No doubt you've seen similar versions of this box joint jig (often called a finger joint jig) in your travels around the internet. It's pretty easy to construct, but many woodworkers have a hard time getting it to work properly without fiddling with it for hours at a time. Some never get it to work at all, and just give up in frustration.
There's a few reasons for this, but by far the biggest problem is getting the spacing between the fingers just right. Without a reliable method for adjusting this spacing, it's a bit of a hit or miss proposition, usually with more misses than hits.
When you consider that just a few thousandths of an inch (or less) can mean the difference between overly loose/tight box joints and those that fit like a well made glove, it seems reasonable that a method of measuring the exact amount of fence adjustment could save a lot of time and frustration.
I've found the simplist way to do this is with a pair of hardwood dowels, one on the back fence and one attached to the main fence. This allows me to use a micrometer or digital calipers to measure how much I'm actually moving the main fence when setting the finger spacing or making small adjustments.
There are other factors that can influence the precision of your box joints. Sloppy miter gauges, poor alignment of the blade to the miter slot, blade sharpness and even the way you push the box joint jig through the dado blade can have some effect on the quality of your box joints.
Fortunately, there are ways around these issues too. I've laid out the methods I use to build and set up my box joint jigs for making accurate box joints in a step by step manner in the rest of this article. The box joint jig being built here will be used to create 1/4" fingers, but the methods described will work for any other size desired.
I've used 3/4" birch plywood for most of this jig, with solid birch stock for the indexing pin and 5/16" hardwood dowels for the measuring pins. Having built these box joint jigs out of mdf before, I found the screws didn't hold very well and would often come loose or strip out after a bit of use. Plywood doesn't have that problem.
There are two fences on this box joint jig, a back fence that will be screwed to the miter gauge and a main fence that will be attached to the back fence. The back fence measures 4" high by 10" long and the main fence is 6" high by 18" long.
The parts of the finger joint jig.
In the photo on the left, the shorter piece of plywood next to the back fence is used to space the main fence dowel even with the other one and is where the index pin will be secured with a screw.
This spacer will be attached to the main fence later with a couple of screws. It is also 4" high like the back fence, but only 3" wide.
The two 5/16" hardwood dowels were cut off a length of dowel stock and are 1 1/2" long. The ends were rounded over slightly with some sandpaper to ease installation.
Your key stock should be made from hardwood to prevent it from wearing down from the constant use it'll see. I cut mine off a length of 3/4" birch, about .005" of an inch wider than the 1/4" I needed.
After The dado blade is set up and a test cut is made, I'll sand it down to it's final size. I cut the key stock long enough to make several pins in case I go overboard with the sanding.
There are three 5/16" holes that will be drilled in the back fence, two are for mounting the main fence and the other will be used to hold one of the hardwood dowels. It's important that the hole for the dowel be straight, so I recommend using a drill press.
With the back fence just touching the blade, it's screwed to the miter gauge. The red arrow shows one of the feeler gauge shims used to eliminate miter gauge side play.
The hole for the dowel is located 1/2" in from the top and side of the top right corner of the fence. The two mounting holes are located 1" down from the top and 1" in from each side.
Once the holes were drilled, I applied a small amount of wood glue to one of the dowels and tapped it in its hole. It should be a snug fit. Wipe off any excess glue right away with a damp rag.
I next set up the dado blade to 1/4" using my digital calipers. This will be close enough for now, until a test cut is made to measure the actual kerf. Before you lower the blade, use a machinist's square to make sure it's at 90 degrees to the table.
As mentioned at the beginning of this article, a sloppy miter gauge can have a negative impact on the quality of your box joints. To get the slop out of mine, I slip in shims from a feeler gauge between the miter gauge bar and the miter slot.
I slip one in ahead of the miter gauge fence and one right at the back of the head. Both are on the left of the bar. The shim at the end near the head is held in with my thumb, the other floats around as the miter gauge is moved back and forth.
The fence being squared to the t slot.
With the side play removed, I set the end of the back fence against the blade and screwed it to the miter gauge with wood screws and small washers. Adjust the fence so that it's just touching the blade and is flat on the table.
Using a framing square, set the miter gauge square to the miter slot. If you're using shims for the miter gauge, make sure they're in place before setting the fence.
With the back fence set up and squared, a test cut will be made to determine the exact kerf (slot) left by the dado blade. The reason for this has to do with table saw alignment. If your blade is not exactly parallel with the miter slot, the blade will tend to cut a wider slot than it's set up for.
Making a test cut to determine slot size.
To make the test, set the blade height close to 3/4". With a scrap piece of the same material you'll be using for your box joint project, make a test cut by clamping it to the back fence and running it through the blade.
Measure the width of the slot left by the blade with your calipers. If the test cut confirms you're at 1/4", you're good to go. If not, add or remove shims and repeat the test until you get as close as your shims will allow. Mine came out to .249", close enough for me.
Now take your index pin stock and cut off a couple of 3" lengths. With a sheet of 150 grit sandpaper laying face up on your saw's table, sand one of the pins until it fits snuggly in your last test cut.
The pin should fit with no gaps or side play.
It's best to do both sides a little bit at a time, checking often with your calipers to make sure you're not getting too thin and that it's the same thickness along it's length.
It really doesn't take long to remove the extra thickness, so take your time and try to get a nice even pin. If you mess up the first one, try again with your spare.
There should be no side play or gaps showing when the pin is inserted in the test cut. If there is, it's too loose. On the other hand, if you have to wrestle the pin in the slot, your box joints will be hard to get together and will want to spread the pins apart, leaving gaps on the ends.
I've found that when you can push the pin into the test cut fully using just your thumb, it's about right. After you've got your pin just right, measure over 3/8" from one end and drill and countersink a hole for a small wood screw (#6) in the middle of the pin for mounting to the fence.
The main fence can be attached now, although it'll only be temporary and will have to be moved again to set the finger spacing. I set the right edge of the fence even with the far edge of my right miter slot and screwed it down to the back fence with 1 1/2" long screws and washers. I used two washers on each screw to be sure they wouldn't pop through the fence face.
The main fence, spacer and second dowel are attached to the box joint jig.
The next piece to go on the box joint jig is the plywood spacer. Before it can be attached, three holes will be drilled -- two mounting holes and one hole for the other hardwood dowel.
The 5/16" hole for the dowel is centered 1/2" from the top and 1/2" from the left side, a mirror image to the one on the back fence. The two mounting holes are drilled down the middle, 1" from the top and bottom and need only be as big as the screws used.
With the dowel glued and in place, butt the spacer up against the back fence and screw it to the rear of the main fence. Both dowels should be straight and parallel with each other at this point.
The distance across the dowels is measured and recorded on the back of the jig.
Before we can cut the slot for the indexing pin, the distance across the dowels must be measured. I write this measurement on the back of the fence for easy reference.
For consistency, it's best to take your measurements from the same spot on the dowels each time you take a measurement. I usually put a pencil mark on the top of the left dowel as a visual aid.
It might be a good idea to do a quick recheck of your fence alignment with your framing square before cutting the slot for the index pin. One time I didn't tighten the miter gauge enough and knocked it out of square while putting in the fence screws.
The index pin is secured to the spacer block with a small wood screw.
Set the blade height a hair higher than the index pin and run the box joint jig through the blade.
Remove the screws holding the main fence, set the index pin even with the back of the spacer and run a small screw in to secure it. Gluing the pin isn't necessary if the slot and pin are a good fit.
You'll find that there's raised bumps on the back of the main fence where the mounting screws went through. Sand these down before remounting the fence in it's new position.
The main fence of the box joint jig must be repositioned to the right exactly twice the thickness of the indexing pin. To do this I multiply my pin thickness by two and then add the distance across the dowels that I wrote on the back of the box joint jig. In my case, the pin measured .249". So, .249 x 2 = .498". Adding the 1.322" from the figure on the jig will give me 1.820".
Adjusting the fence with digital calipers.
An easy way to get close to this mark is to set the digital calipers to the width you need and then lock it with the thumb screw. With the calipers straddling the dowels, move the main fence to the right until the dowels snuggle up against the caliper jaws.
Clamp the fence in this position and measure the distance across the dowels. It should be close, but a bit of adjustment is more than likely necessary. If you only need to move it a few thousandths either way, screw the fence on before adjusting it. The oversize holes in the back fence will allow for a fair bit of adjustment.
To make small adjustments in the fence, loosen the screws just enough where you can tap on the fence with your screwdriver and affect the change. Retighten both screws before taking another measurement. With these tiny amounts of adjustment, it might take a few tries before you hit a home run.
Cutting the second slot in the fence.
Before you cut the second slot in the box joint jig, raise your blade about 1/32" higher than the thickness of the material you'll be using for your box joint jig project.
Don't set it any higher than you need though. If you have to lower the blade after you make your cut, you risk getting blow-out and splintering at the back of your workpiece.
The fingers will protrude a bit when your joints are assembled, but it's better to be a little long than a little short. After the glue has dried, the extra material is easily removed with a sander. Just be sure to factor in this amount when you cut your material to length.
The box joint jig is now complete, but it still needs to be tested by cutting a test joint and making any needed final adjustments before you can put it to work on your project.
In this follow up article on making box joints, I'll show how I dial in the box joint jig and the procedures used to cut accurate box joints.
Before long, you'll be making great looking box joints for your own projects.