Kirk DeHeer: Tool Sharpening

Kirk DeHeer is a widely recognized authority on tool sharpening techniques. He has recently released a commercial video entitled "Sharpening Demystified" that is available for purchase at Craft Supplies (USA).

In this demonstration, Kirk discussed tool characteristics and grinding wheel properties as well as sharpening techniques.

(Note: click any image to see it full-size and sharper.)

Grinding Wheels and Sharpening Systems

Kirk prefers a dry wheel sharpening system for sharpening turning tools. The coarser grit wheels create a serrated cutting edge that is more durable that a finely polished edge. A ten inch diameter bowl turning at 900 rpm is generating shavings at a speed of 28 mph so edge durability is far more important than it would be for hand tools.

A common grinder set up is with a 46 grit coarse wheel and an 80 grit fine wheel. Kirk prefers 8-inch wheels over 6 inch since the larger wheels create less hollow grind curvature in the bevel and also provide longer useful wheel life. Friable ceramic wheels are best for modern high speed steels. The harder the steel, the softer the wheel matrix must be. Wheel hardness is coded alphabetically from A to Z with A being softest. The K grade is mid-range in hardness and is very good for all turning tools except carbides which really require a diamond wheel or hone. Kirk uses a diamond dressing tool to dress the grinding wheels so they are slightly crowned. That provides a well defined contact point between the wheel and the tool edge allowing better control and reducing heat generation while grinding.

Tools being used to cut small diameter (3 inch) work are best sharpened on the 80 grit wheel. Tools being used to cut larger diameter work (higher rim speeds) are best sharpened on the 46 grit wheel. Grinder motor speed is also a factor. Low speed grinders typically turn at about 1750 rpm. Variable speed grinders can be run at 1750-3000 rpm. Higher speeds produce the effect of a finer grit wheel and usually generate more heat.

Turning tools can be sharpened free hand or using jig systems. Freehand sharpening can be very quick, but good freehand sharpening requires much practice and wasted tool steel. Poor freehand sharpening will generate multi-faceted edges with ill defined bevels. That results in difficulty in maintaining a bevel-riding cut and usually leads to excessive scraping that shortens the life of a cutting edge. Kirk recommends use of a sharpening jig system to provide consistent, repeatable bevel angles with a minimum loss of tool steel.

Kirk prefers the One-way Wolverine system but with the Woodcut tool holder. The Wolverine system provides a substantial flat platform as well as the V-arm tool rest. The Woodcut holder will accept shorter tool shafts and larger tool widths than the One-way holder and can also be used for sharpening oval skews. Bevel angles can be adjusted by extending or contracting the V-arm or varying the angle of the platform tool rest. Plywood templates or the Raptor set-up tools can be used to set specific angles. Kirk recommends setting the angle between the control arm of the tool holder and the shaft of the tools to be sharpened at 23°. A convenient gauge for setting that angle can be formed by folding a sheet of paper so the top and side edges are aligned, forming a 45° angle at the corner and then folding it a second time to form a 22½° angle.

Specific Tool Characteristics and Sharpening Techniques

Spindle Roughing Gouges

These tools are ground with a square end and a cutting edge bevel of about 35°. They can be sharpened using the V-arm of the Wolverine jig by adjusting the length of the V-arm to create the desired bevel angle. The handle of the tool is then mounted in the pocket of the V-arm and smoothly rotated against the wheel to create the edge. However, Kirk recommends using the flat platform rest instead. The platform is preset to give the 35° angle. The tool is held flat against the rest with the cutting edge square across the wheel and the axis of shaft perpendicular to the grinding surface. The tool is then rotated smoothly against the rotating wheel without moving the axis of the tool. Before grinding, you can check the platform angle by darkening he bevel of the tool with a magic marker, holding the tool in the proper orientation on the tool rest with the cutting edge resting lightly against the grinding wheel. Turn the wheel manually and then examine the grind mark in the darkened bevel. If the grind mark goes evenly from the cutting edge to the heel of the bevel, you have matched the platform angle to the existing bevel angle of the tool. If the grind mark is primarily at the edge or at the heel of the bevel, the angles do not match and must be adjusted.

This tool is designed for spindle work only and must NEVER be used in bowl turning.

Parting Tools

Parting tools come in several types, e.g., diamond cross section, rectangular cross section, thin blade, etc.

The diamond parting tool is designed so the thickest part of the blade occurs at the mid-line. The intent is to have the cutting edge be the widest point of the tool to minimize side drag on deeper cuts. This only works if the apexes of the diamond cross section are directly opposite one another. Before buying a tool of this type, always examine the tool to verify this symmetry. This tool can be ground in two configurations. The traditional grind is such that the top and bottom bevels are of the same length and the point is an equilateral triangle. Kip Christensen discovered that grinding the upper bevel back further created a more acute angle between the cutting edge and the turning wood thus allowing shavings to come off the wood more easily and smoothly. This asymmetrical grind is now preferred by many turners and should be used with the long bevel up. (Kip uses both side of the bevel it depends on if he wants a peal cut or a scrape.)Both the traditional and asymmetrical grinds are achieved by holding the tool on the platform rest first with the upper edge of the tool down, the tool cross section vertical and the cutting edge square across the wheel. Engage the heel of the bevel against the wheel and raise the handle of the tool until the wheel grinds the full length of the bevel and raises a burr on the cutting edge. The tool is then rolled over so the lower edge of the tool rests on the platform and the same procedure is followed with the lower bevel. This sequence caused the last burr to be on the upper side of the cutting edge where it will be most helpful, at least for an instant. It is very important that the cutting edge be right at the apex of the diamond cross section so the two bevels must be ground until they meet right at the midline of the tool.

The rectangular cross-section parting tool is sharpened in exactly the same way as the diamond tool, but the flat sides make it much less important that the cutting edge be at the mid-line of the tool. In fact, Kirk has ground a rectangular parting tool with a very long upper bevel and very short lower bevel so he can create narrow annular recesses in end-grain boxes to receive epoxy inlays for threading.

Thin blade parting tools are designed with narrow (side to side) but deep (top to bottom) blades. The thinness minimizes the width of kerf and the depth provides tool rigidity. These tools are also ground with a very long upper bevel and very short bottom bevel and are always used with the long bevel up. Once the long upper bevel is established, the tool can be sharpened by lightly grinding only the bottom bevel. The upper bevel will need grinding only when the bottom bevel has been ground back so far that the cutting edge is no longer supported near the tool rest on the lathe. The procedure for grinding the bevels is the same as for the tools already described.

Skews

The proper bevel angle for skews is one that will cause the length of the bevel to be 1.0 to 1.5 times the thickness of the skew blade. The cutting edge should form a 70° angle with the edge of the shaft. To sharpen a skew, set the flat platform rest at an angle that will allow the bevel of the skew to rest evenly against the wheel when the tool shaft is laid flat on the platform with the cutting edge square across the wheel. The same procedure of darkening the bevel with a marker described in the spindle roughing gouge section is useful for matching the existing bevel angle of a skew. While grinding, the tool should be moved smoothly back and forth across the wheel without changing tool orientation on the platform (or the tool skew angle) so the cutting edge always remains square to the wheel. For radius-edge skews it will be necessary to rock the handle back and forth slightly across the platform (using your thumb as a fulcrum) to create the cutting edge radius, but the tool shaft must be kept flat against the rest. Skews with an oval cross section cannot be held flat on the platform. They are best sharpened using the tool holder and V-arm. Similarly, if a skew has such a short blade that it cannot be laid flat against the platform due to ferrule interference you might want to use the tool holder and V-arm. In that technique the skew is mounted in the tool holder with the long point down. (The skew angle is set with the control leg of the tool holder). The V-arm is set at a length that will give the desired bevel angle and the cutting edge is drawn back and forth against the rotating wheel with the end of the control arm of the tool holder engaged in the pocket of the V-arm.

Scrapers

These tools are ground with an almost endless variety of end profiles depending on the type of project being turned. There are square-end, radius-edge, round nose, box end, and on and on. However, almost all are ground with similar bevel angles, usually 60° or more. The principle is to grind enough angle to prevent the heel of the tool from contacting the wood but not enough to threaten the rigidity of the cutting edge. The tool is ground by setting the platform rest to the desired angle, laying the tool blade flat on the platform and moving the tool across the wheel to create the desired end shape. Grinding will create a burr on the upper edge of the tool. The softer the wood being turned the more bold the burr can be. A heavy burr, however, is too aggressive for hard, dense woods. The burr can be removed with a hone rubbed flat against the top surface of the scraper blade. The best rule is: "If in doubt, take it out."

Negative rake scrapers are those where a bevel is ground on the top of the scarper blade also giving the cutting edge of the scraper a negative angle of attack to the wood. These are best used with very hard, dense woods. The upper bevel is ground to an angle of 35°. Kirk prefers to grind negative rake scrapers with both the bottom and top bevels ground at 35° with the top being the longer of the two bevels.

Gouges

Both bowl gouges and spindle gouges are ground similarly using either freehand or jig techniques. The two gouges differ primarily in the depth of the flute. European gouge sizes are measured from flute to flute for bowl gouges and from shaft diameter for spindle gouges. American gouge sizes are all measured by the shaft diameter. A parabolic flute cross section in a bowl gouge tends to provide an easier exit path for shavings and produces cleaner cuts than straight sided flutes.

Bowl gouges are typically ground in three profile types: traditional, fingernail, and modified fingernail. These profile types differ primarily in the extent to which the wings are ground back. In the traditional grind the cutting edge is kept almost square with the axis of the shaft much like a spindle roughing gouge. This can be done using the same technique described earlier for the spindle roughing gouge. The wings might be ground back very slightly to give just a little clearance. The fingernail grind will have the wings ground back enough that the end of the tool will be curved like a fingernail. The modified fingernail grind has the wings ground back somewhat further to provide a long cutting edge on the wing and is most useful in shear scraping or taking very aggressive shavings when roughing a bowl. The extreme of the modified fingernail grind is also called an Irish Grind.

Freehand grinding can be performed with or without a tool rest on the grinder. Kirk demonstrated a freehand grind using the flat platform tool rest. The platform is set at an angle to the wheel that matches the bevel angle desired. The gouge is first ground on the wings and then the tip is blended into the wings. The grinding motion actually swings and rotates the gouge shaft across the platform such that the flute is pointed straight up when the shaft is perpendicular to the grinding surface of the wheel and rolled to the side on each wing so that both the flute angle and the shaft angle on the platform are about the same as the desired bevel angle. For example, if a 45° bevel angle is desired, The shaft will swing from 45° left to 45° right on the platform while the flute is rolled from 45° open on one side of the swing to 45° closed on the other and passes through full open at the midpoint where the shaft is perpendicular to the wheel.

New gouges will have a factory-ground profile, either traditional or fingernail, that is not ready for use. Regrinding is virtually always required. For fingernail type profiles, Kirk recommends new tools first be rough ground to the desired wing shape by laying the top of the flute on the coarse grinding wheel with the axis of the shaft parallel with the side of the grinding wheel. The top corners of the tool end are thus ground away to create the desired curvature and length of the upper edge of the wings. This will also reveal the amount of steel that must be ground away from the side of the wings. Kirk also rough ground by freehand this excess steel from the sides of the wings so there was only a narrow flat facet along the upper edge of the wings.

He then mounted the tool in the tool holder with two inches of the tip of the tool exposed beyond the front of the tool holder. The control arm of the tool holder is adjusted to 23° and the jig V-arm length adjusted to the desired bevel angle as described above in the Grinding Wheels and Sharpening Systems section. He then placed the end of the control arm in the pocket of the V-arm and first sharpened the wings until the facets on the upper edge of the wings disappeared. He then blended the tip into the wings with a smooth swing motion around the end of the tool. When refreshing the edge of a gouge that already is properly profiled, it is only necessary to mount the tool in the tool holder and V-arm and take a couple of passes around the end of the tool on the grinder.

When making cuts on concave surfaces like the interior of a bowl it is often helpful to reduce the length of the bevel to eliminate an annular ring pattern that results from the heel of the gouge embossing the wood as it supports the cutting edge through the curve. This is easily done by merely freehand grinding away part of the heel of the bevel. However, if the bevel is shortened too much by this technique it becomes difficult to locate the bevel and maintain a smooth bevel-riding cut through the curve.

Spindle gouges are typically ground with a bevel angle of 35° to 50° depending on the type of work being done. Bowl gouges are typically ground with bevel angles of 45° to 60° with 60° or even more being fairly common for traditional grinds. The larger angles allow bevel-riding cuts to reach deeper into the interior of a bowl before the shaft of the gouge contacts the lip of the bowl.

It is generally of little value to hone the inner surface of gouge flutes unless the tool was manufactured with surface ridges.

Thread Chasers

Thread chasing tools, both male and female, are basically used as negative rake scrapers. They are sharpened only on the upper face, never on the threads themselves. They are sharpened by placing the upper face of the cutting edge lightly against the fine wheel with the teeth pointed up. It is often helpful to grind off part of the back edge and corner of a new female tool provide better clearance in small openings.