What is Sharp?

A Sharpening Handbook: What Every Woodworker Needs to Know About Sharpening

If there’s one thing to be said about woodworkers, it’s that we’re old school and prefer to pick up a book (or a magazine). With that in mind, I’m thankful for the community of master woodworkers that put forth a great effort to put some of their knowledge down into the pages of a book to share with all. Here is a small sample of a recently published book that I believe you will enjoy as much as we did.

Book: A Sharpening Handbook: What Every Woodworker Needs to Know About Sharpening Shop Now

Author: Richard D Wile

What is Sharp? Ask a dozen people what sharp means and you will get numerous answers; all may be valid, and all based upon their individual frames of reference from their own experience. A common comparison is made to a razor, such as “razor sharp” or “sharp as a razor,” evoking an image of the slicing action of a razor and, for certain generations, that sight of the barber honing the straight razor on the leather strop.

We sharpen an implement to prepare it for a specific task. Abraham Lincoln has been quoted as saying something like, “Give me six hours to chop down a tree, and I will spend the first four sharpening my axe.” You can be certain that while Abe may have spent that much time to sharpen his axe, it would still not be as sharp as the barber’s straight razor, but both would be suited to the task they are designed for. I am not really sure what one could do for four hours to sharpen an axe.

Much has been written about the science of sharp, and many books are devoted to the topic; however, the notion of “sharp enough” is key for woodworkers to understand what they need to do to prepare their tools for use. We have all seen the wood show trick of a plane sharpened to 30,000 grit producing micron-thick wispy shavings of amazing lengths. While very cool, this is not practical, nor useful for most woodworkers.

Sharp is defined as the intersection of two planes at zero radius—well, what does that mean? In practical terms the “pointier” the edge, the sharper it is as it approaches the mythical razor’s edge, which in most cases is just a few atoms wide. But to really understand sharp we must discuss purpose, or what the edge will be used for, before determining if it is sharp enough.

Let’s Talk About Bevel Angles…

When discussing sharpness, one most consider the shape of the bevel, which is heavily influenced by the intended us of the edge. The angle at which the surfaces meet to form the edge of a tool is matched to the tool’s intended purpose. Whether it is a knife, an axe, or a woodworking tool, the geometry of the edge is tailored to its intended use.

In a knife suited to slicing meat or fish, the blade is quite thin and the angle of the edge is as low as 10° per side (20° inclusive angle), while a chopping knife such as a chef’s knife will be thicker and the angle of the blade higher, often in the 20° per side range for edge durability. Kitchen tools designed for chopping or cutting bones, such as a cleaver, are usually 25-30° per side, making the edge very durable.

Axes also have edges of varying shapes and angles depending upon their intended purpose. A hatchet designed for carving or shaping has a thinner blade and is sharpened to a shallow angle, sometimes as low as 20° per side so it can be used as a precision tool, while splitting axes have angles above 25° per side, often a convex shape. The high angle, blade shape, and heavy weight help the splitting axe drive the wood apart and remain sharp while doing so.

Much of the same as other tools such as knives and axes, woodworking tools have varying bevel angles which are adapted to the use of the tool. Most woodworking tools have a single bevel and a flat back, except carving and turning tools, which come in a huge variety of shapes and sizes.

So why not make all tool edges with the same angle to cut the easiest? In a word, durability— the lower the angle of the edge, the thinner the material is near the tip and the more susceptible it is to wear or damage. We know how great a sharp razor feels when shaving, yet it often does not take much use before we feel the pull of a dull blade that needs replacing. This is because a razor blade is very thin; the angle is very low, perhaps as low as 7-8° making a delicate edge that is easily worn in use. Most woodworkers would rather spend time woodworking than sharpening their tools, so the blade angle is often a compromise to ensure the edge is durable enough to be used without frequent re-sharpening. Wood is an abrasive material that will wear down the edge of a blade in use, so we want an edge that will perform the task, yet remain sharp for a reasonable time.

Chisels and plane irons with a relatively low angle of 20° are designed for paring or slicing operations and act most like a knife in slicing the wood fibers. These chisels should never be struck with a mallet. Paring chisels or low-angle block planes designed to pare end grain are often set with a 20° bevel. This angle is ideal for paring or slicing operations in softer woods, but may not be durable enough for hardwoods or hard exotics, necessitating a step up to 25° or more to protect the edge. The most common angles for woodworking tools are 25° and 30°, as the best compromise for slicing wood fibers and edge retention. Bench chisels, spokeshaves, and many bench planes will mostly use a 25° or 30° angle for the primary bevel. Higher angles exist in woodworking tools in special cases where required.  Mortise chisels are often ground to a very durable 30° or 35° angle, because they are less about cutting and more about wedging material out when struck by hammer blows, much like an axe.

The Angle the Blade is Held at Matters too…

The angle the blade is held at also has an impact on performance, measures from the material (wood) to the back of the blade. The effective angle in this case is the sum of the angle from the workpiece to the back of the blade PLUS the angle of the bevel at the cutting edge. When a chisel is used for paring, the angle between blade back and workpiece is close to zero degrees, as the back of the blade rides against the material with a slicing action, as with a knife, to pare away the material in thin slices. In many cases the paring chisel is honed to 20° (effective angle 20°) which emulates a knife’s slicing action even more.

Low-angle planes have become popular again, styled after some of the old-style planes that used this bedding angle. This style of plane has a few advantages, mostly relative to the orientation of the blade, which has the bevel facing up. With the blade completely supported by the plane body, the blade will stay bedded and not move or chatter in use. The angle of the cut can easily be varied by changing the bevel of the blade, allowing the user to tailor the set-up to the task at hand. Bevel down planes, on the other hand, have the angle to the work set by the plane. With a standard angle or pitch of 45°, regardless of bevel angle the effective angle will always be 45°. In a bevel down plane the bevel is unsupported, so in difficult material the blade can chatter in use. A chip breaker is often used to stabilize the blade and to direct the chips or shavings away from the blade.

The blade setup, either bevel-up or bevel down, does not impact how a blade is sharpened. What constitutes a sharp blade and how to achieve it is unaffected by which way it sits in the plane.

Many good resources exist that discuss and debate the bevel and bedding angles of plane irons. Explore these if you wish to change the configuration of a tool to fine-tune it for your own use. Most tools comes with the angles set for optimal use, so most woodworkers will simply go with that.

How Sharp is Sharp Enough?

The angle of the blade determines its suitability for a given task (paring, cutting, chopping). How keen the edge is influences how well the blade performs, and how long it will perform the task before needing to be resharpened. The keenness of the edge is defined by a couple of factors, the fineness of the abrasive grit used to sharpen and the type of steel in the tool. Both of these are explored in detail in future chapters, but generally the use of finer grits of abrasive sharpening media will produce a keener edge. For example, a 10,000 grit stone will produce an edge that is keener (sharper) than an edge sharpened with a 5,000 grit stone. However, much the same as the trade-off with blade angles, what grit to use when honing an edge should be dictated by the anticipated use. A blade’s edge will degrade in use as the wood abrades the edge, so only honing or polishing the edge to the degree necessary for the task will ensure an edge that will last a reasonable amount of time before needing another honing.

Generally speaking, a blade that is considered sharp enough for general woodworking, carpentry, and home improvement tasks would be sharpened on a fine stone (2,000-5,000 grit), while a blade for fine woodworking would be finished on an extra-fine stone (8,000-12,000). Blades for special purposes such as final smoothing or working difficult material may require honing on an extra-fine stone, up to 30,000 grit. See more on choosing the right grits for a given task later in the book.

So What Do I Need to Know?

Sharp is defined as the intersection of two surfaces at a point (zero radius).These surfaces converge at an angle suited to the task the edge is designed to perform. Woodworking tools such as chisels or plane irons mostly have a flat back and a bevel angle of 20° and 50°; exceptions do exist, but this covers most situations the average woodworker will encounter. Decisions on the angle of the blade and keenness to which a blade is prepared are based upon the needs of the task at home. A block plane to trim a pine door casing to fit does not have the same requirements as that same block plane used to trim maple end grain on a dovetailed cabinet project. The following chapters will provide the guidance and examples necessary to help woodworkers get their tools sharp enough. 

All of the topics covered in the book:

1. Why Worry If My Tools Are Sharp?

2. What is Sharp?

3. Steel and Metallurgy: What is steel; Hardness

4. Basics of Sharpening: 4 steps to a sharp edge; Stropping

5. Abrasives and Sharpening MediaAbrasive grits (including grit size); Abrasive media options (including film, paper, oil stones, water stones, ceramic water stones, and diamond stones)

6. Basic Knife Sharpening Basic knife geometry; Abrasive media options for knives; knife blade bevels; basic knife sharpening with stones

7. Card Scraper Sharpening What happens when you prepare the edge of a scraper?; Prepare the sides and edge; Burnish the face and edge; Tilt the Burnisher; Use the scraper

8. Basic Carving Tool Honing Basic carving tool shapes; sharpening stage; honing or stropping stage, maintaining the edges

9. Tips & Tricks Marking knives; Ruler trick; Hollow grind; Removing a nick in an edge; Cambering a Blade

A Final Word from the Author

A need for sharp tools is one of the truisms of woodworking; using tools that have not been properly prepared by sharpening can make woodworking a frustrating craft. My principal objective with this book is to provide a simple and repeatable methods that will get your tools sharp, backed by a solid foundation of understanding. Of course, these are not the only methods that will work, but if you are looking for improvement to your sharpening workflow, these methods should help.

Woodworking technology and techniques have evolved over time, and so have sharpening aids and media choices. Increasingly, expertise has been engineered into guides and jigs, making it easier to achieve sharp edges efficiently and with accuracy. Indeed, efficiency and repeatability, both key aspects of a successful sharpening workflow, are much easier to achieve today with an open-minded approach to this new technology.

I encourage you to try these approaches and techniques and evaluate how they can be incorporated into your own woodworking. As you develop along your own sharpening journey, I trust the tips and insight from this book will increase your enjoyment of the craft. -Richard D Wile