Freehand honing and Hollow grinds
In other places in these pages I consider the problem of freehand honing. Generally I argue that freehand honing produces inferior edges.
In one very special case, freehand honing works. That case is the straight razor. This is the only tool I know of which has its sharpening jig built in - the spine. During honing, the razor rests with the spine and the edge resting on the abrasive. The edge-to-spine distance and the spine thickness determine the included angle at the edge.
Why does this freehand honing work for straight razors but not for other tools?
Because the tool includes its own honing jig!
Straight Razor angles
The typical geometry of a straight razor:
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Distance edge-to-spine is 3 to 5 times the spine thickness [5 to 1 in the model]. In the model the included angle at the edge is very small, much smaller than you would use. Once the model is honed, this angle will increase. As usual, the primary bevel, the angle we use to form the tool before honing, is less than the final honed angle. The total included angle here, before honing, is 5.7 degrees.
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If you hone with the razor spine and edge resting on the abrasive, the included angle at the edge is determined by this ratio. It varies from an included angle of only 8.6 degrees for the 5 to 1 ratio to an included angle of 14.25 for the 3 to 1 ratio. [More later on how to decrease the ratio and thus increase the included angle.] [The brown is the strop!]
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Straight razors are not usually made from rectangular steel stock. They are forged from hot steel in a press to roughly the final shape. The shaping is finished by a series of grinding steps - some dry grinding, some wet - with different sized wheels.
The hollow is not a single curve. Different radius wheels are used - in pairs - to produce the various curves required. This way each wheel serves as the tool rest for the other wheel. The wheels used to finish the part of the hollow nearest the spine have the smallest radius. The wheels spin in opposite directions under a water drip.
The finisher is able to vary the gap between the stones, working different parts of the hollow grind.
Other wheels put the characteristic finish on the hollows, depending on model.
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At the factory, honing is done in three steps. First, a light hone using the side of a large horizontal honing wheel, done dry. This is a preliminary honing required to remove the marks of the earlier steps. This is not required once the edge has been prepared for the first time.
Second, a water stone. The stone looks like a natural stone of European origin called a Coticule stone. The razor if flat on the stone, spine and edge touching. Alternating sides to ensure an even hone, moving the razor in the direction of the edge along the stone.
Third, a clean leather strop. Again, alternating sides but here moving the razor in the direction of the spine along the strop (to avoid catches).
You Don't Grind a Straight Razor
Owners don't grind their razors, they just hone them. Most hand hone with no machine assist, so the honing operation generates no heat. These blades are so thin that any hot grinding operation would quickly over-heat the edge and draw the temper there.
The honed bevel is very small
Generally, the rate of material removal drops with abrasive grit size. Very fine abrasives remove material very slowly.
Very fine abrasives can remove the damage left by the coarser abrasive only if the area being honed is very small. The straight razor honed bevel width at the edge is very small.
Hollow Grinding
A hollow grind is a concave bevel formed when you grind a tool on a convex abrasive. The most common convex abrasive is a grinding wheel. In fact, for most of the history of edge tools a grinding wheel was just about the only option for grinding.
If you have no choice, then a hollow grind will do.
You do have a choice. My pages on grinding with a belt sander and grinding with a bench stone show you two options. Those pages discuss how to grind and explain why each is better than using a grinding wheel.
There are several problems with grinding wheels:
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Lousy tool rests - Grinders are designed for engineering shops that want to hog off metal and are not too interested in angles. Sharpeners are interested in angles, so need a tool rest that can be set to an angle, holds the angle, and is easy to use. Most people end up buying an after-market tool rest and then spending a few days bolting their grinder to the bench and bolting their new tool rest to the bench. Lee Valley sells such a set up for $69. You can make a very good grinder tool rest from Acrylic for a few dollars in an hour or two.
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Setting the angle - it is very hard to set almost any tool rest accurately when working against a curved surface. My grinder page has some tips for setting the grinder tool rest angle.
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Heat - The sparks you see when grinding are burning bits of iron oxide that are at least 600 �C (1,200 �F). The surface speed of a grinding wheel is from 1.7 times (6" wheel) to 2.3 times (8" wheel) that of a belt sander. The heat generated during grinding is exponential in abrasive particle speed. The chances of overheating your tool (and ruining the edge quality) are much higher with a power grinder.
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Speed - Wheels remove metal so quickly that it is easy to take off too much. Except in unusual circumstances, taking off to much means the hollow reaches the edge.
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