UNDERSTANDING SHARPNESS - BACKGROUND
Methods of evaluating sharpness of knives include cutting paper held on edge, cutting the hair off your arm, seeing if the edge will "grab" your thumb nail or just rubbing your thumb over the edge. Subjective tests like these rely on technique and the interpretation of the results by the tester.
There are a few expensive sharpness testing instruments on the market today that give good reproduciable results and are not subjective, but they are very expensive and out of the reach of many of us.
Cutting a piece of paper on the edge with a knife is probably the most widely used test method. Once you get the technique mastered you need to evaluate the resulting and make a judgement call. Did it cut cleanly? Did I use the least amount of force? Is is sharp enough for the entended purpose? With a little practice one can get very good at this and the paper test is a good test. The problem how to record the results so you can repeat so you can get the same results when you sharpen another knife.
Sharpness is affected by lots of things and one should be at least aware of them and take them under consideration when testing sharpness and insuring that you have put the best edge on the knife for its intended purpose.
Item Being Cut
The item being cut obviously plays a big part in evaluating sharpness. It requires less force to cut bologna then pepperoni. It is important to have a good idea of what the intended use of the knife will be. that is to say what it will be cutting. This will allow the sharpener to tailor the sharpening to fit the need.
Side Forces Resistance
The item being cut obviously plays a big part in evaluating sharpness. It requires less force to cut bologna then pepperoni.
Where the item is being cut
Also, where it is being cut also affects the force needed to cut . If you cut a slice of either one of these, the force is much less than if you are attempting to cut it in the middle. The side force, which is the meat in this case, is pushing back on the side of the knife. This force is almost non existent as the knife cuts a thin slice off the end, but is fairly larger when you are trying to cut the meat in half.
Cutting in a confined space
Cutting in a confined space, or with the movement of the material being restricted, will require more force to cut the object. If the material is free to move less force is required It would require a lot more force to cut a ham that is still in its can than if you remove the ham form the can.
Wedge Effect
Effect of the Shape of the Blade from Spine to Edge
A wedge is simply 2 inclined planes that meet to form an edge
The force that is needed to cut through an object is affected by the slope of the side of the knife which is referred to as the wedge slope. A forged knife is usually thicker at the spine and tapers down to the edge forming a wedge.
The spot where the two planes meet, the knife edge, is pushed into the test substance with sufficient force to overcome the resistance of materials and to separate the material. The steeper the slope sides of the wedge, the more force it will take to cut the material. In the diagram below. if both knives are pushed down the same distance, the thinner blade has to move side A & B a smaller amount than the thicker blade. So for the same depth cut, the thinner blade will require less force.
The edge affect number is a calculation based on the slope of the incline plains and the thckeness of the blade
Method of Cutting
Slicing Cut vs. Chopping or Push cutting
Slicing requires much less force than just a simple push cut. The edge of a piece of paper is not very sharp and you would find it almost impossible to push the edge of a piece of paper into anything. Now take that same piece of paper and run it along your finger in a slicing motion and you can get a paper cut.
Angle of Blade
The Angle the Blade is held in Relationship to the Material Being Cut
The position you hold the knife affects how well it cuts. If you push directly down on a object with the blade held parallel to the object, the required force is high. If you do the same thing but hold the knife as an slight angle, the cut force is greatly reduce. Angling the knife causes the blade to act like it is being used in a slicing action. This is how scissors work, except with scissors the angle is constantly changing for large to small and the least little bit being a push cut.
Bevel Angle
The bevel angle is just a very small wedge. It is usually less than 1/4 inch wide. As with any wedge, the smaller the angle the easier it is to move through the material.
Final Honing Grit Size
Which final grit size should be used is very much dependent on what the knife is being used for. The end use of the knife must be taken into consideration when determining what final grit should be used. The smaller the grit, the smoother the edge, so it may be easier to push cut through material. Courser grit leaves larger sharpening lines know as micro serrations on the edge which can act like small teeth to grip the surface of some materials or food which will ultimately require less force to cut the object.