This article discusses sharpening angles. It will show that the sharpening angles of tools which are presented without restricted access should be smaller and lie within a much smaller range than is typically recommended. (The sharpening angle at a point on an edge is the ‘angle between the two surfaces which meet to form that edge measured in a plane running through that point and perpendicular to the edge’. For a hollow-ground or multi-facetted bevel, the sharpening angle is measured to the line on the plane joining the edge to the bevel heel or the nearest facet heel. For a gouge, the stated sharpening angle is usually measured at the bottom of the flute’.)
If you study the many recommendations by different turners for tools used where access is unrestricted, you’ll be struck by the:
- wide range, between 18° and 70°, within which recommended sharpening angles lie
- size of the range of sharpening angles recommended by some turners. For example, it’s common for a turner to recommend a range from 25° to 45°
- increase in recommended sharpening angles during recent decades as tool steels have improved.
I’m not concerned here with what causes different turners to recommend such different sharpening angles for the same and similar tools. Instead I shall show that sharpening angles much bigger than 30° are suboptimal.
Leonard Lee’s important 1995 book The Complete Guide to Sharpening stresses on page 9 the benefit of keeping the sharpening angle “as low [small] as possible consistent with edge retention”. By “edge retention” Lee means ‘not crumbling during the tool’s anticipated use’. By sharpening an edge to the sharpening angle only slightly greater than that at which which it becomes unlikely to crumble in normal usage you:
minimise the force you need to apply to a tool to remove excess wood at a given rate, and
improve the quality of the off-the-tool surface produced.
However these two gains ignore the important aspect of time spent resharpening—surely it’s better to use substantially greater sharpening angles than Lee’s minimums so that only a small proportion of one’s total turning time is spent resharpening. I believe that this logic is wrong.
Research by Steve Elliot on plane blades (http://bladetest.infillplane.com) has shown that as long as the sharpening angle is large enough to prevent an edge crumbling in use, further increasing the sharpening angle actually increases the frequency of resharpenings. Why? Because when keenness is lost only though abrasion, as Figure 1 illustrates, less abrasion is needed to dull an edge with a large sharpening angle than an edge with a small sharpening angle.