2. cutting a too-slender blade shorter—you then have the option of using the off-cut as a bit
3. cutting a slender blade into short lengths (bits) which are fixed into stiff shafts as explained in figure 2.4.
Figure 2.5 The optimum, rotationally-stable detail gouge cross section. The smaller the lever arm between the force exerted by the shaving and the upward reaction force exerted by the toolrest, the smaller the tool’s tendency to unexpectedly rotate and catch. This lever arm is small when there is a semicircle of steel below the base of the flute. Because most cutting is performed by the cutting edge’s shoulders, the lever arm is even smaller if the semicircle is slightly flattened (indicated by the blue, dashed line).
Obviously the stiffness of a blade with this cross section is related to its width.
Figure 2.6 A blade cross section which is somewhat stiffer than one with the same flute radius but the figure 2.5 cross section. However this deeper blade cross section results in a longer lever arm between the two forces and therefore greater rotational instability.
Figure 2.7 A rotationally-stable, stiff blade cross section with a small flute radius. To improve its ability to cut deep, narrow coves, the nose’s width must be reduced by grinding back the flanges as shown in figures 2.10 and 2.11.
Figure 2.8 Top, a 19-mm-wide detail gouge with a ladyfinger nose; bottom, a gouge with a fingernail nose.
2.3.1 Detail gouge noses
There’s less tear-out if you cut cove bottoms with the gouge’s active edge (the length of edge actively cutting) presented at about 45° side rake (skewed at about 45° to the velocity of the wood about to be cut). Therefore of the two gouge noses shown in figure 2.8, the ladyfinger nose at the top is far better than the fingernail nose at the bottom. However if you grind a basic ladyfinger nose like that in figure 2.8 on a bowl gouge or on a blade with a cross section similar to that shown in figure 2.7, you’ll only be able to cut narrow coves a little deeper than semicircular. To cut much deeper narrow coves without lessening the blade’s cross section back from the nose, you need to reduce:
• the sharpening angle
• the height of the flanges in the nose region
• the width of the nose
The effectiveness of reducing the sharpening angle and the height of a blade’s flanges at the nose end of the blade is explained in figure 2.9. Nose grindings which enable the flange height and nose width are shown in figure 2.10. The three types of resulting gouge noses are shown in figures 2.11 and 2.12.
Figure 2.9 Showing that the depth of cove which a gouge can cut without fouling the cove rim increases as the sharpening angle decreases. Therefore sharpen detail gouges at 25°. Also, the smaller the sum of the flange height and blade thickness beneath the bottom of the flute, the deeper the cove which can be cut.
Figure 2.10 Grinding noses on stiff blades to allow the tools to cut deep, narrow coves. The unground cross section has a mauve fill. The outlines with the palest, medium and darkest orange fills represent respectively grinding down the flanges, grinding away nose width, and grinding the nose’s cross section below the bottom of the flute to a semicircle. This third grinding operation is usually optional because the area of the blade which is usually being supported by the toolrest is well back from the nose.
Figure 2.11 A 9.5-mm-wide gouge blade with a flute radius of 1.75 mm, a blade cross section similar to that in figure 2.7. The nose has been ground as shown on the right-hand side of figure 2.10.
The grinding to reduce nose flange heights and widths can be done on a conventional bench grinder with the blade’s axis parallel to the velocity of the grinding wheel’s periphery or parallel to the grinder’s spindle.
Figure 2.12 The noses of two adequately stiff, small-fluted gouges sharpened with ladyfinger noses, and a cove cut by each.
Left, the flute radius of the modified bowl gouge is 2.5 mm, and the cove is 5.5-mm-wide and 9-mm-deep. Right, the blade width of the inserted bit is 6 mm, and its flute radius is 1.5 mm. The cove is 4.5-mm wide and 5-mm deep. The flat on the top of the shaft in which the bit is housed results from a hammer blow to lock the bit into the bored hole in the shaft.
2.4 SHARPENING, HANDLES AND CALLIPERS
Small turning tools need gentler sharpening, and seem more responsive if they’re fitted with lighter handles. The smaller the workpiece, the more apparent small errors in diameter become, and the greater the importance of accurate callipering. This section discusses these three topics.
2.4.1 Sharpening small turning tools
Although a high-speed (3,000 rpm) grinder with, say, an 80-grit wheel is fine for approximate shaping of small woodturning tool noses, it’s rather aggressive for regrinding. A slow-speed grinder with a finer, say 120- or 160-grit wheel would be better. If, like me, you don’t have this facility, a coarse diamond file can be employed for refining the nose form before honing with a fine-grained diamond file or slip stone. (As Alan Lacer and Jeryl Wright have shown in their article “Does Honing Pay Off?”, diamond is the preferred honing abrasive for HSS because of the presence of very hard nonferrous carbides in the steel.)1 Diamond-coated slipstones are available, but you’ll probably have to source an aluminium oxide (figure 2.13) or silicon carbide slipstone for honing flutes with very small radiuses.
Figure 2.13 Hones for small-nosed turning tools. Left to right: a Norton FS-24 aluminium oxide India slipstone, a diamond-coated slipstone with slightly larger edge radii,