November 1, 2017 by SteelWolfKnives
Belt Grinding Styles
There are three major grinding techniques on a belt grinder. There is the flat grind (using the platen), the hollow grind (using a contact wheel), and free grinding (using the slack in a belt). All three are used on a fairly regular basis and all three have distinct advantages. However, the flat grind and hollow grind will be used to remove the bulk of the material.
The flat grind is what you would think of for a kitchen knife. The bevel of the knife starts at some point up the metal, and continues in a straight, flat direction until it hits the cutting edge. This is probably the most common kind of grinding, as it is used for fixed blades and folding knives alike. It is also easiest for starting blade smiths because it is easier to draw or imagine a straight line on the blade. A flat grind is perfect for adding weight to the front half of the knife, as well as for really high quality finishes on the metal.
The flat grind is accomplished by pressing the knife blank against the sanding belt, which is supported by a platen. A platen is a plate of steel on the end of the belt grinder that can support a section of the belt. It is one of the attachments used and can be changed out for different purposes. The platen is used for achieving a bevel, but when coupled with a tool rest (PICTURE), it is also often used to establish a profile for the knife. Having a ninety degree angle allows the bladesmith to create and detail the outline of the knife while keeping the entire piece square.
A hollow grind is created with a contact wheel. Often it is on a large radius (such as a 10”) wheel so that the curve of the grind is not as drastic. A hollow grind is great for lighter knives and sharper edges. Because the hollow grind removes more material than the flat grind if you have the same height bevel, it creates a lighter metal section of the knife, as you can see in the diagram below. The green area highlighted is the material that is removed with a hollow grind but left in with a flat grind.
This extra removal is perfect for when you want a heavier handle to move the balance point further back, as well as when you want to create a light weight knife for something like backpacking or running.
A hollow grind is also great for sharper edges, and is a standard grind for many different blades. It is considered sharper because the smallest part of the blade is a much bigger portion of the bevel. Because the bevel follows a curve instead of a straight line, the effective sharpness of the blade is increased. Unfortunately, this is counteracted by the blade being more fragile because there is less material in that same area. To make up for this, hollow ground blades are often made of harder material, then tempered down to keep from shattering.
Slack grinding does not use any support or backing. It instead uses the “slack” of the sanding belt, or the area between the platend/contact wheel and the support wheels. It is good for rounding and softening edges, as well as getting around some of the harder to reach areas before you hand sand. Because the slack lacks any hard support, it contours to the shape of the wood in a gentle curve, similar to using a small strip of sandpaper to round over a rough edge. This can be very useful in making the handle more comfortable to hold. This is especially good to couple with the feathered belt discussed in the third paragraph of this post.
Which is easiest?
Honestly, it depends on how experienced you are and on personal preference. Starting out it is easier to use a flat grind for three reasons. Firstly, likely you have fewer specialized tools as a novice. Most belt grinders include some straight, flat area even if it is not the ideal platen. Secondly, it is easier to imagine and to correct straight lines than it is to do so with a curve. Lastly, it is much easier to use a sanding block to hand sand a straight bevel than it is to get into the crevices of a curve. You can use a dowel and other tools to get into a curve, as well as progressively higher belts on the belt sander, but it is almost always possible to achieve a nicer finish on a custom blade by hand sanding to some degree.
As for experience, many more experienced blade smiths prefer a hollow grind. Once you have the bevel set, many people find it easier to keep the grind consistent with a hollow grind. You can set the wheel back in the grind and it “locks” into place because there is the indent. In a flat grind, there is nothing that keeps the knife in place except your hands, so it requires slightly more attention and concentration to keep the bevel consistent. That being said, the best kind of grind is the one that you want on that particular knife, and the one that you are comfortable grinding.