Hand Planing Priced in Force: Blade Sharpness, Cut Depth and a Chipbreaker Sizing Rule in Scots Pine

Hand planing of Scots pine with a 50 mm blade is modelled as a function of blade condition — sharp, dulled and blunt — and cut depth from 0.05 to 0.40 mm. At a depth of 0.10 mm the push force is 79.8 N with a sharp blade, 133.8 N with a dulled blade, which already exceeds a stated sustainable push of 120 N, and 209.0 N with a blunt blade, which exceeds even a stated short peak of 200 N. The greatest depth a worker can sustain is 0.162 mm sharp, 0.088 mm dulled and only 0.053 mm blunt, so a blunt blade does not merely work slower: at ordinary finishing depths it does not work at all within human force limits. A chipbreaker sizing rule is derived from a tear-out risk model for interlocked grain: to keep modelled risk at or below 0.10, the chipbreaker edge must sit no farther than 0.146 mm from the cutting edge at 0.05 mm depth, 0.098 mm at 0.10 mm, 0.074 mm at 0.20 mm and 0.062 mm at 0.40 mm. A common loose setting of 0.8 mm carries modelled tear-out risk of 0.562 to 0.999 across those depths. The tighter the cut, the less the chipbreaker setting matters in absolute terms — and the more a careless setting costs.