Making The Advanced Box Joint Jig From MDF Homemade Machines & Jigs
I occasionally get asked if jigs like this can be made with lower cost, easier to find materials. I recommend using the best plywood you can get, but it is possible to use cheaper varieties. The main thing to look for is consistent thickness and flatness.
With that in mind, I thought it would be interesting to build the jig with MDF as a replacement for the plywood, and well seasoned (dried) spruce construction lumber for the solid wood parts.
To start off, here is a video I did showing the assembly of the jig from start to finish:
The materials laid out: 3/4″ and 1/2″ MDF, and a piece of 2″ x 10″ lumber that has been in my shop for several months:
Cutting out the individual parts is the same procedure as with the original, starting with the 3/4″ MDF.
The pine lumber is a nice piece – clear of knots and defects and fairly straight grain:
The key to using this low cost lumber is to have it well dried before using it, and I recommend have a stock of it on hand, ready to use:
The original jig used UHMW plastic for the bearing. This one will use pine, but since it will be coated with water based polyurethane, it should work nearly as well:
Hardwood can be used to make the division plate and locking blade, but I thought it would be interesting to make these from a cheap plastic cutting board.
To cut the notches in the plate, I’ll use a thin kerf 7-1/4″ saw blade in the table saw:
For this jig, I’ve come up with another way to cut the notches accurately. It’s much easier and more precise than the previous method. It starts with drilling a pilot hole and driving a drywall screw part way into the end of the division plate:
It needs to be a drywall screw, or some other screw with a thin head that will fit into the threads.
Next, I taped a piece of 3/8-16 threaded rod down to the surface of my table saw sled. As shown, the head of the drywall screw will lock into the threads of this rod:
On the other side, I taped a ruler to show the spacing. The procedure is to make a cut, move the division plate 1/4″, lock it into the threaded rod and make the next cut. And so on until all of the notches are cut:
It’s important to check the thread spacing on the threaded rod to make sure it’s accurate. I recommend using a reliable measuring tape right in the store where you will buy it to check this.
This picture shows how accurately the notches can be cut with this method:
The locking blade is also cut from the plastic. I used a coping saw to cut the “handle”, then cleaned up the edges on my belt / disk sander:
As shown in the video, I initially cut the bevel on the locking blade with my block plane, then fine tuned it on the disk sander:
The locking blade needs to fit into the notch without bottoming out. This will lock the carriage in place and keep it from moving during the cut:
The advance lever and wooden spring are made from hardwood, and here I’m riving a piece of maple for the spring:
Splitting the wood like this will ensure the piece is as straight grained as possible, to avoid breaking. After it was split, I used the plane to make it the right thickness.
All of the parts, ready for assembly:
Assembly for this jig is the same as the original, so I recommend reading that article for more specific details.
Here’s a tip for lining up the locking blade: put it in place and locked into a notch in the division plate, then mark the hole for the screw. This is easier and more accurate than using the measurements on the drawing:
The locking blade also needs to swing clear of the advance lever beneath it:
The jig fully assembled:
I’m setting this one up to exclusively cut 1/8″ box joints with a single blade and the first step is to line up the edge of the blade with the side fence on the carriage. I can then move the table saw fence over and lock that to act as a guide:
I’m using a piece of maple as the guide bar and put a few dabs of epoxy on it to glue it to the base of the jig until I can get screws driven in:
The first test cut was a bit tight with the blade I was using:
To fix this, I shimmed the blade on one side with a piece of paper. This will make the blade wobble very slightly and cut a wider kerf for a better fit:
This is perfectly safe and acceptable way to fine tune a single blade for a precision fit.
Of course, like the original, the jig is best suited for use with a dado set, using the shims that come with the blade to fine tune the fit.
The cutting board plastic is very tough and I thought it would be interesting to try to make a division plate with notches that are spaced tighter. These are spaced 1/8″ using the thin kerf (0.060″) blade:
The test cut using that thin kerf blade and the new division plate looks pretty good:
This was just a quick test using cheap plywood, but the joint it produces looks really good:
Read here for more information on how to set up and use this jig.