• Nate Large

Perfect Box Joint jig {aka finger joint jig}

Updated: May 14, 2019



A box joint is a robust joint for joining two boards at 90 degrees. The joint is strong because it offers a considerable glue up surface. A fixed box joint jig is better than an adjustable jig because it will not require tedious tuning before each set up. To make and use this jig for the table saw, you will need a table saw that can accept a dado stack and a dado stack.

The written instructions below are intended to work with the information in the video to provide a full picture on the build of the Box Joint Jig.

Sled board

The sled should be sized to support the work pieces that it will cut the box joints into. I recommend using 1/2" plywood for the sled and suggest 7" by 15" for 1/4" box joints and 9" by 22" for 5/8" box joints.

Miter bars

Can be purchased from a shop supply store or made from hardwood. I make miter bars from hardwood, they need to fit snugly in the slots but be free to move back and forth without resistance. They also need to rest below the surface of the table and be long enough to fit on the sled from front to back.

Attaching miter bars

The sled is clamped or held down to the table saw in the orientation that it will be operated in. The bars are slid into the slots under the sled and holes are drilled and countersunk and screwed from the top of the sled into the miter bars.

Next cut a kerf into the sled with the table saw. This is done with the sled running in the miter slots. The kerf should only go at most halfway across the sled.

Support fence

The sled will be held together with a support fence, it needs to have at minimum two adjacent sides that are level and square and be the same length as the sled.


Attach support fence to sled

One of the true sides will be face down on the sled with the other facing forward on the sled. Unlike with other sleds the fence is put in the middle of the sled so that there is adequate space to comfortably

support the work pieces in front of it and room for a 1/2" backer board. I recommend 3" from the front of the sled. Attach the fence on one side with a single countersunk screw from under the sled. Adjust the fence so that it is at a right angle from the kerf in the

sled and secure it with 3 more countersunk screws. evenly spaced and nowhere near the kerf as the kerf will be getting bigger when the dado kerf is cut into it.

Backer board

It should be made from 1/2" plywood and be as long as the sled is and 4"- 6" wide.

*Insert dado blade: always unplug table saw when changing blades.

Insert a stack of dado blades equal the width of the desired box joint.

Cut dado opening in backer board

Cut a spacer opening into the backer board with the dado stack. Set the height to 1/32" higher than the thickness of the material you plan on using your finished jig with. The space opening should be near to the center of the backer board.

*Insert standard blade: always unplug table saw when changing blades.

Cut spacer

The spacer should be made out of hardwood. The width of the spacer should be 0.004" less than the width of the dado cut in the backer board. Its height should be slightly under the height of the opening on the backer board. The length needed is around 3". After making the spacer you can cut it down to 2" long and save the cutoff to be used for dialing in the gap between the space and dado blade.

Chamfer the top two edges of the spacer along its length. This will make it easier to place your fresh cut box joints over the spacer.

Glue the spacer into opening so that it only protrudes on one side of the backer board and sits flush with the bottom of the backer board.

*Insert dado blade: always unplug table saw when changing blades.

Insert a stack of dado blades equal the width of the desired box joint.

Attaching the Backer Board

The backer board will be attached to the support fence first with clamps to approximate the gap between the spacer and the dado stack. My desired gap is 0.006" less than the dado cut. Place the spacer off cut between the spacer and the dado blade to measure a gap that is ~0.004" less than the dado cut. The backer board is then clamped to the sled.

Fine adjusting the gap

Bring the gap into precise specifications will tune the jig to work better.

If the gap is too big: on the side of the sled away from the dado kerf, clamp a blunt pointed offcut to the sled so a long side is pressed against the backer board with the blunt point pressed up to the spacer. Loosen the backer board and slide it away from the clamped off cut, insert desired feeler gauges (thickness of feeler gauges should equal amount of gap to close,) between the blunt end of the off cut and the spacer and slide the backer board back so that the space is pressed up to the bunt point with the feeler gauges between them. Clamp the backer board to the support fence, remove the clamped off cut and feeler gauges to test the new gap.

If the gap is too small: on the side of the sled away from the dado kerf, clamp a blunt pointed offcut to the sled so a long side is pressed against the backer board with the blunt point pressed up to the spacer with desired feeler gauge thicknesses (thickness of feeler gauges should equal amount of gap to expand,) sandwiched between the blunt point and the spacer. Unclamp the backer board, remove the feeler gauges, slide the backer board so the spacer is pressed against the blunt point and reclamp the backer board to the sled, remove the clamped off cut and test the new gap.

Add a Blade guard

A blade guard can be added by gluing a cut off to the back of the sled where the dado blade comes through to the fence. This will add a small measure of safety that just may save a finger or life.


Finished Box joint jig with sample

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