What the jig does
A box joint has square fingers and slots of one width, w, interlocking at a corner; the joint itself is explained in box joints. Cutting a dozen slots by measuring each one would compound every small error. The jig avoids that: it is a fence carried past the cutter by the miter gauge, or by a bar in the miter slot, with a key — a strip exactly w wide — fixed to it a set distance from the cutter. Each new slot is dropped over the key before the next cut, so the spacing is copied from the slot just cut rather than measured.
Search terms mix two joints. Woodworkers often call the box joint a finger joint, but in the Forest Products Laboratory's Wood Handbook a finger joint is something else — interlocking fingers cut in the ends of two boards to join them end to end. This page is about the corner joint.
The geometry: w, w, w and a 2w pitch
- Key = w. The key is the width of one slot. Freud has it milled to fit a test slot cut by the cutter itself.
- Gap = w. The gap between the key and the cutter is one finger width, which leaves a finger between each pair of slots.
- Pitch = 2w. Key, gap and cutter are each w wide, so the centre of the key is 2w from the centre of the cutter, and each cut lands 2w along from the last.
- Board width = an odd number of w. A board 15 widths wide — 15 × 1/4 in = 3-3/4 in — ends in a full finger at both edges of one board (8 fingers, 7 slots) and a full slot at both edges of the other (7 fingers, 8 slots).
- Where each board starts. The first board starts with its edge against the key, so its first cut leaves a finger at the edge. The second starts with a slot at its edge, so its fingers fill the first board's slots.
The same rule gives any size: with w = 3/8 in, a 3-3/8 in board is 9 widths, 5 fingers and 4 slots on one board, 4 and 5 on the other. If a board's width is not an odd number of w, the last finger at one edge comes out partial; plan the width before cutting.
Kinds of box joint jig
| Type | How the spacing is set | Works at | Limits |
|---|---|---|---|
| Shop-made, fixed key | A key glued into a face screwed to the miter gauge, the gap set once | Table saw or router table | One jig per finger width; adjusting means re-fixing the face |
| Shop-made, adjustable | A face carrying the key slides on a carrier and locks | Table saw or router table | Still one face per finger width; the adjustment is only for fit |
| Bought adjustable (e.g. INCRA IBOX) | A lead screw sets pin width and spacing with one knob | Table saw or router table (INCRA) | INCRA: fingers 1/8 to 3/4 in; cut no deeper than 7/8 in |
Two shop-made jigs are drawn and dimensioned in BenchRoot's box joint jig projects. INCRA's figures are for its own jig. Not a ranking.
Measured plans for a fixed jig for 1/4 in fingers and an adjustable jig with swappable faces are in box joint jig projects.
Cutters, with or without a dado set
| Cutter | Finger width | Maker's notes |
|---|---|---|
| Box joint blade set (Freud SBOX8) | 1/4 in or 3/8 in | Both blades always used: back to back for 1/4 in, face to face for 3/8 in |
| Stack dado set | Set by the chippers and shims | INCRA accepts stack dado sets; no wobble dado |
| Single 1/8 in blade | 1/8 in | INCRA: a flat-ground raker tooth or a box joint grind; no 3/32 in thin-kerf blade |
| Two-flute straight bit, router table | The bit's diameter | INCRA: works at the router table with the same set-up routine |
A dado set or cutter set is used only on a saw whose manual allows it, with the throat plate and set-up that manual names. Not a ranking.
Without a dado set, the choices are a single 1/8 in blade with a flat-topped tooth, as INCRA allows, or a router table with a straight bit. Saw compatibility, throat plates and dado set-up are covered in dado blades; router table set-up in the router table guide.
Height and fit
- Height. Freud sets its cutter 1/64 in higher than the stock is thick; INCRA sets the cut slightly through the stock. Either way the fingers stand a little proud and are sanded flush after glue-up.
- Fit. Freud's test: a loose joint with gaps means the key is too close to the cutter; an over-tight one means it is too far. Move it and cut again in stock of the finished size.
- Adjustable jigs. INCRA has its pin plates opened or closed until the test cut fits over them with a little friction; on a shop-made adjustable jig the face is slid on its carrier.
Safe use
Safety— Guarded, clamped, and powered off to adjust
INCRA requires its blade guards in place and the work clamped to the fence with a wooden handscrew before every cut, and the power off with the cutter stopped before any setting is moved; on a SawStop saw it has the saw put in bypass mode only while calibrating and returned to normal mode before cutting. Freud has the power disconnected before each re-clamp and forbids freehand cuts. Read the saw's own manual first; see table saw safety.
The other joinery jigs and tools are mapped in the pocket hole guide.
Before cutting box joints on a jig
- Board width an odd number of finger widths
- Key milled to fit a test slot; gap to the cutter one finger width
- Cutter height just above the stock's thickness
- Saw manual allows the cutter; guards in place
- Work clamped to the jig face for every cut
- Test joint cut in stock of the finished size
Sources
Fit, height and set-up figures are Freud's for its SBOX8 set and INCRA's for its IBOX; other jigs and cutters have their own instructions. The indexing geometry — key, gap and cutter each one finger width, a 2w pitch, an odd number of widths for full fingers at both edges — is arithmetic, and the finger counts quoted are worked from it. 'Finger joint' also names a different, end-to-end joint in the Wood Handbook.
- 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, with a hardwood key milled to fit a test slot; the cutter set 1/64 in higher than the stock; a loose fit meaning the key is too close to the cutter and a tight fit too far; test pieces in stock of the finished size; power disconnected before each re-clamp, no freehand cuts, both blades always used, and the saw's own manual and throat-plate clearance checked first.
- 2.
SBOX8 8" Box Joint Cutter Set. Freud.
Used for: A box joint cutter set cutting 1/4 in or 3/8 in grooves.
- 3.
INCRA IBOX Owner's Manual. INCRA (Taylor Design Group), 2023.
Used for: An adjustable box joint jig for the table saw or router table, from 1/8 in to 3/4 in fingers, with one knob setting pin width and spacing together; a miter bar adjusted to the slot; stack dado sets, two-piece box joint blade sets, 1/8 in blades with a flat-ground raker or box joint grind, or a two-flute straight bit, but no wobble dado, no 3/32 in thin-kerf blade and no cutter under 1/8 in; a cut slightly deeper than the stock, never more than 7/8 in on this jig; blade guards in place, the work clamped to the fence with a wooden handscrew, the power off and the cutter stopped before any adjustment; a SawStop in bypass mode while calibrating and back in normal mode to cut.
- 4.
Chapter 10: Wood Adhesives: Bond Formation and Performance. USDA Forest Service, Forest Products Laboratory — Wood Handbook, FPL-GTR-282, 2021.
Used for: 'Finger joint' in the Wood Handbook's sense: an end-to-end joint that lengthens a board, not the corner box joint.
About this guide
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