SHARPENING – Getting started with the right edge shape
As promised, we’re going to start talking about the number one Gateway skill, sharpening. I’m sure that all experienced woodworkers agree that there are three distinct steps to the sharpening process; shaping (grinding), honing and polishing. Today, we’re going to take a look at the first step, shaping the cutting edge. But there are several things to keep in mind as they’ll influence the way we look at the information. This presentation is based on sharpening edge tools such as chisels, plane irons and knives. The study of various cutting geometries has become a separate discipline within the realms of mechanical and industrial engineering. In order to keep this simple and understandable, we’re going to assume that every tool discussed today has a shaped edge angle of twenty-five (25)degrees. Over the years, toolmakers have settled on the notion that this is an angle that will provide the best performance in the majority of circumstances. However, you should understand that this is a compromise. There are two considerations that dictate this compromise, sharpness and durability. And one will always influence the other. Let’s get started.
Hollow grinding produces an edge that can be honed and polished to extraordinary sharpness in a very short time. The shape is created by grinding on a convex abrasive surface, such as a low or high speed grinder, slow-wet grinder or a foot powered wheel. The depth of the hollow is determined by the diameter of the abrasive media. Honing a hollow ground tool is made easier as you need only to ensure that both the “heel” and “toe” of the ground edge are in contact with the honing surface. The down side of the hollow ground edge is that it is the least durable of the three shapes. The hollow ground shape should be used in situations where there is very little or no impact on the tool, ie paring chisels and other shearing tools. The most exaggerated example of a delicate hollow ground tool is the straight razor, which in fact is “double hollow ground”. Woodwind players who use cane reeds will be familiar with the reed knife, which bears a striking resemblance to the razor (and is, indeed, a very simple tool to make). Anyone who has dropped either of these tools onto a hard surface will be able to testify as to just how fragile the edge can be.
The flat grind provides increased cutting edge durability as well as excellent sharpness. Bench chisels, plane irons and other edge tools that are subject to intermittent, low impact benefit from this shape. The flat grind can be created with coarse hand stones or by using any of the numerous “flat” grinders that are currently available.
The convex or rounded edge shape is the most difficult to create and to maintain. However, it offers superior durability and, when properly maintained, very good cutting capability. This edge shape should be used for tools that see high impact (i.e. mortising chisels) or prying activities (i.e. carving tools, drawknives).
Whenever possible, jigs, guides and toolrests should be used when shaping the cutting edge. A light touch at the grinder is an absolute necessity. Many an edge tool has been ruined by overheating. Keeping a finger in contact with the back of the tool while grinding will serve to notify you if you’re building up too much heat. When it feels hot, it should be cooled by dipping it in water. Another “trick” that will help minimize the possibility of overheating, is to rub paraffin or some other hard wax onto the the grinding wheel. As heat builds during the shaping process, the was begins to melt and draws heat away from the edge being ground. The “wax melt” method can be very useful, especially when large amounts of stock must be removed, but be prepared to deal with the clean-up. It can be a little messy.
Okay, so now you should be able to select the edge shape you want for a particular tool. After we’ve created the shaped edge, we’ll have to hone it. We’ll talk about honing and polishing over the next few days. And remember, “your best work can only be done with sharp tools”.