Lead Screw Box Joint Jig
There is an excellent woodworking site at woodgears.ca. Matthais Wandel documents projects beautifully. I was inspired by his box joint jigs, in particular the video at the end of woodgears.ca/box_joint/jig.html though I thought the gear part was overkill. The premise is to use a lead screw to position the board while cutting the slots. The usual technique for finger joints is to use an indexing pin on a sliding sled. The problem with that method is any error in spacing between the pin and the blade will be cumulative, if you cut 10 pins and they are each off by 2 thousanths, the last one will be off by 20 thousandths. The lead screw should be more accurate, it’s easy to cut multiple pieces at once, and quite large stock can be handled. This fixture will easily handle four inches of material stacked up.
This is the jig mounted on the table saw. It is 11 x 40 inches, will be able to handle 18″ stock in a single pass. Material for the miter slot rails is plastic coated MDF.
The lead screw is 3/8″, 16 Threads Per Inch so one turn of the crank advances the carriage exactly 1/16 inch. In the photo, the block on the right end is fixed to the carriage and has a T-nut to engage the rod. The doubled block also has a T-Nut but is floating (not attached to the carriage). Springs between the fixed block and the floating block take up any backlash in the lead screw. The rest of the blocks support the carriage face and also protect the threaded rod.
At the crank end, a soft rubber washer provides tension on the rod to eliminate backlash at that point. The rod protrudes an extra inch so I can use a 9/16 socket in a drill to rapidly slew the carriage back to the end. 18 inches times 16 TPI is a lot of cranking.
This shot shows how the carriage is mounted on two full extension drawer slides, one on top, one on the rear. Each has a maximum travel of 18 inches, and since they are mounted at opposite ends, when one is at maximum extension the other is at minimum extension.
I was surprised at how rigid this was.
This is the other end of the fixture. You can see the rear mounted drawer slide. I will be cutting a port for the shop vac in the end piece as soon as I figure out how to couple in the hose.
Keeping sawdust out of the ball bearing drawer slides may be a problem. I have an Aluminum extrusion modified to slip over the rear slide for protection. In practice, sawdust has not been a problem after adding the shop vacuum coupling.
This is the face of the carriage. Also shows the board added across the front of the sled for stiffness, and you can see the two runners fitted in the saw’s miter gauge slots.
You can cut all four sides of a box and stack the pieces in the fixture. There’s two clamped up in this photo. If you do all four at once you need to offset two of them by the width of the slot. See the video on Woodgears.ca.
Normally you need a sacrificial backer board between the carriage and the stock because the slot in the jig gets too big.
These were cut on the jigs maiden voyage. Actually the second try, I screwed up the first set. The material is half inch MDF left over from another project.
Part of planning a box joint or dovetail project is figuring out the pin width that leaves a good half pin on the ends. Clearly that didn’t happen here, but it works OK just looks odd.
The box joints fit well though a hair loose. These are half inch pins, two 1/8 chippers in the Freud Dado set. A perfect fit would require narrowing the dado set a few thousandths but it’s good enough as is.
Completed Box Joint Projects
Toy Box for the Granddaughter
This is wood salvaged from an old Pine table, had a nice patina even after planing off the epoxy paint. About 24″x15″x12″.
Toy box opened. Spring loaded holder upper gadget from Menards.
Box joints are half inch pins, one inch pitch. The banding top and bottom was fitted, then cut with one eighth inch box joints. Biscuits reinforce the banding.
It was quite an effort to glue this up. I did all four corners at once with slow setting Titebond which gave just enough time to get every clamp I own on the box.
Garden Planter Frame
The previous owner of this house had made a planter box from treated 2x12s around a back yard tree. The tree grew and pushed the box apart so I replaced the planter with a larger box made of Cedar.
I used box joints with half inch pins to assemble the new planter. The new box was more than four feet on two sides, I had to move the table saw to the driveway because the boards clamped to the carriage hit the roof trusses in the garage.
Each side was formed from two 2×6 Cedar boards. Two opposite sides had one 2×6 ripped in half so adjacent board seams don’t line up when assembled.
I assembled each pair of sides and bored a 3/8 hole all the way down through the pins. Then I inserted a 12 inch long galvanized nail to hold the joint together at each corner. There is no glue.
The final assembled planter. Under the dirt I put 4 inches of gravel inside hoping it will drain better and last longer.