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.

What then is the optimal sharpening angle for general turning use? Elliott’s research with plane blades showed 34°. But as figure 2 shows, the edge of a plane blade is presented with a substantial clearance angle and without side rake (being skewed to the direction of travel of the plane). A plane-blade edge might thus take more of a buffeting than a turning-tool edge which is presented at a near-zero clearance angle and at often considerable side rake so that the wood slides past the edge. Also, the high-speed steels used in turning tools are certainly more resistant to abrasion (and may also be more resistant to crumbling) than the steels used in most plane blades.

My experience with a wide range of woods, including hard hardwoods, is that the following sharpening angles are durable: 25° for skews and detail gouges; and 30° for roughing gouges, parting tools, and  gouges used to rough the outsides of bowls. One exception is when vigorously hollowing in cupchuck turning with a detail gouge with a 25° sharpening angle I’ve broken the last 2 mm of the nose off. I now have a detail gouge with a 35° sharpening angle for this particular cut.

I’ve so far stated three reasons (less effort, better off-the-tool surfaces, and less resharpening) for using small sharpening angles. Below are more:

  • the ability to cut deeper coves (figure 3)
  • the ability to cut better-shaped adjacent beads (figure 4)
  • when cutting vertically with a skew’s short point, usually at the bottom of a bead, the blade has to be rolled less past the vertical
  • the smaller the sharpening angle, the greater the force supporting the bevel, and therefore the longer the safe tool overhangs
  • as figure 5 shows, for a given clearance distance between the toolrest and the workpiece, the less the range of heights at which tool handles need to be held. This lessens or in many cases eliminates the need to raise and lower the toolrest while turning a workpiece.

I hope that the above has convinced you that where access is unimpeded, large sharpening angles are suboptimal. But if you’re unconvinced, sharpen half a roughing gouge edge at 30° and the other half at 45°, and compare the two.

Figure 1 Illustrating that a keen edge with a large sharpening angles needs less abrasion (coloured pink) to become dull than a keen edge with a small sharpening angle.

Figure 2 A plane blade mounted in a plane body is presented to the wood with a considerable, here about 20°, clearance angle.

Figure 3 Illustrating that deeper coves can be cut when a detail gouge has a smaller sharpening angle.

Figure 4 Illustrating that the greater a skew’s sharpening angle, the more the resulting adjacent beads are distorted.

Figure 5 In diagrams A and B the sharpening angles of the two tools are 25° and 45°; in C and D they are 25° and 30°. It’s commonly recommended that turners raise the toolrest for planing with a skew. This figure illustrates that the greater the range of the sharpening angles of the tools used, and the nearer the toolrest is to the workpiece, the greater the range of angles between the two blades and the range of heights at which the tool-handle-holding hand must work. And the greater this range, the more the need to keep adjusting the toolrest height.