Sharpening

Also called: honing, grinding, whetting, stropping

Sharpening is putting a keen cutting edge back onto a tool once ordinary use has dulled or chipped it. A new edge tool is not even ready for its first job until it has been sharpened, and the work generally falls into two stages: grinding, which resets the bevel, the sloped face behind the cutting edge, and honing, which polishes that bevel to a fine finish.

Grinding: resetting the bevel

A workshop grinder reforms the bevel angle on chisels and plane irons and reshapes other hand and machine cutters besides; the same machine restores screwdriver tips and cold and plugging chisels too. A grindstone meant for a carpenter's edge tools should be fine-grained and soft, as Audels (1923) put it, "of fine grit; it should also be soft." A grinding wheel of small diameter leaves chisel and plane irons with a hollow-ground bevel, one ground slightly concave rather than flat, and a wet grinder, a grinding wheel that runs through a water bath, has a wide wheel suited to broad edges such as a bench-plane iron. Saws are ground rather than filed on some machines: one older method sharpens a saw while it is still turning, by touching a piece of hard flagstone lightly against the teeth, as Jones (1920) described it: "The grinding is effected whilst the saw is running by applying a piece of hard flag-stone lightly to the teeth". A spacing jig turns wheel-grinding of saw teeth into a highly accurate process, and grinding a band-saw blade against a rotating wheel is a recognised way to sharpen one; a large, fourteen-inch grinding wheel can sharpen a big band-saw-mill blade of the kind used in heavy sawmill work. Automated saw-sharpening machines seem to win mainly on how many saws they can get through, rather than necessarily cutting a sharper edge than filing by hand. Diamond wheels built for machine grinding cost even more than diamond hand hones.

Honing: the finishing edge

The angle used when honing a blade's edge is commonly called the honing angle, and it is worked on the bevel face rather than the back of the blade, which keeps the edge itself as the tool's lowest point and never lets its clearance drop to zero. Sharpening stones, whether oilstones or waterstones, come in coarse, medium and finish grades and are worked through in that order. On an oilstone the coarse grade is for fast bevel restoration and the medium and fine grades for routine honing; a double-sided combination oilstone, though, is meant only for occasional sharpening rather than everyday use. Waterstones cut noticeably faster than other kinds of stone. A coarse stone opens most sharpening sessions, but routine daily touch-ups skip it and go straight to the medium and finish stones. A diamond plate serves well for truing the face of a sharpening stone flat once it has worn hollow. Diamond hones can sharpen edges that resist ordinary stones, though they carry a high price, and carbide-tipped blades cost more again and resist sharpening by ordinary saw-filing methods altogether.

Sharpening saws

As Hasluck (1907) put it: "Special shapes of files are necessary for sharpening American cross-cut and rip saws." A saw vice grips a handsaw steady while its teeth are filed.

Where the rule bends

A smoothing-plane iron gets barely any curvature at all across its edge, just its corners eased with extra finger pressure so that they do not dig into the work. Plough-plane irons break the usual pattern altogether, sharing one thirty-five-degree angle for both grinding and honing instead of two different ones. Over many sharpenings, enough metal wears away that the job grows slower and the edge's angle can creep well past thirty degrees.