Finger Joint
A finger joint is a woodworking joinery connection made of alternating protruding fingers on one part and matching slots on the mating part, allowing the assembled corner or edge to come together flush with high glue surface area. In CNC routing, it is commonly programmed as a profile cut with inside/outside offsets, requiring precise allowance offsets to achieve a snug slip fit that balances glue space and parts self-registration.
On the shop floor, finger joints are a staple for box corners, cabinet carcasses, crates, and panel edge extensions because the interlocking geometry increases glue surface and self-registers parts during assembly. In CNC routing, the joint is typically laid out so finger width relates to board thickness or tool diameter, then cut in matched pairs to control fit. A typical workflow involves CAD layout, CAM profile generation, and a test cut on scrap to verify fit before production runs. Small changes in tool diameter, bit wear, or allowance offset can swing the fit from press-tight to loose. For flat-sheet work, a square-corner box joint is limited by the round endmill; inside corners require dog-bone, T-bone, or rounded reliefs unless the design is modified for rounded fingers. Production setups target a snug slip fit: tight enough to hold position during dry assembly but loose enough for glue spread and full seating without crushing fibers.
- Tool-radius mismatch: The joint is drawn with square corners, but the endmill leaves rounded inside corners, causing the mating finger to bottom out before full seating, resulting in proud corners, gaps, or forced assembly damage.
- Fit tolerance drift: Machine zero, tool wear, or missing allowance offset makes fingers too tight or too loose; too tight causes splitting or glue starvation, while too loose reduces clamp alignment and creates visible corner steps.
- Poor stock control or fixturing: If the sheet shifts during profiling or tabs are insufficient, the two halves no longer mirror correctly, leading to racking, mis-registration, or a twisted assembly even if the code is dimensionally correct.
Is a finger joint the same as a box joint?
In shop usage, the terms are often used interchangeably for interlocking rectangular fingers in casework and boxes. Some shops use 'finger joint' more broadly, while 'box joint' is the common cabinetmaker term.
Why do CNC finger joints need dog-bones or rounded reliefs?
Because a round endmill cannot cut a perfectly square internal corner. The relief allows the mating rectangular finger to seat fully instead of hanging up on tool-radius residue.
What fit should be targeted for a finger joint?
A test-fit that assembles with gentle taps or light hand pressure is typical, with enough friction to stay together temporarily but not so much that glue-up requires force that distorts the parts.