The two jigs at a glance
| Build | Difficulty | Time | Stock | Tools |
|---|---|---|---|---|
| Fixed jig, 1/4 in fingers | Beginner | An hour, plus glue drying | 3/4 in plywood or MDF, 4 × 18 in; a hardwood offcut for the key | Table saw with a 1/4 in box joint set or dado set its manual allows; miter gauge; drill/driver |
| Adjustable jig, swappable faces | Intermediate | An afternoon | 3/4 in plywood: 3-1/2 × 22 in and 4 × 18 in per face; hardwood for keys | The same, plus a drill press or drill guide and a way to cut two slots |
Both run on a miter gauge at a table saw or router table; how the key indexes the cut is explained in the box joint jig guide.
How the key indexes each cut — key, gap and cutter each one finger width, so each cut lands two widths along — is set out in the box joint jig guide; the joint in box joints. This box joint jig plan pair turns that into parts.
Build 1: the fixed jig
| Part | Qty | Size (in) | Material |
|---|---|---|---|
| Face | 1 | 3/4 × 4 × 18 | Plywood or MDF, flat |
| Key | 1 | 1/4 × 1/2 × 1-3/4 | Hardwood, width milled to the test slot |
| Screws | As the gauge needs | Length to suit the gauge's face plus 1/2 in into the jig | Through the miter gauge's own holes |
Sizes for 1/4 in fingers in 1/2 in stock. For other widths, the key is that width and the gap is that width.
Building the fixed jig
Cut and check the face
Cut the face 3/4 × 4 × 18 in from flat sheet stock, with its bottom edge straight and square to its front.
Set the cutter
With the saw unplugged, fit a 1/4 in box joint set or dado stack the saw's manual allows, and raise it 1/64 in above the stock's thickness — 33/64 in for 1/2 in stock, as Freud sets its cutter.
Cut the key slot
Screw the face to the miter gauge so the cutter will pass about 8-3/4 in from its left end, and push it through once. That slot, 8-3/4 to 9 in from the left end, takes the key.
Make and glue the key
Mill hardwood to slide snugly into the slot, cut it 1-3/4 in long and 1/2 in high, and glue it flush with the face's bottom edge so 1 in stands proud of the front.
Set the gap
Unplug the saw, unscrew the face and move it 1/2 in to the left on the gauge, so a 1/4 in spacer just fits between the key and the cutter; screw it back in new holes. The cutter now passes 9-1/4 to 9-1/2 in from the left end.
Test and adjust
Cut a joint in two pieces of the finished stock. Freud's check: a loose joint means the key is too close to the cutter, a tight one too far. Tap the face over, re-fix and cut again until the joint goes together with slight friction.
Label it
Write the finger width and stock thickness on the face; each finger width needs its own jig.
Build 2: the adjustable jig
| Part | Qty | Size (in) | Material |
|---|---|---|---|
| Carrier | 1 | 3/4 × 3-1/2 × 22 | Plywood, flat |
| Face | 1 per finger width | 3/4 × 4 × 18 | Plywood or MDF |
| Key | 1 per face | 3/8 × 1/2 × 1-3/4 (for 3/8 in fingers) | Hardwood, width milled to the test slot |
| T-nuts | 2 per face | 1/4-20 | Steel |
| Knobs | 2 | 1/4-20, with 1-1/4 in threaded studs | Plastic or metal |
| Washers | 2 | 1/4 in | Steel |
A 1-1/4 in stud passes 3/4 in of carrier and a washer about 1/16 in thick, leaving about 7/16 in in the face — less than the face's 3/4 in, so it never reaches the front.
Building the adjustable jig
Cut the carrier and a face
Carrier 3/4 × 3-1/2 × 22 in, face 3/4 × 4 × 18 in, both with straight, square bottom edges.
Lay out the slots
Mark the carrier's centre and two slot centres 7 in either side of it, 1-3/4 in up from the bottom edge — half the carrier's height.
Cut the slots
Drill a 5/16 in hole at each end of each slot, centres 1-3/16 in apart, so the slot is 1-1/2 in long overall, and remove the waste between them with a router, jigsaw or file.
Fit the T-nuts
Clamp the face to the front of the carrier, bottoms level and centred, and mark the slot centres onto the face. Drill each mark for the T-nut from the back of the face, counterbored so the flange sits flush, and press the T-nuts in.
Assemble and mount
Fit the knobs and washers through the slots into the T-nuts. Screw the carrier to the miter gauge through the gauge's own holes, with the knobs clear of the gauge's face.
Cut the key slot and fit the key
With the face centred on its slots, cut a slot through it with the 3/8 in cutter set 1/64 in above the stock, and glue in a key milled to fit, 1 in proud.
Set and test the gap
Unplug the saw, loosen the knobs, slide the face until a 3/8 in spacer just fits between key and cutter, and tighten. Cut a test joint and adjust by sliding the face: open the gap for a loose joint, close it for a tight one.
For another finger width, make another face with its own key and T-nuts. The 1-1/4 in of travel — the slot's 1-1/2 in less the stud's 1/4 in — is for fine-tuning the gap, not for changing widths.
Using either jig safely
Safety— Clamp the work; unplug to adjust
Clamp each board to the jig face — INCRA uses a wooden handscrew — rather than holding it, and never cut freehand. Unplug the saw before every adjustment or re-clamp, as Freud asks, check that the cutter clears the throat plate, and keep every guard the saw's manual allows with a dado set. The jig must never get in the way of the saw's own safety equipment. See table saw safety and dado blades.
Bought jigs and other cutters are compared in the box joint jig guide; other jigs and joinery tools are mapped in the pocket hole guide and the jig guide.
Before the first box joint
- Face flat, bottom edge straight and square
- Key milled to a snug fit in its own test slot
- Gap between key and cutter one finger width
- Cutter 1/64 in above the stock's thickness
- Board width an odd number of finger widths
- Test joint cut in stock of the finished size
Sources
Both jigs are BenchRoot designs; every dimension is on the drawings and in the cut lists. The key-to-cutter geometry follows Freud's instructions, and fit is checked by Freud's test; the arithmetic — key, gap and cutter each one finger width, slot travel equal to slot length less stud diameter, stud length inside the face — was checked. The designs were not built or tested for this page. No downloadable plan is offered.
- 1.
SBOX8 Box Joint Instructions. Freud (instruction sheet, distributor-hosted copy).
Used for: A shop-made jig on the miter gauge for each pin size; a hardwood key milled to fit a test slot; the cutter 1/64 in higher than the stock; a loose fit meaning the key-to-cutter space is too narrow and a tight one too wide; test pieces of the finished size; power disconnected before each re-clamp and no freehand cuts.
- 2.
SBOX8 8" Box Joint Cutter Set. Freud.
Used for: A box joint cutter set for 1/4 in or 3/8 in fingers.
- 3.
INCRA IBOX Owner's Manual. INCRA (Taylor Design Group), 2023.
Used for: Work clamped to the jig's face with a wooden handscrew for each cut; power off and the cutter stopped before any adjustment; the jig never allowed to interfere with the tool's own safety equipment.
About this guide
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