Sharpening Beveled Blades With Abrasive Sheets

by Spencer Hochstetler and Bill Tindall

Summary

The requirements to achieve a quick, sharp edge are well established. First, the initial grit size must be coarse enough to rapidly establish a new edge. The most common failure in sharpening is failure to remove enough material to establish a fresh edge. To rapidly proceed to a sharp polished edge the grit sizes must be reduced in small increments. One or two bench stone sizes can not meet these requirements. The least cost means to meet these requirements is sharpening with cheap abrasive sheets. A complete set of water stones or diamond stones also meets these requirements, but at a higher initial cost.

Whether a novice or an experienced woodworker, you may find that tool sharpening is one of the most frustrating tasks in all of woodworking. Many people who make the attempt and use the tool with a sub-optimal edge sometimes "get by", while others may yield to frustration and give up on the idea of using hand tools altogether. However, a properly sharpened hand tool can lead to unimagined speed and enjoyment of woodworking. The purpose of this article is to provide straight-forward instructions for sharpening tools with flat bevels, such as chisels and plane irons, and guide the sharpener in the choice of sharpening equipment. The use of jigs and honing techniques using affordable, commonly available abrasive sheets will be discussed.

Definition of Sharpness and General Overview

Basically, an object is sharp when two geometric planes intersect with relatively little radius at the line of intersection. For woodworking, the angle at which the two planes intersect is important. If the angle is too small, the blade will require less cutting force, but the edge will crumble under the cutting forces. If too large, the edge will be very sturdy, but will require a cutting force that makes using the tool difficult or impossible. For blades such as plane irons and chisels, the basic shape is shown in Figure 1A. Figure 1B shows the difference between a sharp edge (near-zero radius) and a dull edge (significant radius). The two geometric planes that are flattened during the sharpening process are the bevel and face. The angle that includes the bevel and face is called the bevel angle. The appropriate bevel angle depends on how the tool will be used, as well as the properties of the metal. In general, the smallest bevel angle that will hold up is best. For plane irons, a bevel angle of 25° is a good starting point. For chisels sharpened for general use in hardwoods 25 to 30° bevel angles are common. Mortise chisels require bevel angles closer to 35° for durability. Additional information on selecting bevel angles and most everything else about sharpening various tools can be found in an excellent sharpening book, The Complete Guide to Sharpening, by Leonard Lee.

Traditionally, sharpening was done with stones. Abrasive sheet sharpening is a faster and cheaper alternative to stones. (Because the abrasives that are glued to abrasive sheets consist of a variety of synthetic materials rather than sand, we prefer the term "abrasive sheet" rather than sandpaper.) The main goal of this article is to instruct how to make the bevel intersect with the face with near zero radius using abrasive sheets. Although the techniques are the same that would be used with stones, abrasive sheet sharpening has several advantages over sharpening with stones. First, abrasive paper is cheap and readily available in a variety of grit sizes and materials. Therefore, it is practical to use the optimum selection of grits to achieve sharpening and optimizing grit selection results in very fast sharpening. In addition, rolls of abrasive enable relatively long lengths of abrasive to be used, particularly in comparison to the relatively short water stones available. This factor is particularly important when using a sharpening jig, because the jig occupies several inches of the abrasive surface. When using a jig the effective length of a water stone can be as little as 5". A 13" length of abrasive sheet doubles this effective length. In either stone or abrasive sheet sharpening the abrasive surface must be flat to achieve a sharp straight cutting edge. A third advantage of flexible abrasive sheets is that they only need to be affixed to flat surface to be used. Worn abrasive sheets are simply rem oved from the flat surface and thrown away. Conversely, a stone will wear and develop curvature with use and must be flattened regularly to yield a flat sharpening surface. Cost and speed are final considerations. With proper selection of grit sizes, metal removal progressing from a rough to a mirror-smooth surface is accomplished far faster with abrasive sheets than with a series of stones. Why this is so will be discussed later. Start-up costs are also significantly lower with abrasive sheet-sharpening. A good set of Japanese water stones can cost as much as several hundred dollars while with some creativity, abrasive sheet sharpening can cost as little as a few dollars to get started.

The method for using abrasive sheets for sharpening has been made popular on the internet www.shavings.net/scary.php and some refer to the method as the "Scary Sharp" system, even though sharpening with abrasive sheets has been used years before the internet. Unlike the original posting of the Scary Sharp method, this article also aims to provide information on selecting grit size and composition as well as descriptions of various equipment to use with abrasive sheets.

Sharpening Jigs

sharpening jig is a great aid to quick and consistent sharpening regardless of the abrasive format used. A few examples of sharpening jigs can be seen in Figure 2. In our opinion, the Lee Valley "Veritas Sharpening System" is definitely worth the $30 price tag. The Veritas jig also helps with aligning the blade edge perpendicular to the honing guide motion. The angle jig, which is a pentagon that can be turned to allow blade-jigging at commonly used sharpening angles, allows one to easily and repeatedly find the same angle when resharpening, however a home made jig provides more accurate setting. Another type of jig clamps the blade from the sides. It is more difficult to align the blade in this jig and the smaller roller makes it more difficult to keep the blade aligned with the abrasive during sharpening. However, this jig can be used to sharpen spoke shave blades.

Flat Surfaces

A flat, rigid surface is an essential component of abrasive sheet sharpening. Flatness to within 0.001 to 0.002" over a foot is acceptable. Further, to minimize the number of strokes one must use on a particular grit, a minimum of a twelve inch length is best. Glass, polished marble tile, Corian, granite surface plates, and, in a pinch, shop machinery beds are all excellent choices for flat surfaces. Glass plates should be at least 3/8" thick to minimize flexing. Occasionally, home improvement stores like Lowes or Home Depot will have close-out sales on 12" x 12" polished marble floor tiles. Bringing a good straight edge, such as a Starrett combination square, to the store can help you select sufficiently flat tiles. By placing the straight edge on edge orthogonally and diagonally across the tile and holding the tile up to the light, you can determine which tiles are flat by looking for light shining through the straight edge and the tile. Tiles with light shining through are rejected. Corian, a synthetic material commonly used in kitchen countertops, has also been used. It is machinable with shop tools and some people have gone so far as to drill an array of holes in a Corian slab, then mount the slab in a box that has a vacuum port on it to make an inverse plenum. When the vacuum is turned on, suction generated at the holes in the Corian is used to hold the abrasive sheets onto the surface. Machinists frequen tly use granite surface plates for layout and jigging in metalworking. Tool room, or Grade B, 12" x 18" surface plates with overall accuracies of ±0.0001" can be purchased for about $30 plus shipping (~$35). While this surface is relatively expensive, it is very durable and could be used for other things that may require an exceptionally flat surface. Finally, a flat machinery surface can be used for experimenting. Jointer beds and iron table saw tops are good surfaces to use, but not very convenient for a long-term sharpening setup. Ideally, one would like to have the whole series of abrasive sheets attached to a readily accessible surface so that touch-ups are quick and easy. We find that a 12" x 12" surface which accommodates 3 to 4 strips of abrasive most convenient.

Abrasive Sheets – The Nitty Gritty

of the information below came from conversations with abrasive specialists at three major abrasive companies. While these companies do not conduct research on sharpening, they do study smoothing and polishing metals. Making the assumption that smoothing and polishing are functionally the same as sharpening, in that two surfaces are smoothed and polished in sharpening, the Sheets – The Nitty Gritty Much of the information below came from conversations with abrasive specialists at three major abrasive companies. While these companies do not conduct research on sharpening, they do study smoothing and polishing metals. Making the assumption that smoothing and polishing are functionally the same as sharpening, in that two surfaces are smoothed and polished in sharpening, the following information can be used to make the best choices in buying abrasive for sharpening.