By Alan Lacer www.alanlacer.com (articles on sharpening and honing)
EQUIPMENT: Wheel grinder (I prefer a 1725 or 1800 rpm) set just below chest height, 8 inch diameter wheels 1 inch wide, 60 or 80 and a 46 grit friable aluminum oxide wheels (usually whit, red, pink, orange or blue) in a J or K hardness OR a SG wheel (same grits and hardness), rock solid tool rests, eye protection, dust mask, wheel dresser (star-wheel, dressing stick, or diamond dresser—all work, but leave different surfaces, my choice is the diamond), diamond slipstone (best for modern tool steels), OR flat stone in diamond plus a tapered or round diamond hone (slipstone replaces these two different hones).
CBN wheels and boron hones also work well—will discuss in the session.
I. Guidelines for sharpening scraping type tools (flat steel, ground on one bevel only, similar to the cabinetmaker’s scraper in its edge—a burr):
- As with any turning tool, shape or profile the tool first. Scrapers are the most readily shaped tools—ground into whatever shape is needed.
- Aim for a bevel angle between 70 and 50 degrees (or think of it as a relief angle of 20 to 40 degrees off of 90 degrees). Set the platform of the grinder to the desired angle.
- Although normally the bevel is not rubbed on the wood—and we do not hone these tools like cutting tools—I still aim for a single faceted tool for developing good habits.
- Start at heel of bevel and lap or grind forward until sparks just begin to appear over the top of the tool, being sure to always leave the tool flat on the rest's platform.
- The burr that is raised from grinding OR raised with a burnisher IS the cutting edge at least 95% of the time. On some woods where the burr is too aggressive, we remove the burr and scrape with a sharp edge.
- Leave the heavy burr from grinding if the intention is to remove considerable material and quickly. If you are using the scraper as a finishing tool, remove the burr with a flat honing tool (face of the slip stone or flat diamond stone). Next raise or pull up a more delicate burr with a burnisher—anything harder than the steel. I use the rounded edge of the slip stone or a cabinetmaker’s burnisher to raise the burr. This is accomplished by tilting the burnisher just about five degrees past 90 (towards the top of the tool) to fold the steel back—and traveling along the full length of the edge. The burr size is most determined by the amount of pressure you apply to raise the burr—more pressure, the heavier the burr. You can successfully raise a burr 4 to 8 times before the edge is too rounded and you must then return to the grinder.
Feel for the burr by running your finger off the edge, not along the edge. When working with a sharp scraper it should also produce small ribbons—if sawdust, then the tool is usually dull.
II. Guidelines for sharpening “cutting” type tools such as gouges, skew chisels, parting tools and hook tools:
OBJECTIVE: Single facet with a slight hollow grind
MENTAL OBJECTIVE: Grind the bevel and not the edge
1. Profile or shape the tool first—don’t be too timid removing large amounts of material to reach desired shape—check a book, video or an experienced turner for recommended shapes/angles.
2. Next, begin to match the desired bevel angle to the profile (to actually sharpen/create an edge on the profile). Start at the heel (back region) of the bevel and gradually lap forward towards the edge.
3. Use light pressure, be slow and deliberate, maintain a relaxed attitude and grip, elbows in, controlled stance.
4. Leave the tool on the wheel, looking at your progress only occasionally—use the spark trail as feedback to determine where you are grinding. Stop grinding when sparks evenly form over the top of the tool edge—further grinding burns away the edge, producing a “saw-toothed” edge. When full bevel is in contact with wheel, tool is sharpened.
5. Avoid heating the tool to such a temperature that you see temper colors developing (yellows, purples, blues). When grinding carbon steel tools, quench in water quite regularly. If using high-speed tools avoid quenching when a tool becomes quite hot to the touch—grind in stages, allowing the tool to air cool between sessions.
SHARPNESS INDICATORS: (1)Under a strong light: if you can see the edge on cutting tools (skews, gouges, parting tools, etc.), there is no edge! (2)The amount of effort or pressure it takes to remove material is a great indicator of sharpness—a sharp tool seems to allow the wood to cut itself, a dull tool requires extra force. (3) Look at the material coming off the tool—dull tools tend to produce dust or short chips, sharp tools tend to produce ribbons and curls even if short. (4) Listen for sharpness: sharp tools make a hissing sound (much like a sharp plane), dull tools sound flat or make a scraping sound. (5) Examine the surface quality—that’s what it’s all about.
HONING GUIDELINES for skews, gouges, parting tools and hook tools:
Honing does two things: refines the edge from grinding and maintains the edge between grindings. It is far easier to keep a sharp tool sharp than it is to use a tool so long that you must return to the grinder frequently. Get in the habit of regular honing, especially before final cuts. For gouges hone the outside ground bevel first by touching the heel of the bevel first, then gently rock into the area just below the edge, still touching the heel of bevel—always a two-point contact. Next, hone the inside flute: hold the curved edge of the slipstone perfectly flat within the flute and move the stone in and out of the flute until the entire edge has been honed. Hone both ground surfaces of the skew and parting tool in a similar two-point strategy. You can only hone a properly ground tool—good grinding is still more critical than honing. RULE: Hone the bevel and not the edge!!