Box Joint

A box joint, also known as a finger joint, is a woodworking corner joinery method where alternating rectangular fingers and slots interlock between two mating parts. It provides a large glue surface and mechanical interlock, commonly used in boxes and casework. On CNC, square end mills cut the pattern, but rounded internal corners require relief cuts for full seating.

On the CNC shop floor, box joints are machined using square end mills in 2D toolpaths. The workflow begins with modeling the interlocking finger pattern, selecting finger width relative to material thickness and cutter diameter, then generating pocket and profile toolpaths. A critical tolerance issue is that the cutter radius leaves rounded internal corners; unless the design adds dogbone-style reliefs, the mating part will bottom out on those corners and the joint will not close fully. To tune the fit, machiners intentionally make one half slightly undersized or oversized due to tool wear, spindle deflection, and stock thickness variation. In production fixtures or vertical-routing systems, specialized setups allow one half to be cut while the board is held vertically. Test-fitting on scrap before production is standard to verify clearance and ensure reliable assembly.

• ⚠️Inside-corner bind-up occurs when square fingers are modeled but CNC leaves radiused corners, causing mating fingers to crash and assembly to stop short or split during glue-up.
• ⚠️Fit too tight from stack-up error: small tool wear, inaccurate stock thickness, or aggressive finishing passes reduce clearance, requiring mallet force that can bruise edges, crush fibers, or bow thin panels.
• ⚠️Weak fingers from poor proportioning: if fingers are too narrow for the cutter or material thickness, the remaining wood becomes fragile and chips, especially in plywood veneers or brittle hardwoods.