Tool Sharpening

SHARPENING

The sharpening of woodworking tools is probably the most fundamental skill an instrument builder or woodworker should possess. In the absence of this skill, accuracy, quality, and the speed with which the work is completed suffer greatly.

It is hoped that the following explanations and illustrations will furnish you with enough information so that you too can "shave with the blade." Like all skills one acquires though, lots of patience and practice are necessary requisites in developing these skills.

1. A chisel blade as it is received from a supplier is rather coarsely finished. It therefore must be ground to an appropriate shape and angle. With a try square, first check to see that the edge is square. If it's not, hold the blade on a tool rest at the level of the center of the grinding wheel and move it left and right (to wear the chisel and stone evenly).

Dip the blade in water frequently to keep the edge cool so it will not lose its temper. If the edge has nicks they must be removed by this same method. If you don't have a grinder, the chisel may be ground on a belt sander or by hand on a coarse stone. You will find that the chisel will move sideways if the edge is not square, making accurate work most difficult.

2. Next, the bottom of the chisel blade is honed so that it is flat. Use a coarse stone and grind away. This will take some time and will require a little patience and endurance. (If you lack these qualities, work on this a little extra each time you sharpen, so that through the course of a year the bottom will be approaching flat). When the bottom is flat the entire length of the cutting edge and for a length great enough so that the blade can be guided accurately, hone the bottom to the same finish as you will the bevel.

3. The blade bevel is then ground to an angle with respect to its purpose. The ultimate edge, regardless of purpose is one that is as keen as possible, with thin point and low sharpness angle, but that has enough metal behind it so that it does not chip, break, or bend.

A blunt bevel increases the strength and durability of the edge but requires greater force (which results in diminishing control) pushing the edge through the wood. A blunt bevel is approximately 2 times the thickness of the blade and amounts to a sharpness angle of about 30 degrees. Such a bevel should be used when doing rough, heavy work in hardwoods. Generally, chisels and plane blades are ground to a medium bevel, which is about 25 degrees for use in most woods, but for instrument building it is worth the time and effort to work for a fine edge. When a finely honed edge of less than 25 degrees is achieved, accuracy and ease of work are greatly facilitated.

The bevel length or angle you want is determined by the angle at which the blade contacts the grinding stone. You should devise a jig or tool rest so that between dipping the blade frequently for cooling and keeping the blade at exactly the same angle to the stone (while moving it right and left) little time is wasted.

When grinding make sure the wheel turns into the cutting edge as there will be less wire hair formed. Do not bear too hard against the stone or the chisel will lose its temper. (If a blue color appears while grinding, the temper is lost). Continue grinding until there is a concave bevel of the length you want (or a flat bevel if using a sanding belt or a bench stone) and a wire hair has been formed on the flat side along the entire length of the edge.

4. Whet the beveled surface of the blade by placing the bevel, with the heel of the bevel slightly raised, in contact with the stone. A back and forth motion is better than a circular motion as it is easier to maintain the correct angle, and a back and forth motion sharpens faster.

It is strongly suggested that an angle guide be used to insure that a correct and exact angle is maintained. (If you use our 5TH Eclipse honing guide, keep pressure on the front of the blade on the forward stroke, not on the wheel.) An angle guide also hastens the sharpening process enormously.

Use a fine sharpening stone such as the 1200 grit or finishing stone and polish both the bevel and the flat side of the blade until both surfaces shine the length of the cutting edge. The finer the stone you use the more polished the edge will become, and it will last longer, too. When whetting the flat side of the blade make sure it is kept perfectly flat so you don't inadvertently produce a bevel.

5. Now the blade is ready for a secondary bevel. This is best applied with a hard stone like a hard Arkansas bench stone or a triangular Arkansas file. A couple of strokes will do it. The secondary bevel should be about 5 degrees greater than the primary bevel and should be about .005". It will show as a bright reflection on the gray background of the honed bevel. The reasons for the secondary bevel are twofold: It increases the durability of the edge without greatly increasing the force required to cut since the change in chip formation is minimal. Also it takes only a few seconds to resharpen the secondary bevel (as opposed to resharpening the entire primary bevel).

6. For final polishing of the blade use an impregnated leather strop. If the blade is of very soft steel a substantial wire hair will develop that is probably best removed by rubbing the edge across the grain of a piece of hardwood. The hair should not be torn off as it will leave a dull or chipped cutting edge. If the blade is of good, hard, and tough steel the honing should have brought the blade to the point where the wire hair is ready to fall off, in which case it is removed in the stropping action. (See drawing) Impregnate the strop with rouge, or sapphire or diamond dust. Hold the blade flat, bear hard, and stroke away from the direction of cutting. Again, use only a few strokes as you will begin to round the edge with any more than a few. When the edge is sharp it won't reflect light, and when you draw your finger across the edge it should slide across and hardly be noticeable. If you can shave a chip off the top of your fingernail, then it's sharp. You should also be able to shave with the blade. (Electrolysis is a more permanent form of depilating, if it's hair removal you're after.)

The foregoing on sharpening procedures holds for plane blades as well.

If your plane has a cap iron make sure it is kept free of burrs and nicks so that chips don't wedge between the two irons and hamper the cutting action. Remove any burrs or nicks by grinding on a stone.

The shape of the plane blade cutting edge is determined by what it is to be used for:

Slightly rounded corners eliminate cutting hard lines at blade edges and assure a smooth surface.

The slightly curved edge speeds wood removal. Especially applicable on blades reserved for cleaning up rough sawn lumber, and other heavy duty applications.

Blades ground with a pronounced curve permit rapid removal of waste wood. (Primary angle may be ground at 30 degrees or even 35 degrees to strengthen edge).

Grind and hone perfectly square with sharp corners.

When roughing out with a gouge, if you grind the center of the edge back there will be less tearing out since the surface wood fibers are cut before those below the surface.

Most experts will agree that, as already stated, the ultimate blade edge, regardless of purpose (and for knives, as well as chisels and planes) is one that is as keen as possible, has an acute a sharpness angle as possible, but is supported well enough so that there is no chipping, bending or breaking. But from expert to expert, methods of achieving the sharp edge may differ. The important thing though is to get the blade sharp with whatever it takes and put it to good use.

One final suggestion: Keep a couple of chisels and knives of what you have found to be an indispensable size handy that have been sharpened at different angles: one for rough work and hard wood, and one for fine paring and soft woods. This will save much time and edge damage.

Text From The Luthiers' Mercantile Catalog - 1993