Edge Joinery
Why can’t a CNC router make a perfect square internal corner in edge joinery?
A round cutting tool leaves an internal radius equal to the tool radius, so the corner geometry must be relieved with features like dogbones or T-bones to allow a square mating part to seat fully.
What tolerance is typically used for tight CNC wood joints?
A commonly cited starting point is 0.005 in per side, with adjustments based on tool wear, wood species, moisture content, and machine accuracy.
Why does stock thickness matter so much in CNC edge joinery?
CNC joinery is programmed to a specific material thickness; variation changes tenon depth, groove depth, and seam alignment, which can prevent proper fit unless stock-to-leave or floor-to-leave strategies are used.
Edge joinery is the technique of joining wood members along their edges to form wider panels or longer assemblies, essential in cabinetmaking for shelf trims, face frames, and glued-up panels. In CNC contexts, it requires precise toolpath compensation and intentional clearance (typically 0.005-0.010 in per side) to account for cutter radius, glue space, and material movement, ensuring proper fit without binding or splitting.
On the shop floor, edge joinery is typically used to build wide stable panels from narrow stock when full-width material is unavailable, and it helps reduce cupping and warping by alternating grain orientation. In CNC machining, the router cutter cannot produce a zero-radius internal geometry, so intentional clearance must be programmed; a commonly cited tolerance is 0.005 in per side for tight wood joints, adjustable to 0.010 in if needed. Parts are cut with inside/outside compensation so the finished edge matches design intent, and tenons are undersized relative to mortises to preserve glue space. Fixturing must be rigid to prevent movement during routing, and sharp tooling with proper chip load limits tear-out on exposed edges, especially in hardwoods and plywood.
Zero-clearance fit: The joint is cut to nominal size without compensating for cutter radius or wood movement, causing parts to bind or split during clamp-up.
Tear-out on exposed edges: The bit exits unsupported fibers, especially on cross-grain or brittle veneer, leading to chipped or fuzzy seams from dull tooling or poor chip load.
Panel distortion after glue-up: Narrow boards edge-joined without proper reference or pressure distribution result in twisted panels and gapped seams from inadequate fixturing or uneven stock thickness.