Finger Joints, Tabs and Slots

Overview#

Flat sheet parts become 3D objects through joinery. For laser, waterjet and router work, the dominant joints are:

• Finger (box) joints: interlocking rectangular fingers along two edges that meet at 90 degrees. Strong, self-aligning, glue-friendly.
• Tab-and-slot: a tab on one panel passes through a slot in another. Used for shelves, internal dividers, and flat-pack furniture.
• T-slot (captive nut) joints: a slot with a nut trap and a screw hole, allowing a bolt to pull panels together without glue.
• Half-lap and cross-lap joints: slots half the width of the material let two panels cross in the same plane.
• Dogbones and relief cuts: not joints, but needed so square-cornered parts fit into router-cut pockets.
• Holding tabs (micro-joints, bridges): small uncut bridges keeping a cut part attached to the sheet. These are fabrication aids and are removed after cutting. They are a different concept from design tabs, and the word "tab" in Fusion's Manufacture workspace refers to holding tabs.

This page shows how to model each in Fusion so that changing the material thickness, finger count or kerf regenerates the whole design.

Where to find it#

• Parameters: Modify, Change Parameters. Create thickness, kerf, fit, finger_w, n_fingers.
• Sketch tools: Rectangle (R), Line (L), Offset (O), Sketch Dimension (D), Rectangular Pattern in the Sketch menu, Mirror.
• Solid tools: Extrude (E) with Cut operation, Rectangular Pattern, Combine for cutting finger bodies out of other bodies.
• Holding tabs in CAM: Manufacture workspace, within a 2D Profile operation, a Tabs checkbox on the Geometry tab with Tab Width, Tab Positioning (By distance or At points) and Tab Distance (per Autodesk help).
• Add-ins: The Autodesk App Store and community scripts offer finger-joint and box generators, such as parametric box makers. Check licensing and review the code before installing. The steps below do not require any add-in.

Step by step#

Parametric finger joint on a box#

1. Create user parameters: thickness = 3 mm, finger_w = 9 mm (target; typically 2 to 3 times the thickness), kerf = 0.18 mm, fit = 0.05 mm.
2. Model the box as a solid: sketch a rectangle 100 x 60 mm, extrude to 40 mm, and shell inward by thickness (or build six separate panels as separate bodies; separate bodies make cutting the fingers easier).
3. Work on the front panel first. On its top edge sketch the first finger as a rectangle of width finger_w and height thickness. Dimension it using parameters.
4. Pattern the finger across the edge. The edge length is usually not an exact multiple of the finger width. Choose an odd count so the corners are fingers on both ends: n = 2*floor(L / (2*finger_w)) + 1, then recompute finger_w = L / n. Fusion parameters support floor and round; verify supported functions in your version.
5. Use Rectangular Pattern with the Distance type set to Spacing or Extent and count n_fingers. Skip every second instance, or use two alternating offsets.
6. Extrude-cut the notches (the gaps) into the front panel through the panel's full thickness direction.
7. For the side panel meeting the front, the fingers must complement: wherever the front has a finger, the side has a notch. The easiest approach is to derive the cut from the front panel's sketch: project the front panel's gap rectangles and use them to cut the fingers from the side panel, or to add them back. Alternatively, mirror the pattern using the mirrored sketch (Create, Mirror) across the plane at the joint corner and offset by one finger width.
8. Apply kerf compensation. For the finger body, add kerf to its width and subtract kerf from the gap. Use the formulas below.
9. Flatten: create a sketch on each panel face and project the outline for export, or use the face directly in the CAM setup. See the export page.

Formulas for a joint where both pieces are cut with the same kerf:

• Finger width as drawn: finger_w + kerf
• Matching gap width as drawn: finger_w - kerf + fit

The finger grows by one kerf because the cut on both sides eats away that much; the gap shrinks. With fit > 0 the joint loosens.

Tab-and-slot#

1. Draw the shelf panel with tabs protruding thickness (or slightly less, 90 percent, so tabs do not show through if the slot goes through the face).
2. Draw the slot rectangle in the upright panel: width thickness - kerf + fit, length equals tab width plus -kerf.
3. Add fillets of 0.3 to 0.5 mm at the slot's corners if material is brittle (acrylic). Laser cuts leave small sharp corners that propagate cracks.
4. Pattern slots with the Rectangular Pattern in a sketch, with the count driven by a parameter.

T-slot (screw and nut)#

1. On the panel with the edge tab, sketch a slot for an M3 nut: nut width across flats (5.5 mm) plus 0.2 mm clearance by nut thickness (2.4 mm) plus 0.2 mm. Orient it so the nut slides in from the face and sits behind the bolt hole.
2. Sketch the screw hole through the edge: diameter 3.2 to 3.4 mm for M3, positioned at the center of the neighbor panel's tab.
3. The nut slot should intersect the screw path at the nut center, so the nut aligns with the screw.
4. Keep material around the nut trap at least 2 mm thick; thin walls split in plywood.

Holding tabs (micro-joints)#

1. Use holding tabs for waterjet and router parts that would otherwise drop free, shift and get caught by the tool or nozzle. On lasers with honeycomb beds, they are rarely needed.
2. In the 2D Profile operation, enable tabs, choose width (commonly 2 to 5 mm for 3 mm plywood on a router, 1 to 2 mm on waterjet), height (router only) and spacing or count.
3. Place tabs on straight edges, away from fit surfaces and visible edges. Edit tab position on screen if your version permits (look for a manual tab placement option; verify in your version).
4. After cutting, trim tabs with a flush-cut saw, chisel or sander.

Options reference#

Tips and pitfalls#

Tip: Model joints on one reference panel and mirror, offset or project them to the others. A single source of truth means that changing finger_w updates every panel.

Tip: Make finger joints at least 2 times the thickness wide, and leave an end margin of at least one thickness at corners to keep the part from snapping.

Tip: On a router, use "fingers-out" designs where the slots are open (no internal sharp corners) or add dogbones; sharp inside corners will not accept square tabs.

Warning: Wood grain direction matters. Fingers parallel to the grain break more easily. Orient long thin tabs across the grain where possible.

Warning: Mirror-image panels (left and right sides) need mirrored finger patterns, not identical patterns, or they will not assemble. Check by assembling the 3D box in Fusion with joints before you cut anything.

Warning: Do not place holding tabs on interlocking surfaces. Tab stubs left on a finger prevent assembly.

Practice exercise#

Design a 100 x 70 x 40 mm open-top box from 3 mm plywood with finger joints on all four vertical corners and a tab-and-slot bottom. Parameters: thickness, kerf, fit, finger_w. Build it as five bodies (four walls and one bottom), assemble them in the Design workspace, and run Interference (Inspect, Interference) to confirm no overlaps. Then change thickness to 4 mm and confirm that all joints regenerate and no interference appears. Expected result: a fully parametric box, flat outlines ready for export, and an interference report showing no intersections.

Related#

• Kerf and tolerances
• Exporting DXF and SVG
• Nesting (placing the panels on sheets)
• Parameters and mirrors in section 04 and 03