WoodEzine:

How To
Sharpen A Chisel

(or A Plane Iron)


 

Woodworkers enjoy the building process a whole lot more when tools are sharp. Trying to clean out a mortise or an inside corner with a dull chisel is frustrating because the wood tears, rather than slices. And the extra effort required often leads to minor injuries when the tool slips. Fortunately, making a chisel sharp is a fairly simple, three-step process. First, the back (unbeveled) face must be made flat. Then the primary bevel is set by grinding. And finally, a secondary bevel is established and sharpened by honing. Don't worry, we'll explain all those terms...
It has been my experience that oilstones deliver the finest edge, but they also take the longest time. High quality water stones are almost as good, and a little bit faster. Diamonds will handle the first 90% very quickly, and then I finish with a very fine oil or water stone. Ceramic stones are comparable to the finest water stones in terms of the edge they deliver.
All stones work better with a jig to hold the chisel when establishing bevels, rather than holding it freehand, because repeatability is very important. Purists may argue with that, but I strongly advise woodworkers with beginning or intermediate skills to use a jig.


Flatten The Back
You don't have to flatten the entire back of a chisel or a plane iron. But the half-inch or so closest to the cutting edge does need to be perfectly flat. The best ways to do that are on bench stones or on a lapping plate, which is a piece of plate glass or perhaps a flat granite floor tile with various grits of wet/dry sandpaper attached to it. Another viable option is to use a slow horizontal wheel, or one can use diamond stones.
The first option (water stones, shown at right) begins with charging various grit stones by immersing them fully in water until there are no more bubbles rising. It's a good idea to use distilled or filtered water, such as is found in bottles of drinking water bought at the store, rather than using tap water. City or well water may have impurities such as chemicals to kill bacteria, added fluorides, salines for water softening, and other treatments. These may or may not be harmful to stones. Most water stones are manmade nowadays, but some older ones (and many of the more expensive Japanese stones) are natural, and the bonds in these sedimentary rocks can be delicate.
 

Begin with the coarsest stone you own or can borrow, and work toward the finest. If you're buying stones, a combination medium/fine is a good choice to start. Diamond stones are usually used with water as a lubricant. Oilstones don't need to be charged. The oil to use is a light kerosene or a dedicated sharpening fluid. In all cases, the liquid is not used as a cleaning agent. It is in fact a lubricant that allows small particles of stone or metal to be moved and contained in an abrasive slurry, called swarf. The swarf speeds up the grinding or honing process.

 

Whatever stone you choose, it will need to be completely flat. Uneven surfaces can be seen when a steel straightedge is held against the top of the stone, with a background of bright light. Check both diagonals (lay the straightedge from one corner to the opposite corner at the other end), and then check across the stone with a square. If it isn't flat, use a special lapping stone that is harder than the bench stone to make it flat (shown at left). A diamond stone can be used to flatten an oil or water stone. That because it is a field of small industrial diamonds suspended in a binder such as nickle, and spread evenly on a very flat steel plate. The diamonds are harder than the sedimentary rock.

You can also use mechanical sharpeners to flatten the back of a chisel. An 8" soft friable stone traveling slowly (about 800 RPM) is a great choice when replacing the standard stone in a slow speed bench grinder. This larger diameter has 33% more surface than a 6" wheel, producing less heat and delivering a shallower hollow grind when using the edge. If the manufacturer allows it (and that's usually just on slow moving stones because a fast one may shatter), you'll be using the side of the stone to flatten the back.

Many grinding wheels lie horizontally, so the wide side of the stone is reachable (shown at right). For grinding bevels on the face, geometry would work against us here. That's because a spot located on the outside edge of a horizontal 8" diameter wheel travels about 25" during every revolution of the wheel. A spot located 3" from the center of the same wheel (that is, along an arc describing a 6" diameter) travels just under 19". So, in the same amount of time that the outside edge of the wheel makes one complete revolution, this inside spot is traveling at about 3⁄4 of that speed. If the bevel of a wide chisel is being sharpened on a horizontal wheel and it is held radially (that is, along an imaginary straight line between the center of the wheel and the perimeter), the outside corner of the cutting edge is being ground more than the inside edge. The ideal solution here would be to hold the chisel at a 90° angle to the radius, which is hard to do. So, most of us end up guessing on a compromise angle and applying more pressure to the trailing edge, which pretty much defeats the purpose of the tool rest. However, if you are just flattening the back of the chisel, that 90° angle is easy to hold and it works perfectly.  

If you need to remove a fair bit of material, begin lapping the back with the most aggressive stone available and work down through the grits to the finest available. A good selection of water stones is 1000, 4000 and 8000. Clean the area and chisel each time you change grits, as residue from a coarser grit can contaminate a finer stone and cause scratches. Using lots of water, hold the chisel flat on the surface so that about 1⁄2" or so rests on the stone. Slide the chisel side to side, checking visually after every few strokes. The surface should have the same scratch pattern across its full width when it has been lapped enough on each stone. Skewing the tool a couple of degrees in alternating directions as each stone is used leaves a more visible scratch pattern. Angle two or three degrees to the right on the coarse stone, a couple of degrees to the left on the medium stone, and then back to the right on the fine one.

  Establishing Bevels
Chisels usually come with just a large primary bevel. Woodworkers like to add a small secondary bevel (often called a micro-bevel, shown at left) at the tip, so that they don't have to grind the entire bevel every time the tool needs a minor touch-up: the secondary bevel is all that needs to be honed. This double bevel system is stronger than honing the entire chisel to a very fine point. Your chisel's primary bevel is usually set at the factory and it depends on the type of tool - paring (bench) chisels, firmers, mortisers, slicks, gouges and so on. The thinner the angle, the quicker an edge will break down. Most butt and bench chisels come with a 25 degree primary, and the new owner then adds a 2 to 5 degree secondary bevel.
Grind the primary bevel on a motorized stone, or use coarse diamond bench stones. Oilstones and water stones simply take too long. A slow, large stone is the best choice. Slow speed helps mitigate heat buildup, and the larger diameter produces less of a hollow grind. Stones are either direct drive (attached to an arbor on the motor), so they travel at whatever speed the motor is rated, or indirect, using pulleys (shown) to slow them down. To determine the speed of a pulley drive, divide the diameter of the motor pulley by the diameter of the grinder pulley and then multiply the result by the motor’s RPM rating. For example, a 1725 RPM motor set up with a 2.5" diameter pulley that is linked to a grindstone arbor equipped with a 6" diameter pulley will turn the stone at the ideal speed of 719 RPM. A stone spinning at 3450 RPM (the speed of most inexpensive bench grinders) is far too fast for sharpening tool steel. The ideal stone is a fine-grit, soft, friable white wheel (shown at right).  
  Charge a hard white cotton buffng wheel with an abrasive such as Flexcut’s Gold Polishing Compound. This is used to remove the wire burr created on the cutting edge by the grindstone. The grinder should have a tool-rest set to deliver a 25° bevel. Keep the chisel moving from side to side across the stone at all times to create an even bevel, and avoid heat buildup. Grind for a few seconds, and then dip the chisel in cool water to dissipate any heat. If a chisel turns blue at the tip, it is essentially ruined. By the time it turns visibly blue, it has already run through a complete spectrum of less visible colors as heat has gradually built up along the leading edge and worked up the chisel, drawing the temper from it. This means that the chisel is now too brittle to hold an edge. Two seconds on the stone and then a quick quenching in water is a sensible sequence. Honing won’t usually repair an error in grinding (unless one hones for several hours), so care must be taken to achieve an even, straight grind with no nicks.
To keep track of the way a stone is grinding a bevel, mark the steel with a permanent marker or machinist’s blue dye. As the edge is dressed, the ink is removed and low spots are easily seen.  
 

Once the primary bevel is ground, the secondary bevel can be established by honing on bench stones. Set a honing guide to 27° and work down through the grits to the finest stone available, using lots of lubricant. Don’t attempt to hone without a jig, as consistency is paramount here. Without a jig, one essentially ends up honing several differently angled bevels at the same time. (The one exception is when a woodworker uses Shapton ceramic stones and faithfully follows Harrelson Stanley’s instruction on the company’s DVD.) When changing from one grit to the next, briefly pass the chisel across a charged cotton buffing wheel to remove the wire burr created on the stone. By the time the last stone has done its work, the bevel will be mirror smooth and bright.

One can also hone an edge quickly on a 1"x 42" belt sander (to my mind, the 30" version seems to build up heat too quickly), using special honing belts available from Klingspor. Set the tool rest table to 27° and work down through the grits, barely touching each time and dipping in water between touches. These gold-colored belts come in 100, 120, 150, 180, 220, 320 and 400 grits.

(Adapted in part from guidelines in the book How To Choose And Use Bench Planes & Scrapers by John English,
published by Linden Publishing in 2010, ISBN 978-1-933502-29-8)


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