at the lathe by Jon Siegel 1

part 1 – tool point geometry Sharpening Woodturning Chisels

Woodturning chisels are subjected Sharpening is the routine at 25 to 30 degrees, that this must be to more wear and tear than any other maintenance of the perfect edge. This good for turning chisels also. However kind of hand held chisels used in operation assumes the correct geometry turning chisels are subjected to much woodworking. Because the wood is has previously been achieved, and we more brutal conditions than hand passing the chisel at 20 miles per hour aim to keep it that way as we sharpen chisels. For this reason, a more blunt for prolonged periods of time, a turning the chisel hundreds or even thousands angle is preferred because it produces an chisel will traverse more wood in a few of times. edge which is more robust, stays sharp minutes than any carving chisel could in longer, and is less likely to overheat. years. Just look at the pile of chips that Basics of Tool Point Geometry Also, unlike carving tools, turning accumulate around a wood lathe and you An edge is formed at the intersection chisels are usually used at a shear will be convinced of this fact. For this of two planes – these planes may be flat, angle. This means that the edge is not reason, sharpening is a more important curved or a complex conical surface. perpendicular to the direction of travel. issue for turning than any other branch The angle of intersection is called the This produces an effective cutting of woodworking. dihedral angle, usually referred to angle which is much less than the Sharpening is not handled well in as the edge angle. It is measured as measured edge angle. To understand most of the turning books I have read, the angle between two lines which are this concept, think about a switch-back because the authors try to cover too perpendicular to the edge at their point road which ascends up a mountain. much at once. I will divide the subject of intersection and lie on the respective The effective angle of climb is reduced into two distinct parts: Tool Point planes. because the road does not go straight up Geometry, which will be covered here, Looking at the chart (Fig. 1), you the mountain, but angles across the face and Sharpening, which will be in the see two main groups of woodturning of the slope. So when we use a turning next issue. chisels — cutting tools and scrapers. chisel at a shear angle, the actual cutting Tool Point Geometry describes Cutting tools are divided into two main angle of the edge is much less than the the shape of the tool. To understand groups — gouges and chisels. As we angle at which we ground the bevel on this topic thoroughly, we need to discuss these types, we will examine two the chisel. For these reasons, I do not explore why the tool is shaped the way aspects of tool point geometry — the think it is necessary to grind turning it is, and how the shape affects the way edge angle and the shape of the line chisels to a more acute angle than 35 to it behaves. which forms the edge. 40 degrees. The best edge angle for woodturning But decisions of tool point geometry Tool Point Geometry chisels is 35 to 40 degrees (Fig. 2) should be based on how they affect the although turners differ greatly in behavior of the chisel in actual use. I working practices. I have seen angles have found that chisels which are too Cutting Tools Scrapers between 25 and 55 degrees used. Many acute are harder to control than those people think that since their cabinet ground at about 40 degrees. Some top Chisels Gouges Spear Point chisels and carving chisels are ground woodturners, teachers and writers, such as Mike Darlow disagree with this and Straight Round Nose use more acute angles. Skewed Form Tools Flat Grind Versus Hollow Grind Rounded Shear Scrapers I am absolutely convinced that wood turning chisels should not be hollow ground — this does not apply to scrapers Bowl Spindle Removable Tip which can be either flat or hollow. I learned this from trial and error, and Side Grind Conical Grind decades of experimentation. When I Ring & Hook Modi ed Fluteless switched to flat grinding, woodturning suddenly became much easier. The Fig 1 – Two main groups Fig 2 – Measuring edge angle reason is because the flat grind defines Photos & drawings by Jon Siegel 2

Scrapers the bevel as a straight line — at least in Gouge Cross Sections the longitudinal plane. This allows better Scrapers are used in bowl turning, control of the chisel. If you want your chuck, and faceplate work, where the chisel to go in a straight line, you need a grain of the wood is perpendicular to the straight bevel. If the bevel is curved in a axis. In this kind of work, the grain is hollow shape (concave), then the chisel constantly changing from end grain, to is always trying to follow this curve, and cross grain, to long grain, and a scraper it is a struggle to keep it going straight. is well suited to the task. Scrapers are almost never used in spindle Nose Radius of Gouges turning. No factor of tool point geometry A scraper does not stay sharp very affects the behavior of a gouge more long even under the best conditions, but than nose radius because a smaller this disadvantage is outweighed by its radius produces a narrower chip and ability to handle variable grain direction results in a smaller cutting force. Figure without lifting or tearing-out the grain. 3 shows five different shapes possible Fig 4 – Two distinct types of gouges A gouge (cutting tool) is the best from the same chisel profile (or cross choice for roughing because it has kinds. For example, the smaller nose positive rake, which removes wood radius allows for easier piercing of the more quickly with less friction, and the surface for initiating cuts (coves) at very tool stays sharp longer (Fig 5). However high vertical angles, while the wider there are many situations where nose chisels generally make it easier to scrapers are necessary — smoothing produce a smooth cut on flat surfaces or and blending curves, fine details, and the bottom of coves. awkward situations where a gouge or During sharpening it is important to maintain the Geometry of a Cutting Tool nose radius to the shape you prefer. Note that there are two distinct types of gouges — those forged from a flat piece of steel which has be bent into the “U” shape common to all gouges, and those created by milling a groove into a Fig 3 – Different nose radii are possible from piece of round steel (Fig. the same chisel profile 4). The milled gouge is the modern convention. section). These differ only in their nose Gouges made from round Fig 5 – Positive rake is best for a gouge radius. The first example is a “straight steel present a cross section across” grind (radius = infinity). This which is thicker in the is the way we grind a roughing gouge middle than at the edges. for spindle work; it has no nose and has Because there is more Geometry of a Scraper sharp outside corners. Until recently this material in the middle, this is the way most English turners ground part resists the grinding their bowl gouges too. (See Frank Paine process more. As a result – The Practical Woodturner, or Peter there is a tendency for Child, – The Craftsman Woodturner) these chisels to become The remaining four examples in the more pointed (toward a illustration show a decreasing nose smaller nose radius) as radius from about one inch down to 1/16 they are sharpened. You inch. must be conscious of this, Besides the straight across gouge and dwell on the grinder already mentioned for roughing and longer in the middle to working into corners, each of these counteract this tendency, variations has certain strengths and and thus maintain the weakness for applications of various correct nose radius. Fig 6 – Scapers are ground with a negative rake at the lathe by Jon Siegel 3 other cutting tool cannot be presented where the cut must be made in a pulling at the correct angle. motion (working toward the mouth). Skew Chisel Cross Sections A scraper is used in a very different The disadvantage of side grind is that manner than a cutting tool, because the angle of the bevel changes whenever the bevel of a scraper does not rub the you rotate the tool. This means that work. Instead a scraper is controlled if you rotate the gouge during the cut by pressure only, and the handle is (to alter the shear angle), the gouge held higher than the edge, giving the reacts by changing its direction. On a tool a negative rake. It is this negative conventionally ground gouge, the angle rake and the ability of the scraper to of the bevel is constant with respect to take microscopically fine shavings that the handle, and you can rotate the gouge reduces tear-out and makes it the best any way you like during the cut (to alter finishing tool for difficult varying grain the shear angle) without changing its situations. direction of cut. Also, it is much easier Fig 8 – Skew chisels are ground on both sides Because the bevel of a scraper does to start a cut when you know that its not rub, the exact angle or shape of the angle is not dependent on the rotation grind (flat or hollow) does not make of the tool. Chisel Edge Shapes much difference. Usually, scrapers are ground at a very blunt angle, Skew Chisels corresponding to an included angle of Skew chisels are part of a whole class 70 to 80 degrees. When the tool is used of spindle turning chisels which are with a negative rake angle of 10 degrees, ground on both sides. Most skews are then the resulting clearance angle is 20 made from rectangular stock, and the to 30 degrees (Figure 6). corners are extremely sharp (Fig 8-A). The chisel cannot successfully be used in Gouges — Axial vs. Oblique Grind this condition, because the corners dig There has been a revolution in the way into the tool rest (not to mention your most turners sharpen their bowl gouges fingers). This will prevent the tool from Fig 9 – Chisel shape can be straight, curved in the last 15 years. To some degree this sliding along the tool rest smoothly, and or angled has changed for spindle gouges too. The eventually will damage the surface of new geometry is usually referred to as the tool rest. a nose radius. Note however that a skew side grind, and is achieved by swinging Such a skew chisel can be put right which is shaped to a curve does not have the handle from side to side during by rounding over the corners (Fig 8-B). a distinct sharp point (the toe of the grinding, instead of simply rotating it. A new type of skew is now available skew) and this is a limitation. What this produces is a conical bevel which has its short sides completely surface which is not referenced to the rounded over; so the cross section takes The Myth of the Original Grind axis of the tool, but oblique to it. The the shape of a race track (Fig 8-C). Many beginners think that they obvious effect of this is that the gouge These are called the “rolled edge” skews, should maintain the geometry that the has longer edges on the sides (or wings) and I highly recommend them. Also, tool had when they bought it. While than a conventional grind (Fig 7). some of the new skew chisels are made this may be true of the Ellsworth These long side edges are useful in from round stock (Fig 8-D), and these signature gouge and a few others, in many bowl turning situations, especially are excellent in the small sizes. Avoid general this is a dangerous assumption. reaching into end grain boxes or goblets the “oval skew” (Fig 8-E) with the long The person who ground the tool at the sides rounded. factory probably never turned a piece Conventional grind The shape of the edge of the chisel of wood in his life. He is just grinding can vary. It may be a straight line which it the quickest possible way so it looks is square across (Fig 9-P) or slightly like it has been sharpened; and this is rounded (Fig 9-Q). OK because it gives you a head start in The standard skew (Fig 9-R) has regrinding the tool the way you want it. a straight edge which is ground to an With all the chisels available and all angle — usually 20 to 40 degrees. This the possible ways to grind them, there is an extremely versatile tool which are too many variations for anyone to offers the choice of using the heel or the master in one lifetime. So I will leave Side grind toe in different situations. you with this final thought — Take The edge may be a curve (Fig 9-S) time to experiment. If you do, you will which creates a narrower chip and soon realize that this article has barely Fig 7 – Side grind produces a conical bevel behaves more like a gouge since it has scratched the surface of this subject.