Folder Blade Geometry: Grinding for the Machine

A folder blade is a knife AND a machine part, and the machine half has opinions about the grinding. Everything from the EDC geometry guide still applies — this guide covers what changes when the blade has to fold.

THE TANG IS A PRECISION PART

The blade’s working end gets ground; the tang gets MACHINED-grade discipline: constant radius around the pivot (scribe from center, grind to the line on a small wheel or spindle), flats dead flat and polished (they are bearing surfaces), and the functional stations — lock face, detent track, stop flats — placed by template and cut deliberately. Heat treat covers the tang too: locking faces and detent tracks want blade hardness (soft tangs dent, per the bearing warning), which means all tang geometry is finish-ground post-HT with the usual cool discipline.

THE THICKNESS STACK

Folder design starts from a thickness budget: blade stock + two washers must equal the pivot barrel’s working length and the channel’s width, with the blade centered in what remains. Common working math: 1/8 or 3/32 blade stock, 0.025-class washers, liners to suit. Change any layer and the stack changes — which is why folder blades get ground to a MEASURED thickness, not a vibe, and why the caliper lives at the folder bench.

GRINDS AT POCKET SCALE

Small blades tighten the geometry game: a 3-inch blade at 7/8 width leaves little room for grind height error, so the grind height calculator earns its keep before every folder bevel. Hollow and flat grinds both serve (the EDC guide’s logic unchanged); what folders add is the plunge discipline — plunge lines sit clear of the pivot area and match side to side, because a folder is inspected closed, where both plunges show at once. Tips get one extra rule: the closed tip must bury below the liner line with margin — a proud tip in the channel is the injury the whole mechanism exists to prevent.

THE SHARPENING CHOIL

Folders want a small sharpening choil (notch between edge heel and ricasso) more than any fixed blade: the edge must end cleanly before the tang’s mechanical zone, and stones need runout room or the heel goes recurved in a year. Cut it with a small wheel or round file before HT; size it so the stone exits, not so a finger enters (finger choils on small folders eat cutting edge the pocket paid for).

CENTERING AND KICK

Closed centering — the blade splitting the channel — is the folder’s handshake inspection, and it is set by the whole system (tang flats, washer equality, liner squareness, pivot fit) rather than one adjustment. The blade’s closed rest position is set by the kick (slipjoints) or stop pin/detent geometry (locking folders): adjust so the EDGE never kisses the backspring or spacer — an edge that touches steel at closing dulls itself with every pocket day, and grinding the kick is the fix, taken slow, because kick steel only removes once. The blade earns its centered, buried, clean-stopped close — and then it is both a knife and a machine part, finished.