Mortise And Tenon
Why do CNC mortise corners look rounded instead of square?
Because the cutter is circular; a router bit cannot machine a perfect internal 90° corner, so the design must include radius reliefs or modified tenon geometry.
Why is tenon width usually made slightly undersize on CNC wood joinery?
To account for tool diameter variation, cutter runout, deflection, surface roughness, and glue clearance so the joint assembles without excessive press force.
What tolerance discipline matters most for CNC mortise-and-tenon joinery?
Datuming, tool radius compensation, and consistent allowances for fit; one manufacturing source reports accuracy within 0.1 mm, while CNC woodworking guidance stresses test cuts to validate real tool size and fit.
A mortise and tenon joint is a traditional woodworking connection where a projecting tenon fits into a matching mortise slot, creating a mechanically interlocking assembly. In CNC and millwork production, the joint is programmed as a pocket-and-protrusion geometry, machined with rotating cutters that leave rounded internal corners requiring design allowances for proper fit.
On the CNC shop floor, a mortise and tenon joint is typically programmed as a pocket-and-protrusion pair, with the mortise cut into one member and the tenon on the mating member. The operator fixtures the workpiece, establishes a consistent reference zero, and runs separate toolpaths aligned to the same datums to ensure accurate registration. Fit is controlled by intentional clearance: the tenon is often made slightly narrower and shorter than the mortise to allow for glue room and to ensure the shoulders seat before the tenon bottoms out. In blind joints, the mortise is a pocket of defined depth. Automated systems can achieve tolerances within 0.1 mm when tooling, fixturing, and machine stability are controlled. This joint is valued for preserving alignment and structural strength without relying solely on fasteners, and it improves repeatability compared to hand-cut joinery, making it ideal for solid-wood furniture, doors, windows, and frames.
Rounded corners prevent seating: A square tenon cannot fully enter if cutter radii exceed corner relief, causing interference, poor bottoming, or shoulder gaps.
Insufficient clearance or glue room: A tenon at nominal size binds during assembly, risking split stock or incomplete closure. Undersizing width and length prevents this.
Registration and setup error between parts: Inconsistent referencing between mortise and tenon causes offset parts, twisted frames, and forced assembly.