register the next position. To get the required slot width, you move the inner sled sideways between each cut.

    To do any micro adjustments, you use a strip of paper to help adjust the stops just enough to get a snug fit of the box joins. Once it’s setup however, it’s rock solid for repeatability. My only complaints of the cut was the fault of my saw blade, which is not a flat top tooth grind, so the bottom of the cuts aren’t perfectly flat. 

    Close up of the finished cuts. (Teeth deliberately cut over length.)

    Why I like it:

    • Works with a single saw blade rather than needing a dado stack. (yes this makes it a little slower as you need to make multiple passes, but not everyone has a dado stack)
    • Is fully adjustable for any tooth width and spacing. You can “fine tune” using a piece of paper between the stops and the cradle to make micro adjustments.
    • Easy to build with all the major parts being simple rectangular cuts.
    • I came up with it 🙂

     (Update: please see my Facebook page for a video of the jig in action if you don’t follow my description)


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