The Ultimate Box Joint Jig is a big step forward in box joint jig design. It is easy to build using commonly available materials, and it uses no complex joinery – all of the parts are easy to make. It has a massive capacity, stock up to 34″ wide can be machined with this jig (in two passes), with a single pass capacity of 17″.

Even with its large capacity, the jig is compact and convenient to store when not in use.

Joints from a tiny 1/16″ (using a thin kerf blade) up to 1″ are possible and the jig will cut joints of any size with just a standard 1/8″ thick saw blade. A dado blade would save time for wider joints, but is not strictly necessary, making this jig perfect for saws that cannot accept a dado stack.

It will cut multiple parts in a single pass, as many as will fit between the fence of the jig and the blade of your saw, so no problem to cut the four pieces of 1-1/2″ thick stock at once.

It’s probably becoming clear why I’m calling this one the “ultimate”!

To begin, I made a video going through the assembly of the jig from beginning to first test cut:

The base of the jig, part A:

Most of the jig is made from 3/4″ plywood, since plywood has better dimensional stability than solid wood, making it less susceptible to seasonal expansion and contraction from changes in humidity. The parts are simple with straight cuts, and the base of the jig is a perfect example – just a simple rectangle.

There was a missing dimension in the plan (now corrected) on part B – please see this detail.

Part B is a bit more complex, in that it has a pair of handle holes that need to be cut out. I used a 1″ forstner bit, then cut out the material between with a jigsaw:

Rounding over the edges of the handle holes and directly in front of them is a good idea, making them a lot more comfortable to grip. I used a round over bit in my router table to do this:

The rest of the parts that make up the base of the jig. Again simple, straight cuts:

No fancy joinery here, just glued and screwed butt joints to make assembly as quick and easy as possible. Save the fancy joinery, and the time it takes to do it, for the projects that need it.

The parts for the carriage are simple as well:

Parts BB and CC have rabbets to receive the 1″ threaded rod:

And the final plywood part is the side fence for the carriage, part EE:

The rabbets in parts BB and CC are cut so that the threaded rod fits in the groove that forms when the two parts are put together, and it’s a good idea to check that before gluing the threaded rod into part CC:

Assembly of the base starts by gluing and screwing part E to part A, then gluing and clamping part D to both. Two things to watch for are that parts A and E are flush with each other on the bottom (very important!), and that part D is oriented correctly – the plywood laminations should be vertical to match part E.

Before gluing on part C, it has to be checked and adjusted so that it will be flush with the top of part B when assembled. This is the only place on the jig where allowances have to be made for the random thickness of plywood from different manufacturers.
Normally, most 3/4″ plywood is less than 3/4″ and you will have to trim a small amount off of part C (red arrow) to make it flush with the top of part B:

With part C adjusted, it can be glued in place on the base, along with the deck support blocks G1 to G4. I used plenty of clamps and left it to dry for several hours before moving on to the next step.

When the glue dried on the previous assembly, part B can be glued and clamped on:

Notice that a lot of the assembly uses just glue, and not screws. These glued joints will be more than strong enough, so no need to add screws. If you decide to use extra screws either to speed up the construction time, or to give in to irrational doubts about the strength of these joints, be mindful of where the blade will cut into the jig and do not put any in this area (important!).

The final step in the assembly of the base is to fasten part F using glue and screws. I shimmed part F above part DD with strips of paper folded over into three layers, and found that to be the perfect thickness.

The carriage was assembled next, being sure to check that the side fence (red arrow) is perfectly square to the fence:

After the base and carriage were fully assembled, I finished them with two coats of water based polyurethane, lightly sanding with fine sandpaper between coats. Water based polyurethane is recommended because it dries fast and leaves a slick, non-sticky surface.
On the carriage, I left the parts that mate with the base unfinished, and applied a thin coat of petroleum jelly instead to act as a lubricant. The petroleum jelly will also act as a low grade finish for those areas, providing some protection against moisture and should be reapplied periodically.

Next, the parts that make the jig work, starting with the locking bar. Made from solid hardwood, the cutout is made so that it will flex (like a spring) more easily:

You may have to file the edge thinner on the locking washer so that it will seat fully in the threads on the rod (red arrow):

Slots are made in the end of the locking bar using a 3/16″ drill bit. These need not be perfect, since they are for a one-time adjustment of the locking bar:

The advance lever is cut to a point from more solid hardwood and a slot is drilled for the pivot screw:

Again, this doesn’t have to be perfect, and will likely require some small adjustment to work correctly.

The lever wing is actually one of the off cuts from the tip of the lever (handy!) and is just glued to the side:

The photo above shows the correct orientation for these parts on the deck of the jig. The wooden spring is a bit less than 1/16″ thick and I’ve found that the perfect thickness for it. Clear, straight grained hardwood is recommended for the spring and it should last for a very long time.

To line up the jig on the saw, I set the carriage to the start position and put that against the blade. I moved the table saw fence over against the jig and used that to square the jig to the saw. The hardwood guide bars are then glued to the bottom of the jig with fast setting epoxy. After the epoxy set, screws were driven through the guide bars and into the base to secure them.
The jig is now ready to use:

I didn’t waste any time trying it out. I put a standard 1/8″ saw blade in my table saw and set the jig to move 1/4″ per click of the advance lever, and cut this perfect 1/8″ box joint in two pieces of maple:

Later, I used a thin kerf 8-1/2″ blade to cut these 1/16″ box joints:

The jig allows the carriage to over-shoot the base on both sides, giving it it’s huge capacity in such a small footprint. The jig is just 18-1/2″ long, 6-1/2″ tall and 10″ deep (not including the guide bars):

 

Set Up And Use The Ultimate Box Joint Jig

Here’s a video going through the set up of the jig a few sample cuts with different blades:

To cut a 1/16″ box joint, the jig is set to advance 1/8″ per click and a thin kerf 7-1/4″ saw blade is used. The kerf of the blade is just a tiny bit bigger than 1/16″, and will cut joints with the correct clearance. If your saw has a lot of runout (wobble), you will need to correct that before trying this joint size, since the slots that it will cut will be too wide:

These were cut in hard maple, 3-1/2″ wide.

Cutting 1/8″ box joints is easy with a standard 1/8″ blade. Again, if your saw has a lot of runout, that will effect the fit of this joint. Here I’ve cut joints into plywood that is 17″ wide:

Setting the carriage travel per click is easy by adjusting the big cam. When you have it done for of the three (1/8″, 1/4″ and 3/8″), making a mark on the cam and the deck will save set up time in the future.

Very nicely fitting 1/4″ joints made with a single blade. It’s a good idea to put a scale on the deck of the jig to make cutting with a single blade easier. A piece of measuring tape or even a printed scale pasted on will work:

My first attempt at 3/4″ joints yielded a loose fit from using the thickest shim that came with the dado set. It’s a case where you will have to trial and error the best fit on a test cut, using different size shims until you have it dialed in. Once you have a good fit, writing down the shims used for that size saves time in the future.

My second try on the 3/4″ joint is much better. When using a dado stack, the shims are used to fine tune the fit of the joints, since the jig operates at fixed points indexed from the threaded rod.:

Tips And Accessories For The Ultimate Box Joint Jig

I made a video going over the accessories and tips for the jig:

As some builders will have a problem finding a 1″ threaded rod to use for this jig, I came up with an easy way to make an alternative from wood, very similar to the division plate in my advanced box joint jig. This will also be very helpful for those in places where only metric size blades are available, since it can be made with a spacing that works with the blade size. For example, if you are using a 2.5mm thick blade, the slots can be cut 5mm apart. This will allow you to cut 2.5mm, 5mm, 7.5mm, 10mm, etc. box joints using that single 2.5mm blade.

The first thing I did was tape my framing square to my saw sled so that it won’t move. I left just enough space between the edge of the square and the fence of the sled for the wooden rod:

The wooden rod is 1″ square and a bit longer than the jig. It is sized to be a perfect fit inside the groove in the carriage on the jig in place of the threaded rod.

The minimum recommended spacing for the slots is 1/4″ (~5mm for metric) and to cut them, I line the end of the rod up with the marks on the square. This will be incredibly accurate and is very easy to do. Fast too, it only took five minutes to make all of the cuts:

When making the slot cuts, I used a thin kerf blade so that a regular 3/16″ washer (used on the end of the locking bar on the jig) will fit in without much play. A small amount of play is ok and unavoidable and will have very little effect on the fit of the box joints. Washers normally vary in thickness, so it’s a good idea to buy a few and find the one the fits best.

Next up, a simple secondary fence for cutting small parts on the jig. It is nothing more than a piece of 3/4″ plywood with a simple side fence and a clamping bar:

The side fence positions the stock 1″ from the jigs fence and is just two pieces of wood glued to the plywood backer. The clamping bar works with two carriage bolts with fender washers and wing nuts. This took less than twenty minutes to make from scraps I had on hand.

To hold it in place, I drove two screws through the fence on the jig. It could also just be clamped on:

It’s a great idea to add a simple pointer to the top of the locking bar that will line up with a scale on the top of the carriage. I just glued this on, and the only thing to watch for is that it doesn’t impede the movement and locking action of the locking bar:

I made a scale by hand on the carriage that lines up with the pointer, but you can download printable measuring tapes (or buy a self adhesive one) that can be glued on.
I prefer the hand-drawn one, since it only has what I need and I find it easier to work with.

Using the secondary fence to cut the parts for a small box, I started with pieces that were wider than I needed and cut the box down to final size after it was glued together:

Starting with wider stock and then cutting it down to size is the best approach, since it avoids problems that can happen when trying to offset the parts and, as you can see, you wind up with perfectly lined up sides and even sized box joints on the top and bottom.

I cut some pieces of walnut 1/16″ thick to make a tiny box. These parts for the sides are 1-1/4″ x 1-1/2″, so pretty small. The fifth one is a backer to prevent chip out.

Glued up, then cut to final size. I even made a bottom and a lid for it:

Those are 1/16″ box joints cut with a single thin kerf 7-1/4″ saw blade.

If you would like to build one of your own (highly recommended!), there are detailed, easy to follow plans available