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.
